System for quantum communication using entangled photons

Abstract

The invention relates to a system (1) implementing quantum communication, comprising: • an emitter of entangled photons (2) comprising a source configured to generate at least one pair of entangled photons comprising a first photon (P1) emitted on a first propagation path (D1) and a second photon (P2) emitted simultaneously on a second propagation path different from the first propagation path; • a first receiver (3) arranged on the first propagation path (D1) and comprising a composite absorber (31) configured to absorb photons in a polarization state chosen from at least two different complementary pairs of states; • a second receiver (4) arranged on the second propagation path (D2) and comprising: - an optical amplifier (40) for multiplying the second photon (P2) while preserving the polarization of the second photon, and a measurement instrument (45) for measuring the average polarization of the multiplied photon (P20).

Description

Technical Field

[0001] This invention relates to quantum communication methods, and more particularly to quantum communication methods using entangled photon pairs. Background Technology

[0002] Currently, information transmission is primarily carried out using electromagnetic waves, regardless of whether the electromagnetic waves are short wavelengths (such as light) or long wavelengths (such as VHF waves).

[0003] The transmitted waves can be guided by metal cables or optical fibers, or they can be transmitted through space.

[0004] These methods use the propagation of waves to transmit energy, thereby transmitting physical particles collected by a receiver that enable the inference of information terms, which are encoded, for example, by their wavelengths or by modulation of the duration of the wave train.

[0005] Recently, a so-called "quantum" communication method has been developed.

[0006] For example, the physical state of a photon, such as its polarization, can be used to transmit information, such as through optical fibers. Bit values ​​are arbitrarily assigned to the direction or polarization mode of a photon. A sequence of polarized photons can then be sent to transmit a binary sequence that forms a message.

[0007] Within the realm of quantum cryptography, the use of entangled photon pairs for secure information transmission is particularly well-known. Entangled photons are those whose quantum states, such as their polarization, are interdependent regardless of the distance between them.

[0008] This quantum entanglement phenomenon has been demonstrated by numerous observations and experiments, such as in the article "Two-photon quantum interference and entanglement at 2.1 μm" by Shasi Prabkhar et al. (Sci Adv, 2020).

[0009] Patent KR101003886B1 describes a system for transmitting encrypted information in which entangled photons are emitted simultaneously, each photon directed toward two targets located at different locations who wish to communicate with each other. The photons received by the two targets form an entangled random sequence of conjugate bits, which is used as the encryption key. This system can receive the same random number sequence almost instantaneously at two different locations, but it cannot transmit information.

[0010] Wang's paper, "Superluminal telecommunication: an observable contradiction between quantum entanglement and relativistic causality," discloses a quantum communication system that can transmit information faster than the speed of light by sending entangled photon pairs toward two receivers that wish to communicate with each other. The photons are entangled, and the linear polarization of the photons arriving at the first receiver along different directions at 45° to each other is measured. At the second receiver, the second photon is multiplied, and the average polarization of the multiplied flux is measured to determine the direction of the first photon.

[0011] However, the polarization of a photon is not necessarily binary, just like the polarization of spin, or even quaternary. It can be specifically represented on a Jones sphere, which characterizes the orientation and ellipticity of polarization: specifically, polarization can be linear, where the electric field is always parallel to an axis perpendicular to the direction of photon propagation; or polarization can be circular, where the electric field rotates around that axis; or polarization can be somewhere in between: the electric field describes an ellipse around the propagation axis. In the case where polarization is not circular, orientation measures the direction of the axis, and eccentricity measures the flatness of the ellipse.

[0012] The aforementioned devices cannot utilize all configurations of polarization, and they are in fact limited in terms of the number and type of information items that can be transmitted.

[0013] Further improvements are needed in quantum communication systems and methods, particularly those capable of transmitting information items, especially continuous values, rapidly, quasi-instantaneously, without delay due to transmission distance between two points. Summary of the Invention

[0014] The present invention aims to meet this need, and according to its first aspect, achieves this by using a quantum communication system comprising:

[0015] • An entangled photon emitter, the emitter comprising a source configured to generate at least one pair of entangled photons, the at least one pair of entangled photons comprising a first photon emitted on a first propagation path and a second photon emitted simultaneously on a second propagation path different from the first propagation path;

[0016] • A first receiver, arranged on the first propagation path, and including a composite absorber configured to absorb photons in a polarization state selected from at least two different complementary polarization state pairs, except for exactly two perpendicular linear polarization pairs, in which the polarization of one pair is at 45° to the polarization direction of the other pair.

[0017] • A second receiver, arranged in the second propagation path, such that after the first photon reaches the first receiver, the second photon reaches the second receiver, the second receiver comprising:

[0018] - An optical amplifier capable of multiplying the second photon while maintaining its polarization, and

[0019] - A measuring instrument arranged downstream of the amplifier, the measuring instrument being capable of measuring the average polarization of the multiplied photons.

[0020] The system according to the invention is arranged to transmit a series of continuous or discrete values ​​quasi-instantaneously between two locations.

[0021] Because the first and second photons are entangled, the instantaneous absorption of the first photon at the first receiver determines the polarization of the second photon, especially before the second photon reaches the second receiver.

[0022] Then, by measuring the average polarization of the photon after it has been multiplied at the second receiver, the polarization state of the absorbed first photon can be detected, and the transmitted information can be inferred from there.

[0023] Therefore, the system is able to transmit information without any delay, regardless of the distance between the two locations that wish to communicate, and by using only photons, it requires almost no energy.

[0024] For example, the system is used for communication and computer networks on Earth, in the sky and / or in space, especially for communicating with systems far from Earth, such as satellites or spacecraft.

[0025] Jones polarization

[0026] The term "complementary polarization state," also referred to below as "complementary absorption polarization state," refers to two polarization states of light in which the polarization of light is modified by linear optical elements, particularly mirrors, birefringent plates, and / or prisms (including quarter-wave plates). For one of these states, light is allowed to be absorbed by a first polarization filter, while for the other, light is allowed to be absorbed by a second filter whose polarization is orthogonal to the polarization that the first filter can absorb.

[0027] In the following text, the terms “polarization state” and “polarization” may be used interchangeably.

[0028] The first receiver is advantageously arranged such that its user can select the polarization state of the absorbed photon from any possible polarization defined by the Jones form, and in particular from a set of elliptical (also called “ellipsoidal”) polarizations characterized by the orientation of polarization and ellipticity.

[0029] The polarization of the photon absorbed by the first receiver can thus be changed in order to transmit a series of information items to the second receiver.

[0030] For example, by selecting 10 different polarization directions, such as those spaced 9° apart from each other, and 21 different phase shifts, also spaced 9° apart, 210 different polarization states can be defined, thereby enabling the transmission of 210 different signals.

[0031] For example, for each pair of entangled photons arriving at the receiver, letters, numbers, and a number of other special characters from the uppercase or lowercase alphabet can be transmitted.

[0032] Composite absorber

[0033] The term "composite absorber" describes the component that receives the first photon, which can determine a complementary polarization pair from a number of complementary polarization pairs and absorb the first photon in one of the two polarization states of the predetermined polarization pair.

[0034] Preferably, the composite absorber of the first receiver is configured to absorb photons in a state of a predetermined complementary polarization pair, said state being selected from the states of at least three different complementary polarization pairs, and preferably from the states of a plurality of different complementary polarization pairs selected according to the number of different values ​​to be transmitted.

[0035] Preferably, the composite absorber includes:

[0036] - At least one absorption instrument, said at least one absorption instrument being capable of absorbing photons in one or the other of two states of a pair of complementary polarizations (e.g., two orthogonal linear polarizations).

[0037] - A polarization modifier, which is disposed upstream of the absorption instrument and configured to convert the polarization of the first photon to a selected polarization along which the absorption instrument absorbs the photon.

[0038] The term "selected polarization" refers to a pair of complementary polarizations predetermined based on the information to be transmitted.

[0039] Polarization modifier

[0040] The linear polarization of a photon can be converted into ellipsoidal polarization as represented by Jones form by first modifying the polarization orientation of the photon with linear polarization and a known direction, thereby distributing an electric field in a predetermined manner between the axis x and the axis y perpendicular to the axis x, and secondly by modifying the phase of the electric field along one of the two perpendicular directions (e.g., the axis y).

[0041] The polarization modifier preferably includes a polarization direction modifier disposed upstream of the polarization phase modifier.

[0042] For example, if θ is the rotation angle of linear polarization, It is the phase shift along the y-axis. If ω is the angular frequency of the wave, then the electric field components after rotation... and for:

[0043]

[0044]

[0045] And after the phase shift:

[0046]

[0047]

[0048] Polarization direction modifier

[0049] The linear polarization direction can be modified in various ways, such as by a half-wave plate or alternatively by a double quarter-wave plate.

[0050] The polarization direction modifier may include two quarter-wave plates arranged one after another along the propagation path of the first photon, at least one of which has a variable orientation.

[0051] The first quarter-wave plate, for example, modifies linear polarization to circular polarization, and the second quarter-wave plate, for example, converts circular polarization to linear polarization, the direction of which is oriented along an axis that depends on the axis of the second quarter-wave plate.

[0052] Therefore, rotation of the axis of the second quarter-wave plate can modify the direction of linear polarization of photons. If the wave plate is a half-wave plate, rotation of the half-wave plate can modify the direction of linear polarization of photons.

[0053] Alternatively, the orientation of the first quarter-wave plate can be modified, the orientation of the second quarter-wave plate can be fixed, or the orientations of both quarter-wave plates can be modified.

[0054] The rotation of a quarter-wave plate or half-wave plate is achieved, for example, by mechanical actuation of a sensor or an electrically adjustable device that enables it to rotate, or by friction on a moving shaft, for example, by a piezoelectric material or an electric motor (e.g., an electric motor operated by direct current).

[0055] Alternatively, multiple devices for rotating linear polarization can be used, each allowing different rotation angles, which may be fixed but are likely modifiable. The selector is able to send photons to one of these rotating devices and, at the exit of the rotating device, conduct the photons toward a common waveguide or common transmission axis.

[0056] These selectors may include mirrors whose axis orientation is adjusted, for example, by electrical means, or alternatively, they may consist of prisms or plates made of materials whose refractive index depends on the electric field: Pockels cells, or transparent materials with a nonlinear refractive index or other luminous flux, such as transverse and preferably having a wavelength different from that of the photons, and unable to produce photons with the same wavelength as the photons. This alters the refractive index of the nonlinear material, thereby adjusting the position and, optionally, the direction of the photons exiting the material. Since the moment of inertia of a quarter-wave plate can be very high, photons can be sequentially sent to different quarter-wave plates, for example, whose orientation has been pre-adjusted by one of the aforementioned mechanical rotation devices, thus allowing time to modify the orientation of each of these quarter-wave plates between two photon passes.

[0057] As a variation, chiral or rotating materials can be used to change the direction of linear polarization.

[0058] A polarization orientation modifier may include a plate or prism made of a chiral or rotatable material that causes a polarization rotation by an angle depending on the position of the wave entering the chiral or rotatable material. Specifically, the material may be arranged between two Pockels.

[0059] The first photon can be projected onto a first plate or prism forming a Pockel cell, the refractive index of which is controlled by an electric field to cause it to exit from different positions, particularly onto an intermediate plate or prism made at least partially of a chiral or rotating material. Before the photon enters a second plate or prism forming the Pockel cell, the intermediate plate or prism rotates the photon's electropolarization axis depending on the angle at which the wave enters the intermediate plate or prism. The refractive index of the second plate or prism is symmetrically adjusted relative to the refractive index of the first plate or prism so that the photon exits in the same direction, as if the refractive indices of both the first and second plates or prisms had fixed values. A third plate, whose refractive index can also be adjusted (e.g., electrically adjusted), is advantageously placed after the second plate or prism to allow the photon's exit position from the plate and prism assembly to be independent of the refractive index of the selected first plate or prism.

[0060] A plate or prism made of an intermediate material may consist of, for example, two adjacent symmetrical prisms having the same refractive index but different chirality or rotational power. The first of the two prisms has, for example, chirality or rotational power, while the second does not have chirality or rotational power, or alternatively, a rotating electric field in the opposite direction to the rotation applied by the first prism.

[0061] One of the two intermediate prisms may comprise a chiral material, such as cadmium selenide (CdSe) nanoparticles with a diameter of approximately 1.4 nm to 2.4 nm, as described by Visheratina, Anastasia, and Nicholas a. Kotov in their article "Inorganic nanostructures with strong chiroptical activity" (CCS Chemistry 2.3 (2020):583-604); the other intermediate prism is preferably not chiral, or alternatively has a chirality opposite to that of the first intermediate prism. Since the rotation of the electric field of a wave passing through the chiral material is proportional to the thickness of the chiral material, the rotation of the electric field passing through the intermediate plate depends on the position of the light entering it.

[0062] One of the two intermediate prisms may also include a “rotating” material, such as an even number of quarter-wave plates stacked on each other in a direction that increases with a predetermined angle on each layer, such that a wave linearly polarized along the axis of the first slice of the material exits from the stack, its polarization rotated by an angle proportional to the thickness of the stack.

[0063] Polarization phase modifier

[0064] A second device, known as a "polarization phase modifier," can also be arranged downstream of the polarization direction modifier to shift the electric field component of the photon by a predetermined angle along one of the two fixed axes.

