QUANTUM COMMUNICATION SYSTEM WITH INTERLOCKED PHOTONS

Abstract

The invention relates to a quantum communication system (1), which comprises: • an entangled photon transmitter (2) comprising a source which is 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 on a second propagation path (D2); • a first receiver (3) which is arranged on the first propagation path (D1) and comprises a first instrument (35), which is arranged to absorb the first photon in one of two complementary polarisations, and an optical selector (30), which is configured either to allow the first photon to pass towards said first measuring instrument (35) or to prevent it from being measured; and • a second receiver (4) which is arranged on the second propagation path (D2) and comprises an optical amplifier (40), which makes it possible to demultiply the second photon while maintaining its polarisation, and a measuring instrument (45), which makes it possible to measure the average quantum state of the demultiplied photons (P20).

Description

[0001] Description

[0002] Title: Quantum communication system using entangled photons Technical field

[0003] The present invention relates to quantum communication methods, and more particularly to those using a pair of entangled photons.

[0004] Prior art

[0005] The transmission of information is currently carried out mainly by electromagnetic waves, whether these are short wavelengths, such as light, or long wavelengths such as VHF waves.

[0006] The transmitted waves can be guided by metal cables, by optical fibers, or transmitted through space.

[0007] These methods use wave propagation to transmit energy and therefore physical particles which, collected by a receiver, allow information to be deduced, this being coded, for example by its wavelength or by modulation of the duration of the wave trains.

[0008] Recently, so-called “quantum” communication methods have been developed.

[0009] For example, the physical state of a photon, such as its polarization, can be used to transmit information, for example, through an optical fiber. The value of a bit is arbitrarily assigned to a direction, or a mode of polarization of the photon. Sequences of polarized photons can then be sent to transmit binary sequences forming a message.

[0010] In quantum cryptography, it is known to use a pair of entangled photons to transmit information securely. Entangled photons are photons whose quantum state, for example their polarization, depends on each other regardless of the distance separating them.

[0011] This phenomenon of quantum entanglement has been observed and demonstrated experimentally on several occasions, such as in the article by Shasi Prabkhar et al. “Two-photon quantum interference and entanglement at 2.1 pm” (Sci Adv, 2020) Patent KR101003886B1 describes an encrypted information transmission system in which entangled photons are emitted simultaneously and each sent to two targets positioned at different locations and wishing to communicate with each other. The photons received by the two targets form random entangled sequences of conjugated bits, these sequences being used as cryptographic encryption keys. This system allows the almost instantaneous reception in two different locations of the same sequence of random numbers but does not allow the transmission of information.

[0012] Wang's paper "Superluminal telecommunication: an observable contradiction between quantum entanglement and relativistic causality" discloses a quantum communication system enabling superluminal information transmission using a pair of entangled photons sent to two receivers wishing to communicate with each other. The photons are entangled, and a linear polarization is measured for the photon arriving at the first receiver from different directions at 45° to each other. At the second receiver, the second photon is demultiplied, and the average polarization of the demultiplied flux is measured to determine in which direction the first photon was measured.

[0013] The polarization of a photon is not necessarily binary, however, like that of a spin or even quaternary. It can be represented on the Jones sphere, which characterizes the orientation and ellipse of the polarization: a polarization can in fact be linear, the electric field always being parallel to an axis perpendicular to the direction of propagation of the photon, or circular, the electric field rotating around this axis, or between the two: the electric field traveling an ellipse around the axis of propagation. In the case where the polarization is not circular, the orientation measures the direction of this axis, and the eccentricity measures the flattening of the ellipse.

[0014] The above-mentioned devices do not allow all polarization configurations to be exploited and are in fact limited in the number and type of information that can be transmitted.

[0015] Statement of the invention

[0016] There is a need to further improve quantum communication systems and methods, particularly those enabling the rapid transmission of information between two points, particularly continuous values, almost instantaneously and without delay due to the transmission distance. Summary of the invention

[0017] The invention aims to meet this need, and it achieves this, according to a first of its aspects, thanks to a quantum communication system comprising:

[0018] • An emitter of entangled photons comprising a source configured to generate at least one pair of entangled photons comprising a first photon emitted on a first propagation path, and simultaneously a second photon emitted on a second propagation path different from the first propagation path;

[0019] • A first receiver disposed on the first propagation path comprising a complex absorber configured to absorb the photon in a polarization state chosen from the states of at least two different pairs of complementary polarization states, with the exception of exactly two pairs of perpendicular linear polarizations of which the polarizations of one of the pairs are at 45° to the polarization directions of the other pair;

[0020] • A second receiver arranged on the second propagation path so as to be reached by the second photon after the first photon has reached the first receiver, said second receiver comprising: an optical amplifier making it possible to demultiply the second photon while maintaining its polarization and, arranged downstream of the amplifier, a measuring instrument making it possible to measure the average polarization of the demultiplied photons.

[0021] The system according to the invention is designed to retransmit almost instantaneously a series of values, continuous or discrete, between two locations.

[0022] Since the first and second photons are entangled, the absorption of the first photon at the first receiver instantly determines the polarization of the second photon, particularly before it reaches the second receiver.

[0023] Measuring the average polarization of the photons multiplied at the second receiver then makes it possible to detect in which polarization state the first photon was absorbed and to deduce the information transmitted. This system therefore makes it possible to transmit information without latency, regardless of the distance between the two places wishing to communicate, and using only photons, which requires little energy.

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

[0025] Jones's Polarizations

[0026] By "complementary polarization states", also referred to hereinafter as "complementary absorption polarization states", is meant two polarization states of a light for which the modification of the polarization of said light by linear optical elements, in particular mirrors, birefringent plates and / or prisms including quarter-wave plates, allows for one, its absorption by a first polarizing filter, and for the other its absorption by a second polarizing filter according to a polarization orthogonal to that according to which the first polarizing filter can absorb.

[0027] In the following, the terms "polarization state" and "polarization" are used interchangeably.

[0028] The first receiver is advantageously arranged so that its user can choose the polarization state in which the photon is absorbed from any of the possible polarizations as defined by the Jones formalism, in particular from a set of elliptical polarizations (also called "ellipsoidal") characterized by the orientation and the ellipse of the polarization.

[0029] We can thus vary the polarization according to which the photons are absorbed by the first receiver in order to transmit a series of information to the second receiver.

[0030] By choosing, for example, 10 different polarization directions, spaced 9° apart from each other, and 21 different phase shifts, also spaced 9° apart from each other, we can define 210 different polarization states and thus transmit 210 different signals.

[0031] For example, uppercase and lowercase letters of the alphabet, numbers, and a number of other special characters can be transmitted for each pair of entangled photons reaching the receivers. Complex absorber

[0032] A "complex absorber" is a set reached by the first photon which makes it possible to determine a pair of complementary polarizations from among a certain number of pairs of complementary polarizations, and to absorb the first photon in one of the two polarization states of the predetermined pair.

[0033] Preferably, G complex absorber of the first receiver is configured to absorb the photon in a state of a predetermined pair of complementary polarizations, chosen from the states of at least three different pairs of complementary polarizations, and preferably from a number of different pairs of complementary polarizations chosen according to the number of different values ​​to be transmitted, as described above.

[0034] Preferably, the complex absorber comprises: at least one absorption instrument making it possible to absorb the photon in one or other of two states of a pair of complementary polarizations, for example two orthogonal linear polarizations a polarization modifier arranged upstream of said absorption instrument and configured to transform the polarization of the first photon towards the chosen polarization according to which said absorption instrument absorbs the photons.

[0035] By "chosen polarization" we mean a pair of complementary polarizations predetermined according to the information to be transmitted.

[0036] Polarization modifier

[0037] The linear polarization of a photon can be transformed into ellipsoidal polarization as represented by the Jones formalism by first modifying the orientation of the polarization of a photon of linear polarization and known direction, thus distributing the electric field in a predetermined manner along an x ​​axis and a y axis perpendicular to the x axis, then, in a second step, by modifying the phase of the electric field along one of two perpendicular directions, for example the y axis.

[0038] The polarization modifier preferably comprises a polarization direction modifier arranged upstream of a polarization phase modifier. For example, if Q is the rotation angle of the linear polarization and f the phase shift along the y axis, w being the pulsation of the wave, the components E x summer y of electric field after rotation are:

[0039] E x = E Cos (0) Cos (wί)

[0040] And after the phase shift:

[0041] E x = E Cos (0) Cos (wί)

[0042] E y = E Sin (0) Cos (wί + ø)

[0043] Polarization direction modifier

[0044] A linear polarization direction can be modified in different ways, for example by a half-wave plate or alternatively by a double quarter-wave plate.

[0045] The polarization direction modifier may comprise two quarter-wave plates arranged one after the other on the propagation path of the first photon, the orientation of at least one of the two plates being variable.

[0046] The first quarter-wave plate, for example, modifies linear polarization into circular polarization and the second quarter-wave plate, for example, transforms circular polarization into linear polarization, the direction of which is oriented along an axis dependent on the direction of the axis of the second quarter-wave plate.

[0047] A rotation of the axis of this second blade therefore makes it possible to modify the direction of the linear polarization of the photon. If the blade is a half-wave blade, the rotation of this so-called half-wave blade makes it possible to modify the direction of the linear polarization of the photon.

[0048] Alternatively, the orientation of the first quarter-wave plate can be changed, that of the second being fixed, or the orientations of the two quarter-wave plates can be changed.

[0049] The rotation of a quarter-wave plate or a half-wave plate is obtained for example by mechanical control of a sensor or an electrically controlled device allowing its rotation, for example by rubbing on an axis set in motion by a piezoelectric material or by an electric motor device, for example direct current.

[0050] Alternatively, several linear polarization rotation devices can be used, each allowing different rotation angles, for example fixed, but which can also be modifiable, selectors making it possible to send the photons to one of these rotation devices and, at the output of said rotation device, to conduct said photon towards a common waveguide or a common transmission axis.

[0051] These selectors may comprise a mirror whose axis direction is controlled for example by an electrical device, or, alternatively, made up of a prism or a blade made of a material whose refractive index depends on an electric field: a Pockels cell, or a transparent material with a non-linear refractive index, another light flux, for example transverse and preferably of a wavelength different from that of the photon and not being able to generate photons of the same wavelength as that of the photon, varying the refractive index of said non-linear material and thus controlling the location and possibly the direction of exit of the photon from said material.Since the rotational inertia of a quarter-wave plate can be significant, the photons can be sent successively towards different quarter-wave plates whose direction will have been adjusted beforehand, for example by one of the mechanical rotation devices described previously, thus allowing time to modify the direction of each of these quarter-wave plates between two photon passages.

[0052] Alternatively, chiral or rotatable materials can be used to modify a linear polarization direction.

[0053] The polarization direction modifier may comprise a blade or prism of chiral or rotating material affecting the rotation of the polarization by an angle depending on the location at which the wave enters said chiral or rotating material. The material may in particular be arranged between two Pockels cells.

[0054] The first photon may be projected onto a first plate or prism forming a Pockels cell whose refractive index is controlled by an electric field to emerge at different locations, in particular onto an intermediate plate or prism made of at least partly chiral or rotating material rotating the electric polarization axis of the photon by an angle depending on the location through which the wave enters said intermediate plate or prism, before entering a second plate or prism forming a Pockels cell whose refractive index is adjusted symmetrically relative to that of the first plate or prism to cause the photon to emerge in the same direction as if the refractive index of the first plate or prism and of the second plate or prism both had a fixed value.A third blade whose refractive index can be adjusted, for example electrically, is advantageously placed after the second blade or prism to make the location of the photon exit from the set of blades and prisms independent of the refractive index chosen for the first blade or prism.

[0055] The blade or prism made of intermediate material is, for example, composed of two symmetrical prisms placed side by side with the same refractive index but having different chiral or rotating powers, the first of the two prisms having, for example, a chiral or rotating power and the second not having one or else rotating the electric field in the opposite direction to the rotation imposed by the first prism.

[0056] One of the two intermediate prisms may comprise a chiral material, for example cadmium selenide (CdsE) nanoparticles with a diameter of approximately 1.4 to 2.4 nm as described in the article by Visheratina, Anastasia, and Nicholas A. Kotov. "Inorganic nanostructures with strong chiroptical activity." (CCS Chemistry 2.3 (2020): 583-604.); the other intermediate prism preferably not being chiral or alternatively being of inverse chirality to that of said first intermediate prism. The rotation of the electric field of waves passing through a chiral material being proportional to the thickness passing through said chiral material, the rotation of the electric field passing through the intermediate plate depends on the location through which it is penetrated by the light passing through it.

[0057] One of the two intermediate prisms may further comprise a "rotating" material, such as a superposition of an even number of quarter-wave plates stacked on top of each other in directions incremented at each layer by a predetermined angle, such that a wave linearly polarized along the axis of the first slice of the material emerges from the stack with a polarization rotated by an angle proportional to the thickness of the stack.

