Quantum communication system using photon polarization correction

Abstract

A receiver (20-m) is proposed for a multiplexed signal (S10-n) and an optical signal (S10-h) transmitted independently through a transmission channel (50), the multiplexed signal comprising a first quantum signal (Qn) and at least one polarization state control signal (Rn1), said optical signal comprising a second quantum signal (Qh), said receiver comprising a processing chain (Cm) and a correlation module (260) associated with a polarization basis of at least one polarization state, said chain being adapted to determine the polarization state of the control signal and to modify the polarization of the multiplexed signal to align the determined state with the state of said basis, the module being configured to perform a correlation measurement between the first and second quantum signals, and to generate, from the measurement, at least one information signal (Imk) of the entanglement state of the first and second quantum signals.Figure for the abridged version: [Fig.6].

Description

[0001] Title of the invention: Quantum communication system using photon polarization correction technical field

[0002] The present invention relates generally to quantum telecommunications, and in particular to a transmitter for transmitting optical signals comprising entangled quantum signals, a receiver for receiving optical signals comprising quantum signals, and a system comprising such transmitters and receivers, and the associated methods implemented.

[0003] The main application of current quantum telecommunication systems is to use quantum information theory to distribute a cryptographic key (or encryption key) between two telecommunication devices (i.e., two users), via specific quantum protocols, with the aim of subsequently encrypting communications between these two devices securely. The resulting secret cryptographic keys have a higher degree of security than keys obtained by classical protocols.

[0004] In a quantum telecommunications system, the two users may be too physically distant to simply use the usual steps of QKD (or 'Quantum Key Distribution') protocols to share such an encryption key. In this context, quantum state teleportation and entanglement sharing protocols allow two distant quantum devices to be linked together to share an encryption key.

[0005] Such quantum protocols of quantum state teleportation and entanglement sharing consist of interfering quantum particles with each other, originating from two entangled quantum signals, each particle belonging to a pair of entangled quantum particles.

[0006] The information used to generate an encryption key is obtained by measuring an encoding variable of quantum particles (also called "qubits"), generally corresponding to photons, which has been previously encoded. This encoding variable has a random value but is identical for the two entangled particles, thus allowing the same information to be shared. An encoding variable of a qubit corresponds to a degree of freedom of the quantum particle and can be the polarization of the photon. However, the polarization state of quantum particles during their propagation between different devices in a system undergoes random rotations. These rotations can be due to the birefringence of the various media traversed or to the displacement of the emitting device relative to the receiving device, such as the movement of a satellite (transmitter or receiver) relative to a ground station in the case of communications via a space segment.

[0007] To avoid such random rotations of polarization state, some known quantum systems use free-space propagation only, in which the polarization of photons is stable during their propagation through a transmission channel (or communication channel). However, in some applications, it is necessary to use guided optics transmission as the transmission channel, for example, for propagation over a ground network or on board a satellite to relax the constraints on payload construction.

[0008] To compensate for (or correct) random polarization state rotations, some known systems use, at the beginning of the quantum protocol implementation, a single polarization reference that allows for the initial estimation of polarization rotations induced during propagation and the alignment of the polarization of the transmitted photons with the measurement bases at reception. However, this single reference does not allow for the correction of new polarization rotations after the initial estimation phase. Alternatively, other existing systems instead use a periodic generation of reference signals from the encoding bases from the source of entangled quantum signals, which are thus time-multiplexed with the qubits, thereby reducing the system's useful throughput.

[0009] There is thus a need for an improved quantum communication system capable of correcting the rotations of the qubit polarization states in real time. Summary of the invention

[0010] To this end, a receiver is proposed that is configured to receive a multiplexed optical signal and an optical signal transmitted independently through a transmission channel, the multiplexed optical signal comprising a first quantum signal, the optical signal comprising a second quantum signal, the multiplexed optical signal further comprising at least one polarization state control signal, the receiver comprising a processing chain associated with a polarization base composed of at least one polarization state and adapted to determine the polarization state of at least one polarization state control signal and to modify the polarization of the multiplexed optical signal so as to align the determined polarization state with respect to one of the at least one polarization state of the associated base,the receiver further comprising a correlation module adapted to perform a correlation measurement between the first quantum signal from the modified polarization multiplexed optical signal and the second quantum signal, the correlation module being associated with the polarization basis, the correlation module being further adapted to generate at least one information signal from the correlation measurement, the signal, information including entanglement information of the polarization states of the first and second quantum signals.