[0065] Preferably, the polarization phase modifier includes a first birefringent waveplate or prism arranged to split the beam into two electromagnetic waves with linear polarization, one along a first axis and the other along a second axis, and a delay plate with a variable refractive index is arranged on the second axis.

[0066] A "beam" refers to a light wave composed of the first photon in the dualistic sense of light.

[0067] The delay plate causes a wave oriented along the second axis to acquire a predetermined phase shift relative to the wave oriented along the first axis before it is mixed again with the wave oriented along the first axis by a new birefringent plate or prism, thereby allowing two waves with perpendicular polarization fields to be combined along the same axis.

[0068] The delay plate may include a Pockel cell or a nonlinear material. The variable refractive index allows for selection of the phase shift applied to the wave oriented along the second axis.

[0069] In a first exemplary embodiment, as described above, the phase modifier rotates the linear polarization at angles spaced 9° apart and varying between -45° and +36°, and changes the phase at angles spaced 9° apart between -90° and +90°, thereby defining 210 possible polarization states. As described below, for example, 209 of the 210 possible polarizations are used to encode data, and the 210th linear polarization is used to absorb any entangled photons received within a predetermined time after receiving the photons used for data encoding.

[0070] In a second exemplary embodiment, the phase modifier rotates the polarization axis by 45° before the photon is absorbed to transmit, for example, 0, and converts the polarization of the photon to circular polarization to transmit, for example, 1.

[0071] The reflection of the first photon

[0072] The present invention also relates to a quantum communication system, comprising:

[0073] • An entangled photon emitter, the emitter comprising a source configured to generate at least one pair of entangled photons, the at least one pair of entangled photons comprising a first photon emitted on a first propagation path and a second photon emitted simultaneously on a second propagation path different from the first propagation path;

[0074] • A first receiver, disposed on the first propagation path, comprising:

[0075] - At least one first absorption instrument, said first absorption instrument being arranged to absorb a first photon in one of two states of a pair of complementary polarizations, said polarization being indeterminate on the observable basis upon which the first instrument absorbs the photon.

[0076] - An optical selector, the optical selector being disposed upstream of the at least one first instrument and configured to allow the first photon to pass toward the at least one first measuring instrument or to prevent the first photon from being measured;

[0077] • A second receiver, arranged in the second propagation path, so that the second photon reaches the second receiver after the first photon reaches the optical selector and / or after a measurement instrument that allows the first photon to reach the first receiver, the second receiver comprising: an optical amplifier capable of multiplying the second photon while maintaining its polarization; and a measurement instrument arranged downstream of the amplifier capable of measuring the average quantum state of the multiplied photon.

[0078] "Observable basis" refers to the basis of two complementary polarizations (e.g., two orthogonal linear polarizations).

[0079] The term "polarization is indeterminate on the observable basis" means that the polarization of a photon observed in one of two predetermined polarizations of the observable basis is undefined, that is, it is random, for example, equally probable.

[0080] Because the first and second photons are entangled, the instantaneous absorption of the first photon at the first receiver determines the polarization of the second photon, especially before the second photon reaches the second receiver.

[0081] Then, by measuring the polarization of the photon after it has been amplified at the second receiver, it is possible to detect whether the first photon has been absorbed, and from this, the information that has been transmitted can be inferred. The information transmitted by this system is, for example, binary.

[0082] The first receiver may include at least one second absorption instrument, and an optical selector is disposed upstream of the first and second instruments and configured to allow the first photon to pass through while directing it toward one or the other of the instruments, or to prevent it from being absorbed.

[0083] Using two instruments, three different information items can be transmitted, such as values ​​0, 1, and 2. Value 0 is assigned to reflection, for example, value 1 is assigned to the first instrument, and value 2 is assigned to the second instrument.

[0084] Optical selector

[0085] The optical selector of the first receiver can be of various types. If necessary, it is advantageously positioned upstream of the measuring instrument to prevent incident photons from reaching the measuring instrument.

[0086] Therefore, the selector preferably includes a reflector, particularly a controlled reflector, such that the first photon can be reflected, for example, on a propagation path different from the incident propagation path of the first photon, especially on a propagation path that does not contain measuring instruments.

[0087] A “controlled reflector” is a device whose characteristics, such as refractive properties or reflection direction, are controlled by an auxiliary device connected to the reflector, which is in particular an electronic device, such as a device that generates an electric field.

[0088] Optical selectors preferably include devices in which the refractive index and / or reflection direction are controlled, particularly by using an electric field or luminous flux.

[0089] Alternatively, optical selectors include, for example, Bragg mirrors, particularly Bragg mirrors whose reflection direction is adjusted by a piezoelectric material, which itself is controlled by an electric field.

[0090] As a variation, the optical selector includes a prism, particularly a Pockel cell, whose refractive index is controlled by an electric field. The application of the electric field enables photons to be directed toward a measuring instrument or one of the measuring instruments or toward a Bragg reflector, which is advantageously tilted relative to the incident direction of the electromagnetic waves arriving at it, such that the waves are reflected toward a position preferably different from the position from which they arrive at the prism entrance.

[0091] In another variation, the optical selector includes one or a series of Bragg filters, some of which are Pockels cells with refractive indices controlled by an electric field. The Bragg filters either reflect light waves or allow them to pass through depending on whether an electric field is applied.

[0092] Alternatively, the Pockel cell used above can be replaced by a nonlinear crystal irradiated by strong light, which can change the refractive index of the medium through which photons pass, and preferably the wavelength of the strong light is different from the wavelength of the photons.

[0093] In particular, the optical selector enables photons to be oriented in three different directions: one direction, for example, makes the photon undetectable; the second direction enables the measurement of the photon's linear polarization; and the third direction enables the measurement of circular polarization.

[0094] Protective devices

[0095] Preferably, the quantum communication system further includes means for protecting the quantum state of at least one photon, particularly means for preventing the measurement or absorption of at least one photon, the means being arranged near the first receiver to protect the quantum state of the first photon after its reflection from being absorbed or measured by one or more measuring instruments, the quantum state of the first photon being protected at least before the second photon is multiplied at the second receiver.

[0096] The device for protecting the quantum state is placed close to the first receiver so as to protect the quantum state of the first photon when the first photon is reflected by the optical selector and / or deflected from its incident propagation path so that it is not observed there, i.e. when it is not desired to be absorbed by the measuring instrument of the first receiver.

[0097] Preferably, the device for protecting the quantum state includes a transparent space, particularly a transparent space and at least one reflector. The transparent space may be empty or filled with gas or liquid.

[0098] Preferably, the device for protecting the quantum state includes a plurality of Bragg mirrors arranged relative to the transparent space to capture the photon received by the device along a path of a suitable length as described above, that is, along a path corresponding to the length of time required for the second photon to be amplified therein to reach the second receiver.

[0099] As a variation, the protection device includes an optical fiber of such length that the first photon can circulate within it while waiting for a second photon that has reached the second receiver to be amplified there.

[0100] In another variation, the device can be a part of space or atmosphere that is either empty or filled with gas, thus ensuring that no objects that reflect or absorb light pass through.

[0101] Instruments for absorbing the first photon

[0102] At the first receiver, the first photon can be absorbed by at least one absorption instrument in one of two states of complementary polarization.

[0103] Preferably, the absorption instrument includes at least one filter that is capable of sending the first photon to at least one photon detector, particularly to one or the other of two photon detectors, according to the polarization state of the first photon. The filter is preferably a prism or plate made of a birefringent material.

[0104] The absorption instrument may also include a polarizing filter and a photon detector located after the polarizing filter.

[0105] The absorption instrument may also include a translucent mirror that reflects circularly polarized photons toward the first photon detector and allows other photons to pass through the second photon detector, as described by Mai, Wending et al. in their article “Broadband transparent chiral mirrors: Design methodology and bandwidth analysis” (AIPA Advances 9.4 (2019): 045305).

[0106] Photon emission

[0107] Photon emitter

[0108] Preferably, the transmitter generates entangled photon pairs through spontaneous parameter downconversion (SPDC), that is, entangled photon pairs are generated according to the process in which the initial photon (also known as the "pump" photon) is split in two and its frequency is halved by a four-wave mixing phenomenon in an optical medium with a nonlinear refractive index, as described in Amanti et al.'s article "Integrated sources of entangled photons at the heart of quantum technologies" (Photoniques, No. 91, 2018).

[0109] This type of photon pair emitter can produce relatively robust entanglement of photons, but its drawback is that it sometimes produces more than one pair of photons at a time, which is undesirable because it is uncontrolled. Auxiliary devices can be implemented to handle these "parasitic" multiple pairs, as described below.

[0110] As a variation, entangled photon pairs can be generated using emitters that incorporate quantum dots, as described in the article cited above. This allows for the acquisition of pairs in a more regular manner. However, the entanglement properties of photons obtained in this way can sometimes be unstable.

[0111] Alternatively, optical fibers made of birefringent crystals can be used, as described by Smith et al. in the article “Photon pairgeneration in birefringent optical fibers” (Optics Express, Vol. 17, Issue 26, 2009).

[0112] Preferably, the transmitter is configured to continuously generate multiple entangled photon pairs.

[0113] Entangled photons can be sent in various ways along the first and second propagation paths at the transmitter exit.

[0114] Emission with predetermined polarization

[0115] Photons are preferably emitted with a predetermined polarization. The term "predetermined polarization" refers to one of two states in a predetermined complementary polarization pair, such as linear polarization with a specific direction. For example, a birefringent plate can separate two entangled photons based on their vertical or horizontal linear polarization.

[0116] The predetermined polarization of the entangled photons sent to each receiver makes it possible to know, in particular, the probability that any chosen complementary polarization pair is in a state of one polarization or its complementary polarization.

[0117] Therefore, a random bit sequence can be generated when the photon is received by the receiver. This sequence is complementary at the two receivers, and the polarization of the photon received at the second receiver is complementary to the polarization observed at the first receiver.

[0118] When a quarter-wave plate is used at the first receiver to rotate the polarization direction of the first photon, a predetermined polarization is also necessary.

[0119] In order to emit photons along a predetermined linear polarization, a plate or prism is arranged at the exit of the emitter, the plate or prism comprising a birefringent optically transparent material, such as lithium niobate or rutile (TiO2), whose optical index depends on the polarization axis through which at least one of the two photons passes.

[0120] Therefore, according to linear polarization, the photon exits the prism at one of two different locations.

[0121] If two photons are emitted from entangled photon sources in the same direction and along an axis perpendicular to the ordinary axis of the birefringent optically transparent material, the first photon is collected at the exit of the prism, and an electromagnetic wave with an electric field parallel to the ordinary axis of the birefringent optically transparent material exits through the exit at that location in order to send the first photon to the first receiver.

[0122] In the same manner, second photons emitted from a location where electromagnetic waves with a field perpendicular to the ordinary axis are emitted are collected so that they can be sent to a second receiver.

[0123] In the case where two entangled photons are generated from emitters at different positions and / or in different directions, each of the two photons can be projected onto the birefringent prism in a direction perpendicular to the ordinary axis of each prism, and each photon can be redirected to a receiver for it. Photons emitted from different positions of the prism are preferably lost or destroyed by being projected onto an absorbing surface.

[0124] For example, for a first entangled photon, a photon emitted from a first birefringent plate or a first birefringent crystal having a linear polarization parallel to a first axis can be used; and for a second entangled photon, a photon emitted with a linear polarization parallel to the first axis of a crystal used for a second plate or a second prism can be used, the second plate or the second prism being adjusted to allow the photon to be emitted along a predetermined polarization corresponding to the polarization of a photon entangled with a photon emitted from the first plate or the first birefringent prism in a direction parallel to the first axis.

[0125] To facilitate the transmission of the photons toward the receiver, a birefringent delay plate that converts the linear polarization of the photons to circular polarization is preferably placed at the exit point where the photons exit the prism.

[0126] Then, preferably, another birefringent delay plate is placed at the entrance of each receiver to convert the circular polarization of the photons back into linear polarization, which allows for precise adjustment of the polarization axis.

[0127] Photon transmission

[0128] Photons can be transmitted from a transmitter to a receiver via space, atmosphere, optical fiber, or a combination of these means.

[0129] Lenses can be used for the transmission of photons, particularly for transmission through space or the atmosphere. Where appropriate, an anti-reflective layer is preferably disposed on the lens. The size of the lens used is preferably matched to the length of the photon's transmission through space or the atmosphere.

[0130] A conjugate mirror that reflects light emitted at the transmitter can be used to adjust the direction of conjugate photons for transmission in space or the atmosphere. For example, a laser transmitter can scan space to detect a receiver there, which reflects the emitted light into space and then adjusts the direction of the conjugate photons to be parallel to or coincide with the direction of the light reflected by the conjugate mirror.

[0131] The light emitted by the laser emitter can have a wavelength close to that of the entangled photon and is introduced into the objective lens used by the photon through a dichroic prism.

[0132] As a variation, if the wavelength is the same as that of the entangled photons, then when the photons are linearly polarized, light can be introduced through a birefringent prism, and the polarization of the light used for guidance is then perpendicular to the polarization of the entangled photons being guided towards the same receiver. When the guiding light arrives, a Pockel cell upstream of the receiver can be used to guide the light toward a conjugate mirror.

[0133] In another variation, the light wave used for aiming can be parallel to the entangled photon emission, but separated by, for example, a few centimeters, so that it can be reflected by the conjugate mirror.