[0058] A second device, called a "polarization phase modifier" can be arranged downstream of the polarization direction modifier, in order to phase shift the component of the photon's electric field by a predetermined angle along one of two fixed axes.

[0059] Preferably, the polarization phase modifier comprises a first birefringent plate or prism arranged to split the beam into two electromagnetic waves of linear polarization, one along a first axis, the other along a second axis, and a variable refractive index retardation plate arranged on the second axis.

[0060] By "beam" we mean the light wave that is the first photon in the sense of the duality of light.

[0061] The delay plate causes the wave oriented along the second axis to acquire a predetermined phase shift relative to the wave oriented along the first axis, before being mixed with it again by a new birefringent plate or prism making it possible to bring together along the same axis the two waves whose polarization fields are perpendicular.

[0062] The delay plate can include a Pockels cell or a non-linear material. The variable refractive index makes it possible to choose the phase shift imposed on the wave oriented along the second axis.

[0063] In a first implementation example, as described above, the phase modifier rotates a linear polarization of angles spaced 9° apart varying between -45° and +36° and varies the phase of angles spaced 9° apart between -90° and +90° thus defining 210 possible polarization states. As described below, 209 of the 210 possible polarizations are for example used to encode data and the 210th linear polarization is used to absorb any entangled photon received within a predetermined time after the reception of a photon having been used to encode data.

[0064] In a second implementation example, the phase modifier rotates the polarization axis by 45° before the photon is absorbed to transmit, for example, a 0, and transforms the photon's polarization into circular polarization to transmit, for example, a 1.

[0065] Reflection of the first photon The invention also relates to a quantum communication system comprising:

[0066] • An emitter of entangled photons comprising a source configured to generate at least one pair of entangled photons comprising a first photon emitted on a first propagation path, and simultaneously a second photon emitted on a second propagation path different from the first propagation path;

[0067] • A first receiver arranged on the first propagation path, comprising: at least one first absorption instrument, arranged to absorb the first photon in one of two states of a pair of complementary polarizations, the polarization of the photon being undetermined in the observable basis according to which the first instrument absorbs the photons, an optical selector arranged upstream of said at least one first instrument and configured to either allow the first photon to pass towards said at least one first measuring instrument, or prevent it from being measured;

[0068] • A second receiver arranged on the second propagation path so as to be reached by the second photon after the first photon has reached the optical selector and / or after it can have reached the measuring instrument of the first receiver, said second receiver comprising an optical amplifier making it possible to demultiply the second photon while retaining its polarization and, arranged downstream of the amplifier, a measuring instrument making it possible to measure the average quantum state of the demultiplied photons.

[0069] By "observable basis" we mean a basis of two complementary polarizations, for example two orthogonal linear polarizations.

[0070] By "undetermined polarization in an observable basis" we mean a photon whose polarization as observed in one of the two predetermined polarizations of an observable basis is not defined, that is to say is random, for example in an equiprobable manner.

[0071] Since the first and second photons are entangled, the absorption of the first photon at the first receiver instantly determines the polarization of the second photon, particularly before it reaches the second receiver. Measuring the polarization of the multiplied photons at the second receiver then makes it possible to detect whether or not the first photon has been absorbed, and to deduce what information has been transmitted. The information transmitted by this system is, for example, of the binary type.

[0072] The first receiver may comprise at least one second absorption instrument, the optical selector being disposed upstream of the first and second instruments and configured to either allow the first photon to pass by directing it towards one or other of the instruments, or to prevent it from being absorbed.

[0073] Using two instruments allows three different pieces of information to be transmitted, for example the values ​​0, 1 and 2, with the value 0 being assigned to the reflection, the value 1 to the first instrument, and the value 2 to the second instrument.

[0074] Optical selector

[0075] The optical selector of the first receiver can be of various types. It is advantageously placed upstream of the measuring instrument in order to be able to prevent the incident photon from reaching the measuring instrument, if this is desired.

[0076] To do this, the selector preferably includes a reflector, in particular a controlled mirror, allowing for example the first photon to be reflected on a propagation path different from its incident propagation path, in particular on a propagation path not including the measuring instrument.

[0077] The term "controlled mirror" means a device of which certain properties, for example the refraction properties or the direction of reflection, are controlled by an ancillary device linked to the mirror, in particular an electronic device, for example generating an electric field.

[0078] The optical selector preferably comprises a device whose refractive index and / or reflection direction are controlled, in particular controlled using an electric field or a luminous flux.

[0079] Alternatively, the optical selector comprises, for example, a Bragg mirror, in particular a Bragg mirror whose direction of reflection is controlled by piezoelectric material itself controlled by an electric field. As a variant, the optical selector comprises a prism, in particular comprising Pockels cells whose refractive index is controlled by an electric field, the application of an electric field being able to direct the photon towards the or one of the measuring instruments or towards a Bragg mirror advantageously inclined relative to the direction of incidence of the electromagnetic waves reaching it, such that said waves are reflected towards a location preferably different from that from which they come at the entrance of the prism.

[0080] In another variant, the optical selector comprises one or a succession of Bragg filters, some layers of which are Pockels cells whose refractive index is controlled by an electric field, the said Bragg filter reflecting or letting the light wave pass depending on whether or not the electric field is applied.

[0081] Alternatively, the Pockels cells used above are replaced by non-linear crystals illuminated by a powerful light capable of varying the refractive index of the medium crossed by the photon, the wavelength of the powerful light preferably being different from the wavelength of the photon.

[0082] The optical selector allows, in particular, the photons to be oriented in three different directions, one direction allowing, for example, the photons not to be detected, a second allowing the measurement of linear polarization of the photons and a third allowing the measurement of circular polarization.

[0083] Protective device

[0084] Preferably, the quantum communication system further comprises a device for protecting the quantum state of at least one photon, in particular a device which prevents the measurement or absorption of at least one photon, arranged close to the first receiver so as to protect the quantum state of the first photon after its reflection, preventing it from being absorbed or measured by one or more measuring instruments, said quantum state of the first photon being protected at least until the second photon has been multiplied at the second receiver.

[0085] The quantum state protection device is placed close to the first receiver so as to protect the quantum state of the first photon when it is reflected and / or deflected from its incident propagation path by the optical selector so as not to be observed there, i.e. when it is not desired that it be absorbed by the measuring instrument of the first receiver.

[0086] Preferably, the quantum state protection device comprises a transparent space, in particular a transparent space and at least one mirror. The transparent space may be empty or filled with gas or liquid.

[0087] Preferably, it comprises several Bragg mirrors, arranged relative to the transparent space so as to trap the photon it receives on a path of appropriate length as described above, that is to say corresponding to the time it takes for the second photon having reached the second receiver to be amplified there.

[0088] Alternatively, the protection device comprises an optical fiber of a length allowing the first photon to circulate there while waiting for the second photon having reached the second receiver to be amplified there.

[0089] In another variant, the device may be a portion of empty or gas-filled space or atmosphere which is ensured not to be crossed by an object reflecting or absorbing light.

[0090] First Photon Absorption Instrument

[0091] At the first receiver, the first photon can be absorbed by at least one absorption instrument in one of two states of a complementary polarization pair.

[0092] Preferably, the absorption instrument comprises at least one filter making it possible to send the first photon to at least one photon detector, in particular to one or the other of two photon detectors depending on the polarization state of the first photon, the filter preferably being a prism or a blade made of birefringent material.

[0093] The absorption instrument may also include a polarizing filter and a photon detector located after said polarizing filter.

[0094] The absorption instrument may also include a semi-transparent mirror reflecting circularly polarized photons to a first photon detector and allowing others to pass through to a second photon detector, as described in the article by Mai, Wending, et al. "Broadband transparent chiral mirrors: Design methodology and bandwidth analysis." (AIP Advances 9.4 (2019): 045305). Photon emission

[0095] Photon pair emitter

[0096] Preferably, the transmitter generates pairs of entangled photons by spontaneous parametric reduction (SPDC), i.e. following a process in which an initial photon, also called a "pump" photon, is split and its frequency halved by a four-wave mixing phenomenon, in an optical medium with a non-linear refractive index, as described in the article by Amanti et. al, "Integrated sources of entangled photons at the heart of quantum technologies" (Photoniques, issue 91, 2018).

[0097] This type of photon pair emitter can produce photons with relatively robust entanglement, but has the disadvantage of sometimes producing more than one pair of photons at a time, which is undesirable because it is uncontrolled. Additional devices can be set up to deal with these multiple "parasitic" pairs, as described below.

[0098] Alternatively, one can use an emitter with a quantum dot to generate entangled photon pairs, as described in the aforementioned article, which allows pairs to be obtained more regularly. The entanglement properties of photons obtained with such a method can, however, sometimes be unstable.

[0099] Birefringent crystal optical fibers can also be used as described in the article by Smith et. al “Photon pair generation in birefringent optical fibers” (Optics Express, Vol 17, Issue 26, 2009).

[0100] Preferably, the transmitter is configured to successively generate a plurality of entangled photon pairs.

[0101] The entangled photons at the transmitter output can be sent on the first and second propagation paths in various ways.

[0102] The photons are preferably emitted according to a predetermined polarization. By "predetermined polarization" is meant one of the two states of a predetermined pair of complementary polarizations, for example a linear polarization of a certain direction. A birefringent plate can, for example, separate the two entangled photons according to their vertical or horizontal linear polarization.

[0103] A predetermined polarization of the entangled photons sent to each of the receivers makes it possible in particular to know the probability of a state according to a polarization or its complementary polarization for any pair of complementary polarizations chosen.

[0104] It is thus possible to generate series of random bits upon reception of said photons by the receivers, said series being complementary to the two receivers, the polarization of a photon received at the second receiver being the complementary polarization of the polarization observed at the first receiver.

[0105] A predetermined polarization is also necessary when using quarter-wave plates at the first receiver to rotate the polarization direction of the first photon.

[0106] To emit photons with a predetermined linear polarization, for example, a plate or prism containing an optically transparent birefringent material, for example lithium niobate or rutile (Ti02), is placed at the emitter output, the optical indices of which depend on the polarization axis of the light through which at least one of the two photons passes.

[0107] The latter then exits the prism at one of two different places, depending on its linear polarization.

[0108] If the source of entangled photons emits the two photons in the same direction and along an axis perpendicular to the ordinary axis of the optically transparent birefringent material, the first photon is collected at the exit of the prism, at the point where the electromagnetic waves whose electric field is parallel to the ordinary axis of the optically transparent birefringent material exit, to send it to the first receiver.

[0109] In the same way, the second outgoing photon is collected from the place where the electromagnetic waves whose field is perpendicular to the ordinary axis exit in order to send it to the second receiver.

[0110] In the case where the two entangled photons are generated from the emitter at different locations and / or in different directions, each of the two photons can be projected onto birefringent prisms in a direction perpendicular to the ordinary axis of each of the prisms and each photon redirected to the receiver intended for it, the photons exiting from different locations of the prisms then preferably being lost or destroyed by projection onto an absorbent surface.

[0111] For example, for the first entangled photon, one can use the one leaving a first birefringent plate or a first birefringent crystal with linear polarization parallel to the first axis, and as the second entangled photon, the one leaving with linear polarization parallel to a first axis of the crystal used for the second plate or the second prism which will have been adjusted to allow the exit of the photons according to a predetermined polarization corresponding to the polarization of the entangled photons to the photons leaving the first birefringent plate or prism with a direction parallel to the first axis.

[0112] To facilitate the transport of said photons to the receivers, birefringent delay plates are preferably placed at the exit of the places where the photons exit the prisms, transforming the linear polarizations of said photons into circular polarizations.

[0113] We then preferably place another birefringent delay plate at the input of each of the receivers to retransform the circular polarizations of the photons into linear polarizations, this transformation into linear polarization allowing the polarization axis to be precisely adjusted.

[0114] Photon transmission

[0115] Photons can be transmitted from transmitters to receivers through space, through the atmosphere, through optical fiber, or a combination of these means.

[0116] Lenses can be used for photon transmission, especially for transmission through space or the atmosphere. If necessary, anti-reflection coatings are preferably provided on said lenses. The size of the lenses used is preferably adapted to the length of the photon transmission through space or the atmosphere.

[0117] To adjust the direction of emission of conjugate photons for spatial or atmospheric transmission, conjugate mirrors can be used to reflect the emitted light back to the transmitter. For example, a laser light transmitter can scan a space to detect the receiver, the receiver reflecting the emitted light back to it, and the direction of the conjugate photons is then adjusted to be parallel or coincident with the direction of the light reflected by the conjugate mirrors.

[0118] The light emitted by the laser light emitter can be of wavelength close to the wavelength of the entangled photons and introduced into the objective lens used by the photons by a dichroic prism.