[0011] In some embodiments, the processing chain may include a module control signal detection and analysis device, the control signal detection module being configured to demultiplex at least one control signal and the first quantum signal from the multiplexed optical signal, the detection module being further configured to route the demultiplexed control signal to the polarization analysis device, the analysis device comprising at least one detection unit adapted to detect the control signal according to at least one polarization state of the associated base.

[0012] In certain aspects, the detection module may further include a processor configured to analyze the determined polarization state and to generate a servo signal applied to a polarization correction module of the multiplexed optical signal.

[0013] In some embodiments, the receiver can be formed from polarization-maintaining fibers and / or single-mode optical fibers.

[0014] The present invention further proposes a transmitter configured to emit optical signals comprising:

[0015] - a signal generator configured to generate a first quantum signal, a a second quantum signal, and a polarization state control signal, the first quantum signal and the second quantum signal being entangled quantum signals,

[0016] - a signal integrator configured to generate a multiplexed optical signal, the multiplexed signal comprising the first control signal and the first quantum signal.

[0017] The transmitter is configured to transmit through a transmission channel, the multiplexed optical signal and an optical signal comprising the second quantum signal.

[0018] The embodiments of the invention thus provide a quantum communication system comprising a plurality of transmitters, and at least one receiver.

[0019] In embodiments, the plurality of emitters may include at least a first emitter and a second emitter, and the system may further include a plurality of auxiliary receivers comprising a first auxiliary receiver configured to receive an optical signal including a quantum signal transmitted by the first emitter, and a second auxiliary receiver configured to receive an optical signal including a quantum signal transmitted by the second emitter, each auxiliary receiver being associated with a biasing base of A measurement consisting of at least one polarization state and adapted to measure the associated quantum signal according to at least one polarization state of the associated measurement polarization basis. Each auxiliary receiver can be configured to receive an entanglement information signal comprising entanglement information of the polarization states of quantum signals transmitted by at least one receiver, each auxiliary receiver being configured to determine a shared quantum encryption key from the measurement of the associated quantum signal and the entanglement information of the polarization states of quantum signals.

[0020] In certain aspects, for one or both auxiliary receivers, the optical signal received by the auxiliary receiver may be a multiplexed optical signal further comprising a polarization state control signal, the auxiliary receiver(s) comprising a processing chain associated with the measurement polarization base and adapted to determine the polarization state of the polarization state control signal and to modify the polarization of the multiplexed optical signal so as to align the determined polarization state with respect to at least one polarization state of the associated measurement polarization base.

[0021] In some embodiments, the multiplexed signals may be frequency-multiplexed signals.

[0022] Advantageously, the absolute value of the wavelength difference between the quantum wavelength of a quantum signal and the reference wavelength of a control signal can be greater than or equal to a minimum wavelength difference value.

[0023] A method is also proposed for determining at least one information signal in response to the reception of a multiplexed optical signal and an optical signal transmitted independently through a transmission channel, the multiplexed optical signal comprising a first quantum signal, the optical signal comprising a second quantum signal, the multiplexed optical signal further comprising at least one polarization state control signal, the method comprising a processing phase, associated with a polarization basis composed of at least one polarization state, for determining the polarization state of at least one polarization state control signal and for modifying the polarization of the multiplexed optical signal so as to align the determined polarization state with respect to one of the at least one polarization state of the associated basis.The method further includes a correlation step comprising a correlation measurement between the first quantum signal from the modified polarization multiplexed optical signal and the second quantum signal, the correlation measurement being associated with the polarization basis, the correlation step further comprising the generation of at least one information signal at . from the correlation measurement, the information signal includes entanglement information of the polarization states of the first and second quantum signals.

[0024] Embodiments of the invention thus make it possible to correct the polarization rotations of the qubits transmitted between transmitters and receivers of entangled quantum signals, in particular to establish a quantum key between two distant receivers.

