QUANTUM COMMUNICATION SYSTEM WITH INTERLOCKED PHOTONS

Description

technical field

[0001] The present invention relates to quantum communication methods, and more particularly those using a pair of entangled photons. Previous technique

[0002] Information transmission is currently done primarily through electromagnetic waves, whether these are short wavelengths, like light, or long wavelengths like VHF waves.

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

[0004] 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 their wavelength or by modulation of the duration of wave trains.

[0005] Recently, so-called "quantum" communication methods have been developed.

[0006] For example, the physical state of a photon, such as its polarization, can be used to transmit information, for instance, via an optical fiber. The value of a bit is arbitrarily assigned to a direction, or polarization mode, of the photon. Sequences of polarized photons can then be sent to transmit binary sequences that form a message.

[0007] In quantum cryptography, it is well 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.

[0008] This quantum entanglement phenomenon 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 µm” (Sci Adv, 2020)

[0009] Patent KR101003886B1 describes a system for transmitting encrypted information in which entangled photons are emitted simultaneously, each sent to two targets located at different points in time that wish to communicate with each other. The photons received by the two targets form entangled random sequences of conjugated bits, these sequences being used as cryptographic encryption keys. This system allows for the near-instantaneous reception of the same sequence of random numbers at two different locations but does not enable the transmission of information.

[0010] Wang's article, "Superluminal telecommunication: an observable contradiction between quantum entanglement and relativistic causality," reveals a quantum communication system enabling superluminal information transmission through 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 multiplied, and the average polarization of the multiplied flux is measured to determine the direction in which the first photon was measured.

[0011] The polarization of a photon is not necessarily binary 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 be linear, with the electric field always parallel to an axis perpendicular to the direction of photon propagation, or circular, with the electric field rotating around this axis, or somewhere in between: the electric field tracing an ellipse around the propagation axis. 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.

[0012] The aforementioned devices do not allow the exploitation of all polarization configurations and are therefore limited in the number and type of information that can be transmitted. Description of the invention

[0013] There is a need to further refine quantum communication systems and methods, particularly those enabling the rapid transmission of information between two points, including continuous values, in a near-instantaneous manner and without delay due to transmission distance. Summary of the invention

[0014] A first aspect, not part of the invention but addressing this need, concerns a quantum communication system comprising: An entangled photon emitter 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; A first receiver disposed on the first propagation path comprising a complex absorber configured to absorb the photon in a polarization state selected from the states of at least two different pairs of complementary polarization states, with the exception of exactly two pairs of perpendicular linear polarizations where the polarizations of one of the pairs are at 45° to the polarization directions of the other pair;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 allowing the second photon to be multiplied while preserving its polarization and, arranged downstream of the amplifier, a measuring instrument allowing the average polarization of the multiplied photons to be measured.

[0015] According to this first aspect, the system is arranged to transmit almost instantaneously a series of values, continuous or discrete, between two locations.

[0016] Since the first and second photons are entangled, the absorption of the first photon at the first receptor instantly determines the polarization of the second photon, especially before it reaches the second receptor.

[0017] Measuring the average polarization of the multiplied photons at the second receiver then makes it possible to detect in which polarization state the first photon was absorbed and to deduce the transmitted information.

[0018] This system therefore allows information to be transmitted without latency, regardless of the distance between the two places wishing to communicate, and using only photons, which requires very little energy.

[0019] 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. Jones' Polarizations

[0020] By "complementary polarization states", also referred to hereafter as "complementary absorption polarization states", we mean 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.

[0021] In the following, the terms "polarization state" and "polarization" are used interchangeably.

[0022] The first receiver is advantageously arranged so that its user can choose the polarization state in which the photon is absorbed from among any of the possible polarizations as defined by the Jones formalism, in particular from among a set of elliptic (also called "ellipsoidal") polarizations characterized by the orientation and ellipse of the polarization.

[0023] This allows us to vary the polarization according to which photons are absorbed by the first receptor in order to transmit a series of information to the second receptor.

[0024] By choosing, for example, 10 different polarization directions, spaced 9° apart, and 21 different phase shifts, also spaced 9° apart, we can define 210 different polarization states and thus transmit 210 different signals.

[0025] For example, we can transmit the letters of the alphabet in uppercase, lowercase, numbers and a number of other special characters, for each of the pairs of entangled photons reaching the receptors. Complex absorber

[0026] A "complex absorber" is defined as a set reached by the first photon which allows a pair of complementary polarizations to be determined from among a number of complementary polarization pairs, and which absorbs the first photon in one of the two polarization states of the predetermined pair.

[0027] Preferably, the 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.

[0028] Preferably, the complex absorber comprises: at least one absorption instrument capable of absorbing the photon in either of two states of a pair of complementary polarizations, for example two orthogonal linear polarizations; a polarization modifier disposed upstream of said absorption instrument and configured to transform the polarization of the first photon to the chosen polarization into which said absorption instrument absorbs photons.

[0029] By "chosen polarization" we mean a pair of complementary polarizations predetermined according to the information to be transmitted. Polarization modifier

[0030] 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 way along an x-axis and a y-axis perpendicular to the x-axis, then, secondly, by modifying the phase of the electric field along one of two perpendicular directions, for example the y-axis.

[0031] The polarization modifier preferably includes a polarization direction modifier arranged upstream of a polarization phase modifier.

[0032] For example, if θ is the rotation angle of the linear polarization and ϕ the phase shift along the y-axis, ω being the angular frequency of the wave, the components Ex And E y The electric field strengths after rotation are: E x = E Cos θ Cos ωt E y = E Sin θ Cos ωt

[0033] And after the phase shift: E x = E Cos θ Cos ωt E y = E Sin θ Cos ωt + ϕ Polarization direction modifier

[0034] A linear polarization direction can be modified in various ways, for example by a half-wave plate or alternatively by a double quarter-wave plate.

[0035] The polarization direction modifier may include 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.

[0036] The first quarter-wave plate, for example, modifies the linear polarization into circular polarization, and the second quarter-wave plate transforms, for example, the circular polarization into linear polarization whose direction is oriented along an axis dependent on the direction of the axis of the second quarter-wave plate.

[0037] Rotating the axis of this second plate allows the direction of the photon's linear polarization to be changed. If the plate is a half-wave plate, rotating it also allows the direction of the photon's linear polarization to be changed.

[0038] Alternatively, the orientation of the first quarter-wave plate can be changed, while that of the second remains fixed, or the orientations of both quarter-wave plates can be changed.

[0039] The rotation of a quarter-wave blade or a half-wave blade is obtained, for example, by mechanical control to 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.

[0040] Alternatively, several linear polarization rotation devices can be used, each allowing different rotation angles, for example fixed, but which can also be modified, selectors allowing the photons to be sent to one of these rotation devices and, at the output of said rotation device, to be conducted to a common waveguide or a common transmission axis.

[0041] These selectors may include a mirror whose axis direction is controlled, for example, by an electrical device, or alternatively, consist of a prism or a plate of material whose refractive index depends on an electric field: a Pockels cell, or a transparent material with a non-linear refractive index, another luminous flux, for example transverse and preferably of a wavelength different from that of the photon and unable 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 exit direction of the photon of said material.Since the rotational inertia of a quarter-wave plate can be significant, photons can be sent successively to different quarter-wave plates whose direction has been adjusted beforehand, for example by one of the mechanical rotation devices described previously, thus allowing time to change the direction of each of these quarter-wave plates between two photon passes.

[0042] Alternatively, chiral or rotating materials can be used to modify a linear polarization direction.

[0043] The polarization direction modifier may include a plate or prism made of chiral or rotating material that affects the polarization rotation by an angle dependent on the point at which the wave enters said chiral or rotating material. The material may, in particular, be arranged between two Pockels cells.

[0044] The first photon can 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 places, notably onto an intermediate plate or prism of material at least partly chiral or rotating causing the axis of electric polarization of the photon to rotate by an angle depending on the place by 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 with respect to that of the first plate or prism to make the photon 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 plate whose refractive index can be adjusted, for example electrically, is advantageously placed after the second plate or prism to allow the location of the photon exiting the set of plates and prisms to be independent of the refractive index chosen for the first plate or prism.

[0045] The intermediate material plate or prism is for example composed of two symmetrical prisms joined together with the same refractive index but having different chiral or rotational powers, the first of the two prisms having for example a chiral or rotational power and the second not having one or rotating the electric field in the opposite direction to the rotation imposed by the first prism.

[0046] One of the two intermediate prisms may contain 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 is preferably not chiral or, alternatively, has the inverse chirality of the first intermediate prism. Since the rotation of the electric field of waves passing through a chiral material is proportional to the thickness traversing said chiral material, the rotation of the electric field through the intermediate prism depends on the location at which it is penetrated by the light passing through it.

[0047] One of the two intermediate prisms may further include a "rotating" material, such as a superposition of an even number of quarter-wave plates stacked one on top of the other in directions incremented at each layer by a predetermined angle, such that a wave polarized linearly along the axis of the first slice of material emerges from the stack with a polarization rotated by an angle proportional to the thickness of the stack. Polarization phase modifier

[0048] A second device, called a "polarization phase modifier", can be placed downstream of the polarization direction modifier, in order to phase shift the electric field component of the photon by a predetermined angle along one of two fixed axes.

[0049] Preferably, the polarization phase modifier includes a first birefringent plate or prism arranged to split the beam into two linearly polarized electromagnetic waves, one along a first axis, the other along a second axis, and a retarder plate with a variable refractive index arranged on the second axis.

[0050] By "beam", we must understand the light wave which is the first photon in the sense of the duality of light.

[0051] 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 again by a new birefringent plate or prism allowing the two waves whose polarization fields are perpendicular to be brought together along the same axis.

[0052] The delay plate can incorporate a Pockels cell or a nonlinear material. The variable refractive index allows the phase shift imposed on the wave oriented along the second axis to be chosen.