[0119] Alternatively, if the wavelength is the same as that of the entangled photons, the light can be introduced by a birefringent prism when said photons are linearly polarized, the polarization of the light used for guidance then being perpendicular to the polarization of the entangled photons directed towards the same receiver. A Pockels cell upstream of the receivers can be used to guide the light towards the conjugate mirrors when the guide light arrives.

[0120] In another variant, the light waves used for aiming can be emitted parallel to the entangled photons but separated, for example, by a few centimeters, to be reflected by conjugate mirrors.

[0121] In another variant, one may not use a conjugate mirror but instead aim at the receivers or targets close to them, the information according to which targets are received being communicated by another means of communication, in particular by radio signal or by quantum transmission.

[0122] The area to be scanned can be identified by map recognition of the area in which the said receivers are likely to be located.

[0123] The aforementioned phase modifier can also be used when the receivers are in relative rotation with respect to the transmitter, in particular due to the rotation of the Earth and the satellites, to allow photons emitted in a fixed direction with respect to the transmitter to enter each of the receivers with a fixed and determined direction with respect to it.

[0124] Calibration procedures

[0125] The different media through which the photons travel from their place of emission to the two receivers can give rise to changes in their phase that are not necessarily desired or planned. Furthermore, these changes can be different depending on whether the photons are directed towards the first or second receiver. Finally, these changes can vary over time depending in particular on the weather or the temperature of the materials during their use.

[0126] The system according to the invention can be periodically calibrated by having the photons absorbed by the first receiver in polarizations belonging to predetermined pairs of complementary polarizations and by observing for each of these pairs one of the two possible polarizations in which the entangled photon is detected by the second receiver.

[0127] Thus, the invention also relates to a method for calibrating a system as defined above making it possible to determine the polarization state of a photon reaching the second receiver as a function of the polarization state in which its entangled photon was absorbed at the first receiver, a method comprising the steps consisting of:

[0128] Generating pairs of entangled photons from an emitter, the first photon of the pair being emitted to a first receiver and the second photon of the pair being emitted simultaneously to a second receiver, the first and second photons being entangled according to their polarization state,

[0129] Absorb the first photons at the first receiver in a predetermined pair of polarizations, memorize in which of the two possible polarizations each photon was absorbed and stop sending photons by the transmitter as soon as a predetermined number of each of the different polarizations have been observed,

[0130] At the second receiver, duplicate the second photons into streams of multiplied photons using an amplification device, each multiplied photon having retained the polarization state of the second photon,

[0131] Measure the polarization states, i.e. the polarization directions and phase shifts of each flux of multiplied photons, and store these measurements as well as their reception times,

[0132] Transmit the list of polarizations and times of reception of the stored photons from the first receiver to the second receiver, Delete from the list of photons received at the second receiver the photons not corresponding to photons received at the first receiver and, likewise, delete from the list of photons received at the first detector the photons whose corresponding photon has not been received at the second receiver, and Calculate using on the one hand: knowledge of the state of polarization of two photons detected in complementary polarizations at the first receiver and whose entangled photons have reached the second receiver, and on the other hand knowledge of the state of polarization of the corresponding entangled photons received at the second receiver, the Jones matrix allowing to deduce from the polarization of the photons received at the first receiver the polarization of the photons received at the second receiver.

[0133] The storage of states as well as the calculation of the Jones matrix are carried out by any suitable electronic device, for example a microcontroller.

[0134] The Jones matrix thus calculated makes it possible to calculate the polarization of the second entangled photon received at the second receiver as a function of the absorption polarization of the first photon by the complex absorber located at the first receiver.

[0135] A second calibration can be made by determining the probability for any photon arriving at the second receiver whose entangled photon was absorbed during its transit to the first detector to belong to each range of pairs of complementary polarizations detectable or identifiable by the second receiver; the range of a P polarization being defined as the set of polarizations assimilated by the polarization measuring instrument as being of P polarization.

[0136] Thus, the invention also relates to a method for calibrating a system as defined above, making it possible to determine the probability of losing a photon during its transit from the transmitter to the first receiver, consisting of:

[0137] Configure G complex absorber of the first receiver to absorb photons received at the first receiver in one of two complementary polarizations of an absorption polarization pair,

[0138] Successively generating several pairs of entangled photons from an emitter, the first photon of the pair being emitted towards a first receiver and the second photon of the pair being emitted simultaneously towards a second receiver, the first and second photons being entangled according to their polarization state,

[0139] Count the number of photons received at the second receiver in each of the polarization ranges detectable by the second receiver other than one of the two polarization ranges of the entangled photons of the photons absorbed by the first detector.

[0140] It is thus possible to detect ranges of polarizations in which the entangled photons of photons lost during transmission during their transit to the first detector arrive in priority, or in the most probable way and thus preferably avoid using the corresponding absorption polarizations to transmit data from the first to the second receiver.

[0141] We can also calculate a transmission rate of photons between the transmitter and the second receiver by calculating the ratio between the number of photons received at the second receiver with one of the polarizations corresponding to one of the two absorption polarizations at the first receiver.

[0142] Second receiver measuring instrument

[0143] The measuring instrument of the second receiver preferably comprises at least one photon detector arranged to measure the polarization of the light resulting from the multiplication of the second photon.

[0144] The measuring instrument of the second receiver comprises, for example, a succession of semi-reflecting blades arranged downstream of the optical amplifier, said blades directing the flux of multiplied photons, i.e. the luminous flux, with equal intensity towards polarization measuring instruments arranged, for example, to measure the intensity of the flux along two perpendicular axes and / or the phase shift of the light between these same two axes.

[0145] The light can also be directed towards various polarizing filters after passing through one or a succession of lenses which enlarge the section of the light beam, thus allowing several mirrors or lenses to direct portions of this light beam towards the various polarization measuring instruments.

[0146] The various polarization measuring instruments preferably allow:

[0147] • measure the intensity of the component of the electric field of light along a first axis x,

[0148] • measure the intensity of the electric field component of the light along a second y axis perpendicular to the first x axis,

[0149] • measure the phase shift between the light along its x axis and the light along its y axis,

[0150] • and, preferably, measure the phase shift between the light along its x' axis, bisector of x and y and the light along its y' axis.

[0151] The measurement of the intensity of light along two perpendicular axes is done, for example, by separating the light along two perpendicular axes by a birefringent plate or prism followed by two light intensity sensors placed at the outputs of said plate or prism. The measurement of the phase shift between two perpendicular components is done, for example, by separating the light along two perpendicular axes by a birefringent plate or prism followed, for the wave polarized along one of the two axes, by a rotation of this polarization axis by 90°, for example using a rotating or chiral material or by the succession of two quarter-wave plates, then a joint projection through Young's slits of these two waves, one of which has undergone the rotation of its electric field, onto a screen, the interference of the two light sources drawing fringes whose positions depend on said phase shift.

[0152] Optical amplifier

[0153] The term "optical amplifier" refers to a device that allows a photon introduced into it to be duplicated, in particular the second photon of a pair of entangled photons, while preserving its polarization state.

[0154] Preferably, the optical amplifier is a doped fiber amplifier.

[0155] For example, we can use an Erbium amplifier (EDFA), for example 4m long, into which the photon to be amplified is introduced at the same time as an amplifying wave of shorter wavelength, which makes it possible to amplify the wave corresponding to the introduced photon, with gains which can be of the order of 37db / m.

[0156] Alternatively, a doped fiber amplifier (DFA) can be used using a dopant other than Erbium.

[0157] Alternatively, the optical amplifier is, for example, a vertical cavity amplifier (VCSOA), or a semiconductor amplifier (SOA).

[0158] Anti-reflective coatings

[0159] To avoid the loss of photons, anti-reflection layers are preferably provided at the interfaces between adjacent transparent media of different indices crossed by the photons, as well as at the interfaces of prisms and birefringent plates crossed by photons, the anti-reflection layer being preferably adapted to the index(es) of the material and to the angle(s) of incidence and direction(s) of polarization as well as to the wavelength of the photon which must cross it.

[0160] Dichroic filters

[0161] The first and / or the second receiver preferably comprise one or more dichroic filters allowing only photons of a given wavelength to pass through, in particular a prism made of a transparent dispersive material, the filter(s) preferably being arranged in front of the measuring instrument(s), or in front of the optical selector for the first receiver, and this in particular if the refractive indices of the non-linear materials are modified by the application of powerful light fluxes.

[0162] Assigning photons to information with synchronized clocks

[0163] When the photons arrive at the second receiver, it is useful to be able to differentiate whether they should be assigned to the transmitted information, whether it is information transmitted several times or whether it is information that would not have been coded on a pair of entangled photons. Indeed, photon emissions are sometimes irregular, for example an SPDS type transmitter as mentioned above, can emit a double pair of photons without this being desired.

[0164] The transmitter and each of the receivers preferably include a clock, the clocks of the transmitter and the receivers being synchronized with each other.

[0165] This allows, when combined with knowledge of the travel time of the photons between the transmitter and each of the receivers, to determine coding and reception periods, preferably repetitive, during which a photon can be coded at the first receiver for the coding periods and its entangled photon arrive at the second receiver during the reception periods.

[0166] If several photons are received during a reception period, the first receiver maintains an unchanged state until a time between, for example, a quarter of the non-emission time after the end of the reception period and half of the non-emission time before the next reception period.

[0167] The second receiver preferably records the bits received for each reception period. It then advantageously transmits to the first receiver from time to time, in particular if the device uses reflection to transmit a bit or a trit, for example every 3000 reception intervals, or every 100 reception intervals without photon, a list of the reception periods for which it has not received a photon, the transmitted photon having been lost or no photon having been emitted by the transmitter for this period, without the first transmitter knowing whether it has reflected photons during these same periods, in particular if the device does not have a photon detector.

[0168] The transmission of this list can be done by instant communication using the method according to the invention described below, or by a conventional means of communication. Upon receipt of this list, the first receiver then transmits the bits corresponding to these transmission periods for which no photon has reached the second receiver.

[0169] Alternatively, the first receiver uses only the two different measuring instruments to transmit information, one detecting linear polarizations, the other circular polarizations. The first receiver then retransmits each bit by selecting the measuring instrument appropriate to the bit it wants to transmit for that transmission period, and preferably keeps the selector in the same state until the end of the transmission period for which it receives a photon, allowing it to select the measuring instrument to be used for the transmission of the next bit.

[0170] The second receiver, receiving the photons during the transmission periods, and if it receives any, measures for each transmission period their state of polarization and deduces the bit that it must add to the list of received bits.

[0171] If the transmission line between the transmitter and the first receiver has a certain opacity and can absorb a certain proportion of the photons passing through it, some photons may arrive at the second receiver during reception periods with random polarizations.

[0172] The second receiver can then be configured to recognize at least a portion of the photons arriving in a polarization state which cannot be that of an entangled photon absorbed or reflected at the first receiver.

[0173] Clock shift

[0174] Preferably, the clocks of the system components, namely the transmitter and the two receivers, are configured to take into account the phenomenon of flow differing from the time at different locations, including different altitudes of each of the components.

[0175] Synchronization of transmission periods between receivers

[0176] A method can be implemented to allow the first and second receivers to synchronize their clocks.

[0177] For example, when measuring a first photon, the first receiver notes using its own clock the time at which the photon hit the measuring instrument.

[0178] The noted time is subsequently transmitted to the second receiver together with, preferably, the references of the transmission interval, in particular the start time and possibly the duration during which the photon was expected. The second receiver notes the time of reception of the second photon in order to adjust its own clock so that the offset between the start of the transmission period and the time when the first photon struck the measuring instrument of the first receiver is the same as the offset between the start of the period of reception of the second photon at the second receiver and the time given by the clock for the reception of the second photon.

[0179] If the transmission lines between the transmitter and one or other of the receivers absorb certain photons, the transmitter can send photons in a predetermined number, or for a predetermined duration, and each of the two receivers can note the average time of reception of each of the photons and then one of the receivers can communicate this said average time to the other receiver;

[0180] The average time can be calculated by averaging the reception times of each photon or only the first and last photons received. The predetermined number of photons sent or the duration of their sending are preferably adjusted in particular according to the rate of transmission / loss of photons on each of the two paths between the transmitter and the receivers, and the average frequency of sending the photons.

[0181] Since the transit times of the photons between the transmitter and each of the receivers are different, the synchronization of the clocks between each of the receivers may be different depending on whether the information is transmitted in one direction or the other. Each of the receivers may then have a synchronization register used to increment or, on the contrary, subtract from the time of the clock of said receiver a time lapse adjusted during synchronization, to deduce the time of a synchronized clock for the reception of information from the other receiver.

[0182] A method can be implemented to synchronize the periods of sending photons to the receivers and the periods of receiving them at the receivers, these periods being time intervals during which photons can leave the transmitter or reach the receiver.