[0025] In particular, embodiments of the invention provide optical signal emitters, associated with pairs of entangled quantum particles, enabling robust integration of one or more polarization state reference signals of these quantum particles.

[0026] Such references can be generated at any power level, independently of qubit generation, to form an efficient and affordable solution in terms of hardware complexity. A guided-optics emitter, according to embodiments of the invention, advantageously offers reduced volumetric and mass compactness, as well as optimized size and robustness. Furthermore, frequency multiplexing of such references to the qubits makes it possible to maintain a high transmission rate of useful information (i.e., the qubits).

[0027] The receiver(s) according to the embodiments of the invention allow for real-time correction of the polarization rotations undergone by the qubits before detection. Such receivers make it possible, in particular, to analyze the qubits and reference signals independently, in order to align the polarization of the qubits as closely as possible with the receiver's measurement bases. Description of the figures

[0028] Other features, details and advantages of the invention will become apparent from the description made with reference to the accompanying drawings given by way of example.

[0029] [Fig.1] Fig.1 is a diagram representing a quantum communication system, according to embodiments of the invention.

[0030] [Fig.2] The [Fig.2] is a diagram representing a quantum communication system, according to embodiments of the invention.

[0031] [Fig.3] The [Fig.3] is a diagram representing a transmitter of a quantum communication system, according to embodiments of the invention.

[0032] [Fig.4] The [Fig.4] is a diagram representing a transmitter of a quantum communication system, according to embodiments of the invention.

[0033] [Fig.5] The [Fig.5] is a diagram representing a signal generator of a transmitter of a quantum communication system, according to embodiments of the invention.

[0034] [Fig.6] The [Fig.6] is a diagram representing an intermediate receiver of a quantum communication system, according to embodiments of the invention.

[0035] [Fig.7] Fig.7 is a diagram representing an intermediate receptor of a quantum communication system, according to embodiments of the invention

[0036] [Fig.8] Fig.8 is a diagram representing a processing chain of an intermediate receiver of a quantum communication system, according to embodiments of the invention

[0037] [Fig.9] Fig.9 is a diagram representing a signal detection module control of an intermediate receiver of a quantum communication system, according to embodiments of the invention

[0038] [Fig. 10] The [Fig. 10] is a diagram representing a control signal detection module for an intermediate receiver of a quantum communication system, according to embodiments of the invention

[0039] [Fig. 11] The [Fig. 11] is a diagram representing a final receiver of a quantum communication system, according to embodiments of the invention

[0040] [Fig. 12] The [Fig. 12] is a diagram representing a quantum photon analysis module of an end receiver of a quantum communication system, according to embodiments of the invention.

[0041] [Fig. 13] The [Fig. 13] is a flowchart representing a method of emitting optical signals implemented by a transmitter of a quantum communication system, according to embodiments of the invention.

[0042] [Fig. 14] The [Fig. 14] is a flowchart representing an intermediate optical signal reception method implemented by a receiver of a quantum communication system, according to embodiments of the invention.

[0043] [Fig. 15] The [Fig. 15] is a flowchart representing a final optical signal reception method implemented by a receiver of a quantum communication system, according to embodiments of the invention.

[0044] Identical reference numerals are used in the figures to designate identical or analogous elements. For clarity, the elements shown are not to scale. Detailed description

[0045] Figures 1 and 2 schematically represent a quantum communication system 1 comprising a first set of devices 10, a second set of devices 20 and a third set of devices 30, capable of communicating with each other, according to embodiments of the invention.

[0046] The first set of devices 10 of system 1 comprises a plurality of emitting devices 10-n. The index 'n' is associated with the nth emitting device of the system 1 and is an integer between 1 and N, the value of N being greater than or equal to 2.

[0047] The second set of devices 20 of system 1 comprises one or more receiving devices 20-m, also called 'intermediate receivers'. The index 'm' is associated with the m-th receiving device of system 1 and is an integer between 1 and M, the value of M being greater than or equal to 1.

[0048] The third set of devices 30 of system 1 includes two receiving devices, also called 'final receivers', denoted 30-1 and 30-2 (or more generally 30-k, the index 'k' being an integer equal to 1 or 2).