[0053] In a first implementation example, as described above, the phase modifier rotates a linear polarization by 9° intervals between -45° and +36° and varies the phase by 9° intervals between -90° and +90°, thus defining 210 possible polarization states. As described below, 209 of the 210 possible polarizations are used, for example, 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 used to encode data.

[0054] 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 polarization of the photon into circular polarization to transmit, for example, a 1. Reflection of the first photon

[0055] The invention relates to a quantum communication system comprising: An entangled photon emitter 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; A first receiver disposed 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 indeterminate in the observable basis according to which the first instrument absorbs photons, an optical selector disposed upstream of said at least one first instrument and configured to either allow the first photon to pass to said at least one first measuring instrument, or to prevent it from being measured; • 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 reach the measuring instrument of the first receiver, said second receiver comprising an optical amplifier allowing to multiply the second photon while preserving its polarization and, arranged downstream of the amplifier, a measuring instrument allowing to measure the average quantum state of the multiplied photons.

[0056] By "observable basis" we mean a basis of two complementary polarizations, for example two orthogonal linear polarizations.

[0057] By "indeterminate 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 way.

[0058] Since the first and second photons are entangled, the absorption of the first photon at the first receptor instantly determines the polarization of the second photon, especially before it reaches the second receptor.

[0059] Measuring the polarization of the multiplied photons at the second receiver allows us to detect whether the first photon was absorbed or not, and thus deduce what information was transmitted. The information transmitted by this system is, for example, binary in nature.

[0060] The first receiver may include at least one second absorption instrument, the optical selector being positioned upstream of the first and second instruments and configured to either allow the first photon to pass through by directing it towards one or the other of the instruments, or to prevent it from being absorbed.

[0061] 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 reflection, the value 1 to the first instrument, and the value 2 to the second instrument. Optical selector

[0062] The optical selector of the first receiver can be of various types. It is advantageously placed upstream of the measuring instrument so as to prevent the incident photon from reaching the measuring instrument, if desired.

[0063] To achieve this, the selector preferably includes a reflector, in particular a controlled mirror, allowing for example to reflect the first photon onto a propagation path different from its incident propagation path, in particular onto a propagation path not containing the measuring instrument.

[0064] By "controlled mirror" we mean a device in which certain properties, for example refraction properties or direction of reflection, are controlled by an auxiliary device linked to the mirror, in particular an electronic device, for example generating an electric field.

[0065] The optical selector preferably includes a device whose refractive index and / or direction of reflection are controlled, in particular controlled by means of an electric field or a luminous flux.

[0066] Alternatively, the optical selector includes, for example, a Bragg mirror, in particular a Bragg mirror whose reflection direction is controlled by piezoelectric material itself controlled by an electric field.

[0067] Alternatively, the optical selector includes 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 with respect to the direction of incidence of the electromagnetic waves reaching it, so that the said waves are reflected towards a place preferably different from that from which they come at the entrance of the prism.

[0068] In another variant, the optical selector includes 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 through depending on whether or not the electric field is applied. Alternatively, the Pockels cells used above are replaced by nonlinear crystals illuminated by a powerful light that can vary the refractive index of the medium through which the photon passes, the wavelength of the powerful light preferably being different from the wavelength of the photon.

[0069] The optical selector allows, in particular, the orientation of photons in three different directions, one direction allowing, for example, the photons not to be detected, a second allowing the measurement of a linear polarization of the photons and a third allowing the measurement of a circular polarization. Protective device

[0070] Preferably, the quantum communication system also includes a device for protecting the quantum state of at least one photon, in particular a device that prevents the measurement or absorption of at least one photon, arranged near 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.

[0071] 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 we do not want it to be absorbed by the measuring instrument of the first receiver.

[0072] Preferably, the quantum state protection device includes 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.

[0073] Preferably, it includes several Bragg mirrors, arranged with respect to the transparent space so as to trap the photon it receives on a path of appropriate length as described above, i.e. corresponding to the time it takes for the second photon having reached the second receiver to be amplified there.

[0074] Alternatively, the protection device includes an optical fiber of sufficient length to allow the first photon to travel through it while waiting for the second photon, having reached the second receiver, to be amplified there.

[0075] In another variant, the device can be a portion of empty or gas-filled space or atmosphere which is ensured is not traversed by an object reflecting or absorbing light. First photon absorption instrument

[0076] At the first receiver, the first photon can be absorbed by at least one absorption instrument in one of two states of a pair of complementary polarizations.

[0077] Preferably, the absorption instrument includes at least one filter enabling the first photon to be sent 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 plate of birefringent material.

[0078] The absorption instrument may also include a polarizing filter and a photon detector located after said polarizing filter.

[0079] The absorption instrument may also include a semi-transparent mirror reflecting circularly polarized photons towards a first photon detector and letting others pass 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). Emission of photons Photon pair emitter

[0080] Preferably, the emitter generates pairs of entangled photons by spontaneous parametric reduction (SPDC), that is, by a process in which an initial photon, also called the "pump" photon, is split and its frequency divided by two by a four-wave mixing phenomenon, in an optical medium with a nonlinear refractive index, as described in the article by Amanti et al., "Integrated sources of entangled photons at the heart of quantum technologies" (Photonics, issue 91, 2018).

[0081] This type of photon pair emitter produces photons with relatively robust entanglement, but has the disadvantage of sometimes producing more than one photon pair at a time, which is undesirable because it is uncontrolled. Additional devices can be implemented to handle these multiple "parasitic" pairs, as described below.

[0082] Alternatively, an emitter incorporating a quantum dot can be used to generate entangled photon pairs, as described in the aforementioned article, which allows for more regular pair formation. However, the entanglement properties of photons obtained with such a method can sometimes be unstable.

[0083] 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).

[0084] Preferably, the emitter is configured to successively generate a plurality of entangled photon pairs.

[0085] The entangled photons at the emitter output can be sent along the first and second propagation paths in various ways. Broadcast according to a predetermined polarization

[0086] Photons are preferably emitted with a predetermined polarization. By "predetermined polarization," we mean in one of the two states of a predetermined pair of complementary polarizations, for example, a linear polarization in a certain direction. A birefringent plate can, for example, separate the two entangled photons according to their vertical or horizontal linear polarization.

[0087] A predetermined polarization of the entangled photons sent to each of the receptors 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.

[0088] We can thus 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.

[0089] A predetermined polarization is also necessary when using quarter-wave plates at the first receiver to rotate the polarization direction of the first photon.

[0090] To emit photons with a predetermined linear polarization, for example, at the output of the emitter, a blade or prism containing an optically transparent birefringent material, for example Lithium Niobate or Rutile (TiO2), whose optical indices depend on the axis of polarization of the light through which at least one of the two photons passes.

[0091] The latter then exits the prism at one of two different locations, depending on its linear polarization.

[0092] 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 emerge, to send it to the first receiver.

[0093] Similarly, the second photon exiting from the point where electromagnetic waves whose field is perpendicular to the ordinary axis emerge is collected in order to send it towards the second receiver.

[0094] 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 prism and each photon redirected to its intended receiver, the photons exiting from different locations of the prisms then being preferably lost or destroyed by projection onto an absorbing surface.

[0095] For example, we can use for the first entangled photon the one coming out of 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 coming out 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 photons according to a predetermined polarization corresponding to the polarization of the photons entangled with the photons coming out of the first plate or the first birefringent prism with a direction parallel to the first axis.

[0096] To facilitate the transport of said photons to the receivers, birefringent delay plates are preferably placed at the exit of the places from which the photons emerge from the prisms, transforming the linear polarizations of said photons into circular polarizations.

[0097] We then preferably place another birefringent delay plate at the entrance of each of the receivers to transform the circular polarizations of the photons back into linear polarizations, this transformation into linear polarization allowing the polarization axis to be precisely adjusted. Transmission of photons

[0098] Photons can be transmitted from emitters to receivers through space, through the atmosphere, via optical fiber, or a combination of these means.

[0099] Lenses can be used for photon transmission, particularly for transmission through space or the atmosphere. If necessary, anti-reflective coatings are preferably applied to these lenses. The size of the lenses used is preferably matched to the length of the spatial or atmospheric transmission of the photons.

[0100] Conjugate mirrors, which reflect the emitted light back to the emitter, can be used to adjust the emission direction of conjugated photons for spatial or atmospheric transmission. For example, a laser light emitter can scan a space to detect the receiver, which then reflects the emitted light. The direction of the conjugated photons is then adjusted to be parallel to, or coincide with, the direction of the light reflected by the conjugate mirrors.

[0101] The light emitted by the laser light emitter can be of a wavelength close to the wavelength of the entangled photons and introduced into the lens used by the photons through a dichroic prism.

[0102] Alternatively, if the wavelength is the same as that of the entangled photons, the light can be introduced through a birefringent prism when these photons are linearly polarized. The polarization of the guiding light is then 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 upon the arrival of the guiding light.

[0103] 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.

[0104] In another variant, one can not use a conjugate mirror but aim in the same way at the receivers or targets close to them, the information according to which the targets are received being communicated by another means of communication, in particular by radio signal or by quantum transmission.

[0105] The area to be scanned can be identified by mapping the area in which said receivers are likely to be located.

[0106] The aforementioned phase modifier can also be used when the receivers are in relative rotation with respect to the emitter, particularly due to the rotation of the earth and satellites, to allow photons emitted in a fixed direction with respect to the emitter to enter each of the receivers with a fixed and determined direction with respect to it. Calibration methods

[0107] The different media through which photons travel from their point of emission to the two receivers can give rise to modifications of their phase that are not necessarily desired or expected.

[0108] Furthermore, these modifications can differ depending on whether the photons are directed towards the first or second receptor. Finally, these modifications can vary over time, depending in particular on the weather or the temperature of the materials during their use.

[0109] The system according to the invention can be periodically calibrated by having the first receiver absorb photons in polarizations belonging to predetermined pairs of complementary polarizations and observing for each of these pairs one of the two possible polarizations in which the entangled photon is detected by the second receiver.