[0134] In another variant, a conjugate mirror may not be used, but the receiver or a target near the receiver can still be observed, and information about the target can be transmitted by another communication device, such as via radio signals or quantum transmission.

[0135] The area to be scanned can be identified by mapping and reconnaissance of the area where the receiver may be located.

[0136] Furthermore, when the receiver rotates relative to the transmitter, especially due to the rotation of the Earth or a satellite, the aforementioned phase modifier can be used to allow photons emitted in a fixed direction relative to the transmitter to enter each receiver in a fixed direction determined relative to the receiver.

[0137] Calibration method

[0138] The various media through which photons travel from their emission location to the two receivers can cause a change in their phase, which is not necessarily expected or anticipated.

[0139] Furthermore, these modifications can vary depending on whether the photons are directed toward the first or second receiver. Finally, these modifications may change over time, particularly depending on the weather or temperature during the material's use.

[0140] The system according to the invention can be periodically calibrated by having a photon absorbed by a first receiver with a polarization belonging to a predetermined complementary polarization pair, and for each of these pairs, periodically calibrated by observing one of the two possible polarizations of the entangled photon detected by a second receiver.

[0141] Therefore, the present invention also relates to a method for calibrating a system as described above, such that the polarization state of a photon arriving at a second receiver can be determined based on the polarization state of the entangled photon absorbed at the first receiver, the method comprising the following steps:

[0142] - An entangled photon pair is generated from the transmitter, wherein a first photon in the pair is emitted toward a first receiver, and a second photon in the pair is emitted toward a second receiver, the first photon and the second photon being entangled with respect to their polarization states.

[0143] - The transmitter absorbs the first photon at the first receiver with a predetermined polarization pair, stores one of the two possible polarizations of each absorbed photon, and stops transmitting photons once a predetermined number of different polarizations have been observed.

[0144] - At the second receiver, the second photon is replicated using an amplification device to form a multiplied photon flux, with each multiplied photon retaining the polarization state of the second photon.

[0145] - Measure the polarization state, that is, the polarization direction and phase shift of each multiplied photon flux, and store these measurements along with their reception time.

[0146] - The list of stored photon polarizations and reception times is transferred from the first receiver to the second receiver.

[0147] - Remove photons from the list of photons received at the second receiver that do not correspond to photons received at the first receiver, and similarly, remove photons from the list of photons received at the first detector whose corresponding photons were not received at the second receiver.

[0148] - On the one hand, by using the knowledge of the polarization states of two photons detected at the first receiver with complementary polarization and whose entangled photons have reached the second receiver, and on the other hand, by using the knowledge of the polarization states of the corresponding entangled photons received at the second receiver, the Jones matrix is ​​calculated so that the polarization of the photons received at the second receiver can be inferred from the polarization of the photons received at the first receiver.

[0149] The storage of states and the calculation of the Jones matrix are performed by any suitable electronic device, such as a microcontroller.

[0150] The Jones matrix calculated in this way allows the polarization of the second entangled photon received at the second receiver to be calculated based on the polarization of the first photon absorbed by the recombination absorber located at the first receiver.

[0151] A second calibration can be performed, which includes determining the probability that any photon arrives at the second receiver, whose entangled photons have been absorbed during its journey toward the first detector, and which belong to each range of complementary polarization pairs detectable or identifiable by the second receiver; the range of polarization P is defined as the set of polarizations assimilated by the polarization measurement instrument to have polarization P.

[0152] Therefore, the present invention also relates to a method for calibrating a system as described above, the method enabling the determination of the probability of a photon being lost during its journey from the transmitter to the first receiver, the method comprising:

[0153] - Configure the composite absorber of the first receiver to absorb photons received at the first receiver that are polarized in one of a pair of complementary absorption polarizations.

[0154] - Multiple entangled photon pairs are continuously generated from the transmitter, wherein the first photon of the pair is emitted toward the first receiver, and the second photon of the pair is emitted toward the second receiver, and the first photon and the second photon are entangled with respect to their polarization states.

[0155] - Count the number of photons received at the second receiver that fall within each polarization range detectable by the second receiver, the polarization range being different from one of the two polarization ranges of entangled photons of the photons absorbed by the first detector.

[0156] Therefore, a polarization range can be detected in which entangled photons that are lost during transmission toward the first detector arrive preferentially, or are most likely to arrive, and thus the corresponding absorption polarization is preferably avoided in order to transmit data from the first receiver to the second receiver.

[0157] Therefore, the transmission ratio of photons between the transmitter and the second receiver can be calculated by taking the quotient between the number of photons received at the second receiver and one of the two absorption polarizations corresponding to the first receiver.

[0158] The measuring instrument for the second receiver

[0159] The measuring instrument of the second receiver preferably includes at least one photon detector, which is arranged to measure the polarization of the multiplied light originating from the second photon.

[0160] The measuring instrument for the second receiver includes, for example, a series of semi-reflective plates arranged downstream of an optical amplifier, which guide the multiplied flux of photons, i.e., the luminous flux, toward a polarization measuring instrument with equal intensity. The polarization measuring instrument is arranged, for example, to measure the intensity of the flux along two perpendicular axes and / or the phase shift of the light between these two axes.

[0161] Light can also be directed toward various polarization filters after passing through one or more lenses that magnify the cross-section of the beam, thus allowing multiple mirrors or lenses to direct a portion of the beam toward various polarization measuring instruments.

[0162] Various polarization measurement instruments preferably enable:

[0163] • Measure the intensity of the electric field component of light along the first axis x.

[0164] • Measure the intensity of the electric field component of light along a second axis y that is perpendicular to the first axis x.

[0165] • Measure the phase shift of light along its x-axis and along its y-axis.

[0166] • And, preferably, the phase shift of the light along its axis x' (the bisector of x and y) and the light along its axis y' is measured.

[0167] The intensity of light along two perpendicular axes is measured, for example, by separating the light along the two perpendicular axes using a birefringent plate or prism, followed by two light intensity sensors placed at the exit of said plate or prism. The phase shift between the two perpendicular components is measured, for example, by separating the light along the two perpendicular axes using a birefringent plate or prism, then, for a wave polarized along one of the axes, by rotating that polarization axis by 90° (e.g., by using a rotating or chiral material or by rotating a series of quarter-wave plates), and then jointly projecting the two waves, one of which has undergone an electric field rotation, through a Young's slit onto a screen. The interference of the two light sources produces fringes, the positions of which depend on the phase shift.

[0168] Optical amplifier

[0169] An "optical amplifier" is a device that can replicate a photon introduced therein, particularly the second photon in a pair of entangled photons, while maintaining its polarization state.

[0170] Preferably, the optical amplifier is a doped fiber amplifier.

[0171] For example, an erbium amplifier (EDFA) with a length of 4m can be used to simultaneously introduce the photon to be amplified along with an amplified wave with a shorter wavelength into the erbium amplifier. This allows the wave corresponding to the introduced photon to be amplified, with a gain of approximately 37dB / m.

[0172] Alternatively, a doped fiber amplifier (DFA) using dopants other than erbium can be used.

[0173] As a variant, an optical amplifier is, for example, a vertical cavity amplifier (VCSOA) or a semiconductor-type amplifier (SOA).

[0174] Anti-reflective layer

[0175] To avoid photon loss, the anti-reflective layer is preferably disposed at the interface between adjacent transparent media with different refractive indices through which the photon passes, and at the interface between the birefringent prism and the plate through which the photon passes. The anti-reflective layer is preferably matched with one or more refractive indices, one or more incident angles and polarization directions of the material, and the wavelength of the photon intended to pass through it.

[0176] Dichroic filter

[0177] The first receiver and / or the second receiver preferably include one or more dichroic filters that enable only photons with a given wavelength to pass through, and in particular prisms made of dispersive transparent materials. These one or more filters are preferably arranged in front of one or more measuring instruments or the optical selector of the first receiver, especially when modifying the refractive index of a nonlinear material by applying a strong luminous flux.

[0178] Distribute photons to information with a synchronized clock.

[0179] When photons arrive at the second receiver, it is useful to be able to distinguish whether they should be assigned to the transmitted information, whether they are information items that have been transmitted multiple times, or whether they are information items that have not yet been encoded into a pair of entangled photons.

[0180] This is because photon emission is sometimes irregular, and emitters of the SPDS type, as described above, can emit two photon pairs, which is not desirable.

[0181] The transmitter and each receiver preferably include a clock, and the clocks of the transmitter and receiver are synchronized with each other.

[0182] When combined with knowledge of the time of flight of a photon between the transmitter and each receiver, this makes it possible to determine preferred repeating encoding and receiving periods during which a photon can be encoded at the first receiver during the encoding period and its entangled photon can reach the second receiver during the receiving period.

[0183] If multiple photons are received during the receiving period, the first receiver remains in the same state until a time is reached, for example, between a quarter of the non-emission time after the end of the receiving period and half of the non-emission time δt' before the next receiving period.

[0184] The second receiver preferably records the bits received in each reception period. Then, advantageously, the second receiver periodically sends to the first receiver, for example, every 3000 or 100 reception intervals without photons (especially if the device uses reflection to transmit bits or ternary digits), a list of reception periods in which no photons were received, where the transmitted photons may have been lost, or the transmitter did not transmit photons during that period, and the first transmitter is unaware of whether it reflected photons during these periods, especially if the device does not have a photon detector.

[0185] The list can be transmitted via instantaneous communication using the method according to the invention described below, or via conventional communication means. Upon receiving the list, the first receiver then transmits bits corresponding to the transmission periods during which no photons reach the second receiver.

[0186] As a variant, the first receiver uses only two different measuring instruments to transmit information: one instrument detects linear polarization and the other detects circular polarization.

[0187] Then, the first receiver retransmits each bit while selecting a suitable measuring instrument for each bit it will transmit during that transmission period, and preferably keeps the selector in the same state until the transmission period for receiving photons ends, thereby allowing it to select a measuring instrument for the transmission of the next bit.

[0188] When the second receiver receives photons during a transmission period, if it receives some of them, it measures the polarization state of those photons in each transmission period and derives from that the bit that should be added to the list of received bits.

[0189] If the transmission line between the transmitter and the first receiver has a certain degree of opacity and can absorb a certain proportion of photons passing through it, some photons may arrive at the second receiver with random polarization during the receiving period.

[0190] The second receiver can then be configured to identify at least some of the photons arriving with a polarization state that cannot be the polarization state of entangled photons that were absorbed or reflected at the first receiver.

[0191] Clock deviation

[0192] Preferably, the clocks of the system components, namely the clocks of the transmitter and the two receivers, are configured to take into account the phenomenon of running at different times at different locations, especially at different heights of each component.

[0193] Synchronization of transmission periods between receivers

[0194] A method can be implemented that allows the first and second receivers to synchronize their clocks.

[0195] For example, when measuring the first photon, the first receiver uses its own clock to record the time when the photon hits the measuring instrument.

[0196] The recorded time, along with preferably referenced transmission intervals, particularly the start time and optionally the expected duration of the photons, is then sent to a second receiver.

[0197] The second receiver records the reception time of the second photon in order to adjust its own clock so that the deviation between the start of the transmission period and the time when the first photon strikes the measuring instrument of the first receiver is the same as the deviation between the start of the reception period of the second photon at the second receiver and the time given by the clock for receiving the second photon.

[0198] If the transmission line between the transmitter and one or the other of the two receivers absorbs some photons, the transmitter can send photons in a predetermined number or for a predetermined duration, and each of the two receivers can record the average reception time of each photon, and then one receiver can transmit the average time to the other receiver.

[0199] The average time can be calculated by averaging the reception times of each photon, or by averaging the reception times of only the first and last received photons. The predetermined number of photons transmitted or the duration of their transmission is preferably adjusted specifically based on the transmission / loss ratio of each photon on each of the two paths between the transmitter and receiver, as well as the average transmission frequency of the photons.

[0200] Because the travel time of photons between the transmitter and each receiver may differ, the clock synchronization between each receiver may also differ, depending on whether information transmission is involved in one direction or the other. Therefore, each receiver can have a synchronization register for adding or subtracting a timelapse period from its clock time, which can be adjusted during synchronization to infer the time required to synchronize the clock for receiving information from the other receiver.

[0201] Transmission between transmitter and receiver

[0202] A method can be implemented that enables the synchronization of the time periods during which photons are sent to the receiver and the time periods during which they are received at the receiver, which are the time intervals during which photons can leave the transmitter or arrive at the receiver.

[0203] For this purpose, the transmitter can emit a brief first optical signal, followed or preceded by other optical signals that form the message. These optical signals are preferably signed with a digital signature that indicates the precise time the signal was sent on its clock.

[0204] Upon receiving the first optical signal, the receiver records the reception time, then reads the signal transmission time, takes the difference between the signal transmission time and the start time of the photon transmission period (when the signal is transmitted during the photon transmission period) (or, if the signal is transmitted outside the transmission period, the start time of the previous transmission period), and performs the same processing on the signal reception time and the start time of the reception period.

[0205] If the difference between the signal transmission date and the start of the transmission period is greater than the difference between the reception date and the start of the reception period, the receiver can move the start of the transmission period forward, or vice versa.

[0206] The calibration is advantageously repeated multiple times, and the results are averaged, thereby enabling the clock to be adjusted with higher precision than the inaccuracy of the transmission and reception times of the optical signal.

[0207] The wavelength used for the optical signal is preferably the same as the wavelength of the entangled photon, so that it can be transmitted at the same speed as the entangled photon.