[0183] To this end, the transmitter may emit a first brief light signal followed or preceded by other light signals composing a message, preferably signed by a digital signature indicating the precise time, on its clock, of sending the signal. Upon receipt of said first light signal, the receiver notes the time of reception then reads the time of sending of the signal, differentiates between the time of sending of the signal and the time of the start of the photon transmission period, during which the signal was sent, or if it was sent outside a transmission period, the start of the previous transmission period, and does the same with the time of reception of the signal and the time of the start of the reception period.

[0184] If the difference between the date of transmission of the signal and the start of the transmission period is greater than the difference between the date of reception and the start of the reception period, the receiver can advance the start of the transmission period, or vice versa in the opposite case.

[0185] The calibration is advantageously repeated several times and the results are averaged, thus making it possible to adjust the clocks with a precision greater than the imprecision of the times of sending and receiving said light signals.

[0186] The wavelength used for the light signal is preferably the same as the wavelength of the entangled photons, so that it is transmitted at the same speed as the entangled photons.

[0187] Assigning photons to information without a synchronized clock

[0188] As an alternative to synchronized clocks, the first receiver may comprise detector-transmitters and a waiting element, for example a reflector arranged to reflect the first photon with a predetermined polarization different from the polarizations detectable by the detector-transmitters, or an element arranged to absorb the first photon in a predetermined polarization different from the polarizations of the detector-transmitters.

[0189] The polarizations of the detector-transmitters are preferably complementary so that any photon directed towards an absorbing waiting element is absorbed.

[0190] The detector-transmitters are preferably arranged to detect the polarization of photons in at least four different polarizations, these polarizations being grouped into complementary pairs such that any photon directed towards a detector-transmitter is thus detected and absorbed regardless of its initial polarization.

[0191] The detector-transmitter(s) of the first receiver are used to send the information by choice of the pair of detectors used.

[0192] Each photon received by the first receiver during a predetermined period, also referred to as a "rest period", after the reception of a photon used to send information is sent to the waiting element, the duration of this predetermined period being preferably fixed and preferably greater than twice the inaccuracy of the clock of the receivers, and the start of this rest period being determined by the or one of the two sensors absorbing the photon used to send the information.

[0193] The second receiver is preferably arranged to, on the one hand,

[0194] • ignore photons paired with photons sent to the waiting element, on the other hand,

[0195] • interpret two successive photons not paired with photons sent to the waiting element as successive bits if these are separated by at least the rest period from which the inaccuracy of the receiver clock is subtracted, and finally,

[0196] • interpret two photons received successively in a time less than or equal to, for example, three-quarters of the rest period as representing the same bit.

[0197] The second receiver can also detect an error in the transmission of information if: two consecutive photons of different polarizations not paired with photons sent to the waiting element are received in a time less than the rest period minus the inaccuracy of the receivers' clock.

[0198] This method has the advantage of not requiring a switch at the emitter to restrict the emission of entangled photons, nor clock synchronization.

[0199] The transmitting detector(s) of the first receivers can also serve as a waiting element, in particular if the polarization in which they detect the photons is adjustable and can therefore be adjusted to the polarization used by the waiting element to absorb the photons.

[0200] Optical amplifiers tend to emit photons in the opposite direction of the propagating signal, either during signal amplification or in the absence of a signal, after molecules or atoms in the amplifying medium have been excited by a pump signal.

[0201] The second receiver includes, for example, a switch placed in front of the optical amplifier, in order to limit the number of photons emitted by the amplifier towards the source of entangled photons of the device.

[0202] The switch of the second receiver is for example configured to absorb or reflect the photon(s) subsequent to a first photon reaching said second receiver within a predetermined time interval.

[0203] The switch of the second receiver can be connected to the measuring instrument of the second receiver, so as to no longer allow light to pass in one direction or the other from the arrival of a first photon and until the next photon is expected or a predetermined time before this moment, for example half of the rest period defined above.

[0204] Alternatively, the switch of the second receiver may only allow photons to pass during periods when they are expected, in particular during the aforementioned photon processing time intervals.

[0205] Assignment of photons to information without a synchronized clock using non-transparent optical transmission means

[0206] Optical fibers, although transparent, do not transmit all the photons presented at their input. A loss of 0.20dB per kilometer is common, for example, losing the OdB in 50 km, or 90% of the photons.

[0207] The effect of photons lost in transit between the transmitter and the first receiver is however different from the effect of the loss of lost photons going to the second receiver; the entangled photons of the photons lost between the transmitter and the first receiver can arrive at the second receiver with any polarization corresponding to the polarization of the entangled photon absorbed in the line going to the first receiver, whereas the loss of a photon in the line leading to the second receiver only causes the disappearance of a photon potentially carrying information, that is to say one whose entangled photon has reached the first receiver.

[0208] Moreover, the polarizations of photons arriving at the second receiver after their entangled photons have been absorbed in transit to the first receiver are not necessarily equally distributed among all possible observable polarizations.

[0209] Therefore, for the transmission of information, we preferably use pairs of complementary polarizations for which corresponding polarizations observed at the second receiver are the least numerous when the entangled photon of the photon detected at the second receiver has been absorbed during its transit to the first receiver.

[0210] The invention also relates to a method for calibrating a system as defined above, making it possible to determine two Jones matrices, one of which makes it possible to calculate the polarization of a photon arriving at the second receiver as a function of the polarization of a photon absorbed at the first receiver, in which, preferably, the quantity of different polarizations observable at the second receiver is greater than twice the inverse of the photon transmission rate between the transmitter and the first receiver, the method consisting of:

[0211] Configure G complex absorber of the first receiver to absorb photons received at the first receiver in one of two complementary polarizations of an absorption polarization pair,

[0212] Successively generating several pairs of entangled photons from an emitter, the first photon of the pair being emitted towards a first receiver and the second photon of the pair being emitted simultaneously towards a second receiver, the first and second photons being entangled according to their polarization state,

[0213] Count for each polarization in which a photon is received at the second receiver the number of photons having reached said second receiver with this same polarization, Stop sending photons when a predetermined number of photons is received at the second receiver,

[0214] Determine the two polarizations in which the photons were received most often at the second receiver, these polarizations being considered as corresponding to the absorption polarizations of the photons at the first receiver,

[0215] Calculate the two possible Jones matrices of polarization transformation allowing the polarization of a photon received at the first receiver to be deduced from the polarization of a photon received at the second receiver.

[0216] The recording of the polarization of at least one photon received at the first receiver and that of its entangled photon received at the second receiver, the correspondence being made for example thanks to their respective reception times, also makes it possible to choose from the two calculated Jones matrices, which makes it possible to calculate the polarization of the photons received at the second receiver as a function of the polarization of their entangled photons possibly received at the first receiver.

[0217] The following method can also be used to transmit information from the first to the second receiver, which method comprises the steps of:

[0218] Successively generating several pairs of entangled photons from an emitter, the first photon of the pair being emitted towards a first receiver and the second photon of the pair being emitted simultaneously towards a second receiver, the first and second photons being entangled according to their polarization state,

[0219] For each information to be transmitted, configuring the complex absorber of the first receiver to absorb a number NPT of transmission photons in a predetermined complementary polarization pair called an "absorption polarization pair" corresponding to the information to be transmitted, then, if the next information to be transmitted is not already known or is the same as the information just transmitted, configuring the absorber in a polarization pair called a "standby polarization pair", and, if said information is the same as the information just transmitted, counting at least NPT photons absorbed in this "standby polarization pair".

[0220] At the second receiver, count for each pair of complementary polarizations the number of photons received in one of the two polarizations of said polarization pairs since the last possible reception of a signal. As soon as this counter exceeds for one of the pairs a predetermined NSP threshold number and the pair of complementary polarizations is different from that of the last signal received, consider the information corresponding to this pair of polarizations as a new signal, and if this last information does not correspond to the waiting pair of polarization, add this information to the list of information received.

[0221] Alternatively, at the first receiver, the polarization absorber is systematically configured to absorb at least NPT photons in one of the two standby polarizations after sending any information.

[0222] For example, to transmit information, we will use:

[0223] • transmission lines between the transmitter and each of the receivers presenting an attenuation of 10db, i.e. a loss of 90% of the photons.

[0224] • transmission lines between the complex absorber configured to absorb photons in 450 pairs of complementary polarizations, these pairs representing 80% of the photons received at the second receiver during absorption of their entangled photons in their transits towards the first receiver,

[0225] • we can choose for NPT=200 photons and 6 for NSP which allows to obtain, according to the inventor's calculations, a transmission error rate which would then be less than 1 in 10,000.

[0226] In another example, with an error rate of 97% corresponding to a signal attenuation of 15db, using 450 pairs of complementary polarizations, each polarization and phase shift being separated from each other by approximately 5°, the coding of 1200 photons at the first receiver (therefore NPT=1200) and the threshold number NSP being set at 14 makes it possible to obtain, according to the inventor's calculations, an error rate of less than 1 in 150,000 if the photons resulting at the second receiver from photons absorbed during their transits towards the first receiver are equally distributed over all the observable polarizations.

[0227] Processing of pairs of photons sent simultaneously

[0228] The transmitter may sometimes send pairs of entangled photons very close to each other. The photon detectors located at the first receiver therefore preferably count the number of absorbed photons and not just the number of photon impacts on the said first receiver. This counting can, for example, take into account the intensity of the electromagnetic wave hitting the photon detectors.

[0229] Groups of photons can also arrive almost simultaneously at the second receiver, the latter being unable to discriminate the polarizations of the photons from one another. Since the polarization detector can detect an average polarization of all the photons detected 'simultaneously', the polarization of the groups of photons arriving simultaneously at the second receiver, i.e. whose generated light intensity is for example 50% greater than the intensity generated by a single photon, advantageously does not give rise to the incrementation of the counters of photons received in various polarizations. However, it may sometimes happen that several photons reach the first receiver but that only one of their entangled photons reaches the second receiver and this is then counted.

[0230] Device performance

[0231] The use of lossless or low-loss lines between the transmitter and the first receiver, for example allowing photons to pass through a vacuum, makes it possible to reduce or avoid photons received at the second receiver with random polarization.

[0232] Similarly, the use of precise measuring instruments at the second receiver, allowing either the number of pairs of complementary polarizations to be increased or a large number of photons with random polarizations to be ignored, can allow:

[0233] • to increase the transmission distance of photons,

[0234] • to increase the throughput by reducing the number of NPT transmission photons.

[0235] An increased speed of switching the polarization of the complex absorber of the first receiver makes it possible to increase the frequency of sending entangled photons sent by the transmitter. An increased temporal precision of the polarization detectors of the second receiver which makes it possible to discern the interferences of various photons received very close in time to each other also makes it possible to use a larger flux of entangled photons, but also to reduce the number of receptions of photons received 'simultaneously' as described above.

[0236] This method has the advantage of not requiring a switch at the emitter to restrict the emission of entangled photons, nor clock synchronization.

[0237] Processing of photon return to the emitter

[0238] Photons are sometimes reflected back to the transmitter from the receivers. To prevent them from being reflected back from the transmitter to one of the receivers, the cavity or material in which the entangled photons are produced by wave mixing, whose refractive index is nonlinear, is preferably surrounded or covered with a material that absorbs light of a wavelength equal to that of the entangled photons.

[0239] Quantum communication processes

[0240] The invention also relates to a quantum communication method using the system defined above, comprising the steps consisting of:

[0241] Generating a pair of entangled photons from a transmitter, the first photon of the pair being transmitted to a first receiver and the second photon of the pair being transmitted simultaneously to a second receiver, the first and second photons being entangled, the second receiver being located on the propagation path of the second photon further from the transmitter than the first receiver, such that it arrives there later,

[0242] Modifying, by means of a polarization modifier, the polarization state of the first photon when it reaches the first receiver into a polarization state dependent on the information to be transmitted, the state being chosen from at least two different pairs of complementary absorption polarizations, with the exception of exactly two pairs of perpendicular linear polarizations of which the polarization directions of one of the pairs are at 45° to the polarization directions of the other pair,

[0243] Absorb, using an absorption instrument, the first photon in one of the two complementary polarizations of the chosen pair,

[0244] At the second receiver, duplicate the second photon into a stream of multiplied photons using an amplification device, the light thus created having retained the polarization state of the second photon,

[0245] Measure the average polarization state of the light flux and determine from this measurement the polarization state of the first photon, in order to deduce the information transmitted by the first receiver.

[0246] The pair of complementary polarizations is preferably chosen from at least three different pairs of complementary polarizations.

[0247] The pair of complementary polarizations is for example chosen from 210 distinct pairs of absorption polarizations, in particular from polarizations spaced 9° apart in their direction of polarization and phase shifted by 9°.