[0110] Thus, a calibration method, not part of the present invention, for a system according to the first aspect, allowing the determination of the polarization state of a photon reaching the second receptor as a function of the polarization state in which its entangled photon was absorbed at the first receptor, a method comprising the steps of: Generate entangled photon pairs from an emitter, with the first photon of the pair emitted towards a first receiver and the second photon of the pair emitted simultaneously towards a second receiver. The first and second photons are entangled according to their polarization state. Absorb the first photons at the first receiver in a predetermined pair of polarizations. Record in which of the two possible polarizations each photon was absorbed and stop the emission of photons from the emitter as soon as a predetermined number of each of the different polarizations have been observed. At the second receiver, duplicate the second photons into multiplied photon streams using an amplification device. Each multiplied photon retains the polarization state of the second photon. Measure the polarization states, i.e., the polarization directions and phase shifts of each multiplied photon stream.and to store these measurements along with their reception times; to transmit the list of polarizations and reception times of the stored photons from the first receiver to the second receiver; to remove from the list of photons received at the second receiver the photons that do not correspond to photons received at the first receiver, and similarly, to remove from the list of photons received at the first detector the photons whose corresponding photon was not received at the second receiver; and to calculate, using on the one hand: the knowledge of the polarization state of two photons detected in complementary polarizations at the first receiver and whose entangled photons reached the second receiver, and on the other hand, the knowledge of the polarization state of the corresponding entangled photons received at the second receiver, the Jones matrix allowing the polarization of the photons received at the second receiver to be deduced from the polarization of the photons received at the first receiver.

[0111] The storage of states as well as the calculation of the Jones matrix can be carried out by any suitable electronic device, for example a microcontroller.

[0112] The Jones matrix thus calculated allows the polarization of the second entangled photon received at the second receptor to be calculated as a function of the absorption polarization of the first photon by the complex absorber located at the first receptor.

[0113] A second calibration can be made consisting of determining the probability for any photon arriving at the second receiver whose entangled photon was absorbed during its transit to the first detector of belonging to each range of complementary polarization pairs 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.

[0114] Thus, a calibration method, not part of the present invention, for a system according to the first aspect, allowing the determination of the probability of losing a photon during its transit from the emitter to the first receiver, consists of: Configure the complex absorber of the first receiver to absorb the photons received at the first receiver in one of two complementary polarizations of an absorption polarization pair. Generate successively 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. Count the number of photons received at the second receiver in each of the ranges of polarizations detectable by the second receiver other than one of the two ranges of polarizations of the entangled photons of the photons absorbed by the first detector.

[0115] We can thus detect ranges of polarizations in which entangled photons of lost photons during transmission on their way to the first detector arrive first, or most likely, and thus preferably avoid using the corresponding absorption polarizations to transmit data from the first to the second receiver.

[0116] We can also calculate a photon transmission rate between the emitter and the second receiver by taking 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. Second receiver measuring instrument

[0117] The measuring instrument of the second receiver preferably includes at least one photon detector arranged to measure the polarization of the light resulting from the multiplication of the second photon.

[0118] The measuring instrument of the second receiver includes, for example, a series of semi-reflective plates arranged downstream of the optical amplifier, said plates 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 two same axes.

[0119] Light can also be directed to various polarizing filters after passing through one or a succession of lenses which enlarge the cross-section of the light beam, thus allowing several mirrors or lenses to direct portions of this light beam to the various polarization measuring instruments.

[0120] The various polarization measurement instruments preferably allow for: measure the intensity of the component of the electric field of light along a first x-axis, measure the intensity of the component of the electric field of light along a second y-axis perpendicular to the first x-axis, measure the phase shift between the light along its x-axis and the light along its y-axis, 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.

[0121] The measurement of light intensity along two perpendicular axes is carried out, for example, by splitting the light along two perpendicular axes using 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 difference between two perpendicular components is carried out, for example, by splitting the light along two perpendicular axes using a birefringent plate or prism, followed, for the wave polarized along one of the two axes, by a 90° rotation of that polarization axis, for example, using a rotating or chiral material or by a 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 creating fringes whose positions depend on said phase difference. Optical amplifier

[0122] An "optical amplifier" is a device that allows the duplication of a photon introduced into it, in particular, the second photon of a pair of entangled photons, while preserving its polarization state.

[0123] Preferably, the optical amplifier is a fiber-doped amplifier.

[0124] For example, an Erbium amplifier (EDFA), for example 4m long, can be used, in which the photon to be amplified is introduced at the same time as an amplifying wave of shorter wavelength, which allows the wave corresponding to the introduced photon to be amplified, with gains that can be on the order of 37db / m.

[0125] One can also use a doped fiber amplifier (DFA) using a dopant different from Erbium.

[0126] Alternatively, the optical amplifier is, for example, a vertical cavity amplifier (VCSOA), or a semiconductor-type amplifier (SOA). Anti-reflective coatings

[0127] To avoid the loss of photons, anti-reflective layers are preferably used at the interfaces between adjacent transparent media of different index through which photons pass, as well as at the interfaces of prisms and birefringent plates through which photons pass, the anti-reflective layer being preferably adapted to the index(es) of the material and to the angles of incidence and directions of polarization as well as to the wavelength of the photon which must pass through it. Dichroic filters

[0128] The first and / or second receiver preferably includes one or more dichroic filters allowing only photons of a given wavelength to pass through, in particular a prism made of a dispersive transparent material, the filter(s) preferably being placed in front of the measuring instrument(s), or in front of the optical selector for the first receiver, especially if the refractive indices of nonlinear materials are modified by the application of powerful light fluxes. Assigning photons to information using synchronized clocks

[0129] 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 has not been encoded on a pair of entangled photons.

[0130] Indeed, photon emissions are sometimes irregular; for example, an SPDS type emitter, as mentioned above, may emit a double pair of photons unintentionally.

[0131] The transmitter and each of the receivers preferably include a clock, with the clocks of the transmitter and the receivers being synchronized with each other.

[0132] This allows, when combined with knowledge of the travel time of photons between the emitter 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.

[0133] If several photons are received during a reception period, the first receiver maintains an unchanged state until a time between, for example, one quarter of the non-emission time after the end of the reception period and half of the non-emission time δt' before the next reception period.

[0134] The second receiver preferably records the bits received for each reception period. It then advantageously transmits to the first receiver from time to time, especially if the device uses reflection to transmit a bit or a trit, for example every 3000 reception intervals, or every 100 photon-free reception intervals, a list of reception periods for which it did not receive a photon, the transmitted photon having been lost or no photon having been emitted by the transmitter for that period, without the first transmitter knowing whether it reflected photons during these same periods, especially if the device does not have a photon detector.

[0135] The transmission of this list can be done by instant communication using the method according to the invention described below, or by conventional communication. Upon receiving this list, the first receiver then transmits the bits corresponding to those transmission periods during which no photon reached the second receiver.

[0136] Alternatively, the first receiver uses only two different measuring instruments to transmit information, one detecting linear polarizations, the other circular polarizations.

[0137] 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 use for the transmission of the next bit.

[0138] The second receiver, upon receiving photons during the transmission periods, and if it receives any, measures for each transmission period their polarization state and deduces the bit that it must add to the list of received bits.

[0139] If the transmission line between the transmitter and the first receiver has a certain opacity and can absorb a certain proportion of the photons that pass through it, some photons may arrive at the second receiver during the reception periods with random polarizations.

[0140] The second receiver can then be configured to recognize at least some of the photons arriving in a polarization state that cannot be that of an entangled photon absorbed or reflected at the first receiver. Clock discrepancy

[0141] Preferably, the clocks of the system components, namely the transmitter and the two receivers, are configured to take into account the phenomenon of different flow of time in different locations, including the different altitudes of each of the components. Synchronization of transmission periods between receivers

[0142] A process can be implemented allowing the first and second receivers to synchronize their clocks.

[0143] For example, when measuring a first photon, the first receiver notes, using its own clock, the time at which said photon struck the measuring instrument.

[0144] The recorded time is then transmitted to the second receiver along with, preferably, the references to the transmission interval, including the start time and possibly the duration during which the photon was expected.

[0145] The second receiver records 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 hit the measuring instrument of the first receiver is the same as the offset between the start of the reception period of the second photon at the second receiver and the time given by the clock for the reception of the second photon.

[0146] If the transmission lines between the transmitter and either receiver absorb some photons, the transmitter can send a predetermined number of photons, or for a predetermined duration. Each receiver can then record the average time of reception of each photon, and one receiver can then communicate this average time to the other. 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 transmission, is preferably adjusted based on factors such as the transmission / loss rate of photons on each of the two paths between the transmitter and the receivers, and the average frequency of photon transmission.

[0147] Since the transit times of photons between the transmitter and each receiver vary, the clock synchronization between each receiver can differ depending on whether information is being transmitted in one direction or the other. Each receiver can therefore have a synchronization register used to increment or subtract a time interval from its clock, adjusted during synchronization, to determine the time of a synchronized clock for receiving information from the other receiver. Transmission between transmitter and receiver

[0148] We can implement a process to synchronize the periods of sending photons to the receivers and the periods of reception at the receivers, these periods being time intervals during which photons can leave the emitter or reach the receiver.

[0149] 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, according to its clock, of the sending of the signal.

[0150] Upon receiving the said first light signal, the receiver notes the time of reception, then reads the time of sending the signal, calculates the difference between the time of sending 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 of a transmission period, the start of the previous transmission period, and does the same with the time of receiving the signal and the time of the start of the reception period.

[0151] If the difference between the date of signal transmission 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 the opposite in the case of the difference.

[0152] The calibration is advantageously repeated several times and the results are averaged, thus allowing the clocks to be adjusted with a precision greater than the imprecision of the sending and receiving times of said light signals.

[0153] 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. Assignment of photons to information without a synchronized clock

[0154] As an alternative to synchronized clocks, the first receiver may include 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.

[0155] The polarizations of the detector-transmitters are preferably complementary so that any photon directed towards an absorbing waiting element is absorbed.