[0208] Distributing photons to information without a synchronized clock

[0209] As a variation of the synchronization clock, the first receiver may include a detector-transmitter and a waiting element, such as a reflector arranged to reflect the first photon at a predetermined polarization different from that detectable by the detector-transmitter, or alternatively an element arranged to absorb the first photon at a predetermined polarization different from that of the detector-transmitter.

[0210] The polarizations of the detector and transmitter are preferably complementary, such that any photons directed toward the absorber waiting element are absorbed.

[0211] The detector-transmitter is preferably arranged to detect the polarization of photons with at least four different polarizations, which are grouped into complementary pairs, such that any photon guided toward the detector-transmitter is thus detected and absorbed, regardless of its initial polarization.

[0212] One or more detector-transmitters are used with the first receiver so that information can be sent by selecting the detector pair used.

[0213] After receiving a photon for transmitting an information item, each photon received by the first receiver during a predetermined period (also known as a "rest period") is sent to a waiting element. The duration of the predetermined period is preferably fixed and preferably more than twice the amount of clock inaccuracy of the receiver. The start of the rest period is determined by a sensor or one of two sensors that have been used to transmit information and have absorbed photons.

[0214] The second receiver is preferably arranged in sequence.

[0215] • Ignore photons paired with photons sent toward the waiting element; on the other hand...

[0216] • Interpret two consecutive photons that are not paired with photons sent toward the waiting element as consecutive bits (if they are at least separated from the rest period), subtract the receiver's clock inaccuracy from the rest period, and finally...

[0217] • For example, two photons received consecutively for less than or equal to three-quarters of the rest period will be interpreted as representing the same bit.

[0218] Therefore, if two consecutive photons with different polarizations that are not paired with photons sent toward the waiting element are received within a time period less than the time of the receiver's clock inaccuracy during the rest period, the second receiver can detect the error in information transmission.

[0219] The advantage of this method is that it does not require a switch at the transmitter to limit the emission of entangled photons, nor does it require clock synchronization.

[0220] Therefore, one or more detector-transmitters of the first receiver can be used simultaneously with the waiting element, especially if the polarization of the photons they detect is adjustable and can therefore be adjusted to the polarization used by the waiting element in order to absorb the photons.

[0221] Noise reduction device

[0222] During signal amplification or in the absence of a signal, after the molecules or atoms of the amplifier medium have been excited by the pump signal, the optical amplifier has a tendency to emit photons in the direction opposite to the propagating signal.

[0223] The second receiver includes, for example, a switch arranged in front of the optical amplifier to limit the number of photons emitted by the amplifier toward the entangled photon source of the device.

[0224] For example, the switch of the second receiver is configured to absorb or reflect one or more photons after the first photon arriving at the second receiver within a predetermined time interval.

[0225] The switch of the second receiver can be connected to the measuring instrument of the second receiver so as to allow light to pass in only one direction or the other after the arrival of the first photon, until the next photon is expected, or a predetermined time before that moment, such as half of the rest period described above.

[0226] Alternatively, the switch of the second receiver can allow photons to pass through only during their intended time period, particularly during the aforementioned photon processing interval.

[0227] In the absence of a synchronized clock, photons are distributed to information using an optical transmission device with low transparency.

[0228] Although optical fibers are transparent, they cannot transmit all photons present at their entry point. A loss of 0.20 dB per kilometer is common; for example, a loss of 10 dB, or 90% of photons, would occur over 50 km.

[0229] However, the effect of photons lost during the journey between the transmitter and the first receiver is different from the effect of photons lost during the journey toward the second receiver; the entangled photons of photons lost between the transmitter and the first receiver can reach the second receiver at any polarization corresponding to the polarization of the entangled photons absorbed in the path toward the first receiver, while the loss of photons in the path toward the second receiver necessarily leads to the disappearance of photons that potentially carry information, that is, the photons whose entangled photons have reached the first receiver.

[0230] Furthermore, after the entangled photon arriving at the second receiver is absorbed on its journey toward the first receiver, the polarization of the photon arriving at the second receiver is not necessarily uniformly distributed among all possible observable polarizations.

[0231] For information transmission, complementary polarization pairs are preferred, where less corresponding polarization is observed at the second receiver when the entangled photons of the photons detected at the second receiver are absorbed during their journey toward the first receiver.

[0232] The present invention also relates to a method for calibrating a system as described above, such that two Jones matrices can be determined, one of which allows the polarization of photons arriving at a second receiver to be calculated based on the polarization of photons absorbed at a first receiver, wherein, preferably, the amount of different polarizations observable at the second receiver is more than twice the reciprocal of the photon transmission ratio between the transmitter and the first receiver, the method comprising:

[0233] - Configure the composite absorber of the first receiver to absorb photons received at the first receiver that are polarized in one of a pair of complementary absorption polarizations.

[0234] - Multiple entangled photon pairs are continuously generated from the transmitter, wherein a first photon in each pair is emitted toward a first receiver, and a second photon is emitted toward a second receiver. The first and second photons are entangled with respect to their polarization states.

[0235] For each polarity of a photon received at the second receiver, count the number of photons that have arrived at the second receiver with that same polarity.

[0236] - Stop sending photons when a predetermined number of photons have been received at the second receiver.

[0237] - Identify the two polarizations of photons that are most frequently received at the second receiver; these polarizations are considered to correspond to the absorption polarization of photons at the first receiver.

[0238] - Calculate two possible Jones matrices for transforming polarization, such that the polarization of a photon received at the second receiver can be inferred from the polarization of the photon received at the first receiver.

[0239] Furthermore, recording the polarization of at least one photon received by the first receiver and the polarization of its entangled photon received by the second receiver, for example by corresponding them using their respective reception times, makes it possible to select one of two calculated Jones matrices, which allows the polarization of the photon received at the second receiver to be calculated based on the polarization of the entangled photon of the photon received at the second receiver that may be received at the first receiver.

[0240] To transmit information items from the first receiver to the second receiver, the following method can also be used, which includes the following steps:

[0241] - Multiple entangled photon pairs are continuously generated from the transmitter, wherein a first photon in each pair is emitted toward a first receiver, and a second photon is emitted toward a second receiver, the first and second photons being entangled with respect to their polarization states.

[0242] For each information item to be transmitted, the composite absorber of the first receiver is configured to absorb NPT transmission photons corresponding to the information item to be transmitted, which are in a predetermined complementary polarization pair (referred to as the "absorbed polarization pair"). Then, if the next information item to be transmitted is unknown or is the same as the information item just transmitted, the absorber is configured with a polarization pair referred to as the "waiting polarization pair". And if the information item is the same as the information item just transmitted, at least NPT photons absorbed with the "waiting polarization pair" are counted.

[0243] At the second receiver, for each complementary polarization pair, the number of photons received at one of the two polarizations since the last possible signal has been received is counted. Once the counter exceeds a predetermined threshold number NSP for one of the pairs, and the complementary polarization pair is different from the complementary polarization pair of the last received signal, the information item corresponding to that polarization pair is considered a new signal, and if the last information item does not correspond to a waiting polarization pair, the information item is added to the list of received information items.

[0244] Alternatively, at the first receiver, a polarization absorber is systematically configured to absorb at least NPT photons in one of two waiting polarizations after any information has been transmitted.

[0245] For example, the following would be used to transmit information items:

[0246] • The transmission line between the transmitter and each receiver has a 10dB attenuation, which translates to a 90% photon loss.

[0247] • A transmission line between composite absorbers is configured to absorb 450 pairs of complementary polarized photons, representing 80% of the photons received at the second receiver, in the case that entangled photons of the photons received at the second receiver are absorbed as they travel toward the first receiver.

[0248] • NPT can select 200 photons, while NSP can select 6 photons. According to the inventors' calculations, this makes it possible to obtain a transmission error ratio of less than 1 / 10000.

[0249] In another example, with an error ratio of 97% corresponding to a signal attenuation of 15 dB, according to the inventors' calculations, by using 450 pairs of complementary polarizations, each polarization and phase shift separated from each other by about 5°, encoding 1200 photons at the first receiver (that is, NPT = 1200), and fixing the threshold number NSP at 14, it is possible to obtain an error ratio of less than 1 / 150000 if the photons generated at the second receiver from the photons absorbed during their journey to the first receiver are uniformly distributed across all observable polarizations.

[0250] Processing of simultaneously transmitted photon pairs

[0251] The transmitter may sometimes send entangled photon pairs very close together. Therefore, the photon detector located at the first receiver preferably counts the number of photons absorbed, rather than just the number of photon impacts on the first receiver. This counting allows, for example, the intensity of the electromagnetic waves impacting the photon detector to be taken into account.

[0252] Therefore, the photon group can arrive at the second receiver almost simultaneously, and the second receiver cannot distinguish the polarizations of the photons from each other. The polarization detector is able to detect the average polarization of all the photons detected "simultaneously," that is, the polarization of the photon group arriving at the second receiver simultaneously. In other words, the intensity of the light produced is, for example, 50% greater than that produced by a single photon. This advantageously avoids causing an increase in the counter of photons received with different polarizations. However, sometimes multiple photons may arrive at the first receiver, but only one of their entangled photons arrives at the second receiver, and then it is counted.

[0253] Device performance

[0254] Using lossless or low-loss lines between the transmitter and the first receiver, for example, allows photons to travel in a vacuum, making it possible to reduce or avoid randomly polarized photons received at the second detector.

[0255] Similarly, using precise measuring instruments at the second receiver makes it possible to increase the number of complementary polarization pairs or ignore a large number of photons with random polarization, which allows for:

[0256] • Increase the transmission distance of photons,

[0257] • Increase the baud rate by reducing the number of transmitted photons (NPT).

[0258] The increased switching speed of the polarization of the composite absorber in the first receiver enables an increase in the transmission frequency of entangled photons emitted by the transmitter.

[0259] As described above, the improved timing accuracy of the polarization detector in the second receiver enables the differentiation of parasites from different photons received very close in time, allows for the use of higher entangled photon flux, and also reduces the number of times photons are received "simultaneously".

[0260] The advantage of this method is that it does not require a switch at the transmitter to limit the emission of entangled photons, nor does it require clock synchronization.

[0261] Photon return processing towards the emitter

[0262] Photons sometimes reflect from the receiver toward the transmitter. To prevent them from being reflected back to one of the receivers by the transmitter, the cavity or material (with a nonlinear refractive index) that generates entangled photons by wave mixing is preferably surrounded or covered by a material that absorbs light with a wavelength equal to that of the entangled photons.

[0263] Quantum communication methods

[0264] The present invention also relates to a quantum communication method using the above system, comprising the following steps:

[0265] - An entangled photon pair is generated from the transmitter. The first photon in the pair is emitted toward the first receiver, and the second photon in the pair is emitted toward the second receiver. The first and second photons are entangled. The second receiver is farther from the transmitter than the first receiver and is located in the propagation path of the second photon, causing the second photon to arrive at the second receiver later.

[0266] - By using a polarization modifier, the polarization state of the first photon upon arrival at the first receiver is modified to a polarization state dependent on the information to be transmitted. This polarization state is selected from at least two different pairs of complementary absorption polarizations, except for exactly two pairs of perpendicular linear polarizations, in which one pair of perpendicular linear polarizations forms a 45° angle with the polarization direction of the other pair.

[0267] - By using an absorption instrument, the first photon, which is in one of the two complementary polarizations of a selected pair, is absorbed.

[0268] - At the second receiver, the second photon is replicated using an amplification device to form a multiplied photon flux, and the resulting light retains the polarization state of the second photon.

[0269] - Measure the average polarization state of the light flux and determine the polarization state of the first photon based on the measurement, so as to deduce the information transmitted by the first receiver.

[0270] The complementary photon pair is preferably selected from at least three different complementary polarization pairs.

[0271] The complementary polarization pair is selected, for example, from 210 different absorption polarization pairs, particularly from polarizations that are spaced 9° apart in their polarization directions and have a phase shift of 9°.

[0272] The present invention also relates to a quantum communication method using the above system, comprising the following steps:

[0273] • An entangled photon pair is generated from the transmitter. The first photon in the pair is emitted toward the first receiver, while the second photon in the pair is emitted toward the second receiver. The first and second photons are entangled. The second receiver is farther from the transmitter than the first receiver and is located in the propagation path of the second photon, causing the second photon to arrive at the second receiver later.

[0274] • Depending on the information to be transmitted, when the first photon arrives at the first receiver, an optical selector is used to selectively absorb or not absorb the first photon in one of two complementary polarization pairs. This optical selector directs the photon toward or away from one or more instruments.

[0275] • By selectively not absorbing photons with a predetermined polarization, the first photon is trapped in a protective device such that absorption of the first photon is avoided at least until the second photon reaches the second receiver.

[0276] • At the second receiver, the second photon is replicated using an amplification device to form a flux of multiplied photons, each of which retains the quantum state of the second photon.

[0277] • Measure the average quantum state of the multiplied photon flux, and determine, based on the measurement and / or using the instrument, whether the first photon has been absorbed at the first receiver, so as to infer the information transmitted by the first receiver.

[0278] Preferably, the polarization of the entangled photon pairs is indeterminate in one or more polarizations of complementary polarization pairs, and the instrument of the first receiver absorbs these entangled photon pairs.