[0049] The quantum communication system 1 can be used in various applications. For example, and without limitation, the quantum communication system 1 can be used in the space domain and comprise a 10⁻ⁿ transmitter and / or a receiver (20⁻¹² and / or 30⁻¹²) mounted on a satellite. In such an example of the space-domain application of the invention, the system 1 can also comprise a 10⁻ⁿ transmitter and / or a receiver (20⁻¹² and / or 30⁻¹²) on the ground, which can be carried in one or more ground-based devices. The system 1 can also be used in avionics applications, at least one of the 10⁻ⁿ transmitter and / or receiver (20⁻¹² and / or 30⁻¹²) devices then being an avionics device. System 1 can also be used in fiber optic network applications, in which at least one of the 10-n transmitting devices and / or one receiver (20-m and / or 30-k) is a device for fiber optics integrated into a ground network.

[0050] A device of system 1 can be fixed or in motion relative to another device with which it communicates.

[0051] By way of non-limiting example, devices of the quantum communication system 1 may be quantum computers or quantum sensor networks.

[0052] A 10-n transmitter includes a signal generator 120 (also called a 'signal generation module') and at least one signal integrator (also called a 'signal integration module'), as shown in Figures 3 and 4, which illustrate embodiments of the invention.

[0053] As used here, an 'optical signal' (also simply called a 'signal') results from one or more coherent light pulses originating from an optical source, such as, for example, a laser beam. A laser beam can be characterized, in particular, by its pulse rate f and by a laser pulse (i.e., the signal) defined by its frequency te, its intensity T, its polarization P, and its phase. The 'frequency œ' of the laser beam denotes the 'optical frequency of the laser pulse multiplied by 2tt' defined as a function of the wavelength of the beam A, such that XX denotes the speed of light.

[0054] A transmitter 10-n of the first set of devices 10 of the system 1 is configured to generate and transmit, through a transmission channel 50, two distinct optical emission signals, denoted Si 0 ni and Si 0 n2 (also referred to respectively as 'first optical emission signal' and 'second optical emission signal').

[0055] The transmission channel 50 can be, for example, a free space or a fiber (or guided optics) information transport device using, for example, optical fiber elements for communication, depending on the field of application of the invention.

[0056] Each optical emission signal delivered by a 10-n emitter comprises a quantum signal, denoted Qni and Qn2 respectively. The two quantum signals Qni and Qn2 are entangled with each other, each photon of a quantum signal originating respectively from an entangled pair of photons generated by the signal generator 120. The first quantum signal Qni is associated with a polarization state, denoted PQ nb and the second quantum signal Qn2 is associated with a polarization state, denoted PQ n2.

[0057] As used here, the expression 'quantum signal' refers to an optical signal comprising at least one photon that is entangled with another photon of a different 'quantum signal'. These two quantum signals are then said to be 'entangled quantum signals'. An 'entangled pair of photons' refers to two photons forming a bound system and exhibiting quantum states that are dependent on each other regardless of the distance separating them. There are correlations between the physical properties (in particular between their polarization states), which can be measured, of these distinct particles. The entanglement of a pair of photons arises from the fact that these photons, each included in a specific quantum signal, are both generated from the same pump photon. Entangled quantum signals can be either pulsed or continuous optical signals.Measuring a quantum signal provides a measurement of the detection of a photon (or 'particle') that depends on a "detection probability" of that photon.

[0058] In some embodiments, the polarization states PQni and PQn2 of the first and second quantum signals Qni and Qn2, respectively, may be identical (i.e., correlated) and / or orthogonal to each other (i.e., anticorrelated). Advantageously, the polarization states PQni and PQn2 of the entangled quantum signals are not well-defined individual polarization states at the time of signal generation and can only be defined during an entanglement measurement.

[0059] The first SiO ni optical emission signal delivered by the 10-n transmitter is a multiplexed optical signal (also called a 'multiplexed optical signal' or 'signal of multiplexed communication') comprising the first quantum signal Qni and a first integrated optical polarization control signal, denoted Rni. Such a signal is also called the 'first control signal' or 'first reference signal'.