[0156] Detector-transmitters are preferably arranged to detect the polarization of photons in at least four different polarizations, these polarizations being grouped in complementary pairs such that any photon directed towards a detector-transmitter is thus detected and absorbed regardless of its initial polarization.

[0157] The detector-transmitter(s) of the first receiver are used to send information by choosing the pair of detectors used.

[0158] Each photon received by the first receiver during a predetermined period, also referred to as the "rest period", after the receipt 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 receivers' clock, and the start of this rest period being determined by the one or one of the two sensors absorbing the photon that were used to send the information.

[0159] The second receptor is preferably arranged so that, on the one hand, ignore photons paired with photons sent to the waiting element, on the other hand, 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 less the inaccuracy of the receivers' clock, and finally, 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.

[0160] 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 towards the waiting element are received in a time less than the rest period less the imprecision of the clock of the receivers.

[0161] This method has the advantage of not requiring a switch at the emitter to restrict entangled photon emissions, nor clock synchronization.

[0162] The transmitting detector(s) of the first receivers can also serve as a waiting element, especially if the polarization in which they detect photons is adjustable and can therefore be adjusted to the polarization used by the waiting element to absorb photons. Noise reduction device

[0163] Optical amplifiers tend to emit photons in the opposite direction to the propagation signal, either during signal amplification or in the absence of a signal, after molecules or atoms of the amplifying medium have been excited by a pump signal.

[0164] The second receiver includes, for example, a switch positioned in front of the optical amplifier, in order to limit the number of photons emitted by the amplifier towards the entangled photon source of the device.

[0165] 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.

[0166] The switch of the second receiver can be connected to the measuring instrument of the second receiver, so as to prevent light from passing in either direction from the arrival of a first photon until the next photon is expected or a predetermined time before that moment, for example half of the rest period defined above.

[0167] Alternatively, the switch of the second receiver can allow photons to pass only during the periods when they are expected, in particular during the aforementioned photon processing time intervals. Assignment of photons to information without a synchronized clock using low-transparency optical transmission methods

[0168] Optical fibers, although transparent, do not transmit all the photons presented at their input. A loss of 0.20 dB per kilometer is common, resulting, for example, in a 10 dB loss over 50 km, or 90% of the photons.

[0169] The effect of lost photons in transit between the emitter and the first receiver is, however, different from the effect of the loss of lost photons going towards the second receiver; the entangled photons of the lost photons between the emitter 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 towards the first receiver, whereas the loss of a photon in the line leading to the second receiver only makes disappear a photon potentially carrying information, that is to say, whose entangled photon reached the first receiver.

[0170] Furthermore, the polarizations of photons arriving at the second receptor after their entangled photons have been absorbed in transit to the first receptor are not necessarily equally distributed among all possible observable polarizations.

[0171] Therefore, for information transmission, we preferentially use pairs of complementary polarizations for which corresponding polarizations observed at the second receptor are the fewest when the entangled photon of the photon detected at the second receptor has been absorbed during its transit to the first receptor.

[0172] A calibration method for a quantum communication system, not part of the present invention, is described below. This method allows for the determination of two Jones matrices, one of which calculates the polarization of a photon arriving at the second receiver as a function of the polarization of a photon absorbed at the first receiver. Preferably, the number of different polarizations observable at the second receiver is greater than twice the inverse of the photon transmission rate between the emitter and the first receiver. The method consists of: Configure the complex absorber of the first receiver to absorb the photons received at the first receiver in one of two complementary polarizations of an absorption polarization pair. Successively generate several pairs of entangled photons from an emitter, with the first photon of the pair emitted towards a first receiver and the second photon of the pair emitted simultaneously towards a second receiver, the first and second photons being entangled according to their polarization state. Count, for each polarization in which a photon is received at the second receiver, the number of photons that reached said second receiver with that same polarization. Stop sending photons when a predetermined number of photons are received at the second receiver. Determine the two polarizations in which the photons were received most often at the second receiver.Given that these polarizations are considered to correspond to the absorption polarizations of photons at the first receptor, calculate the two possible Jones transformation matrices of the polarizations that allow us to deduce, from the polarization of a photon received at the first receptor, the polarization of a photon received at the second receptor.

[0173] Recording 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 times of reception also makes it possible to choose among the two calculated Jones matrices which one 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.

[0174] The following method can also be used to transmit information from the first to the second receiver, a method comprising the steps of: Generate successively 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. For each piece of information to be transmitted, configure the complex absorber of the first receiver to absorb a number NPTof transmission photons in a predetermined complementary pair of polarizations called the "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, configure the absorber in a pair of polarizations called the "waiting polarization pair", and, if said information is the same as the information just transmitted, count at least NPT photons absorbed in this "waiting polarization pair". 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 signal reception. The counter should be activated as soon as it exceeds a threshold number for one of the pairs. NSPpredetermined and that the pair of complementary polarizations is different from that of the last received signal, consider the information corresponding to this pair of polarizations as a new signal, and if this last information does not correspond to the waiting polarization pair, add this information to the list of received information.

[0175] 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.

[0176] For example, to transmit information, we would use: Transmission lines between the transmitter and each receiver exhibit an attenuation of 10dB, representing a 90% loss of photons. Transmission lines between the complex absorber are configured to absorb photons in 450 pairs of complementary polarizations. These pairs represent 80% of the photons received at the second receiver. During absorption of their entangled photons in their transits to the first receiver, we can choose NPT=200 photons and NSP=6, which, according to the inventor's calculations, results in a transmission error rate of less than 1 in 10,000.

[0177] 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 about 5°, the encoding of 1200 photons at the first receiver (therefore NPT=1200) and the threshold number NSP being fixed 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 to the first receiver are equally distributed over all observable polarizations. Processing of pairs of photons sent simultaneously

[0178] The emitter can sometimes send pairs of entangled photons very close to each other. Therefore, the photon detectors at the first receiver preferably count the number of absorbed photons, and not just the number of photon impacts on that first receiver. This count can, for example, take into account the intensity of the electromagnetic wave striking the photon detectors.

[0179] Groups of photons can also arrive almost simultaneously at the second receiver, as it cannot distinguish the polarizations of individual photons. Since the polarization detector can detect an average polarization of all photons detected 'simultaneously', the polarization of groups of photons arriving simultaneously at the second receiver—that is, those whose generated light intensity is, for example, 50% greater than the intensity generated by a single photon—does not advantageously result in the counting of photons received in various polarizations being incremented. However, it is sometimes possible that several photons reach the first receiver, but only one of their entangled photons reaches the second receiver, and this one is then counted. Device performance

[0180] The use of lossless or low-loss lines between the transmitter and the first receiver, allowing photons to pass through a vacuum, for example, makes it possible to reduce or avoid photons received at the second receiver with random polarization.

[0181] Similarly, using precise measuring instruments at the second receiver, which either increase the number of complementary polarization pairs or ignore a large number of photons with random polarizations, can allow: to increase the transmission distance of photons, to increase the throughput by decreasing the number of NPT transmission photons.

[0182] 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 emitter.

[0183] Increased temporal accuracy of the polarization detectors of the second receiver, which makes it possible to distinguish interference from various photons received very close together in time, also allows the use of a larger flux of entangled photons, but also reduces the number of photon receptions received 'simultaneously' as described above.

[0184] This method has the advantage of not requiring a switch at the emitter to restrict entangled photon emissions, nor clock synchronization. Processing of the return of photons to the emitter

[0185] Photons are sometimes reflected back to the emitter from the receivers. To prevent them from being reflected back by the emitter to one of the receivers, the cavity or material in which the entangled photons are produced by wave mixing, whose refractive index is non-linear, is preferably surrounded or covered with a material that absorbs light of the same wavelength as the entangled photons. Quantum communication processes

[0186] A quantum communication method, not part of the present invention, using the system according to the first aspect, comprises the steps of: Generate a pair 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, the second receiver being located further from the emitter than the first receiver on the propagation path of the second photon than the first receiver, such that it arrives there later; Modify, 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, excluding exactly two pairs of perpendicular linear polarizations where the polarization directions of one pair are at 45° to the polarization directions of the other pair; Absorb, by means of an absorption instrument,The first photon is in one of the two complementary polarizations of the chosen pair. At the second receiver, the second photon is duplicated into a stream of multiplied photons using an amplification device; the light thus created retains the polarization state of the second photon. The average polarization state of the light stream is measured, and the polarization state of the first photon is determined from this measurement, in order to deduce the information transmitted by the first receiver.

[0187] The pair of complementary polarizations is preferably chosen from at least three different pairs of complementary polarizations.

[0188] The pair of complementary polarizations is chosen for example from among 210 distinct pairs of absorption polarizations, including polarizations spaced 9° apart in their polarization direction and phase-shifted by 9°.

[0189] The invention also relates to a quantum communication method using the system according to the present invention, comprising the steps of: Generate a pair 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, the second receiver being located further from the emitter than the first receiver on the propagation path of the second photon than the first receiver, such that it arrives there afterward. Choose, according to 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 by means of an optical selector directing said photon towards one or more instruments. In the case where it is chosen not to absorb the photon in a predefined polarization, trap said first photon in a protective device allowing its absorption to be prevented at least until the second photon has reached the second receiver.At the second receiver, duplicate the second photon into a stream of multiplied photons using an amplification device, each multiplied photon retaining the quantum state of the second photon. Measure the average quantum state of the multiplied photon stream and determine from 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.

[0190] Preferably, the polarization of the entangled photon pair is indeterminate in the observable basis or bases in which the instruments of the first receiver absorb them.

[0191] The pair of entangled photons reaches, for example, the receptors with a linear entangled polarization, one of the two complementary absorption polarizations being circular, the second receptor being arranged to distinguish whether the average polarization of the multiplied photon flux is circular or linear, and to determine according to this distinction whether the first photon has been measured or not.

[0192] Alternatively, the entangled photon pair reaches the receptors with a circularly entangled polarization, the quantum state measured at the first receptor being a linear polarization, the second receptor being arranged to distinguish whether the average polarization of the multiplied photon flux is circular or linear, and to determine according to this distinction whether the first photon has been measured or not.