[0248] The invention also relates to a quantum communication method using the system defined above, comprising the steps consisting of:

[0249] • Generating a pair of entangled photons from a transmitter, the first photon of the pair being transmitted to a first receiver and the second photon of the pair being transmitted simultaneously to a second receiver, the first and second photons being entangled, the second receiver being located on the propagation path of the second photon further from the transmitter than the first receiver, such that it arrives there later,

[0250] • Choose, depending on the information to be transmitted, whether or not to absorb the first photon in one of two complementary polarization pairs when it reaches the first receiver using an optical selector directing, or not, said photon towards one or more instruments,

[0251] • In the case where it is chosen not to absorb the photon in a predefined polarization, trap said first photon in a protection device making it possible to avoid its absorption at least until the second photon has reached the second receiver,

[0252] • At the second receiver, duplicate the second photon into a stream of multiplied photons using an amplification device, each multiplied photon having retained the quantum state of the second photon,

[0253] • Measure the average quantum state of the flux of multiplied photons and determine according to this measurement whether the first photon was absorbed at the first receiver, and / or with which instrument, in order to deduce the information transmitted by the first receiver.

[0254] Preferably, the polarization of the pair of entangled photons is undetermined in the observable basis(s) in which the instruments of the first receiver absorb them.

[0255] The pair of entangled photons reaches, for example, the receivers with a linear entangled polarization, one of the two complementary absorption polarizations being circular, the second receiver being arranged to distinguish whether the average polarization of the flux of demultiplied photons is circular or linear, and to determine according to this distinction whether the first photon has been measured or not.

[0256] Alternatively, the entangled photon pair reaches the receivers with a circular entangled polarization, the quantum state measured at the first receiver being a linear polarization, the second receiver being arranged to distinguish whether the average polarization of the multiplied photon flux is circular or linear, and determine according to this distinction whether the first photon has been measured or not.

[0257] Alternatively, the entangled photon pair reaches the receivers with a vertical or horizontal linear polarization, the quantum state measured at the first receiver being a linear polarization at 45° or -45° from the vertical or horizontal, the second receiver being arranged to distinguish whether the average linear polarization of the multiplied photon flux (P20) is at 45 or -45°, or vertical or horizontal, and determine according to this distinction whether the first photon has been measured or not.

[0258] Transmission of information The system according to the invention and the quantum communication method described above makes it possible to transmit information between the first and second receivers, either of binary type, in the form of bits, or of discrete or continuous values.

[0259] Preferably, several pairs of entangled photons are generated successively by the transmitter, each pair of photons allowing the transmission of information, for example of the binary type, from the first receiver to the second receiver.

[0260] For example, we can choose to absorb the first photon at the first receiver to transmit a bit 1 and to reflect it to transmit a bit 0. The measurement at the second receiver of the average quantum state of the flux of multiplied photons then makes it possible to determine whether a bit 1 or a bit 0 is transmitted from the first receiver almost instantaneously.

[0261] Several processes can be implemented to secure communication and avoid transmission errors, due for example to double pairs of photons emitted simultaneously.

[0262] For example, during time intervals previously established using the clocks described above, only the first photon reaching a receiver is preferably considered, with subsequent photons being ignored.

[0263] As described above, photons can be counted as they arrive at a polarization detector. If the count reveals the arrival of more than one photon during a predefined time interval, the bit is not transmitted during this interval, and is for example transmitted in the next time interval, or, preferably, the same bit is retransmitted again.

[0264] At the second receiver, counting the photons arriving in the time interval where they are expected can advantageously allow the creation of a temporary list of unreceived bits.

[0265] Two-way communication

[0266] To enable two-way communication, i.e. to enable each of the two receivers to transmit information to the other receiver, several transmitters can be used. The system according to the invention may in particular comprise a second transmitter capable of generating one or more pairs of entangled photons, the second transmitter being located closer to the second receiver than to the first receiver.

[0267] Alternatively, at least some of the photons can be made to travel an indirect path so as to lengthen their transport time to one of the receivers, for example by reflecting the photons off one or more intermediate mirrors, or by making them pass through media with a high refractive index, or by transporting them in optical fibers of varying lengths.

[0268] This elongated path can for example alternate at a fixed or variable rate depending on needs, with the non-elongated path so as to be able to use the entangled photons sometimes to transmit the information from one point to another, sometimes in the other direction. Optical switches upstream of the receivers and synchronized with the switch affixed to the transmitter capable of sending the photons on an extended path, can be installed to send said photons to receivers of the first receiver type as described above, or on the contrary of the second receiver type.

[0269] Method of transmitting a key and verifying that this transmission has not been listened to

[0270] It is possible to verify that information transmitted by a stream of photons from the transmitter to the second receiver has not been listened to by implementing the following steps: the second receiver establishes a first list of the dates of reception of the photons having carried the information as well as the relative polarization designating that of the two complementary polarizations in which each photon was received; the second receiver generates a message comprising the list collected in the previous step, creates an electronic signature of this list and transmits said list as well as the signature to the receiver; the first receiver receives the list and the signature then verifies said signature; the first receiver makes a second list consisting of the elements of the first list whose relative polarizations of the photons received by the first and second receivers are equal, or whose entangled photon has never reached the first receiver;two entangled photons having different relative polarizations; if the number of elements of the second list restricted to pairs of entangled photons each of which has reached its respective receivers is less than the product of a predetermined ratio by the count of the elements of the first list restricted to pairs of entangled photons each of which has reached its respective receivers, then the information is declared to have been transmitted unheard.;

[0271] We can then have the following steps: if the information is declared to have been transmitted unheard, then the second list is sent signed by the first receiver to the second receiver, then a third list is created by the first receiver, consisting of the relative polarizations of the photons appearing in the first list and not appearing in the second list, upon receipt of the second list by the second receiver and after verification of the signature, the third list is recreated in the second receiver using the first and second lists, then a signed message, confirming the correct reception of the second list, is sent by the second receiver to the first receiver; and the second receiver uses the third list as a key shared with the first receiver;upon receipt by the first receiver of the message transmitted by the second receiver in the previous step, the first receiver uses the third list as a shared key in exchanges with the second receiver.;

[0272] A polarizing filter can be placed upstream of the second receiver.

[0273] Brief description of the drawings

[0274] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, and by examining the attached drawing, in which:

[0275] [Fig IA] partially and schematically represents a quantum communication system according to the invention,

[0276] [Fig IB] partially and schematically represents another quantum communication system according to the invention,

[0277] [Fig 2A] partially and schematically represents a linearly polarized photon,

[0278] [Fig 2B] partially and schematically represents a circularly polarized photon,

[0279] [Fig 3] Figure 3 schematically represents details of photon transmission from the transmitter to the receivers,

[0280] [Fig 4A] partially and schematically represents an example of an optical selector according to the invention,

[0281] [Fig 4B] partially and schematically represents a variant of the selector of figure 4A,

[0282] [Fig 4C] partially and schematically represents another example of an optical selector according to the invention,

[0283] [Fig 5A] [Fig 5B] Figure 5A and Figure 5B schematically represent an example of a polarization direction modifier comprising quarter-wave plates,

[0284] [Fig 5C] Figure 5B schematically represents another example of a polarization direction modifier comprising a chiral material,

[0285] [Fig 6] Figure 6 schematically represents an example of a polarization phase modifier,

[0286] [Fig 7] partially and schematically represents an example of an instrument for absorbing a linearly polarized photon,

[0287] [Fig 8] partially and schematically represents another example of an instrument for absorbing a linearly polarized photon, [Fig 9] partially and schematically represents an example of a receiver of the second photon comprising a doped fiber amplifier,

[0288] [Fig 10A] partially and schematically represents an example of a measuring instrument for the second receiver comprising a succession of semi-reflecting blades,

[0289] [Fig 10B] partially and schematically represents an example of a Young interferometer used for the measuring instrument of the second receiver,

[0290] [Fig 11 A] is a block diagram partially illustrating an example of how a quantum communication system operates to transmit a 0 bit,

[0291] [Fig 11B] is a block diagram partially illustrating an example of how a quantum communication system works to transmit a 1 bit,

[0292] [Fig 12] is a block diagram partially illustrating an example of how a quantum communication system works to transmit a series of discrete values,

[0293] [Fig 13] is a block diagram partially illustrating an example of a method for timestamping the arrival of a photon in a receiver,

[0294] [Fig 14] illustrates in a partial and schematic way the possibility of placing an “anti-noise” device in front of the amplifier,

[0295] [Fig 15] partially and schematically represents the possibility of using the system according to the invention to establish two-way communication between two locations, and

[0296] [Fig 16] illustrates an alternative implementation of the invention.

[0297] Detailed description

[0298] Figure 1A illustrates a quantum communication system 1 according to the invention. The system comprises an emitter 2 emitting a pair of entangled photons (PI, P2), the first photon PI propagating on a propagation path DI and the second photon D2 propagating on a propagation path D2 different from the path D 1. The photons PI and P2 are emitted simultaneously.

[0299] Due to the nature of light and the wave-particle duality of the photon, the terms "wave", "photon" and "particle" are used interchangeably in the following to designate the product emitted by emitter 2. The terms "measuring instrument" and "absorption instrument" are used interchangeably to designate an instrument absorbing a photon according to a certain polarization.

[0300] The system 1 comprises a first receiver 3, located on the propagation path DI of the first photon PI, and a second receiver 4, located on the propagation path D2 of the second photon P2.

[0301] Receiver 3 is closer to emitter 2 than receiver 4, so that photon PI reaches it before photon P2 reaches receiver 4.

[0302] In the example considered, the receiver 3 comprises a complex absorber 31 comprising a polarization modifier 32 and an absorption instrument 35.

[0303] The complex absorber 31 is configured to absorb the photon in a polarization state selected from several pairs of different complementary polarizations, preferably at least three pairs.

[0304] The polarization modifier 31 is configured to transmit the photon P1 according to the chosen pair of complementary polarizations and the absorption instrument 35 makes it possible to determine in which of the two states of the pair is the photon P1. Before entering the absorption instrument 35, the polarization state of the photon P1 is a superposition of the two states of the chosen pair, weighted by certain probabilities, for example in an equiprobable manner.

[0305] Figure 1B illustrates another example of a quantum communication system 1 according to the invention. In this example, the receiver 3 comprises two measuring instruments 35 arranged to measure the quantum state of the first photon Pl, and an optical selector 30 arranged upstream of the measuring instruments 35 and configured to send the first photon Pl either to a birefringent prism 36 which leads the photon, according to the linear direction of its polarization on the paths D1' or D1” then to one or other of the measuring instruments 35, or prevents it from being measured, for example by reflecting it on a propagation path D3 different from the path D1.

[0306] The direction D3 is notably chosen so that the photon Pl is not absorbed before its entangled photon is multiplied at the second receiver 4. The system 1 preferably comprises, as illustrated, a device 5 for protecting the quantum state of a photon arranged near the receiver 3, notably on the propagation path D3 in the example considered.

[0307] This device makes it possible, for example, to "trap" the photon PI in the case where it is reflected by the optical selector 30 on the path D3, in order, for example, to prevent it from being measured or absorbed at least until the second photon P2 has reached the second receiver 4. A 5-D photon detector (not shown) can be placed at the end of the device 5 to detect the photons which have been captured there, after their entangled photons have reached the second receiver.

[0308] The second receiver 4 comprises an optical amplifier 40 making it possible to multiply the second photon P2 while maintaining its polarization and a measuring instrument 45 making it possible to measure the average polarization of the multiplied photons.

[0309] The transmission of PI and P2 photons can occur in different ways and in different media. For example, photons propagate in an optical fiber or a waveguide, or freely in space, whether the space is empty or filled with gas.

[0310] Photons can pass through several media, with different refractive indices. For example, anti-reflection plates can be inserted between two media crossed, if necessary, in order to avoid unwanted optical phenomena, in particular the Fresnel reflection of the waves emitted by emitter 2.

[0311] The emitter 2 comprises, for example, one or more lenses chosen of sufficient size so that the photons reach their respective receiver with low diffraction, for example allowing at least 99.99% of the emitted wave to reach the receiver.

[0312] The emitter 2 may further comprise a system, in particular an electronic device, making it possible to adjust the initial directions in which the photons P1 and P2 are respectively emitted.

[0313] This adjustment can, for example, take into account the different refractive indices of the media crossed, and the modification of the trajectory of the photons which can result, for example for a photon emitted from space which enters the atmosphere.

[0314] The wavelength of the emitted wave is, for example, chosen according to the medium(s) to be crossed; for example, photons in the infrared range will preferably be used when they must pass through the atmosphere or air. The emitter 2, for example, generates pairs of entangled photons using the spontaneous parametric conversion (SPDC) method. The emitter 2 is, for example, configured to emit on average less than one pair per unit of time, for example one pair per nanosecond, which corresponds to a photon transmission frequency of 1 GHz.