[0060] In some embodiments, the polarization state of the first control signal Rni can be defined in a polarization basis BQ ni (also called the 'first control polarization basis'). For example, and without limitation, the polarization basis BQ ni can be the H / V polarization basis comprising a linear polarization state of type H, i.e., 'horizontal', and a linear polarization state of type V, i.e., 'vertical'. In other embodiments, the polarization basis BQ ni can be the D / A polarization basis comprising a linear polarization state of type D, i.e., 'diagonal', and a linear polarization state of type A, i.e., 'anti-diagonal'.

[0061] In certain embodiments, the first multiplexed optical signal SiO ni delivered by the 10-n transmitter may further comprise a second integrated optical polarization control signal, denoted Rn2. Such a signal is also called the 'second control signal' or 'second reference signal'. Advantageously, the polarization state of the second control signal Rn2 may be defined in a polarization basis BQ n2 (also called the 'second control polarization basis'). In particular, the polarization basis BQ n2 may be a polarization basis that is not orthogonal to the polarization basis BQ ni of the first control signal Rnb

[0062] In some embodiments, the second optical emission signal Si0. n2 delivered by the transmitter 10-n can also be a multiplexed optical signal comprising, in addition to the second quantum signal Qn2, the first control signal Rni and / or the second control signal Rn2.

[0063] An intermediate receiver 20-m of the second set of devices 20 of the system 1 is configured to receive, from the transmission channel 50, on the one hand a multiplexed optical signal, SiO n (that is to say a signal Si0. ni or Si0. n 2), transmitted by a first transmitter 10-n of the first set of devices 10, and on the other hand an optical transmission signal, Sio h (corresponding to a signal Si0. h 1 or Si0. h 2) transmitted by a second transmitter, denoted 10-h, of the first set of devices 10 and different from the first transmitter 10-n, the index 'h' being an integer between 1 and N and different from the index 'n'. Thus, as illustrated in Figures 1 and 2, the intermediate receiver 20-1 for example is configured to receive on the one hand the first optical emission signal transmitted by the first transmitter 10-1, and on the other hand an optical emission signal transmitted by the second transmitter 10-2.The first optical emission signal received by the intermediate receiver 20-1 is the first optical emission signal Sio-n transmitted by the first transmitter 10-1 corresponding to a multiplexed optical signal. including a quantum signal Qn (or Qni) and at least one first control signal Rni. In addition, the optical emission signal transmitted by the second transmitter 10-2 can be either the first optical emission signal S10-21 corresponding to a multiplexed optical signal, or the second optical emission signal S10-22 also corresponding to a multiplexed optical signal, or comprising only a quantum signal Qh (or Q22, as in the example illustrated in Figures 1 and 2).

[0064] The intermediate receiver 20-m is then configured to perform an estimation of the first control signal Rn[ received, via the optical transmission signal transmitted by the first transmitter 10-n, which provides a first estimated control signal, denoted Rm nb

[0065] In embodiments where the multiplexed optical signal Sio-n received by the intermediate receiver 20-m, transmitted by the first transmitter 10-n, includes a second control signal Rn2, the intermediate receiver 20-m can further be configured to perform an estimation of the second control signal Rn2 received, transmitted by the first transmitter 10-n, which provides a second estimated control signal, denoted Rm n2.

[0066] In certain embodiments where the optical transmission signal Sio_h transmitted by the second transmitter 10-h and received by the intermediate receiver 20-m is a multiplexed optical signal comprising a first control signal Rh[ and / or a second control signal Rh2, the intermediate receiver 20-m can further be configured to perform an estimation of the first and / or second control signal Ru and / or Rh2 received, transmitted by the second transmitter 10-h, which provides a third and / or fourth estimated control signal, denoted respectively Rmh[ and Rmh2.

[0067] Furthermore, the intermediate receiver 20-m is configured to perform a correlated measurement of quantum signals, relative to the quantum signal Qn of the received multiplexed optical signal Si0.n, transmitted by the first transmitter 10-n, and to the quantum signal Qh (i.e., Qh1 or Qh2) of the received optical emission signal Sio_h (i.e., Si0.h1 or Sio_h2, respectively), transmitted by the second transmitter 10-h. The correlated measurement of quantum signals is further performed using the first control signal estimated Rmni by the intermediate receiver 20-m.