[0193] 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 the vertical or the horizontal, and to determine according to this distinction whether the first photon has been measured or not. Information transmission

[0194] The system according to the invention and the quantum communication method described above allows the transmission between the first and second receiver of information either of binary type, in the form of bits, or of discrete or continuous values.

[0195] Preferably, several pairs of entangled photons are generated successively by the emitter, each pair of photons enabling the transmission of information, for example of a binary type, from the first receiver to the second receiver.

[0196] 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 multiplied photon flux then makes it possible to determine whether a bit 1 or a bit 0 is transmitted from the first receiver in a quasi-instantaneous manner.

[0197] Several methods can be implemented to secure communication and avoid transmission errors, such as those caused by double pairs of photons emitted simultaneously.

[0198] 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.

[0199] As described above, photons can be counted when 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 that interval, and is instead transmitted, for example, in the next time interval, or preferably the same bit is retransmitted.

[0200] 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. Two-way communication

[0201] To enable bidirectional communication, that is, to allow each of the two receivers to transmit information to the other receiver, several transmitters can be used.

[0202] The system according to the invention may in particular include a second emitter capable of generating one or more pairs of entangled photons, the second emitter being located closer to the second receiver than to the first receiver.

[0203] Alternatively, at least some of the photons can be made to travel an indirect path in order to lengthen their transport time to one of the receivers, for example by reflecting the photons on one or more intermediate mirrors, or by passing them through media with high refractive index, or by transporting them in optical fibers of varying lengths.

[0204] This extended path can, for example, alternate at a fixed or variable rate, depending on the needs, with the non-extended path, so that entangled photons can sometimes be used to transmit information from one point to another, and sometimes in the opposite direction. Optical switches upstream of the receivers, synchronized with the switch attached to the transmitter, can send the photons along an extended path. These switches can be installed to send said photons to first-receiver type receivers, as described above, or conversely, to second-receiver type receivers.

[0205] Method for transmitting a key and verifying that this transmission is not being intercepted. The interception of information transmitted by a stream of photons from the transmitter to the second receiver can be verified by implementing the following steps: The second receiver 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 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; the first receiver receives the list and the signature and 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 never reached the first receiver; two entangled photons having different relative polarizations;If the number of elements in the second restricted list of entangled photon pairs, where each of the two photons has reached its respective receptors, is less than the product of a predetermined ratio and the count of elements in the first restricted list of entangled photon pairs, where each of the two photons has reached its respective receptors, then the information is declared to have been transmitted but not intercepted.

[0206] The following steps can then be taken: If the information is declared to have been transmitted unlistened to, 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 good 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 shared key 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.

[0207] A polarizing filter can be placed upstream of the second receiver. Brief description of the drawings

[0208] The invention will be better understood upon reading the detailed description that follows, the non-limiting examples of its implementation, and upon examination of the attached drawing, on which: [ Fig 1A ] represents in a partial and schematic way a quantum communication system according to the first aspect, which is not part of the invention, [ Fig 1B ] represents in a partial and schematic way a quantum communication system according to the invention, [ Fig 2A ] partially and schematically represents a linearly polarized photon, [ Fig 2B ] represents in a partial and schematic way a circularly polarized photon, [ Fig 3 ] there figure 3 schematically represents details of photon transmission from the emitter to the receivers, [ Fig 4A] represents in a partial and schematic way an example of an optical selector according to the invention, [ Fig 4B ] represents, in a partial and schematic way, a variant of the selector of the figure 4A , [ Fig 4C ] represents in a partial and schematic way another example of an optical selector according to the invention, [ Fig 5A ] ] Fig 5B ] there figure 5A and the figure 5B schematically represent an example of a polarization direction modifier comprising quarter-wave plates, [ Fig 5C ] there figure 5B schematically represents another example of a polarization direction modifier involving a chiral material, [ Fig 6 ] there figure 6 schematically represents an example of a polarization phase modifier, [ Fig 7 ] represents in a partial and schematic way an example of an instrument for absorbing a linearly polarized photon, [ Fig 8] represents in a partial and schematic way another example of an instrument for absorbing a linearly polarized photon, [ Fig 9 ] represents in a partial and schematic way an example of a second photon receiver comprising a doped fiber amplifier, [ Fig 10A ] represents in a partial and schematic way an example of a measuring instrument for the second receiver comprising a succession of semi-reflective plates, [ Fig 10B ] represents in a partial and schematic way an example of a Young interferometer used for the second receiver measurement instrument, [ Fig 11A ] is a block diagram partially illustrating an example of how a quantum communication system works to transmit a bit 0, [ Fig 11B ] is a block diagram partially illustrating an example of how a quantum communication system works to transmit a bit 1, [ Fig 12] is a block diagram partially illustrating an example of how a quantum communication system works to transmit a series of discrete values, [ Fig 13 ] is a block diagram partially illustrating an example of a time-stamping process for the arrival of a photon at a receiver, [ Fig 14 ] partially and schematically illustrates the possibility of placing a "noise-canceling" device in front of the amplifier, [ Fig 15 ] represents in a partial and schematic way the possibility of using the system according to the invention to establish bidirectional communication between two locations, and [ Fig 16 ] illustrates a variant implementation of the first aspect. Detailed description

[0209] We illustrated at the Figure 1Aa quantum communication system 1 according to the first aspect, not forming part of the present invention. The system comprises an emitter 2 emitting a pair of entangled photons (P1, P2), the first photon P1 propagating along a propagation path D1 and the second photon D2 propagating along a propagation path D2 different from path D1. The photons P1 and P2 are emitted simultaneously.

[0210] 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 refer to the product emitted by the emitter 2.

[0211] The terms "measuring instrument" and "absorption instrument" are used interchangeably to refer to an instrument that absorbs a photon according to a certain polarization.

[0212] System 1 includes a first receiver 3, located on the propagation path D1 of the first photon P1, and a second receiver 4, located on the propagation path D2 of the second photon P2.

[0213] Receiver 3 is closer to emitter 2 than receiver 4, so that photon P1 reaches it before photon P2 reaches receiver 4.

[0214] In the example considered, the receiver 3 comprises a complex absorber 31 including a polarization modifier 32 and an absorption instrument 35.

[0215] The complex absorber 31 is configured to absorb the photon in a polarization state chosen from several different complementary polarization pairs, preferably at least three pairs.

[0216] The polarization modifier 31 is configured to transmit the photon P1 according to the chosen complementary polarization pair, and the absorption instrument 35 determines which of the two states of the pair the photon P1 is in. 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.

[0217] We illustrated at the figure 1Ban 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 P1, and an optical selector 30 disposed upstream of the measuring instruments 35 and configured to send the first photon P1 either towards a birefringent prism 36 which guides the photon, according to the linear direction of its polarization on the paths D1' or D1" and then towards one or the 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.

[0218] The direction D3 is specifically chosen so that the photon P1 is not absorbed before its entangled photon is multiplied at the second receiver 4.

[0219] The system 1 preferably includes, as illustrated, a device 5 for protecting the quantum state of a photon located near the receiver 3, in particular on the propagation path D3 in the example considered.

[0220] This device allows, for example, to "trap" the photon P1 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.

[0221] The second receiver 4 includes an optical amplifier 40 allowing the second photon P2 to be multiplied while preserving its polarization and a measuring instrument 45 allowing the average polarization of the multiplied photons to be measured.

[0222] The transmission of photons P1 and P2 can occur in different ways and in different media. Photons propagate, for example, in an optical fiber or a waveguide, or even freely in space, whether the space is empty or filled with gas.

[0223] Photons can pass through several media with different refractive indices. For example, anti-reflective plates can be inserted between two media, if necessary, to avoid unwanted optical phenomena, particularly Fresnel reflection of the waves emitted by emitter 2.

[0224] The emitter 2 includes, for example, one or more lenses chosen to be 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.

[0225] The emitter 2 may also include a system, in particular an electronic device, for adjusting the initial directions in which the photons P1 and P2 are emitted respectively.

[0226] This adjustment can, for example, take into account the different refractive indices of the media traversed, and the modification of the trajectory of photons that can result, for example for a photon emitted from space that enters the atmosphere.

[0227] The wavelength of the emitted wave is chosen, for example, according to the medium or media to be traversed; for example, photons in the infrared range are preferably used when they have to pass through the atmosphere or air.

[0228] Transmitter 2, for example, generates entangled photon pairs using the spontaneous parametric conversion (SPDC) method. Transmitter 2 is configured, for example, 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.

[0229] When emitted, photons P are, for example, linearly polarized, meaning that the corresponding electromagnetic wave has an electric field whose direction is perpendicular to their direction of propagation D. If applicable, the polarization of the photons can be vertical V or horizontal H, as illustrated in the figure 2A .

[0230] The quantum state corresponding to the polarization of photons is sometimes indeterminate until it has been measured or absorbed. Before being measured, the quantum state of the photon is therefore sometimes considered as the superposition of possible states, namely, in the example considered, as the superposition of a polarization at an angle of 45° and a polarization at an angle of -45°.

[0231] In the variant illustrated at the 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 magnitude remains constant.

[0232] Where applicable, the polarization of photons is defined by the direction of rotation of the electric field, either clockwise C1 or counterclockwise C2. Before being measured, linearly polarized photons are in an indeterminate quantum state, considered as the superposition of two circular polarization states with opposite directions of rotation.

[0233] In some cases, two quarter-wave plates 6 and 8 can be introduced into the photon propagation paths between the emitter 2 and the receivers 3 and / or 4, as illustrated in the figure 3 .

[0234] 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 with respect to the waves having an electric field parallel to said ordinary axis.

[0235] 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 linearly polarized fields, before propagating to the receivers 3 or 4.

[0236] This transformation of the polarization of the emitted photons makes it possible, in particular, not to have to take into account at the level of the receivers the direction of the polarization of the photons emitted by the emitter.