[0279] For example, the entangled photon pair arrives at the receiver with linearly entangled polarization, one of the two complementary absorption polarizations being circular, and the second receiver is arranged to distinguish whether the average polarization of the multiplied photon flux is circular or linear, and to determine whether the first photon has been measured based on this distinction.

[0280] As a variation, entangled photon pairs arrive at the receiver with circularly entangled polarization, and the absorption quantum state of the photon is linearly polarized. A second receiver is arranged to distinguish whether the average polarization of the multiplied photon flux is circular or linear, and to determine whether the first photon has been measured based on this distinction.

[0281] As a variation, entangled photon pairs arrive at the receiver with vertical or horizontal linear polarization. The quantum state measured at the first receiver is linearly polarized at 45° or -45° to the vertical or horizontal direction. The second receiver is arranged to distinguish whether the average linear polarization of the multiplied photon flux (P20) is at 45° or -45°, or vertical or horizontal, and to determine whether the first photon has been measured based on this distinction.

[0282] Information transmission

[0283] The system and quantum communication method according to the present invention enable the transmission of binary, bit-based, or discrete or continuous information items between a first receiver and a second receiver.

[0284] Preferably, the transmitter continuously generates multiple entangled photon pairs, each photon pair enabling the transmission of information items, such as binary information, from a first receiver to a second receiver.

[0285] For example, a choice can be made to absorb a first photon at a first receiver to transmit bit 1 and reflect it to transmit bit 0. Measurement of the average quantum state of the multiplied photon flux at a second receiver makes it possible to determine whether bit 1 or bit 0 was transmitted quasi-instantaneously from the first receiver.

[0286] Various methods can be implemented to ensure secure communication and avoid transmission errors, such as those caused by simultaneously emitted two-photon pairs.

[0287] For example, during the time interval pre-established using the aforementioned clock, it is preferable to consider only the first photon arriving at the receiver and ignore subsequent photons.

[0288] As described above, when a photon arrives at the polarization detector, the photons can be counted. If the count shows that more than one photon has arrived within a predetermined time interval, then that bit is not transmitted during that time interval, and is transmitted, for example, in the next time interval, or preferably, the same bit is transmitted again.

[0289] At the second receiver, counting the expected photons arriving within the time interval can advantageously enable the creation of a temporary list of unreceived bits.

[0290] Two-way communication

[0291] To allow bidirectional communication, that is, to allow each of two receivers to send information to the other receiver, multiple transmitters can be used.

[0292] The system according to the invention may specifically include a second transmitter capable of generating one or more entangled photon pairs, the second transmitter being located closer to the second receiver than the first receiver.

[0293] As a variation, at least some of the photons can be made to travel along an indirect path in order to extend their transmission time toward one of the receivers, for example by reflecting the photons on one or more intermediate mirrors, or by passing them through a medium with a high refractive index, or alternatively by extending their transmission time by transmitting them in optical fibers of different lengths.

[0294] For example, this extended path can alternate with the non-extended path at a fixed or variable rate as needed, so that entangled photons can sometimes be used to transmit information from one point to another, and sometimes in another direction. An optical switch located upstream of the receiver and synchronized with a switch attached to the transmitter can be installed, enabling photons to be transmitted along the extended path to send the photons to a receiver of the first receiver type as described above, or conversely, to a receiver of the second receiver type.

[0295] A method for transmitting keys and verifying whether the transmission has been eavesdropped on.

[0296] By performing the following steps, it can be verified that the information transmitted from the transmitter to the second receiver via photon flux has not been eavesdropped on:

[0297] - The second receiver establishes a first list of the reception dates of photons that convey the information item, and specifies the relative polarization of the polarizations in the two complementary polarizations of each photon received;

[0298] - The second receiver generates a message containing the list collected in the previous step, creates an electronic signature for the list, and sends the list and signature to the receiver;

[0299] - The first receiver receives the list and signatures, and then verifies the signatures;

[0300] - The first receiver creates a second list consisting of the elements of the first list, for which the relative polarization of photons received by the first receiver and the second receiver is equal, or the entangled photons never actually reach the first receiver and the two entangled photons have different relative polarizations;

[0301] - If the number of elements in the second list that are restricted to entangled photon pairs (each of the two photons in the entangled photon pair arrives at its respective receiver) is less than a predetermined ratio multiplied by the product of the count of elements in the first list that are restricted to entangled photon pairs (each of the two photons in the entangled photon pair arrives at its respective receiver), then the information item is declared to have been transmitted and not eavesdropped on.

[0302] Then you can proceed with the following steps:

[0303] If the information item is declared to have been transmitted without being eavesdropped on, the second list is signed by the first receiver and sent to the second receiver, and then the first receiver creates a third list, which consists of the relative polarizations of photons that appear in the first list but not in the second list.

[0304] - After the second receiver receives the second list and verifies the signature, it recreates the third list in the second receiver using the first and second lists. Then, the second receiver sends a signature confirmation message to the first receiver acknowledging successful receipt of the second list.

[0305] The second receiver uses the third list as a shared key with the first receiver, and when the first receiver receives a message transmitted by the second receiver in the previous step, the first receiver uses the third list as a shared key exchanged with the second receiver.

[0306] The polarizing filter can be placed upstream of the second receiver. Attached Figure Description

[0307] The invention will be more clearly understood by reading the following detailed description of non-limiting exemplary embodiments of the invention and by studying the accompanying drawings, in which:

[0308] [ Figure 1A [Partially and schematically illustrating a quantum communication system according to the present invention,]

[0309] [ Figure 1B [Partially and schematically illustrating another quantum communication system according to the present invention,]

[0310] [ Figure 2A [Partially and schematically representing linearly polarized photons,]

[0311] [ Figure 2B [Partially and schematically representing a circularly polarized photon]

[0312] [ Figure 3 ] Figure 3 This diagram partially and schematically illustrates the details of photon transmission from the transmitter to the receiver.

[0313] [ Figure 4A [This section schematically illustrates an example of an optical selector according to the invention.]

[0314] [ Figure 4B Partially and schematically represented Figure 4A Variations of the selector

[0315] [ Figure 4C [This section schematically illustrates another example of an optical selector according to the invention.]

[0316] [ Figure 5A ][ Figure 5B ] Figure 5A and Figure 5B This schematically illustrates an example of a polarization direction modifier that includes a quarter-wave plate.

[0317] [ Figure 5C ] Figure 5C This schematically illustrates another example of a polarization orientation modifier that includes a chiral material.

[0318] [ Figure 6 ] Figure 6 An example of a polarization phase modifier is illustrated schematically.

[0319] [ Figure 7 This image partially and schematically illustrates an example of an instrument used for absorbing linearly polarized photons.

[0320] [ Figure 8 This section schematically illustrates another example of an instrument used for absorbing linearly polarized photons.

[0321] [ Figure 9 This image partially and schematically illustrates an example of a receiver that includes a second photon and a doped fiber amplifier.

[0322] [ Figure 10A This is a partial and schematic illustration of an example of a measuring instrument comprising a second receiver with a series of semi-reflective plates.

[0323] [ Figure 10B This is a partial and schematic illustration of an example of a Young's interferometer used for measuring instruments in a second receiver.

[0324] [ Figure 11A [This is a block diagram that partially illustrates an example of the operation of a quantum communication system for transmitting bit 0.]

[0325] [ Figure 11B This is a block diagram that partially illustrates an example of how a quantum communication system can operate to transmit bit 1.

[0326] [ Figure 12 This is a block diagram that partially illustrates an example of how a quantum communication system operates to transmit a series of discrete values.

[0327] [ Figure 13 [This is a block diagram that partially illustrates an example of a method for timestamping the arrival of photons in a receiver.]

[0328] [ Figure 14 The possibility of placing a "noise suppression" device in front of the amplifier is partially and schematically shown.

[0329] [ Figure 15 [Partially and schematically illustrating the possibility of establishing bidirectional communication between two locations using the system according to the invention, and]

[0330] [ Figure 16 ] Figure 16 A variation of the implementation of the present invention is shown. Detailed Implementation

[0331] Figure 1A A quantum communication system 1 according to the present invention is shown. The system includes a transmitter 2 that emits a pair of entangled photons (P1, P2), wherein the first photon P1 propagates along a propagation path D1, and the second photon D2 propagates along a propagation path D2 different from path D1. Photons P1 and P2 are emitted simultaneously.

[0332] Due to the nature of light and the wave-particle duality of photons, the terms “wave,” “photon,” and “particle” are used interchangeably below to refer to the products emitted by emitter 2.

[0333] The terms "measuring instrument" and "absorption instrument" are used interchangeably to refer to instruments that absorb photons according to a specific polarization.

[0334] System 1 includes a first receiver 3 located on the propagation path D1 of the first photon P1 and a second receiver 4 located on the propagation path D2 of the second photon P2.

[0335] Receiver 3 is closer to transmitter 2 than receiver 4, so that photon P1 reaches receiver 3 before photon P2 reaches receiver 4.

[0336] In the example under consideration, receiver 3 includes a composite absorber 31, which includes a polarization modifier 32 and an absorption instrument 35.

[0337] The composite absorber 31 is configured to absorb photons in polarization states selected from a plurality of different complementary polarization pairs (preferably at least three pairs).

[0338] The polarization modifier 32 is configured to transmit photon P1 according to a selected complementary polarization pair, and the absorption instrument 35 enables determination of which of the two states of that pair photon P1 is in. Before entering the absorption instrument 35, the polarization state of photon P1 is, for example, a superposition of the two states of the selected pair with equal probability weights.

[0339] Figure 1B Another example of a quantum communication system 1 according to the present invention is shown. In this example, the receiver 3 includes two measuring instruments 35 and an optical selector 30. The measuring instruments 35 are arranged to measure the quantum state of a first photon P1. The optical selector 30 is arranged upstream of the measuring instruments 35 and is configured to send the first photon P1 toward a birefringent prism 36 (which guides the photon according to the linear direction of photon polarization and then guides the photon toward one or the other of the measuring instruments 35 along path D1' or D1''), or to prevent the first photon P1 from being measured, for example by reflecting the first photon P1 onto a propagation path D3 different from path D1.

[0340] Specifically, direction D3 is chosen such that photon P1 is not absorbed before its entangled photon is multiplied at the second receiver 4.

[0341] As shown in the figure, system 1 preferably includes a device 5 for protecting the quantum state of photons, which is arranged near receiver 3, particularly on propagation path D3 in the example under consideration.

[0342] This device makes it possible, for example, to “capture” photon P1 if it is reflected onto path D3 by optical selector 30, to prevent photon P1 from being measured or absorbed, at least until the second photon P2 has reached the second receiver 4. A 5D photon detector (not shown) can be placed at the end of the device 5 to detect the photon captured in the device 5 after the entangled photon has reached the second receiver.

[0343] The second receiver 4 includes an optical amplifier 40 and a measuring instrument 45. The optical amplifier 40 enables the multiplication of the second photon P2 while maintaining its polarization, and the measuring instrument 45 enables the measurement of the average polarization of the multiplied photon.

[0344] The transmission of photons P1 and P2 can occur in various ways and in various media. For example, photons can propagate in optical fibers or waveguides, or propagate freely in space, whether the space is empty or filled with gas.

[0345] Photons can pass through a variety of media with different refractive indices. For example, to prevent undesirable optical phenomena, particularly Fresnel reflection of waves emitted by emitter 2, an anti-reflective plate can be appropriately inserted between the two media through which the photons pass.

[0346] The transmitter 2 includes, for example, one or more lenses, which are selected to be of sufficient size to allow photons to reach their respective receivers with minimal diffraction, such as allowing at least 99.99% of the emitted waves to reach the receivers.

[0347] The transmitter 2 may also include a system, particularly electronic devices, that enables the adjustment of the initial directions of photons P1 and P2, respectively.

[0348] This adjustment can take into account, in particular, the different refractive indices of the medium through which the photon passes, and the resulting possible modifications to the photon's trajectory, for example, for photons emitted from space into the atmosphere.

[0349] The wavelength of the emitted wave is selected, for example, based on one or more media through which it will pass; for example, these photons are preferred when photons in the infrared range must pass through the atmosphere or air.

[0350] Transmitter 2 generates, for example, entangled photon pairs using a spontaneous parameter downconversion (SPDC) method. Transmitter 2 is configured, for example, to emit less than one pair per unit time on average, such as one pair per nanosecond, which corresponds to a photon transmission frequency of 1 GHz.

[0351] When they are emitted, photons P are, for example, linearly polarized, meaning that the corresponding electromagnetic wave has an electric field perpendicular to its propagation direction D. In appropriate cases, the polarization of the photon can be vertical V or horizontal H, such as... Figure 2A As shown.

[0352] The quantum state corresponding to the polarization of a photon is sometimes indeterminate, as long as it is not measured or absorbed. Therefore, before being measured, the quantum state of a photon is sometimes considered as a superposition of possible states, i.e., in this example, as a superposition of polarizations at 45° and -45°.

[0353] exist Figure 2B In the variant shown, photon P is circularly polarized when emitted, meaning that the direction of the corresponding electric field changes during rotational motion, while its norm remains unchanged.

[0354] Under appropriate conditions, the polarization of a photon is defined by the direction of rotation of the electric field, either clockwise (C1) or counterclockwise (C2). Before being measured, a linearly polarized photon is in an indeterminate quantum state, considered as a superposition of two circularly polarized states with opposite rotation directions.

[0355] In some cases, such as Figure 3 As shown, two quarter-wave plates 6 and 8 can be introduced into the photon propagation path between transmitter 2 and receiver 3 and / or 4.