[0315] When emitted, the photons P are for example rectilinearly polarized, that is to say that the corresponding electromagnetic wave has an electric field whose direction is perpendicular to their direction of propagation D. If necessary, the polarization of the photons can be vertical V or horizontal H, as illustrated in Figure 2A.

[0316] The quantum state corresponding to the polarization of photons is sometimes undetermined until it has been measured or absorbed. Before being measured, the quantum state of the photon is therefore sometimes considered as the superposition of the possible states, namely, in the example considered, as a superposition of a polarization at an angle of 45° and a polarization at an angle of -45°.

[0317] In the variant illustrated in Figure 2B, the P photons when emitted are circularly polarized, that is to say that the direction of the corresponding electric field changes according to a rotational movement, while its norm remains constant.

[0318] If applicable, the polarization of the photons is defined by the direction of rotation of the electric field, either clockwise Cl or counterclockwise C2. Before being measured, the linearly polarized photons are in an indeterminate quantum state, considered as the superposition of two circular polarization states with opposite directions of rotation.

[0319] In certain cases, two quarter-wave plates 6 and 8 can be introduced into the photon propagation paths, between the transmitter 2 and the receivers 3 and / or 4, as illustrated in figure 3.

[0320] The blade 6, depending on its orientation, transforms for example the rectilinear polarization of a photon into circular polarization by advancing or delaying the electromagnetic waves propagating along an axis perpendicular to the ordinary axis of the crystal with an electric field perpendicular to the same axis compared to the waves having an electric field parallel to said ordinary axis.

[0321] The photons emerging from the plate 6 are for example transmitted by an optical fiber 7 to another quarter-wave plate which converts the circularly polarized fields into rectilinearly polarized fields, before propagating towards the receivers 3 or 4. This transformation of the polarization of the emitted photons makes it possible in particular to not have to take into consideration at the receivers the direction of the polarization of the photons emitted by the transmitter.

[0322] The optical selector 30 can be implemented in various ways, some examples of which are illustrated in Figures 4A to 4D.

[0323] The optical selector 30 comprises, for example, a liquid crystal controlled mirror as illustrated in FIG. 4A, comprising a plate 310 enclosing liquid crystals. For example, electrodes 315 and 320 are placed on opposite sides of the plate 310 in order to subject the liquid crystals to an electric field, making it possible to control its refractive index ne.

[0324] Depending on the refractive index ne , the incident photon PI can be reflected on a propagation path D3, or it can pass through the plate 310 and be transmitted on a propagation path Dl', which can be identical or different from the path Dl, towards the measuring instrument 35.

[0325] In a variant illustrated in Figure 4B, the photon PI passes through the liquid crystal plate 310 and comes out along D3 or Dl', the directions Dl, Dl' and D3 being, in the example considered, parallel.

[0326] A mirror 325, in particular a Bragg mirror, can be placed on the propagation axis D3 in order to reflect the photon in another direction, for example towards the protection device 5

[0327] Anti-reflection blades can advantageously be placed on each of the faces of the blade 310 and adjusted according to the entry and exit angles of the photon when it passes through it.

[0328] In the variant illustrated in Figure 4C, the optical selector 30 is a mirror controlled by non-linear fibers. It comprises two optical fibers 330 and 335. The fiber 335 is for example formed from a non-linear material.

[0329] The first photon PI, upon arrival at the first receiver, enters fiber 330. We can choose to simultaneously illuminate fiber 335 with a light signal F, also called a “control signal”.

[0330] The signal F is a high-intensity light wave, for example emitted by a laser, and of a wavelength different from that of the photon P1. If the fiber 335 is illuminated by the signal F, the photon P1 remains in the fiber 330 before, for example, passing through a prism 345 at the fiber outlet, arranged on a propagation path D3. Similar to what was described above, a mirror 325, in particular a Bragg mirror, can be placed on the propagation axis D3 in order to reflect the photon in a desired direction, for example towards the protection device 5 or, straight back into the fiber 330.

[0331] If the fiber 335 is not illuminated by the signal F, the photon PI is transmitted from the fiber 330 to the fiber 335. The photon PI emerges from the fiber 335 and passes through, for example, a prism 340 before being transmitted, for example, to the measuring instrument 35. The prism 340 is preferably made of a dispersive material making it possible to bring out the illumination light from a different location and in a different direction from that of the photon. A Bragg mirror (not shown) returning only said illumination light and allowing the photon to pass may be placed between the fiber 335 and the prism 340, for example to reflect said illumination light in a direction other than toward the prism 340 or even into the fiber.

[0332] Anti-reflection plates (not shown) can be placed at the entrance and exit of prisms 340 and 345 and adjusted to the wavelength of the photon Pl.

[0333] We will now describe various ways of implementing the polarization modifier 32 of the system of Figure 1B.

[0334] The polarization modifier 32 preferably comprises a polarization direction modifier 32a disposed upstream of a phase modifier 32b.

[0335] In a first exemplary embodiment of polarization direction modification, the polarization direction modifier 32a comprises a stack of plates 510, in particular quarter-wave plates, oriented differently from one another, as illustrated in FIG. 5A so that a circular wave 508 penetrating one of these quarter-wave plates emerges as a wave 511 linearly polarized in a direction associated with said plate.

[0336] These blades can be designed to operate with waves 508 penetrating them with a direction parallel to the bisector of their edges 521 and 522. These blades are preferably composed of uniaxial birefringent crystals such as, for example, rutile. This birefringent crystal can compose their entire mass or be concentrated on a slice, for example on one of the edges 523 crossed by the wave 508, as illustrated in FIG. 5A.

[0337] These blades can be juxtaposed with each other to form a structure 524, as illustrated in FIG. 5B, such that the outgoing waves 511 or 512 have a linear polarization whose direction of the electric field depends on the location of penetration into the assembly 524 of two circularly polarized waves 508 or 509 with a direction parallel to the bisector of the edges 521 and 522.

[0338] Thus, a wave 505 of determined direction and linear polarization penetrating into the quarter-wave plate 501 crosses it in the direction 507 to become circularly polarized there before penetrating into a crystal 502 whose refractive index is adjustable, for example under the effect of an electric field generated by electrodes 516, then preferably enters another crystal 503 whose refractive index is also adjustable, for example under the effect of an electric field generated by electrodes 517 generating an electric field perpendicular to the field generated by the electrodes 516.

[0339] Two circularly polarized waves 508 and 509 then emerge from the crystal 503 and enter the assembly 524 at different locations depending on the choice of the refractive index imposed on the crystal 503. The direction of the light ray being different depending on these locations, the direction of the edges 520 and 521 is preferably different for each element 510 of the assembly 524.

[0340] In the diagram illustrated in Figure 5B, waves 511 and 512 are therefore two possibilities for the light ray to pass through assembly 524. These waves leave assembly 524 linearly polarized in different directions and having the same direction of propagation but at different locations on the edge 514 of assembly 524. They enter another crystal 504 whose refractive index is electrically adjustable by electrodes 518 then, preferably, yet another crystal 505 whose refractive index is also adjustable by electrodes 519 creating an electric field perpendicular to the field created by electrodes 518, and whose voltages are adjusted as a function of the refractive index chosen for crystals 502 and 503 so that the light waves always leave as a wave 513 in the same direction from crystal 505 at the same location 515.The crystals 502, 503, 504 and 505 are, for example, E7 liquid crystals from Merck, to the terminals of which the electrodes apply, for example, voltages between 0 and 5,000 V if the distance between the electrodes of each pair is 5 mm, making it possible to vary their refractive indices between 1.5 and 1.67. The electrodes are preferably covered with a dielectric film. These refractive indices modified by the electric fields can be different depending on the relative direction of the electric field of the light and the direction of the electric field, the application of successive perpendicular electric voltages makes it possible to modify the refractive index for each of the two components of said light, this being the superposition of the waves whose electric field is perpendicular to Figure 5B and the waves whose electric field is perpendicular to the direction of propagation and in the plane of the figure.

[0341] In a second exemplary embodiment (not shown) of the modification of the polarization direction, the linearly polarized wave in a determined direction enters a first quarter-wave plate transforming its polarization into circular polarization, then a second quarter-wave plate transforming this circular polarization into a linear polarization in an adjustable direction depending on the orientation of this second quarter-wave plate. The orientation of the second quarter-wave plate is obtained for example by mechanical control of a sensor, or an electrically controlled device allowing its rotation, for example by rubbing on an axis set in motion by a piezoelectric material or by an electric motor device, for example direct current.

[0342] In a third example illustrated in Figure 5C, liquid crystals are used to control the rotation of the linear electric field of the incident photons 526 linearly polarized in the same direction of polarization, which begin for example by crossing, preferably perpendicularly, one of two transparent electrodes 527 of 0.2 pm coated on one of its faces 528 with a material orienting the liquid crystal particles 529 adjacent to said face 527 in the direction of the polarization of the incident photons 526, then crossing the liquid crystal 529 over 2 pm before crossing the second transparent electrode 527 whose face 528, preferably parallel to the first electrode, is also coated with a material making it possible to align the liquid crystals but in a direction perpendicular to the direction imposed by the coating of the first electrode.The direction of the liquid crystals between the two electrodes is thus gradually modified from 0° to, for example, 60° relative to their initial orientation. The application of an electrical voltage between, for example, 0 and IV gradually reorients the liquid crystals to impose a direction on them in the axis of the direction of propagation of the photons, thus gradually reducing the difference in refractive index of the liquid crystals between the two polarizations perpendicular to the displacement of the photons and the rotary power of the device. This device therefore makes it possible to continuously choose the output direction of the polarization of the incident photon between 0 and 60°.

[0343] Devices such as that illustrated in Figure 5B are advantageously used in a device similar to that described above and illustrated in Figure 5A, for example placed between the blades 524 and 504, each of for example 4 elements 510 making it possible to place the linear polarization of an incident photon in one of four precise directions, for example -90°, -45°, 0° and +45°, then each of the devices described in Figure 5B placed in succession, and preferably aligned with the direction of propagation of the photos leaving the devices 510 which precede them, making it possible to add to this rotation, a rotation of any angle between for example 0 and 45°.

[0344] Figure 6 illustrates a polarization phase modifier 32b according to the invention. The phase modifier 32b comprises a first birefringent plate or prism 310 arranged to split the incident photon PI of polarization field E into two electromagnetic waves of linear polarization Ei and E2 on two different axes xi and X2 .

[0345] On the second axis x2, a delay plate 321 of variable refractive index is placed, for example comprising a Pockels cell such as Lithium Niobate, controlled by electrodes 322 or a non-linear material, so as to cause the wave E2 oriented along the second axis X2 to acquire a predetermined phase shift relative to the wave Ei oriented along the first axis xi. .

[0346] Another birefringent plate 310, for example made of paratellurite, is arranged at the output of the phase modifier 32b making it possible to combine along the same axis x the two waves whose polarization fields Ei and E2 are perpendicular into a wave or photon PI of field E' having acquired a phase shift with respect to the field E. Alternatively, to take advantage of the different modification of the refractive index of the crystal according to the direction of the electric field of the light relative to the direction of the electric field modifying the refractive index, a simple Pockels cell321 can be used with the electrodes 322 but without the plates 310.

[0347] The measuring instruments 35 of the first receiver 3 may be of various types and comprise various elements. These depend in particular on the nature of the polarization of the photon PI when it is emitted by the transmitter 2. Certain examples are illustrated in figures 7 and 8 and described below.

[0348] In the example illustrated in Figure 7, the measuring instrument 35 comprises a polarizing filter 350 arranged on the propagation path DI of the photon PI, and a photon detector 355 arranged on the same propagation path, downstream of the filter 350.

[0349] The 350 polarizing filter only allows photons with a certain linear polarization to pass through and absorbs those with a polarization perpendicular to it. It therefore allows the selection of photons with a rectilinear polarization of a certain direction.

[0350] In the example considered, the polarizing filter 350 is a grid formed of wires, for example metallic, vertical. It only lets through photons having a horizontal rectilinear polarization H.

[0351] In a variant illustrated in Figure 8, the measuring instrument 35 comprises an anisotropic plate 360, for example a birefringent plate, a birefringent prism or two birefringent prisms joined together. The plate is for example made of barium beta borate (BaB204, BBO).

[0352] The photon PI whose quantum state is to be measured reaches the plate 360 ​​on an incident propagation path D1, and is transmitted on one of two propagation paths DI1 or D12, depending for example on whether its polarization is in the plane of plate 360 ​​(i.e., perpendicular to the plane of figure 8), or normal to the plane of plate 360.

[0353] The measuring instrument 35 further comprises two detectors 355, each placed on a propagation path D11 or D12 of the photon Pl.

[0354] In this example and unlike the polarizing filter, all photons can be detected by the 355 detectors, regardless of their polarization.

[0355] In some embodiments, a quarter-wave plate is placed upstream of the anisotropic plate 360 ​​in order to transform a circular polarization of a photon into linear polarization, and thus detect the direction of rotation of a circularly polarized photon. The second receiver 4 may comprise different types of optical amplifier 40 and measuring instrument 45.