[0237] The optical selector 30 can be made in different ways, some examples of which are illustrated in figures 4A to 4D.

[0238] The optical selector 30, for example, includes a liquid crystal controlled mirror as illustrated in the figure 4Acomprising 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, allowing their refractive index to be controlled. n C .

[0239] According to the refractive index n C , The incident photon P1 can be reflected on a propagation path D3, or it can pass through the blade 310 and be transmitted on a propagation path D1', which can be identical or different from the path D1, to the measuring instrument 35.

[0240] In an illustrated variant at the figure 4B , the photon P1 passes through the liquid crystal plate 310 and emerges along D3 or D1', the directions D1, D1' and D3 being, in the example considered, parallel.

[0241] 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. Anti-reflective blades can advantageously be placed on each of the faces of the blade 310 and adjusted according to the angles of entry and exit of the photon when it passes through it.

[0242] In the variant illustrated at the figure 4C The optical selector 30 is a mirror controlled by nonlinear fibers. It comprises two optical fibers 330 and 335. Fiber 335, for example, is made of a nonlinear material.

[0243] The first photon P1, upon its arrival at the first receiver, enters the fiber 330. We can choose to simultaneously illuminate the fiber 335 with a light signal F, also called the "control signal".

[0244] The signal F is a high-intensity light wave, for example emitted by a laser, and with a wavelength different from that of the photon P1.

[0245] 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 output, 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, in a straight line back into the fiber 330.

[0246] If fiber 335 is not illuminated by signal F, photon P1 is transmitted from fiber 330 to fiber 335. Photon P1 exits fiber 335 and passes, for example, through a prism 340 before being transmitted, for example, to the measuring instrument 35. Prism 340 is preferably made of a dispersive material that allows the illumination light to exit at a different point and in a different direction than that of the photon. A Bragg mirror (not shown) that reflects only the illumination light and allows the photon to pass through can be placed between fiber 335 and prism 340, for example, to reflect the illumination light in a direction other than towards prism 340 or even back into the fiber.

[0247] Anti-reflective plates (not shown) can be placed at the input and output of prisms 340 and 345 and adjusted to the wavelength of photon P1.

[0248] We will now describe various ways of implementing the polarization modifier 32 of the system of the Figure 1A .

[0249] The polarization modifier 32 preferably includes a polarization direction modifier 32a arranged upstream of a phase modifier 32b.

[0250] In a first example of polarization direction modification implementation, the polarization direction modifier 32a comprises a stack of plates 510, including quarter-wave plates, oriented differently relative to each other, as illustrated in the figure 5A such that a circular wave 508 penetrating one of these quarter-wave plates emerges as a wave 511 linearly polarized along a direction associated with said plate.

[0251] These plates can be designed to function with waves 508 penetrating them with a direction parallel to the bisector of their edges 521 and 522. These plates are preferably composed of uniaxial birefringent crystals such as, for example, rutile. This birefringent crystal can constitute 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 the figure 5A .

[0252] These blades can be placed side by side to form a 524 structure, as illustrated in the figure 5B , such that the outgoing waves 511 or 512 have a linear polarization whose direction of the electric field depends on the point of penetration into the assembly 524 of two circularly polarized waves 508 or 509 in a direction parallel to the bisector of the edges 521 and 522.

[0253] Thus, a wave 505 of determined linear direction and polarization entering the quarter-wave plate 501 passes through it in the direction 507 to become circularly polarized before entering 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 electrodes 516.

[0254] 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. Since the direction of the light ray is different at these locations, the direction of the edges 520 and 521 is preferably different for each element 510 of the assembly 524.

[0255] On the diagram illustrated in the figure 5BWaves 511 and 512 are therefore two possibilities for the light ray to pass through assembly 524. These waves exit assembly 524 linearly polarized along 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 and 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 according to the refractive index chosen for crystals 502 and 503 so that the light waves always emerge as a wave 513 of the same direction from crystal 505 at the same location 515.

[0256] Crystals 502, 503, 504, and 505 are, for example, Merck E7 liquid crystals across whose electrodes voltages between 0 and 5000 V are applied if the distance between the electrodes of each pair is 5 mm, allowing their refractive indices to vary between 1.5 and 1.67. The electrodes are preferably coated with a dielectric film. Since these refractive indices, modified by electric fields, can differ 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 allows the refractive index to be modified for each of the two components of said light, the latter being the superposition of waves whose electric field is perpendicular to the figure 5B and waves whose electric field is perpendicular to the direction of propagation and in the plane of the figure.

[0257] In a second example (not shown) of modifying the polarization direction, the linearly polarized wave, polarized in a specific direction, enters a first quarter-wave plate, transforming its polarization into circular polarization. It then enters a second quarter-wave plate, transforming this circular polarization back into linear polarization along an adjustable direction that depends on the orientation of this second quarter-wave plate. The orientation of the second quarter-wave plate is achieved, for example, by mechanical control of a sensor, or by an electrically controlled device that allows its rotation, for example, by rubbing against an axis set in motion by a piezoelectric material or by an electric motor, for example, a direct current motor.

[0258] In a third example illustrated at figure 5CLiquid crystals are used to control the rotation of the linear electric field of incident photons 526 linearly polarized along the same polarization direction, which begin for example by passing, preferably perpendicularly, through one of two transparent electrodes 527 of 0.2µm 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 passing through the liquid crystal 529 over 2 µm before passing through the second transparent electrode 527 whose face 528, preferably parallel to the first electrode, is also coated with a material allowing the liquid crystals to be aligned 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. Applying an electrical voltage, for example, between 0 and 1V, gradually reorients the liquid crystals to align them with the direction of photon propagation, thereby progressively reducing the difference in refractive index between the two polarizations perpendicular to the photon propagation and the rotational power of the device. This device therefore allows continuous selection of the output direction of the incident photon polarization between 0° and 60°.

[0259] Devices such as the one illustrated in the figure 5B are advantageously used in a device similar to that described above and illustrated in the figure 5A, for example placed between the 524 and 504 plates, each of for example 4 elements 510 allowing 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 fifre 5B placed in succession, and preferably aligned with the direction of propagation of the photos coming out of the preceding 510 devices, allowing to add to this rotation, a rotation of any angle between for example 0 and 45°.

[0260] We illustrated at the figure 6 a polarization phase modifier 32b. The phase modifier 32b comprises a first birefringent plate or prism 310 arranged to split the incident photon P1 of polarization field E into two linearly polarized electromagnetic waves E1 and E2 on two different axes x1 and x2.

[0261] On the second axis x2, a delay plate 321 of variable refractive index, for example comprising a Pockels cell such as Lithium Niobate, controlled by electrodes 322 or a nonlinear material, is placed so as to make the wave E 2 oriented along the second axis x 2 acquire a predetermined phase shift with respect to the wave E 1 oriented along the first axis x 1.

[0262] Another birefringent plate 310, for example made of paratellurite, is placed at the output of the phase modifier 32b, allowing the two waves whose polarization fields E1 and E2 are perpendicular to be joined along the same x-axis into a wave or photon P1 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 depending on the direction of the electric field of the light relative to the direction of the electric field modifying the refractive index, a simple Pockels cell 321 can be used with the electrodes 322 but without the plates 310.

[0263] The measuring instruments 35 of the first receiver 3 can be of various types and comprise different elements. These depend in particular on the nature of the polarization of the photon P1 when it is emitted by the emitter 2. Some examples are illustrated in figures 7 and 8 and described below.

[0264] In the example illustrated in the figure 7 , the measuring instrument 35 includes a polarizing filter 350 arranged on the propagation path D1 of the photon P1, and a photon detector 355 arranged on the same propagation path, downstream of the filter 350.

[0265] The 350 polarizing filter only allows photons with a specific linear polarization to pass through, while absorbing those with perpendicular polarization. It therefore allows the selection of photons with linear polarization in a particular direction.

[0266] In the example considered, the 350 polarizing filter is a grid made of vertical wires, for example metallic ones. It only allows photons with horizontal linear polarization (H) to pass through.

[0267] In an illustrated variant at the figure 8The measuring instrument 35 comprises an anisotropic plate 360, for example a birefringent plate, a birefringent prism, or two birefringent prisms placed side by side. The plate is, for example, made of barium beta borate (BaB2O4, BBO).

[0268] The photon P1, whose quantum state is to be measured, reaches plate 360 ​​on an incident propagation path D1, and is transmitted on one of two propagation paths D11 or D12, depending, for example, on whether its polarization is in the plane of plate 360 ​​(that is, perpendicular to the plane of the figure 8 ), or normal to the plane of the 360 ​​blade.

[0269] The measuring instrument 35 further comprises two detectors 355, each placed on a propagation path D11 or D12 of the photon P1.

[0270] In this example, and unlike the polarizing filter, all photons can be detected by the 355 detectors, regardless of their polarization.

[0271] In some embodiments, a quarter-wave plate is placed upstream of the 360 ​​anisotropic plate in order to transform a circular polarization of a photon into a linear polarization, and thus detect the direction of rotation of a circularly polarized photon.

[0272] The second receiver 4 can include different types of optical amplifier 40 and measuring instrument 45.

[0273] For example, a fiber-doped amplifier, such as the one illustrated in the figure 9 .

[0274] In this example, the amplifier 40 includes a fiber 400, specifically a fiber made of nonlinear material, into which the second photon P2 is introduced after passing through a dichroic prism 401 when it reaches the receiver 4.

[0275] An electromagnetic control wave F used to provide 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.

[0276] The control wave F is preferably of high light intensity and preferably of shorter wavelength than that of the photon P2.

[0277] During its passage through the fiber 400 and under the influence of the luminous flux F, the photon P2 is multiplied into N photons P20. These N photons and the flux F then 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, which allows the polarization of the luminous flux to be determined. Examples of its implementation are given in Figure 10A And 10B.

[0278] We can deduce from the measurements of the measuring instrument 45 the information transmitted by the first receiver, because the amplification of the photon P2 by the device 40 preserves its polarization.