[0356] Depending on its orientation, waveplate 6 converts, for example, the linear polarization of a photon into circular polarization by causing an electromagnetic wave propagating along an axis perpendicular to the ordinary axis of the crystal to advance or delay relative to a wave having an electric field parallel to the ordinary axis using an electric field perpendicular to the same axis.

[0357] Photons emitted from waveplate 6 are transmitted, for example, through optical fiber 7 toward another quarter-wave plate, which converts the circularly polarized field into a linearly polarized field before propagating toward receiver 3 or 4.

[0358] This conversion of the polarization of emitted photons makes it possible to receive the photons at the receiver without having to consider the polarization direction of the photons emitted by the transmitter.

[0359] The optical selector 30 can be manufactured in various ways, some examples of which are as follows: Figures 4A to 4C As shown.

[0360] For example, optical selector 30 includes, as Figure 4A The controlled liquid crystal reflector shown includes a plate 310 surrounding the liquid crystal. For example, electrodes 315 and 320 are placed on opposite sides of the plate 310 to subject the liquid crystal to an electric field, thereby enabling control of its refractive index n. C .

[0361] According to the refractive index n CThe incident photon P1 can be reflected onto the propagation path D3, or it can be transmitted through the plate 310 and along the propagation path D1' toward the measuring instrument 35, which can be the same as or different from path D1.

[0362] exist Figure 4B In the variant shown, photon P1 passes through liquid crystal panel 310 and exits along D3 or D1'. In the example under consideration, directions D1, D1' and D3 are parallel.

[0363] The reflector 325, especially the Bragg reflector, can be placed on the propagation axis D3 to reflect photons in another direction, for example, towards the protective device 5.

[0364] The anti-reflective plate can be advantageously placed on each surface of the plate 310 and adjusted according to the photon's entry and exit angles as the photon passes through.

[0365] exist Figure 4C In the variant shown, the optical selector 30 is a mirror controlled by a nonlinear optical fiber. It comprises two optical fibers 330 and 335. Fiber 335 is formed, for example, from a nonlinear material.

[0366] When the first photon P1 reaches the first receiver, the first photon P1 enters fiber 330. Fiber 335 can be selectively illuminated simultaneously with the optical signal F (also known as the "control signal").

[0367] Signal F is, for example, a high-intensity light wave emitted by a laser, whose wavelength is different from that of photon P1.

[0368] If fiber 335 is illuminated by signal F, photon P1 is retained in fiber 330, for example, at the fiber exit before passing through prism 345 arranged on the propagation path D3. In a similar manner as described above, reflector 325, particularly a Bragg reflector, can be placed on the propagation axis D3 to reflect photons in a desired direction, for example, towards protective device 5, or to reflect photons directly back into fiber 330.

[0369] If fiber 335 is not illuminated by signal F, photon P1 is transmitted from fiber 330 to fiber 335. Photon P1 exits from fiber 335 and passes through prism 340, for example, before being transmitted toward measuring instrument 35. Prism 340 is preferably composed of a dispersive material that allows illumination light to pass through different positions and exit in a direction different from that of the photon. A Bragg reflector (not shown) that only returns the illumination light and allows photons to pass can be placed between fiber 335 and prism 340, for example, to reflect the illumination light in a direction other than toward prism 340, or even reflect it back into the fiber.

[0370] An anti-reflective plate (not shown) can be placed at the entrance and exit of prisms 340 and 345 and adjusted to the wavelength of photon P1.

[0371] The description will now be generated. Figure 1B Various methods of the polarization modifier 32 of the system.

[0372] The polarization modifier 32 preferably includes a polarization direction modifier 32a disposed upstream of the phase modifier 32b.

[0373] In a first exemplary embodiment of polarization direction modification, the polarization direction modifier 32a includes a stack of waveplates 510, particularly quarter-wave plates, which are oriented differently from each other, such as... Figure 5A As shown, a circular wave 508 entering one of these quarter-wave plates is emitted as a wave 511 linearly polarized along the direction associated with the wave plate.

[0374] These plates can be designed to operate in conjunction with a wave 508, which enters the plates in a direction parallel to the bisectors of the edges 521 and 522 of these waveplates. These waveplates are preferably composed of a uniaxial birefringent crystal, such as rutile. This birefringent crystal can form the entire mass of these waveplates, or it can be concentrated on a single slice, for example, concentrated on one of the edges 523 through which the wave 508 passes, such as... Figure 5A As shown.

[0375] These waveplates can be juxtaposed to form structure 524, such as Figure 5B As shown, the outgoing wave 511 or 512 is linearly polarized, and the direction of its electric field depends on the position of the two circularly polarized waves 508 or 509 entering the component 524, and the direction of the outgoing wave is parallel to the bisectors of the edges 521 and 522.

[0376] Therefore, the wave 5050 with a defined direction and linear polarization that enters the quarter-wave plate 501 passes through it along direction 507 and becomes circularly polarized therein before entering the crystal 502 (for example, the refractive index of the crystal 502 is adjustable under the action of the electric field generated by the electrode 516), and then preferably enters another crystal 503, for example, whose refractive index is also adjustable under the action of the electric field generated by the electrode 517, which is perpendicular to the electric field generated by the electrode 516.

[0377] Circularly polarized waves 508 and 509 then exit from crystal 503 and enter assembly 524 at different locations depending on the choice of the refractive index applied to crystal 503. The direction of the light rays varies depending on these locations, and the directions of edges 520 and 521 are preferably different for each element 510 of assembly 524.

[0378] Therefore, in Figure 5B In the diagram shown, waves 511 and 512 represent two possibilities for light rays passing through component 524. These waves exit component 524, are linearly polarized in different directions, and have the same propagation direction, but at different positions on the boundary 514 of component 524. They enter another crystal 504 whose refractive index can be electrically adjusted via electrode 518, and then preferably enter another crystal 505 whose refractive index can also be adjusted via electrode 519, the electric field generated by electrode 519 being perpendicular to the electric field generated by electrode 518. The voltage of electrode 518 is adjusted according to the refractive indices of the selected crystals 502 and 503, such that all light waves exit crystal 505 as waves 513 having the same direction at the same position 515.

[0379] Crystals 502, 503, 504, and 505 are, for example, Merck E7 liquid crystals. For instance, if the distance between each pair of electrodes is 5 mm, a voltage of 0 to 5000 V is applied to the terminals of these crystals, allowing their refractive indices to be changed between 1.5 and 1.67. The electrodes are preferably covered with a dielectric film. These refractive indices modified by the electric field can vary depending on the relative direction and direction of the electric field of the light. Applying a continuous perpendicular voltage allows modification of the refractive index of each of the two components of the light, where the electric field is perpendicular to the direction of the light. Figure 5B The superposition of the wave and the electric field perpendicular to the propagation direction and in the plane of the graph.

[0380] In a second exemplary embodiment (not shown) of polarization direction modification, a wave linearly polarized along a defined direction enters a first quarter-wave plate whose polarization is converted to circular polarization, and then enters a second quarter-wave plate, which converts the circular polarization back to linear polarization along an adjustable direction depending on the orientation of the second quarter-wave plate. The orientation of the second quarter-wave plate is obtained, for example, by mechanical actuation of a sensor or an electrically adjustable device that enables its rotation, or by friction set on a moving shaft by a piezoelectric material or an electric motor device (e.g., DC-operated).

[0381] In the third example, such as Figure 5CAs shown, liquid crystal is used to control the rotation of the linear electric field of incident photons 526 linearly polarized along the same polarization direction. The incident photons 526 begin, for example, by passing, preferably perpendicularly, through one of two 0.2 μm transparent electrodes 527. This transparent electrode has a surface 528 coated with a material that orients liquid crystal particles 529 adjacent to the surface 528 in the polarization direction of the incident photons 526. The incident photons 526 then pass through liquid crystal particles 529 larger than 2 μm before passing through a second transparent electrode 527. The surface 528 of this second transparent electrode is preferably parallel to the first electrode and is also coated with a material that allows the liquid crystals to align in a direction perpendicular to the direction in which the coating is applied to the first electrode. Therefore, the orientation of the liquid crystal between the two electrodes gradually changes from 0° to, for example, 60° relative to its initial orientation. Applying a voltage, for example, between 0 and 1 V, gradually reorients the liquid crystal to apply an orientation along the axis of photon propagation, thereby gradually reducing the refractive index difference between the two polarizations perpendicular to the photon motion and the rotational power of the device. Therefore, the device can continuously select the departure direction of the polarization of the incident photon between 0° and 60°.

[0382] Such as Figure 5B The apparatus shown is advantageously used for applications similar to those described above and Figure 5A In the apparatus shown, for example, placed between plates 524 and 504, each of the four elements 510 enables the linear polarization of the incident photon to be placed in one of four precise directions, such as -90°, -45°, 0°, and +45°. Then, Figure 5B Each of the devices described herein is placed at the rear, preferably aligned with the direction of propagation of photons emitted from the preceding device 510, such that rotation at any angle, for example, from 0° to 45°, can be added to that rotation.

[0383] Figure 6 A polarization phase modifier 32b according to the present invention is shown. The phase modifier 32b includes a first birefringent plate or prism 310, which is arranged to split an incident photon P1 having a polarization field E into two electromagnetic waves having linear polarizations E1 and E2 on two different axes x1 and x2.

[0384] A delay plate 321 with a variable refractive index, such as a Pockel cell of lithium niobate, which is adjusted by an electrode 322 or a nonlinear material, is arranged on a second axis x2 such that a wave E2 oriented along the second axis x2 acquires a predetermined phase shift relative to a wave E1 oriented along the second axis x1.

[0385] Another birefringent plate 310, for example composed of parachute, is arranged at the exit of the phase modifier 32b, such that two waves with polarization fields E1 and E2 perpendicular to each other can be combined into a single wave or photon P1 on the same axis x, the field E' of which has acquired a phase shift relative to field E. Alternatively, in order to modify the crystal refractive index according to the direction of the electric field of light relative to the direction of the electric field that modifies the refractive index, a simple Pockels cell 321 with electrodes 322 but without plate 310 can be used.

[0386] The measuring instrument 35 of the first receiver 3 can be of various types and includes various components. These components depend in particular on the polarization properties of photon P1 when it is emitted by emitter 2. Some examples are as follows: Figure 7 and Figure 8 As shown, and described below.

[0387] exist Figure 7 In the example shown, the measuring instrument 35 includes a polarization filter 350 arranged on the propagation path D1 of photon P1 and a photon detector 355 arranged on the same propagation path downstream of the filter 350.

[0388] The polarization filter 350 allows only photons with a specific linear polarization to pass through and absorbs photons with polarization perpendicular to it. Therefore, it is possible to select photons with linear polarization in a specific direction.

[0389] In the example considered, the polarizing filter 350 is a grid formed by vertical straight lines (e.g., metal wires). It only allows photons with horizontal linear polarization H to pass through.

[0390] In a variant, such as Figure 8 As shown, the measuring instrument 35 includes an anisotropic panel 360, such as a birefringent plate, a birefringent prism, or two birefringent prisms attached to each other. For example, the panel is made of barium β-borate (BaB2O4, BBO).

[0391] The photon P1 of the quantum state to be measured arrives at panel 360 on the incident propagation path D1 and propagates on one of the two propagation paths, D11 or D12, depending on whether the polarization of photon P1 is in the plane of panel 360 (that is, perpendicular to the plane of the plate). Figure 8 (The plane) or the plane perpendicular to the 360° plane of the plate.

[0392] The measuring instrument 35 also includes two detectors 355, each of which is placed on the propagation path D11 or D12 of photon P1.

[0393] In this example, unlike polarization filters, all photons can be detected by detector 355, regardless of their polarization.

[0394] In some implementations, a quarter-wave plate is placed upstream of the anisotropic plate 360 ​​to convert the circular polarization of photons into linear polarization, thereby detecting the rotation direction of the circularly polarized photons.

[0395] The second receiver 4 may include various types of optical amplifiers 40 and measuring instruments 45.

[0396] For example, using Figure 9 The doped fiber amplifier shown.

[0397] In this example, amplifier 40 includes optical fiber 400, specifically an optical fiber made of a nonlinear material, which introduces the second photon P2 into the optical fiber 400 after passing through dichroic prism 401 when the second photon P2 arrives at receiver 4.

[0398] The control electromagnetic wave F, which provides energy and multiplies photon P2, is introduced into the same dichroic prism 401 through optical fiber 410. Its introduction point and direction make it exit from optical fiber 400 like photon P2. Photon P2 and control wave F have different wavelengths.

[0399] The control wave F preferably has a high luminescence intensity and preferably has a wavelength shorter than that of photon P2.

[0400] When photon P2 passes through optical fiber 400, under the influence of luminous flux F, photon P2 is multiplied into N photons P20. These N photons and luminous flux F then pass through the second dichroic prism 402, exiting from the prism in different directions. The N photons are then advantageously guided toward the measuring instrument 45, thereby enabling the determination of the polarization of the luminous flux. Figure 10A and Figure 10B An exemplary implementation of the polarization of luminous flux is given.

[0401] The information transmitted by the first receiver can be derived from the measurement of the measuring instrument 45, because the amplification of photon P2 by device 40 maintains its polarization.