[0356] For example, a doped fiber amplifier is used as illustrated in Figure 9.

[0357] In this example, the amplifier 40 comprises a fiber 400, in particular a fiber made of non-linear material, into which the second photon P2 is introduced after having passed through a dichroic prism 401 when it reaches the receiver 4.

[0358] An electromagnetic control wave L used to supply energy and multiply the photon P2 is introduced by a fiber 410 into the same dichroic prism 401 at a point and with a direction such that it emerges in the fiber 400 just like the photon P2, the photon P2 and the control wave F having different wavelengths.

[0359] The control wave F is preferably of high light intensity and preferably of a wavelength shorter than that of the photon P2.

[0360] During its passage through the fiber 400 and under the effect of the luminous flux F, the photon P2 is multiplied into N photons P20, then these N photons and the flux F pass through a second dichroic prism 402 from which they emerge in different directions. The N photons are then advantageously directed towards a measuring instrument 45, making it possible to determine the polarization of the luminous flux and examples of embodiment of which are given in figures 10A and 10B.

[0361] The information transmitted by the first receiver can be deduced from the measurements of the measuring instrument 45, because the amplification of the photon P2 by the device 40 preserves its polarization.

[0362] In particular, if the photon P2 is the photoentangled of a photon PI absorbed in one of two complementary polarizations known at the first receiver, the N demultiplied photons P20 are measured with a polarization which can be deduced from the absorption polarization of the photon PI by the Jones matrix calculated during a calibration as described above.

[0363] The measuring instrument 45 may comprise a succession of semi-reflecting plates 452 and a mirror 453, as illustrated in FIG. 10A. The plates 452 and the mirror 453 direct the multiplied photons P20, which form a luminous flux, towards, for example, prisms made of birefringent materials 350 with preferably equal intensity. The filters 350 divide the luminous flux into two fluxes of orthogonally polarized light, so that the detectors 455 can determine either the intensity of said flux in each of the orthogonal directions, or determine the phase shift between the two orthogonal components of the flux, for example by means of a Michelson interferometer or a Young slit interferometer.

[0364] Interferometers are preferably arranged so that the light paths of the two orthogonal components are identical, the light flux resulting from the multiplication of a single photon being very brief. The use of several pairs of orthogonal directions, for example offset by 45° from each other, advantageously makes it possible to measure the polarization, i.e. the direction of the polarization, as well as the phase shift between the two directions, several times, thus allowing greater precision in the measurement.

[0365] Figure 10B shows an example of such a Young's slit interferometer. Two waves 460 and 463 come from the prism 350. The wave 460 passes for example through a prism 461 straightening its direction of propagation so that it becomes parallel to the wave 463 at 462. The wave 463 passes for example through a half-wave plate making it possible to align the electric field of said wave 465 with that of the wave 462. The waves 462 and 465 then pass through two holes perforated in a screen 466 before interfering with each other to form fringes on a screen 467 observed by a camera or equipped with photosensitive sensors making it possible to determine the position on said screen 467 of the brightest fringe.

[0366] Information, in particular binary information, can be transmitted between the receivers 3 and 4 of a system 1 such as that described in Figure 1B, for example by following the steps illustrated in Figures 11A and 11B.

[0367] The correspondence protocol between the information to be transmitted, for example, between sending a bit "0" or a bit "1", and the measurement, or not, of the first PI photon, is determined before the start of the transmission.

[0368] As an example, we choose to subsequently not measure the first photon PI to transmit a "0", and to measure it to transmit a "1". Of course, the opposite choice, or any other suitable correspondence, would also be valid. In step 10, two entangled photons PI and P2 are emitted simultaneously from an emitter 2 to receivers 3 and 4, respectively, with receiver 3 placed closer to emitter 2 than receiver 4, as described above.

[0369] The emitted photons PI and P2 exhibit an indeterminate quantum state in a predetermined pair of complementary polarizations, for example a 45° polarization if they are linearly polarized (the complementary polarizations being 0° and 90°).

[0370] At step 11, the photon PI reaches the optical selector 30 of the receiver 3.

[0371] If a bit “0” must be transmitted, the photon PI is for example reflected by the optical selector, on a perpendicular path in the example illustrated in figure 1 1A (but any other path is possible), so as not to be able to reach the measuring instrument 35. The photon PI can in particular be trapped at step 12 in a device for protecting its quantum state 5, in order to avoid its absorption, at least until the second photon P2 has reached the second receiver 4.

[0372] The PI photon, for example, retains its polarization at 45°.

[0373] If a bit “1” is to be transmitted, the photon PI passes for example through the optical selector 30 to go towards the measuring instrument 35, as illustrated in figure 1 IB.

[0374] The quantum state of the photon PI is then measured by the measuring instrument 35, in step 13.

[0375] Photon PI now has a specific quantum state, for example, vertical polarization (90°), or horizontal polarization (0°). Instantly, the measurement made in step 13 projects the entangled photon P2 into a specific state.

[0376] At step 15, whatever the information to be transmitted, the photon P2 reaches the optical amplifier 40 of the second receiver 4, where it is duplicated into a flow of multiplied photons P20, each photon P20 having retained the polarization of the photon P2.

[0377] In step 16, the polarization of the photon flux P20 is measured by a measuring instrument 45.

[0378] If, on average, an intermediate state is obtained, for example a 45° polarization, it is deduced that the first PI photon was not measured, and a bit "0" is received. If a quantum state corresponding to a measurement in the first receiver 3 is obtained, for example a vertical (90°) or horizontal (0°) polarization, it is deduced that the first PI photon was measured, and a bit "1" is received.

[0379] It is still possible to transmit series of discrete values ​​between receivers 3 and 4 of a system 1 such as that described in Figure 1A, for example by following the steps illustrated in Figure 12.

[0380] In step 10, two entangled photons PI and P2 are emitted simultaneously from an emitter 2 to receivers 3 and 4, respectively, for example with linear polarization.

[0381] At step 17, the PI photon reaches the polarization modifier 32, which transforms the linear polarization of the PI photon to a chosen pair of complementary polarizations, the pair having been chosen according to the Jones formalism and corresponding to a discrete value to be transmitted.

[0382] In step 13, the photon PI is absorbed by the instrument 35 in one of the two complementary states of the chosen pair, at the same time projecting the entangled photon P2 into its complementary state.

[0383] The following steps 15 and 17 are similar to those described above; the polarization of photon P2 is measured by amplification then absorption, and the transmitted discrete value is deduced.

[0384] The invention is not limited to the measurement of a linear polarization of photons. Other types of quantum state can be measured, and / or other measurement means can be used, in particular measurements in other observable bases.

[0385] The PI and P2 photons are for example emitted with a circular entangled polarization, and the measuring instrument 35 projects, by measuring a linear polarization, the PI photon and by entanglement the P2 photon, into a linear polarization base.

[0386] Alternatively, the PI and P2 photons are emitted with a linear entangled polarization, and the measuring instrument 35 projects, by measuring a circular polarization, the PI photon and by entanglement the P2 photon according to this observable basis.

[0387] In another example, the photons P1 and P2 are emitted with a vertically or horizontally entangled linear polarization and the measuring instrument 35 measures a linear polarization at 45° or -45° from the vertical or horizontal. In a variant, the receiver 3 comprises two measuring instruments 35, one measuring a linear polarization, the other measuring a circular polarization. The optical selector sends, for example, the photon P1 to the first measuring instrument if a bit “1” is to be transmitted, to the second measuring instrument if a bit “2” is to be transmitted, and prevents it from being measured if a bit “0” is to be transmitted.

[0388] In each of the above examples, the measuring instrument 45 of the second receiver 4 can be configured to detect whether the first photon PI has been measured, or not, and, if so, with which polarization it was measured at the first receiver.

[0389] Gyroscopes can further be used at transmitter 2 and receivers 3 and 4 to determine the polarization direction of the P1 and P2 photons, if these are emitted and transported to the receivers with linear polarization.

[0390] As mentioned above, receivers 3 and 4 as well as device 5 can be equipped with photon counting devices during their absorption, and possibly after they wait for the absorption of their entangled photon, which can be accompanied by a timestamp of their arrival, as illustrated in Figure 13. This can make it possible, for example, to check the correspondence between the pairs of entangled photons and the transmitted bits.

[0391] Each photon triggers for example the time-stamping process by arriving at the receiver in step 80, in particular by reaching one of the measuring instruments 35 or 5D. Following this triggering event, the receiver 3 copies for example in step 82 the current time H to the clock of said receiver, for example written in a register R, on a free memory register M, preferably after having subtracted from the arrival time at the detector 5-D the transit time of the photon in the waiting device 5.

[0392] A similar device (not shown) may be arranged at the second receiver 4.

[0393] Preferably, the clocks enabling the time stamping which has just been described are synchronized for the two receivers 3 and 4, which possibly makes it possible to define time intervals for processing the photons common to the two receivers, and to process the “multiple” double pairs of photons emitted by the transmitter 2.

[0394] The second receiver 4 may also comprise a switch 50 placed in front of the optical amplifier 40, and configured to absorb or reflect any unwanted photon(s) PE2 which reach the receiver 4 after the photon P2 within the same processing time interval, as shown in FIG. 14.

[0395] The switch 50 can also absorb any photons PA2 emitted by the amplifier 40 towards the emitter 2 during the amplification of the signal, or during the de-excitation of molecules or atoms from the amplifying medium.

[0396] The switch 50 is for example controlled by an electronic mechanism (not shown) itself controlled by the measuring instruments 45 when these or one of them detects photons.

[0397] In certain embodiments, the system according to the invention may comprise several transmitters arranged differently relative to the receivers, in particular in order to establish two-way communication.

[0398] In the example illustrated in Figure 15, system 1 comprises two transmitters 21 and 22, each emitting pairs of entangled photons towards two receivers 91 and 92.

[0399] The transmitter 21 is closer to the receiver 91 than to the receiver 92, and the transmitter 22 is closer to the receiver 92 than to the receiver 91. This arrangement allows the receivers 91 and 92 to communicate bidirectionally; the photon P11 emitted by the transmitter 21 arrives first at the receiver 91, which can then act as the first receiver 3, that is, transmit information to the receiver 92 which receives its entangled photon P12 and acts as the second receiver 4.

[0400] Conversely, the photon P21 emitted by the transmitter 22 arrives first at the receiver 92, which this time can transmit information to the receiver 91 which receives the entangled photon P22. The mechanism for transmitting information, in particular binary information, is for example similar to what was described above, in figures 11 and 12.

[0401] Reflecting devices 93 and 94 may be disposed near receivers 91 and 92, respectively, to reflect photons possibly reflected by receivers 91 and 92.

[0402] We will now describe with reference to Figure 16 an alternative implementation of the invention, intended to verify non-listening to information transmitted by the transmitter 2 and, if necessary, to generate a shared key for the exchange of information between the receivers 3 and 4, from the transfer of information that is known not to have been listened to. The system of Figure 16 is modified by the addition of at least one polarizing filter 46 at the input of the receiver 4, this filter having a polarization direction that corresponds to that expected for the photons having traveled from the transmitter to the receiver 4. The polarizing filter 46 is thus arranged upstream of the change in polarization of the photons initiated by the receiver 3 transmitting the information to the receiver 4.

[0403] The filter 46 prevents a fraudulent receiver from replacing the authentic receiver 4, from observing the exact polarization of the entangled photon after multiplication of said photon, then from emitting to the authentic receiver 4 a fraudulent photon, in replacement of the authentic photon, to be necessarily observed by the receiver 4 in the same relative polarization as the authentic photon, because on the one hand if the latter photon is not entangled with another photon its polarization will not change after having passed the filter 46 and will therefore have the polarization imposed by the filter 46 and not that of the authentic photon, and on the other hand if the fraudulent photon is entangled with another photon called the second fraudulent photon, it is not possible to impose the relative polarization of these fraudulent photons nor moreover to destroy or stop said fraudulent photon after it has passed the filter 46,if the verification of the relative polarization of its entangled photon is then verified as inadequate, by a third-party device.,

[0404] It is therefore extremely unlikely that all or some of the photons in a group of entangled photons observed by both receivers 3 and 4 have been observed elsewhere, if their relative polarizations are always different.

[0405] We can statistically verify the equality of the quantum state of the photons used for the transmission of information arriving at each of the two receivers 3 and 4 and deduce that the probability that some of the photons were observed is low, by applying the following method:

[0406] Upon reception of photons by each of the receivers 3 and 4, the receivers note the time of reception as well as the exact polarization of the photons by the use of photon detectors, and not only to which pair of complementary polarizations they belong. The times and polarizations of the detected photons too close to each other to have their polarization or arrival time measured independently are, however, preferably not noted and the two receivers 3 and 4 preferably have synchronized clocks.After a determined number of photons carrying information I have been received, the second receiver 4 generates a message comprising the precise time of detection and the polarization for each of the photons whose attributes have been noted and have been retained as transmitting information and therefore belonging to a group of NSP photons or more having a given polarization or the complementary polarization of this polarization; NSP is defined above as the predetermined threshold number of photons received in a given polarization or its complementary polarization making it possible to identify the reception of information.