[0279] In particular, if photon P2 is the photoentangled image of a photon P1 absorbed in one of two known complementary polarizations at the first receiver, the N multiplied photons P20 are measured with a polarization that can be deduced from the absorption polarization of photon P1 by the Jones matrix calculated during a calibration as described above. The measuring instrument 45 may comprise a succession of semi-reflective plates 452 and a mirror 453, as illustrated in the Figure 10AThe 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, preferably with equal intensity. The filters 350 split the luminous flux into two orthogonally polarized light streams, so that the detectors 455 can determine either the intensity of said flux along 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's slit interferometer.

[0280] Interferometers are preferably arranged so that the light paths of the two orthogonal components are identical, as the light flux resulting from the multiplication of a single photon is very short. Using several pairs of orthogonal directions, for example, offset by 45° from each other, advantageously allows for the measurement of polarization—that is, the direction of polarization—as well as the phase difference between the two directions, multiple times, thus enabling greater measurement precision.

[0281] There figure 10BThis represents an example of such a Young's slit interferometer. Two waves, 460 and 463, originate from the prism 350. Wave 460 passes, for example, through a prism 461, which straightens its direction of propagation so that it becomes parallel to wave 463 at point 462. Wave 463 then passes, for example, through a half-wave plate, which aligns the electric field of wave 465 with that of wave 462. Waves 462 and 465 then pass through two holes drilled in a screen 466 before interfering with each other to form fringes on a screen 467. This screen is observed by a camera or equipped with photosensitive sensors, allowing the position of the brightest fringe on the screen 467 to be determined.

[0282] Information, including binary information, can be transmitted between receivers 3 and 4 of a system 1 such as the one described in the figure 1B , for example by following the steps illustrated in Figures 11A And 11B .

[0283] The protocol for matching the information to be transmitted, for example, between sending a bit "0" or a bit "1", and the measurement, or not, of the first photon P1, is determined before the start of the transmission.

[0284] As an example, we choose for the following purposes not to measure the first photon P1 to transmit a "0", and to measure it to transmit a "1". Of course, the reverse choice, or any other suitable correspondence, would also be valid.

[0285] In step 10, two entangled photons P1 and P2 are emitted simultaneously from an emitter 2 to receivers 3 and 4, respectively, with receiver 3 being placed closer to emitter 2 than receiver 4, as described above.

[0286] The emitted photons P1 and P2 exhibit an indeterminate quantum state in a predetermined pair of complementary polarizations, for example a polarization at 45° if they are linearly polarized (the complementary polarizations being 0° and 90).

[0287] At step 11, the photon P1 reaches the optical selector 30 of the receiver 3.

[0288] If a "0" bit is to be transmitted, the photon P1 is, for example, reflected by the optical selector, along a perpendicular path in the example illustrated in the figure 11A (but any other path is possible), so as not to be able to reach the measuring instrument 35. The photon P1 can in particular be trapped at step 12 in a device protecting its quantum state 5, in order to avoid its absorption, at least until the second photon P2 has reached the second receptor 4.

[0289] For example, the P1 photon retains its 45° polarization.

[0290] If a bit "1" is to be transmitted, the photon P1 passes, for example, through the optical selector 30 to head towards the measuring instrument 35, as illustrated in the figure 11B .

[0291] The quantum state of photon P1 is then measured by measuring instrument 35, in step 13.

[0292] The photon P1 now has a definite quantum state, for example, vertical polarization (90°) or horizontal polarization (0°). Instantly, the measurement performed in step 13 projects the entangled photon P2 into a definite state.

[0293] 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 stream of multiplied photons P20, each photon P20 having retained the polarization of the photon P2.

[0294] In step 16, the polarization of the P20 photon flux is measured by a measuring instrument 45.

[0295] If, on average, an intermediate state is obtained, for example a polarization at 45°, it is deduced that the first photon P1 has not been measured, and a bit "0" is received.

[0296] 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 photon P1 has been measured, and a bit "1" is received.

[0297] It is still possible to transmit series of discrete values ​​between receivers 3 and 4 of a system 1 such as the one described in the Figure 1A , for example by following the steps illustrated in the figure 12 .

[0298] In step 10, two entangled photons P1 and P2 are emitted simultaneously from an emitter 2 to receivers 3 and 4, respectively, for example with linear polarization.

[0299] At step 17, the photon P1 reaches the polarization modifier 32, which transforms the linear polarization of the photon P1 into 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.

[0300] At step 13, the photon P1 is absorbed by the instrument 35 in one of the two complementary states of the chosen pair, simultaneously projecting the entangled photon P2 into its complementary state.

[0301] The following steps 15 and 17 are similar to those described above; the polarization of the P2 photon is measured by amplification and then absorption, and the discrete transmitted value is deduced.

[0302] The invention is not limited to the measurement of linear polarization of photons. Other types of quantum states can be measured, and / or other measurement methods can be used, including measurements in other observable bases.

[0303] Photons P1 and P2 are emitted, for example, with circular entangled polarization, and the measuring instrument 35 projects, by measuring linear polarization, photon P1 and, by entanglement, photon P2, into a basis of linear polarization.

[0304] Alternatively, the photons P1 and P2 are emitted with linear entangled polarization, and the measuring instrument 35 projects, by measuring a circular polarization, the photon P1 and by entanglement the photon P2 according to this observable basis.

[0305] In another example, photons P1 and P2 are emitted with linear entangled polarization vertically or horizontally and measuring instrument 35 measures linear polarization at 45° or -45° from the vertical or horizontal.

[0306] In one variant, the receiver 3 comprises two measuring instruments 35, one measuring linear polarization, the other measuring circular polarization. The optical selector, for example, sends 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.

[0307] In each of the above examples, the measuring instrument 45 of the second receiver 4 can be configured to detect whether the first photon P1 has been measured, or not, and, if so, with what polarization it was measured at the first receiver.

[0308] Gyroscopes can also be used at emitter 2 and receivers 3 and 4 to determine the polarization direction of photons P1 and P2, if these are emitted and transported to the receivers with linear polarization.

[0309] 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 have waited for the absorption of their entangled photon, which can be accompanied by a timestamp of their arrival, as illustrated in the figure 13 This can, for example, allow control of the correspondence between entangled photon pairs and transmitted bits.

[0310] Each photon, for example, triggers the timestamping process upon arriving at the receiver in step 80, specifically upon reaching one of the measuring instruments 35 or 5D. Following this triggering event, the receiver 3, for example, in step 82, copies the current time H from the clock of said receiver, for example stored in a register R, onto a free memory register M, preferably after subtracting the transit time of the photon in the waiting device 5 from the arrival time at detector 5-D.

[0311] A similar device (not shown) can be arranged at the second receiver 4.

[0312] Preferably, the clocks enabling the timestamping just described are synchronized for both receivers 3 and 4, which makes it possible to define time intervals for processing photons common to both receivers, and to process the "multiple" double pairs of photons emitted by emitter 2.

[0313] The second receiver 4 may also include a switch 50 placed in front of the optical amplifier 40, and configured to absorb or reflect any unwanted photon(s) PE2 that reach the receiver 4 after photon P2 within the same processing time interval, as shown in the figure 14 .

[0314] The switch 50 can also absorb any PA2 photons emitted by the amplifier 40 towards the emitter 2 during signal amplification, or during the de-excitation of molecules or atoms of the amplifying medium.

[0315] 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.

[0316] In some embodiments, the system according to the invention may include several transmitters arranged differently relative to the receivers, in particular to establish bidirectional communication.

[0317] In the example illustrated in the figure 15 , system 1 has two emitters 21 and 22, each emitting pairs of entangled photons towards two receivers 91 and 92.

[0318] Transmitter 21 is closer to receiver 91 than to receiver 92, and transmitter 22 is closer to receiver 92 than to receiver 91. This arrangement allows receivers 91 and 92 to communicate bidirectionally; the photon P11 emitted by transmitter 21 arrives first at receiver 91, which can then act as the first receiver 3, i.e. transmit information to receiver 92 which receives its entangled photon P12 and acts as the second receiver 4.

[0319] Conversely, the photon P21 emitted by the emitter 22 arrives first at the receiver 92, which can then transmit information to the receiver 91, which receives the entangled photon P22. The mechanism for transmitting information, particularly binary information, is similar, for example, to what was described above, in Figures 11 And 12 .

[0320] Reflecting devices 93 and 94 can be arranged near receptors 91 and 92, respectively, in order to reflect photons possibly reflected by receptors 91 and 92.

[0321] We will now describe, with reference to the figure 16 an implementation variant of the first aspect, intended to verify that information transmitted by sender 2 has not been listened to and, if necessary, to generate a shared key for the exchange of information between receivers 3 and 4, from the transfer of information which is known not to have been listened to.

[0322] The system of the 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 which corresponds to that expected for the photons which have travelled from the emitter 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 emitting the information to the receiver 4.

[0323] Filter 46 prevents a fraudulent receiver from replacing the genuine receiver 4, from observing the exact polarization of the entangled photon after multiplication of said photon, and then from emitting a fraudulent photon towards the genuine receiver 4, in place of the genuine photon, to be necessarily observed by receiver 4 in the same relative polarization as the genuine photon, because on the one hand, if this latter photon is not entangled with another photon, its polarization will not change after passing through filter 46 and will therefore have the polarization imposed by filter 46 and not that of the genuine photon, and on the other hand, if the fraudulent photon is entangled with another photon, called a 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 through filter 46.if the verification of the relative polarization of its entangled photon is then found to be inadequate by a third-party device.

[0324] It is therefore extremely unlikely that all or some of the photons in a group of entangled photons observed by the two receptors 3 and 4 have been observed elsewhere, if their relative polarizations are always different.

[0325] The equality of the quantum state of the photons used to transmit information arriving at each of the two receivers 3 and 4 can be statistically verified, and the probability that some of the photons were observed can be deduced to be low, by applying the following method: Upon reception of the photons by each of the receivers 3 and 4, the receivers record the time of reception as well as the exact polarization of the photons using photon detectors, and not only which complementary polarization pair they belong to. The times and polarizations of detected photons that are too close to each other to have their polarization or arrival time measured independently are, however, preferably not recorded, and the two receivers 3 and 4 preferably have synchronized clocks.