[0068] In embodiments the correlated measurement of quantum signals can further be carried out from the second estimated control signal Rmn2, the third estimated control signal Rmhi and / or the fourth estimated control signal Rmh2.

[0069] Each of the quantum particles from the two quantum signals, i.e., Qn and Qh, received separately by the intermediate receiver 20-m, is associated with an independently generated entangled pair of photons. Furthermore, the particles of the first quantum signal Qn 1 of the multiplexed optical signal Si0-ni transmitted by the first emitter 10-n to the receiver 20-m belong to an associated entangled pair to the particles of the second quantum signal Qn2 of the optical emission signal Si0.n2 emitted by the first emitter 10-n. Similarly, the particles of the quantum signal Qh, for example and without limitation of the first quantum signal Qu, of the optical emission signal Sio hi transmitted by the second emitter 10-h to the receiver 20-m, belong to an entangled pair associated with the particles of the second quantum signal Qh2 of the optical emission signal Sio h2 emitted by the second emitter 10-h.

[0070] Advantageously, the correlated measurement of quantum signals performed by the intermediate receiver 20-m can be a Bell measurement, projecting these received quantum particles, Qn and Qh, into an entangled Bell state in polarization. Such a projection induces an entanglement, called 'resultant entanglement', between these received particles.This resulting entanglement consequently induces an entanglement called 'teleported entanglement' (or 'induced entanglement' or 'consequent entanglement'), between the entangled quantum particles and the Qn and Qh particles received by the 20-m receiver, that is respectively between: .

[0071] - the particles of the second quantum signal Qn2 of the optical emission signal Sion2 emitted by the first emitter 10-n (i.e., not received by the receiver 20-m), and the particles of the second quantum signal Qh2 of the optical emission signal Sio h2 emitted by the second emitter 10-h (i.e., not received by the receiver 20-m), if the intermediate receiver 20-m is configured to receive the multiplexed optical signal Sio-hi, or

[0072] - the particles of the second quantum signal Qn2 of the optical emission signal Sio n2 emitted by the first transmitter 10-n (i.e., not received by the receiver 20-m), and the particles of the first quantum signal Qhi of the multiplexed optical signal Sio-hi emitted by the second transmitter 10-h (i.e., not received by the receiver 20-m), if the intermediate receiver 20-m is configured to receive the optical emission signal Si0_h2-

[0073] . In some embodiments, the two final receivers 30-1 and 30-2 (also called "additional receivers" or "auxiliary receivers") of the third set of devices 30 of the system 1 can be configured to each receive a distinct optical emission signal from the transmission channel 50. The two optical emission signals, each received by a final receiver 30-k, are transmitted independently by two distinct transmitters from the first set of devices 10. These optical emission signals are thus not received beforehand by an intermediate receiver. 20-m, any, from the second set of devices 20..

[0074] In embodiments, the first final receiver 30-1 can then be configured to receive an optical emission signal SiO n (and specifically Sio. n2) transmitted by a transmitter 10-n, while the second final receiver 30-2 can be configured to receive an optical emission signal Sio h (and specifically Sio h 2 or Sio-hi) transmitted by another transmitter 10-h of the first set of devices 10. Each 30-k final receiver can thus be configured to perform an estimation of the quantum signal from the received optical emission signal, which provides an estimated received quantum signal, denoted SQk respectively (i.e. SQi for the first 30-1 final receiver or Sq2 for the second 30-2 final receiver).

[0075] For example and without limitation, as illustrated in Figures 1 and 2, the first final receiver 30-1 can be configured to receive the optical emission signal Sim 2 (being optionally a multiplexed optical signal) transmitted by the first transmitter 10-1, and thus be configured to determine the estimated received quantum signal SQ i (i.e. estimation of the second quantum signal Qn from the signal Sim2).