[0407] This message is signed, preferably using the random hash technique described in application US20210165914 A1, by the receiver 4, then is sent to the receiver 3 with said signature, preferably by implementing a quantum transmission means as described above, then the signature of the received message is verified, as well as the correspondence of the relative polarizations of the photons received by the two receivers for the pairs of photons of which the two photons have reached the receivers.This information I can be declared not to have been listened to during its transmission between the two receivers if the verification of the signature makes it appear as authentic and the proportion of polarizations not corresponding is lower than a given threshold, for example 1% if 450 pairs of different polarizations are used, or a multiple such as 2 of the probability that two pairs of different entangled photons are emitted at dates indistinguishable by the receivers.

[0408] Furthermore, a relative polarization in each of the pairs of polarizations of the photons having been retained for having transmitted the information I, if this relative polarization has been verified as being different for each of the photons arriving at each of the receivers, can be used to form a sequence of bits forming a randomly generated key known only to each of the receivers; the receiver 3 having carried out the verification of concomitance of the relative polarizations will send to the other receiver 4 a signed message, preferably by the random hash technique, formed from the list of the reception dates of the photons for which the relative polarizations do not correspond or for which the entangled photon has never reached the receiver 3. Thus, in an example, the following steps are implemented.

[0409] Step 1

[0410] The receiver 4 receives from the transmitter 2 a flow of photons carrying information and establishes a first list of the dates of reception of the photons used to determine this information as well as the relative polarization designating that of the two complementary polarizations in which the photon was received.

[0411] Step 2

[0412] Receiver 4 forms a message consisting of the list of information collected in the previous step, creates an electronic signature of this list and transmits said list as well as the signature to receiver 3.

[0413] Step 3

[0414] Receiver 3 receives the list and the signature and then verifies the signature.

[0415] Step 4

[0416] Receiver 3 makes a second list consisting of the elements of the first list whose relative polarizations of the photons received by receivers 3 and 4 are the same or whose entangled photon has never reached receiver 3, said receiver 3 having recorded in a register the relative polarization of each photon used for the transmission of information as well as its date of reception. Knowing the difference in travel time of the entangled photons between transmitter 2 and each of the two receivers 3 and 4, it can find for each photon of the first list the relative polarization of the photon that it received on the date of reception of the photon by receiver 4, from which the travel time has been subtracted.This second list is preferably made up of two distinct parts, a first of these parts grouping the photons whose entangled photon has never reached the first receiver 3 and a second part grouping the photons whose two entangled photons have reached their respective receivers but with equal relative polarizations. Step 5.

[0417] If the number of elements in the second list restricted to pairs of entangled photons in which each of the two photons has reached its respective receivers is less than the product of a predetermined ratio by the count of the elements in the first list restricted to pairs of entangled photons in which each of the two photons has reached its respective receivers, then the information is declared to have been transmitted unheard.

[0418] Step 6

[0419] If the information is declared to have been transmitted unheard, then the second list is sent signed by receiver 3 to receiver 4, then a third list is created by receiver 3, consisting of the relative polarizations of the photons appearing in the first list and not appearing in the second list, that is to say concerning the photons for which the relative polarizations were observed to be different.

[0420] Otherwise, that is to say if the aforementioned product is greater than said threshold, a message indicating that the transmission is likely to have been listened to is sent by receiver 3 to receiver 4.

[0421] Step 7

[0422] Upon receipt of the second list by receiver 4 and after verification of the signature, the third list is recreated in receiver 4 using the first and second lists, then a signed message, confirming the correct receipt of the second list, is sent by receiver 4 to receiver 3.

[0423] Step 8

[0424] Receiver 4 can use the third list as a shared key with receiver 3; Upon receipt by receiver 3 of the message transmitted in step 7, receiver 3 can use the third list as a shared key with receiver 4.

Claims

Demands 1. Quantum communication system (1) comprising: • An entangled photon emitter (2) comprising a source configured to generate at least one pair of entangled photons comprising a first photon (PI) emitted on a first propagation path (D1), and simultaneously a second photon (P2) emitted on a second propagation path (D2) different from the first propagation path; • A first receiver (3) disposed on the first propagation path (Dl), comprising: at least one first instrument (35) arranged to absorb the first photon (PI) in one of two complementary polarizations, the polarization of the photon (PI) being indeterminate in the observable basis according to which the first instrument absorbs the photons, an optical selector (30) disposed upstream of said at least one first instrument (35) and configured to either allow the first photon (PI) to pass to said at least one first measuring instrument (35), or to prevent it from being measured; • A second receiver (4) disposed on the second propagation path (D2) so as to be reached by the second photon (P2) after the first photon (PI) has reached the optical selector (30) and / or after it can have reached the instrument (35) of the first receiver (3), said second receiver (4) comprising an optical amplifier (40) allowing the second photon (P2) to be multiplied while preserving its polarization and, disposed downstream of the amplifier, a measuring instrument (45) allowing the average quantum state of the multiplied photons (P20) to be measured.

2. System according to the preceding claim, the first receiver (3) comprising at least a second instrument (35), the optical selector (30) being disposed upstream of the first and second instrument (30) and configured to either allow the first photon (PI) to pass by directing it towards one or the other of the instruments, or to prevent it from being absorbed.

3. System according to any one of the preceding claims, the emitter (2) being configured to successively generate a plurality of entangled photon pairs.

4. System according to any one of the preceding claims, the optical selector (30) comprising a reflector (310), in particular a controlled mirror.

5. System according to any one of the preceding claims, the optical selector comprising a device (310; 330) whose refractive index and / or direction of reflection are controlled, in particular by means of an electric field or a luminous flux.

6. System according to any one of the preceding claims, comprising a device (5) for protecting the quantum state of at least one photon, in particular a device which prevents the measurement or absorption of at least one photon, disposed near the first receiver (3) so as to allow the quantum state of the first photon (PI) to be protected in the case where it is prevented from being measured by the optical selector (30), said quantum state of the first photon (PI) being protected at least until the second photon (P2) has been multiplied at the second receiver (4).

7. System according to the preceding claim, the quantum state protection device (5) comprising a transparent space, in particular a transparent space and at least one mirror.

8. System according to any one of the preceding claims, at least one instrument (35) of the first receiver comprising at least one filter (350; 360) enabling the first photon to be sent to at least one photon detector (355), in particular to one or the other of two photon detectors (355) depending on the polarization state of the first photon, the filter preferably being a prism or a plate of birefringent material.

9. System according to any one of the preceding claims, the instrument (45) of the second receiver (4) comprising at least one photon detector (455), arranged to measure the polarization of the light (P20) resulting from the multiplication of the second photon (P2).

10. System according to any one of the preceding claims, the measuring instrument (45) of the second receiver (4) comprising a succession of semi-reflective blades arranged downstream of the optical amplifier (40), said blades directing the flux of multiplied photons (P20) with equal intensity towards polarizing filters downstream of which detectors are arranged.

11. System according to any one of the preceding claims, the optical amplifier (40) being a doped fiber amplifier.

12. System according to any one of the preceding claims, the first receiver (3) and / or the second receiver (4) comprising one or more dichroic filters (420), said filters being preferably arranged in front of the instrument(s) (35; 45), or in front of the optical selector (30) for the first receiver.

13. System according to any one of the preceding claims, the transmitter (2) and each of the receivers (3;4) comprising a clock, the clocks of the transmitter and the receivers being synchronized with each other.

14. System according to any one of the preceding claims, the second receiver (4) comprising a switch (50) disposed in front of the optical amplifier (40), configured to absorb or reflect the photon(s) subsequent to a first photon reaching said second receiver in a predetermined time interval.

15. System according to any one of the preceding claims, comprising a second emitter (22) capable of generating one or more pairs of entangled photons, the second emitter being located closer to the second receiver (92) than to the first receiver (91).

16. A quantum communication method using the system of any one of claims 1 to 15, comprising the steps of: • Generate a pair of entangled photons from an emitter (2), the first photon (PI) of the pair being emitted towards a first receiver (3) and the second photon (P2) of the pair being emitted simultaneously towards a second receiver (4), the first and second photons being entangled, the second receiver (4) being located on the propagation path (D2) of the second photon (P2) further from the emitter (2) than the first receiver (3), such that it arrives there later, • Choose, according to the information to be transmitted (“0”, “1”, “2”), whether or not to absorb the first photon (PI) in one of two complementary polarization pairs when it reaches the first receiver (3) by means of an optical selector (30) directing, or not, said photon (PI) towards one or more instruments (35), • In the case where it is chosen not to absorb the photon in a predefined polarization, trap said first photon (PI) in a protection device (5) allowing to avoid its absorption at least until the second photon (P2) has not reached the second receiver (4). • At the second receiver (4), duplicate the second photon (P2) into a stream of multiplied photons (P20) using an amplification device (40), each multiplied photon (P20) having retained the quantum state of the second photon (P2), • Measure the average quantum state of the multiplied photon flux (P20) and determine from this measurement whether the first photon (PI) was absorbed at the first receiver (3), and / or with which instrument, in order to deduce the information transmitted (“0”, “1”) by the first receiver (3).

17. Method according to the preceding claim, the polarization of the entangled photon pair being indeterminate in the observable basis or bases in which the instruments of the first receiver (35) absorb them, 18. Method according to claim 16 or 17, the pair of entangled photons reaching the receivers (3;4) with a linear entangled polarization, one of the two complementary absorption polarizations being circular, the second receiver (4) being arranged to distinguish whether the average polarization of the multiplied photon flux (P20) is circular or linear, and to determine according to this distinction whether the first photon has been measured or not.

19. Method according to claim 16 or 17, the pair of entangled photons reaching the receivers (3;4) with a circularly entangled polarization, the quantum state measured at the first receiver (3) being a linear polarization, the second receiver (4) being arranged to distinguish whether the average polarization of the multiplied photon flux (P20) is circular or linear, and to determine according to this distinction whether the first photon (PI) has been measured or not.

20. Method according to claim 16 or 17, the pair of entangled photons reaching the receivers with a vertical or horizontal linear polarization, the quantum state measured at the first receiver (3) being a linear polarization at 45° or -45° from the vertical or the horizontal, the second receiver (4) being arranged to distinguish whether the average linear polarization of the multiplied photon flux (P20) is at 45 or -45°, or the vertical or the horizontal, and to determine according to this distinction whether the first photon (PI) has been measured or not.

21. A method according to any one of claims 16 to 20, wherein several pairs of entangled photons are generated successively by the emitter (2), each pair of photons allowing the transmission of information (“0”, “1”, “2”) from the first receiver (3) to the second receiver (4).

22. A method according to any one of claims 16 to 21, wherein the non-eavesdropping of information transmitted by a stream of photons from the transmitter (2) to the receiver (4) is verified by implementing the following steps: the second receiver (4) establishes a first list of the reception dates of the photons that carried the information, as well as the relative polarization designating which of the two complementary polarizations each photon was received in; the second receiver (4) generates a message containing the list collected in the previous step, creates an electronic signature of this list and transmits said list and the signature to the receiver (3); the first receiver (3) receives the list and the signature and then verifies said signature;the first receiver (3) makes a second list consisting of the elements of the first list whose relative polarizations of the photons received by the first and second receivers (3) and (4) are equal, or whose entangled photon never reached the first receiver (3); two entangled photons having different relative polarizations; if the number of elements of the second list restricted to pairs of entangled photons where each of the two photons reached their respective receivers is less than the product of a predetermined ratio by counting the elements of the first list restricted to pairs of entangled photons where each of the two photons reached their respective receivers, then the information is declared to have been transmitted unheard.

23. A method according to claim 22, wherein: if the information is declared to have been transmitted unlistened to, then the second list is sent signed by the first receiver (3) to the second receiver (4), then a third list is created by the first receiver (3), consisting of the relative polarizations of the photons appearing in the first list and not appearing in the second list, Upon receipt of the second list by the second receiver (4) and after verification of the signature, the third list is recreated in the second receiver (4) using the first and second lists, then a signed message, confirming the good receipt of the second list, is sent by the second receiver (4) to the first receiver (3); and the second receiver (4) uses the third list as a shared key with the first receiver (3); upon receipt by the first receiver (3) of the message transmitted by the second receiver in the previous step, the first receiver (3) uses the third list as a shared key in exchanges with the second receiver (4).

24. Method according to any one of claims 22 and 23, wherein a polarizing filter (46) is arranged upstream of the second receiver (4).