[0326] After a determined number of photons carrying information I have been received, the second receiver 4 generates a message containing the precise time of detection and the polarization for each of the photons whose attributes have been noted and 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 allowing the identification of the reception of information.

[0327] This message is signed, preferably using the random hashing technique described in application US20210165914 A1, by receiver 4, and then sent to receiver 3 with said signature, preferably by implementing a quantum transmission means as described above, and 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 whose two photons have reached the receivers.This information 1 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 to be authentic and the proportion of non-matching polarizations is less than a given threshold, for example 1% if 450 different polarization pairs are used, or a multiple such as 2 of the probability that two different pairs of entangled photons are emitted at times indistinguishable by the receivers.

[0328] Furthermore, a relative polarization in each of the pairs of polarizations of the photons having been retained to have 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 and known only to each of the receivers; the receiver 3 having carried out the verification of the 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 never reached the receiver 3.

[0329] Thus, in one example, the following steps are implemented. Step 1

[0330] Receiver 4 receives from transmitter 2 a stream of photons carrying information and establishes a first list of the dates of reception of the photons which were used to determine this information as well as the relative polarization designating which of the two complementary polarizations in which the photon was received. Step 2

[0331] 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 and the signature to receiver 3. Step 3

[0332] Receiver 3 receives the list and the signature and then verifies said signature. Step 4

[0333] Receiver 3 creates a second list consisting of the elements from the first list whose relative polarizations of the photons received by receivers 3 and 4 are the same, or whose entangled photon never reached receiver 3. Receiver 3 has recorded in a register the relative polarization of each photon used for information transmission, as well as its reception date. Knowing the difference in travel time of the entangled photons between transmitter 2 and each of the two receivers 3 and 4, it can retrieve, for each photon in the first list, the relative polarization of the photon it received at the time of reception of the photon by receiver 4, minus the travel time.This second list is preferably made up of two distinct parts, the first part grouping the photons whose entangled photon has never reached the first receptor 3 and the second part grouping the photons whose two entangled photons have reached their respective receptors but with equal relative polarizations. Step 5

[0334] If the number of elements in the second restricted list of entangled photon pairs, where each of the two photons has reached their respective receptors, is less than the product of a predetermined ratio by counting the elements in the first restricted list of entangled photon pairs, where each of the two photons has reached their respective receptors, then the information is declared to have been transmitted but not listened to. Step 6

[0335] If the information is declared to have been transmitted unlistened to, 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 have been observed to be different.

[0336] Otherwise, if the aforementioned product is above said threshold, a message indicating that the transmission is likely to have been listened to is sent by receiver 3 to receiver 4. Step 7

[0337] 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 good receipt of the second list, is sent by receiver 4 to receiver 3. Step 8

[0338] 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

1. Quantum communication system (1) comprising: • an emitter (2) of entangled photons, comprising a source configured in order to generate at least one pair of entangled photons comprising a first photon (P1) emitted on a first propagation path (D1) and simultaneously a second photon (P2) emitted on a second propagation path (D2) different to the first propagation path; • a first receiver (3) arranged on the first propagation path (D1), comprising: - at least one first instrument (35) arranged in order to absorb the first photon (P1) in one of two complementary polarizations, the polarization of the photon (P1) being indeterminate in the observable basis according to which the first instrument absorbs the photons, - an optical selector (30) arranged upstream of said at least one first instrument (35) and configured in order either to allow the first photon (P1) to pass through towards said at least one first measuring instrument (35) or to prevent it from being measured; • a second receiver (4) arranged on the second propagation path (D2) so as to be reached by the second photon (P2) after the first photon (P1) has reached the optical selector (30) and / or after it has been able to reach the instrument (35) of the first receiver (3), said second receiver (4) comprising an optical amplifier (40) making it possible to multiply the second photon (P2) while preserving its polarization, and, arranged downstream of the amplifier, a measuring instrument (45) making it possible to measure the average quantum state of the multiplied photons (P20).

2. System according to the preceding claim, the first receiver (3) comprising at least one second instrument (35), the optical selector (30) being arranged upstream of the first and second instruments (30) and configured in order either to allow the first photon (P1) to pass through while directing it towards one or other of the instruments or to prevent it from being absorbed.

3. System according to either of the preceding claims, the emitter (2) being configured in order to generate a plurality of pairs of entangled photons successively.

4. System according to any one of the preceding claims, the optical selector (30) comprising a reflector (310), in particular a controlled mirror, and / or the optical selector comprising a device (310; 330) whose refractive index and / or reflection direction are controlled, in particular by using an electric field or a light flux.

5. 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, which is arranged in proximity to the first receiver (3) so as to make it possible to protect the quantum state of the first photon (P1) in the event that it is prevented from being measured by the optical selector (30), said quantum state of the first photon (P1) being protected at least until the second photon (P2) has been multiplied at the second receiver (4), in particular the device for protecting the quantum state (5) comprising a transparent space, in particular a transparent space and at least one mirror.

6. 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) making it possible to send the first photon towards at least one photon detector (355), in particular towards one or other of two photon detectors (355) according to the polarization state of the first photon, the filter preferably being a prism or plate made of a birefringent material.

7. System according to one of the preceding claims, the instrument (45) of the second receiver (4) comprising at least one photon detector (455) arranged in order to measure the polarization of the light (P20) originating from the multiplication of the second photon (P2), and / or the measuring instrument (45) of the second receiver (4) comprising a succession of semireflective plates arranged downstream of the optical amplifier (40), said plates directing the flux of multiplied photons (P20) with an equal intensity towards polarizing filters, downstream of which detectors are arranged.

8. System according to any one of the preceding claims, the optical amplifier (40) being a doped-fibre amplifier, and / or the second receiver (4) comprising a switch (50) arranged in front of the optical amplifier (40) and configured in order to absorb or reflect the photon or photons subsequent to a first photon reaching said second receiver in a predetermined time interval.

9. 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 preferably being arranged in front of the instrument or instruments (35; 45), or in front of the optical selector (30) for the first receiver, and / or the emitter (2) and each of the receivers (3; 4) comprising a clock, the clocks of the emitter and of the receivers being synchronized with one another.

10. 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).

11. Quantum communication method using the system according to any one of Claims 1 to 10, comprising the steps consisting in: • generating a pair of entangled photons from an emitter (2), the first photon (P1) 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 away from the emitter (2) than the first receiver (3), so that it arrives there later, • according to the information to be transmitted ("0", "1", "2"), selecting to absorb or not absorb the first photon (P1) in one of two pairs of complementary polarizations when it reaches the first receiver (3), by using an optical selector (30) which does or does not direct said photon (P1) towards an instrument (35) or towards one of a plurality of instruments (35), • in the event that the selection is made not to absorb the photon in a predefined polarization, trapping said first photon (P1) in a protection device (5) making it possible to avoid its absorption at least so long as the second photon (P2) has not reached the second receiver (4), • at the second receiver (4), duplicating the second photon (P2) to form a flux of multiplied photons (P20) by using an amplification device (40), each multiplied photon (P20) having preserved the quantum state of the second photon (P2), • measuring the average quantum state of multiplied photons (P20) and determining according to this measurement whether the first photon (P1) has been absorbed at the first receiver (3), and / or with which instrument, in order to deduce therefrom the information transmitted ("0", "1") by the first receiver (3).

12. Method according to the preceding claim, the polarization of the pair of entangled photons being indeterminate in the observable basis or bases, in which the instruments of the first receiver (35) absorb them.

13. Method according to Claim 11 or 12, 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 in order to distinguish whether the average polarization of the flux of multiplied photons (P20) is circular or linear, and to determine according to this distinction whether or not the first photon has been measured, or the pair of entangled photons reaching the receivers (3; 4) with a circular entangled polarization, the quantum state measured at the first receiver (3) being a linear polarization, the second receiver (4) being arranged in order to distinguish whether the average polarization of the flux of multiplied photons (P20) is circular or linear, and to determine according to this distinction whether or not the first photon (P1) has been measured, or 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° to the vertical or the horizontal, the second receiver (4) being arranged in order to distinguish whether the average linear polarization of the flux of multiplied photons (P20) is at 45° or -45°, or the vertical or the horizontal, and to determine according to this distinction whether or not the first photon (P1) has been measured.

14. Method according to any one of Claims 11 to 13, a plurality of pairs of entangled photons being generated successively by the emitter (2), each pair of photons making it possible to transmit an item of information ("0", "1", "2") from the first receiver (3) to the second receiver (4).

15. Method according to one of Claims 11 to 14, wherein it is verified that an item of information transmitted by a flux of photons from the emitter (2) to the receiver (4) has not been eavesdropped by implementing the following steps: - the second receiver (4) establishes a first list of the reception dates of the photons that conveyed the item of information, and the relative polarization designating that of the two complementary polarizations in which each photon was received; - the second receiver (4) generates a message containing the list collected in the preceding 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 then verifies said signature; - the first receiver (3) makes a second list consisting of elements of the first list for which the relative polarizations of the photons received by the first and second receivers (3) and (4) are equal, or indeed for which the 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 each of the two photons of which reached their respective receivers is lower than the product of a predetermined ratio multiplied by the count of the elements of the first list restricted to pairs of entangled photons each of the two photons of which reached their respective receivers, then the item of information is declared to have been transmitted without having been eavesdropped, in particular - if the item of information is declared to have been transmitted without having been eavesdropped, 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), this list consisting of the relative polarizations of the photons featuring in the first list but not featuring in the second list, - on reception 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 successful 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 shared key with the first receiver (3), and, on reception by the first receiver (3) of the message transmitted by the second receiver in the preceding step, the first receiver (3) uses the third list as shared key in exchanges with the second receiver (4), a polarizing filter (46) being in particular placed upstream of the second receiver (4).