[0402] Specifically, if photon P2 is an entangled photon of photon P1 that absorbs with one of the two complementary polarizations known to the first receiver, then the polarization of the N multiplied photons P20 can be derived from the absorption polarization of photon P1 using the Jones matrix calculated during calibration, as described above.

[0403] Measuring instrument 45 may include a series of semi-reflective plates 452 and reflectors 453, such as Figure 10AAs shown. Plate 452 and mirror 453 guide the photons P20, which have multiplied the luminous flux, toward prism 350, which is made of, for example, a birefringent material and preferably has equal intensity. Filter 350 splits the luminous flux into two orthogonally polarized luminous fluxes, such that detector 455 can determine the intensity of the luminous flux along each orthogonal direction, or determine the phase shift between the two orthogonal components of the luminous flux, for example, by means of a Michelson interferometer or a Young's slit interferometer.

[0404] The interferometer is preferably arranged such that the optical paths of the two orthogonal components are identical, and the luminous flux multiplied from a single photon is very short. Using multiple pairs of orthogonal directions, such as orthogonal directions offset from each other by 45°, it is advantageous to measure polarization multiple times, that is, the direction of polarization and the phase shift between the two directions, thus allowing for higher measurement accuracy.

[0405] Figure 10B This illustrates an example of such a Young's slit interferometer. Two waves, 460 and 463, originate from prism 350. Wave 460 passes, for example, through prism 461, which restores its propagation direction such that at 462 it becomes parallel to wave 463. Wave 463 passes, for example, through a half-wave plate, making it possible to make the electric field of wave 465 parallel to the electric field of wave 462. Waves 462 and 465 then pass through two apertures formed in screen 466 before interfering with each other, so as to form fringes on screen 467, which is observed by a camera or equipped with a photosensitive sensor, allowing the determination of the position of the brightest fringe on screen 467.

[0406] Information items, especially binary information items, can be received by receivers 3 and 4 in system 1 (e.g., Figure 1B Transmission between receivers 3 and 4 shown, for example by following Figure 11A and Figure 11B The steps shown are for transmission.

[0407] The correspondence protocol between the information items to be transmitted, such as the correspondence protocol between sending bit "0" or bit "1" and measuring or not measuring the first photon P1, is determined before transmission begins.

[0408] For example, the following choices can be made: not measuring the first photon P1 to transmit "0", or measuring the first photon P1 to transmit "1". Of course, the opposite choice or any other suitable correspondence is also valid.

[0409] In step 10, two entangled photons P1 and P2 are emitted simultaneously from transmitter 2 toward receivers 3 and 4, respectively. As mentioned above, receiver 3 is placed closer to transmitter 2 than receiver 4.

[0410] The emitted photons P1 and P2 have uncertain quantum states in a predetermined complementary polarization pair, for example, if they are linearly polarized, they have a polarization of 45° (complementary polarizations are 0° and 90°).

[0411] In step 11, photon P1 reaches the optical selector 30 of receiver 3.

[0412] If we want to transmit bit "0", for example, photon P1 is... Figure 11A The optical selector on the vertical path (though there may be other paths) in the example shown reflects light, thus preventing it from reaching the measuring instrument 35. In particular, photon P1 can be trapped in the device 5 for protecting its quantum state in step 12 to prevent its absorption, at least as long as the second photon P2 has not yet reached the second receiver 4.

[0413] For example, photon P1 maintains its polarization at 45°.

[0414] If a bit "1" is to be transmitted, photon P1 is guided toward measuring instrument 35, for example, through optical selector 30. Figure 11B As shown.

[0415] Then, in step 13, the quantum state of photon P1 is measured using measuring instrument 35.

[0416] Photon P1 now has a defined quantum state, such as vertically polarized (90°) or horizontally polarized (0°). The measurement performed in step 13 instantaneously projects entangled photon P2 into the defined state.

[0417] In step 15, regardless of the information to be transmitted, photon P2 arrives at the optical amplifier 40 of the second receiver 4, where it is replicated to form a flux of multiplied photons P20, each photon P20 maintaining the polarization of photon P2.

[0418] In step 16, the polarization of the flux of photon P20 is measured by measuring instrument 45.

[0419] If an intermediate result is obtained by averaging, such as a polarization of 45°, it can be inferred that the first photon P1 has not been measured and bit "0" has been received.

[0420] If a quantum state corresponding to the measurement in the first receiver 3 is obtained, such as vertical (90°) or horizontal (0°) polarization, it can be inferred that the first photon P1 has been measured and bit "1" has been received.

[0421] In addition, receivers 3 and 4 in system 1 (such as...) Figure 1A A series of discrete values ​​are transmitted between receivers 3 and 4 as shown, for example, by following Figure 12The steps shown are for transmission.

[0422] In step 10, two entangled photons P1 and P2 are emitted simultaneously from transmitter 2 toward receivers 3 and 4, respectively, for example, with linear polarization.

[0423] In step 17, photon P1 reaches polarization modifier 32, which converts the linear polarization of photon P1 into a selected complementary polarization pair, which is selected according to the Jones form and corresponds to the discrete value to be transmitted.

[0424] In step 13, photon P1 is absorbed by instrument 35 in one of the two complementary states of the selected pair, thereby projecting entangled photon P2 into its complementary state.

[0425] The subsequent steps 15 and 16 are similar to those described above; the polarization of photon P2 is measured by amplification and then absorption, and the discrete values ​​of the transmission are derived.

[0426] This invention is not limited to the measurement of the linear polarization of photons. Other types of quantum states can be measured and / or other measurement methods can be used, particularly measurements with other observable bases.

[0427] Photons P1 and P2 are emitted, for example, with circularly entangled polarization, and by measuring linear polarization, measuring instrument 35 projects the entangled photons P1 and P2 into a linear polarization basis.

[0428] As a variant, photons P1 and P2 are emitted with linearly entangled polarization, and by measuring circular polarization, measuring instrument 35 projects entangled photons P1 and P2 according to the observable basis.

[0429] In another example, photons P1 and P2 are emitted with vertical or horizontal entangled linear polarization, and measuring instrument 35 measures the linear polarization at 45° or -45° to the vertical or horizontal.

[0430] In one variant, receiver 3 includes two measuring instruments 35, one measuring linear polarization and the other measuring circular polarization. For example, if bit "1" is to be transmitted, the optical selector sends photon P1 toward the first measuring instrument; if bit "2" is to be transmitted, photon P1 is sent toward the second measuring instrument; and if bit "0" is to be measured, measurement of photon P1 is prevented.

[0431] In each of the above examples, the measuring instrument 45 of the second receiver 4 can be configured to detect whether the first photon P1 has been measured, and if so, what polarization is measured at the first receiver.

[0432] Furthermore, gyroscopes can be used at transmitter 2 and receivers 3 and 4 to determine the polarization directions of photons P1 and P2 if they are emitted with linear polarization and transmitted toward the receiver.

[0433] As described above, receivers 3 and 4 and device 5 may be provided with means for counting photons during photon absorption and optionally after they have waited to absorb their entangled photons, which may be time-stamped with their arrival, such as... Figure 13 As shown. For example, this allows control over the correspondence between entangled photon pairs and transmitted bits.

[0434] For example, each photon triggers a timestamping process by arriving at the receiver in step 80, specifically by arriving at one of the measuring instruments 35 or 5D. Following this triggering event, in step 82, the receiver 3 copies, for example, the current time H on the receiver's clock, into register R on the free memory register M, preferably after the travel time of the photon in the waiting device 5 has been subtracted from the time of arrival at the 5D detector.

[0435] A similar device (not shown) can be arranged at the second receiver 4.

[0436] Preferably, the clock that makes the timestamp function described above possible is synchronized for the two receivers 3 and 4, which optionally makes it possible to define a photon processing time interval shared by the two receivers and to process “multiple” two-photon pairs emitted by transmitter 2.

[0437] Furthermore, the second receiver 4 may include a switch 50, which is positioned in front of the optical amplifier 40 and configured to absorb or reflect one or more possible undesired photons PE2 arriving at the receiver 4 after photon P2 within the same processing time interval, such as... Figure 14 As shown.

[0438] Switch 50 can also absorb possible photons PA2 emitted by amplifier 40 toward transmitter 2 during signal amplification or during relaxation of molecules or atoms in the amplifier medium.

[0439] The switch 50 is regulated, for example, by an electronic mechanism (not shown), which is itself controlled by the measuring instrument 45 when one or more measuring instruments 45 detect photons.

[0440] In some embodiments, the system according to the invention may include multiple transmitters arranged differently relative to the receiver, particularly for establishing bidirectional communication.

[0441] exist Figure 15In the example shown, system 1 includes two transmitters 21 and 22, each of which emits entangled photon pairs toward two receivers 91 and 92.

[0442] Transmitter 21 is closer to receiver 91 than receiver 92, and transmitter 22 is closer to receiver 92 than receiver 91. This arrangement enables receivers 91 and 92 to communicate bidirectionally; the photon P11 emitted by transmitter 21 arrives at receiver 91 first, and receiver 91 can then act as a first receiver 3, that is, it can transmit information items to receiver 92, and receiver 92 receives its entangled photon P12 and acts as a second receiver 4.

[0443] Conversely, the photon P21 emitted by transmitter 22 first reaches receiver 92, this time enabling receiver 92 to send an information item to receiver 91, which in turn receives the entangled photon P22. For example, the mechanism for transmitting information, particularly binary information, is similar to the one described above. Figure 11A , Figure 11B and Figure 12 The mechanism described.

[0444] Reflecting devices 93 and 94 can be arranged near receivers 91 and 92 respectively, so as to reflect photons that may be reflected by receivers 91 and 92.

[0445] refer to Figure 16 A variation of the present invention will now be described, which is designed to verify that the information transmitted by transmitter 2 has not been eavesdropped on, and, where appropriate, to generate a shared key for information exchange between receivers 3 and 4 based on the transmission of information known to have not been eavesdropped on.

[0446] By adding at least one polarizing filter 46 before entering receiver 4, for Figure 16 The system was modified so that the polarization direction of the filter corresponds to the expected polarization direction of the photons traveling from the transmitter to the receiver 4. Therefore, the polarization filter 46 is placed upstream of the photon polarization change caused by the information sent from the receiver 3 to the receiver 4.

[0447] Filter 46 prevents a deceptive receiver from replacing the real receiver 4, prevents the observation of the precise polarization of entangled photons after photon multiplication, and prevents the transmission of deceptive photons instead of real photons to the real receiver 4. Receiver 4 will necessarily observe that the deceptive photons have the same relative polarization as the real photons because, on the one hand, if the photon is not entangled with another photon, its polarization will not change after passing through filter 46, and therefore it will have the polarization imposed by filter 46 instead of the polarization of the real photon. On the other hand, if the deceptive photon is entangled with another photon, referred to as the second deceptive photon, it is impossible to set the relative polarization of these deceptive photons if the relative polarization of the entangled photons is not sufficiently verified by a third device, nor is it possible to destroy or stop the deceptive photons after they pass through filter 46.

[0448] Therefore, if the relative polarizations of a set of entangled photons observed by receivers 3 and 4 are still different, it is extremely unlikely that all or some of these entangled photons will be observed elsewhere.

[0449] By applying the following method, the equality of the quantum states of photons used to transmit information terms arriving at each of the two receivers 3 and 4 can be statistically verified, and it can be inferred that the probability of observing some photons is low:

[0450] When each of receivers 3 and 4 receives a photon, the receiver records the reception time and the precise polarization of the photon by using a photon detector, not just the complementary polarization pair to which they belong. However, preferably, the time and polarization of detected photons that are too close to each other to independently measure their polarization or their arrival time are not recorded, and the two receivers 3 and 4 preferably have a synchronized clock.

[0451] After a certain number of photons carrying information I have been received, the second receiver 4 generates a message containing the precise detection time and polarization of each photon. These properties of the photons are recorded and considered to be transmitting information, and therefore belong to a group of NSP photons with a given polarization or actually a complementary polarization of that polarization; NSP is defined above as a predetermined threshold number of photons received with a given polarization or its complementary polarization, thereby allowing the reception of identification information.

[0452] The message is signed by receiver 4, preferably using the random hashing technique described in patent application US 20210165914 A1, and then sent along with the signature to receiver 3, preferably using the quantum transmission device described above. The signature of the received message is then verified, and for photon pairs where both photons arrive at the receivers, the correspondence of the relative polarizations of the photons received by the two receivers is also verified. If the signature verification indicates that the message I is genuine and the proportion of non-corresponding polarizations is below a given threshold (e.g., 1% if using 450 different polarization pairs, or a multiple of the probability that two different pairs of entangled photons are emitted at a time indistinguishable to the receivers, e.g., 2), then the message I can be declared as not having been eavesdropped on during transmission between the two receivers.

[0453] Furthermore, a relative polarization of each pair of photons in the transmitted information I is considered to be different for each photon arriving at each receiver. This relative polarization can be used to form a bit sequence that forms a randomly generated key known only to each receiver. Receiver 3, having verified the comorbidity of the relative polarization, will send a signed message to another receiver 4. This signed message is preferably signed using a random hash technique and is formed from a list of the reception dates of photons whose relative polarizations do not correspond or a list of the reception dates of photons whose entangled photons have never been received by receiver 3.

[0454] Therefore, in one example, the following steps are performed.

[0455] Step 1

[0456] Receiver 4 receives the photon flux that conveys information from transmitter 2 and establishes a first list for defining the reception dates of the photons that convey the information, as well as the relative polarizations of the two complementary polarizations of the received photons.

[0457] Step 2

[0458] Receiver 4 forms a message consisting of a list of information collected in the previous step, creates an electronic signature for the list, and sends the list and signature to receiver 3.

[0459] Step 3

[0460] Receiver 3 receives the list and signatures, and then verifies the signatures.

[0461] Step 4

[0462] Receiver 3 creates a second list consisting of elements from the first list. For the second list, photons received by receivers 3 and 4 have the same relative polarization, or entangled photons never reach receiver 3. Receiver 3 stores in a register the relative polarization of each photon used for information transmission and its reception date. Knowing the difference in travel time (path time difference) of entangled photons along the path between transmitter 2 and each of the two receivers 3 and 4, receiver 3 can determine the relative polarization of the photon received on the date of reception by receiver 4 minus the path time for each photon in the first list. This second list preferably consists of two separate parts: the first part contains photons whose entangled photons never reach the first receiver 3, and the second part contains photons whose two entangled photons both reach their respective receivers but have equal relative polarization.

[0463] Step 5

[0464] If the number of elements in the second list that are restricted to entangled photon pairs (each of which has two photons arriving at its respective receiver) is less than the product of a predetermined ratio and the count of elements in the first list that are restricted to entangled photon pairs (each of which has two photons arriving at its respective receiver), then the information is declared to have been transmitted without being eavesdropped.

[0465] Step 6

[0466] If the information is declared to have been transmitted and not eavesdropped, the second list is signed by receiver 3 and sent to receiver 4, and then receiver 4 creates a third list, which consists of the relative polarizations of photons that appear in the first list but not in the second list, i.e., photons with different observed relative polarizations.

[0467] Conversely, if the product is higher than the threshold, a message indicating that the transmission may be eavesdropped is sent from receiver 3 to receiver 4.

[0468] Step 7

[0469] After receiving the second list and verifying the signature, receiver 4 recreates the third list in receiver 4 using the first and second lists, and then receiver 4 sends a signature message to receiver 3 confirming successful receipt of the second list.

[0470] Step 8

[0471] Receiver 4 can use the third list as a shared key with receiver 3, and receiver 3 can use the third list as a shared key with receiver 4 when receiver 3 receives the message sent in step 7.

Claims

1. A quantum communication system (1), comprising: • An entangled photon emitter (2), the emitter comprising a source configured to generate at least one pair of entangled photons, the at least one pair of entangled photons comprising a first photon (P1) emitted on a first propagation path (D1) and a second photon (P2) emitted simultaneously on a second propagation path (D2) different from the first propagation path. • A first receiver (3; 91) is arranged on the first propagation path (D1) and includes a composite absorber (31) configured to absorb photons in a polarization state selected from at least two different pairs of complementary polarization states, except for exactly two perpendicular linear polarization pairs in which the polarization of one pair is at 45° to the polarization direction of the other pair. • A second receiver (4; 92), arranged on the second propagation path (D2) so that after the first photon (P1) reaches the first receiver (3), the second photon (P2) reaches the second receiver, the second receiver (4) comprising: - An optical amplifier (40) for multiplying the second photon (P2) while maintaining its polarization, and - A measuring instrument (45) arranged downstream of the optical amplifier is used to measure the average polarization of the multiplied photons (P20).

2. The system according to claim 1, wherein, The composite absorber (31) is configured to absorb photons in a predetermined polarization state, which is selected from at least three different complementary polarization pairs.

3. The system according to claim 1, wherein, The composite absorber (31) includes: - At least one instrument (35) for absorbing photons in one or the other of two complementary polarization states. - A polarization modifier (32) is arranged upstream of the instrument (35) and configured to convert the polarization of the first photon (P1) to a selected polarization, the instrument (35) absorbing the photon along the selected polarization.

4. The system according to claim 3, wherein, The polarization modifier (32) includes a polarization direction modifier (32a) disposed upstream of the polarization phase modifier (32b).

5. The system according to claim 4, wherein, The polarization direction modifier (32a) includes two quarter-wave plates (310) arranged one after another in the propagation path of the first photon (P1), at least one of which has a variable orientation.

6. The system according to claim 4, wherein, The polarization direction modifier (32a) includes a plate or prism made of a chiral or rotating material (316) that causes a polarization rotation by an angle depending on the position of the wave entering the chiral or rotating material.

7. The system according to claim 4, wherein, The polarization phase modifier (32b) includes a first birefringent plate or prism that splits the beam into two electromagnetic waves with linear polarization, one electromagnetic wave along a first axis and the other electromagnetic wave along a second axis, and a delay plate with a variable refractive index is arranged on the second axis.

8. The system according to claim 3, wherein, The at least one instrument (35) includes at least one filter (350; 360) for transmitting the first photon to one or the other of two photon detectors (355) according to the polarization state of the first photon.

9. The system according to claim 1, wherein, The measuring instrument (45) of the second receiver (4) includes at least one photon detector (455) which is arranged to measure the polarization of a photon (P20) multiplied from the second photon (P2).

10. The system according to claim 1, wherein, The measuring instrument (45) of the second receiver (4) includes a series of semi-reflective plates arranged downstream of the optical amplifier (40), which guide the flux of multiplied photons (P20) toward the polarization measuring instrument with equal intensity. The polarization measuring instrument is arranged to measure the intensity of the flux of multiplied photons (P20) along two vertical axes and the phase shift of the light between the two axes.

11. The system according to claim 1, wherein, The optical amplifier (40) is a doped fiber amplifier.

12. The system according to claim 1, wherein, The transmitter (2) is configured to continuously generate multiple entangled photon pairs.

13. The system according to claim 1, wherein, The transmitter (2) and each of the receivers (3; 4) include a clock, and the clocks of the transmitter and the receiver are synchronized with each other.

14. The system according to claim 1, wherein, The second receiver (4) includes a switch (50) arranged in front of the optical amplifier (40), the switch being configured to absorb or reflect one or more photons following a first photon arriving at the second receiver within a predetermined time interval.

15. The system of claim 1 further includes a second transmitter (22) capable of generating one or more entangled photon pairs, the second transmitter being closer to the second receiver (92) than to the first receiver (91).

16. A quantum communication method using the system according to claim 1, comprising: - An entangled photon pair is generated from the transmitter (2), in which the first photon (P1) of the entangled photon pair is emitted toward the first receiver (3), and at the same time the second photon (P2) of the entangled photon pair is emitted toward the second receiver (4). The first photon and the second photon are entangled. The second receiver (4) is farther away from the transmitter (2) than the first receiver (3) and is located on the second propagation path (D2) of the second photon (P2), so that the second photon arrives at the second receiver later. - By using a polarization modifier (32), the polarization state of the first photon (P1) when it arrives at the first receiver (3) is modified to a polarization state that depends on the information to be transmitted. This polarization state is selected from at least two different complementary absorption polarization pairs, except for exactly two pairs of perpendicular linear polarizations, in which the polarization direction of one pair of perpendicular linear polarizations is at 45° to the polarization direction of the other pair of perpendicular linear polarizations. - The first photon (P1) in one of the two complementary polarizations of the selected complementary linear polarization pair is absorbed by using an absorption instrument (35). - At the second receiver (4), the second photon (P2) is replicated using an optical amplifier (40) to form a flux of a multiplied photon (P20). The resulting multiplied photon (P20) maintains the polarization state of the second photon (P2). - Measure the average polarization state of the flux of the multiplied photon (P20), and determine the polarization state of the first photon (P1) based on the measurement, so as to derive the information transmitted by the first receiver (3).

17. The method according to claim 16, wherein, The absorption polarization pair is selected from at least three different absorption polarization pairs.

18. The method according to claim 16, wherein, The absorption polarization pair is selected from at least 210 different absorption polarization pairs.

19. The method of claim 16, wherein, The transmitter (2) continuously generates multiple entangled photon pairs, each photon pair enabling the transmission of information items from the first receiver (3) to the second receiver (4).

20. The method of claim 16, wherein, By performing the following steps, it is verified that the information transmitted by the photon flux from the transmitter (2) to the second receiver (4) has not been eavesdropped on: - The second receiver (4) establishes a first list of the reception dates of the photons that convey the information item, and specifies the relative polarization of the polarizations in the two complementary polarizations of each photon received; - The second receiver (4) generates a message containing the first list collected in the previous step, creates an electronic signature of the first list, and sends the first list and the electronic signature to the first receiver (3). - The first receiver (3) receives the first list and the electronic signature, and then verifies the electronic signature; - The first receiver (3) makes a second list consisting of the elements of the first list, for which the relative polarization of the photons received by the first receiver (3) and the second receiver (4) is equal, or the entangled photons never actually reach the first receiver (3), and the two entangled photons have different relative polarizations; - If the number of elements in the second list that are restricted to entangled photon pairs where each of the two photons arrives at its respective receiver is less than a predetermined ratio multiplied by the count of elements in the first list that are restricted to entangled photon pairs where each of the two photons arrives at its respective receiver, then the information item is declared to have been transmitted without being eavesdropped.

21. The method of claim 20, wherein: - If the information item is declared to have been transmitted without being eavesdropped, the second list is signed by the first receiver (3) and sent to the second receiver (4), and then the first receiver (3) creates a third list consisting of the relative polarizations of photons that appear in the first list but not in the second list. - After the second receiver (4) receives the second list and verifies the signature, the third list is recreated in the second receiver (4) using the first list and the second list, and then the second receiver (4) sends a signature message confirming successful receipt of the second list to the first receiver (3); as well as - The second receiver (4) uses the third list as a shared key with the first receiver (3), and when the first receiver (3) receives a message transmitted by the second receiver in the previous step, the first receiver (3) uses the third list as a shared key exchanged with the second receiver (4).

22. The method according to claim 20, wherein, The polarizing filter (46) is placed upstream of the second receiver (4).

23. A method for calibrating a system as claimed in claim 1, for determining the polarization state of a photon arriving at a second receiver based on the polarization state of an entangled photon being absorbed by a first receiver, the method comprising: - An entangled photon pair is generated from the transmitter (2), wherein the first photon (P1) of the entangled photon pair is emitted toward the first receiver (3), and the second photon (P2) of the entangled photon pair is emitted toward the second receiver (4), wherein the first photon and the second photon are entangled with respect to their polarization states. - At the first receiver, the first photon in a predetermined polarization pair is absorbed; one polarization of each absorbed photon in two possible polarizations is stored; and once a predetermined number of different polarizations have been observed, photon transmission by the transmitter is stopped. - At the second receiver (4), the second photon (P2) is replicated using an optical amplifier (40) to form a flux of multiplied photons (P20), each multiplied photon (P20) retaining the polarization state of the second photon (P2). - Measure the polarization state, that is, measure the polarization direction and phase shift of the flux of each multiplied photon, and store these measurements and their reception time. - The list of stored polarizations and reception times of the photons is transmitted from the first receiver to the second receiver. - Remove photons from the list of photons received at the second receiver that do not correspond to photons received at the first receiver, and remove photons from the list of photons received at the first receiver whose corresponding photons were not received at the second receiver. - On the one hand, by using the knowledge of the polarization states of two photons detected at the first receiver with complementary polarization and whose entangled photons have reached the second receiver, and on the other hand, by using the knowledge of the polarization states of the corresponding entangled photons received at the second receiver, the Jones matrix is ​​calculated so that the polarization of the photons received at the second receiver can be inferred from the polarization of the photons received at the first receiver.

24. A method for calibrating a system as claimed in claim 1, for determining the probability that a first photon (P1) is lost during its journey from the transmitter to the first receiver, the method comprising: - Configure the composite absorber (31) of the first receiver to absorb photons of one of two complementary polarizations in a pair of absorption polarizations received at the first receiver (3). - Multiple entangled photon pairs are continuously generated from the transmitter (2), wherein the first photon (P1) of the entangled photon pair is emitted toward the first receiver (3), and the second photon (P2) of the entangled photon pair is emitted toward the second receiver (4), wherein the first photon and the second photon are entangled in relation to their polarization states, and - Count the number of photons received at the second receiver that fall within each polarization range detectable by the second receiver, the polarization range being different from one of the two polarization ranges of entangled photons of photons absorbed by the first receiver.

25. A method for calibrating the system of claim 1, comprising determining two Jones matrices, wherein one Jones matrix enables the calculation of the polarization of photons arriving at the second receiver based on the polarization of photons absorbed at the first receiver, wherein, The amount of different polarizations observable at the second receiver is more than twice the reciprocal of the photon transmission ratio between the transmitter and the first receiver, the method comprising: - Configure the composite absorber of the first receiver to absorb photons received at the first receiver that are polarized in one of a pair of complementary absorption polarizations. - Multiple entangled photon pairs are continuously generated from the transmitter (2), wherein the first photon (P1) of the entangled photon pair is emitted toward the first receiver (3), and the second photon (P2) of the entangled photon pair is emitted toward the second receiver (4), wherein the first photon and the second photon are entangled in terms of their polarization states. For each polarity of a photon received at the second receiver, count the number of photons that have arrived at the second receiver with the same polarization. - Stop sending photons when a predetermined number of photons have been received at the second receiver. - Determine the two polarizations of the photons most frequently received at the second receiver, these polarizations being considered to correspond to the absorption polarization of the photons at the first receiver. - Calculate two Jones matrices to transform the polarization, such that the polarization of the photon received at the second receiver can be inferred from the polarization of the photon received at the first receiver.

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