[0076] In the example illustrated in [Fig. 1], the second final receiver 30-2 can be configured to receive the optical emission signal S10-22 (optionally being a multiplexed optical signal) transmitted by the second transmitter 10-2, while in the example illustrated in [Fig. 2], the second final receiver 30-2 can be configured to receive the optical emission signal S10-32 (optionally being a multiplexed optical signal) transmitted by a third transmitter 10-3 of the first set of devices 10. The second final receiver 30-2 can also be configured to determine the estimated received quantum signal SQ2 (i.e., estimation of the second quantum signal Q22 from the signal Si0.22, or Q32 from the signal Si0_32, for example).

[0077] Furthermore, in embodiments where the second set of devices 20 of system 1 comprises a single intermediate receiver, then denoted 20-1, as shown in [Fig. 1], the single receiver 20-1 can be further configured to generate two information signals, each comprising the result of the correlated measurement of quantum signals performed by the single receiver 20-1 (i.e., comprising the result of the projection of the independently received quantum particles into a Bell polarization state). Each information signal, denoted In or I22, comprising the resulting entanglement information from the correlated measurement performed, can be transmitted respectively to one or two final receivers 30-1 or 30-2.

[0078] Alternatively, in embodiments where the second set of devices 20 of system 1 comprises several intermediate receivers, as shown in [Fig. 2], a predetermined receiver 20-m from the second set 20 can be configured to generate a first information signal comprising the result of the correlated measurement of quantum signals, performed by the predetermined receiver 20-m, while another receiver, denoted 20-p, also predetermined from the second set 20, can be configured to generate a second information signal comprising the result of the correlated measurement of quantum signals performed by the other predetermined receiver 20-p. In this case, the index 'p' is an integer between 1 and M and different from the index 'm'. The first information signal, denoted for example Imk, includes resulting entanglement information from the correlated measurement, performed by the predetermined 20-m receiver, and can be transmitted to a 30-k receiver of the two final receivers, while the second information signal, denoted for example Ipq, includes resulting entanglement information from the correlated measurement, performed by the predetermined 20-p receiver, and can be transmitted to the other 30-q receiver of the two final receivers of system 1. In this embodiment, the index 'q' is an integer equal to 1 or 2, and different from the index 'k'.

[0079] Information signals, generally denoted 1^, can be transmitted from an intermediate receiver to a final receiver of system 1, through the transmission channel 50. Furthermore, a resulting entanglement information value, to be included in an information signal to be transmitted, can correspond, for example and without limitation, to an entanglement value equal to 1, associated with photons of received quantum signals Qn and Qh having identical polarization states (i.e. correlated), or alternatively to an entanglement value equal to 0 associated with received photons having different polarization states (i.e. anticorrelated).

[0080] Thus, each final receiver 30-k (30-1 and 30-2) of the third set of devices 30 of the system 1 can be configured to receive a single entanglement information signal Imk transmitted by an intermediate receiver 20-m of the second set of devices 20, and to determine (i.e. deduce) the resulting associated entanglement information or information.

[0081] According to one aspect of the invention, the first final receiver 30-1 and the second final receiver 30-2 of the third set of devices 30 of the system 1 can be configured to determine (i.e., establish) a quantum encryption key, using the received quantum signal SQi estimated by the first final receiver 30-1 and the received quantum signal SQ2 estimated by the second final receiver 30-2. In particular, such a quantum distribution of key(s) is also performed from the entanglement information signals Imi and Im2 (or for example Imi and Iq2) received by the final receivers 30-k, and thus from the associated resulting entanglement information, the two received estimated quantum signals SQk being associated respectively with a teleported entanglement generated by one or more resulting entanglements from one or more intermediate receivers 20-m.System 1 can thus be a quantum key distribution system for encryption via quantum "teleportation". That is to say, System 1 can be configured to perform a quantum distribution of key(s) using one or more quantum repeaters, corresponding respectively to one or more intermediate receivers configured to "repeat" the resulting entanglement information initially originating from several transmitters. of independently generated entangled photon pairs. Quantum key distribution can notably be implemented within a space or terrestrial communication service in order to ensure the security of part or all of the communications exchanged between end receivers, for example.

[0082] Figures 3 and 4 schematically represent a 10-n transmitter of the first set of devices 10 configured to form at least one multiplexed optical signal, according to embodiments of the invention.

[0083] The multiplexed optical signal transmitted by the 10-n transmitter is generated via a signal integrator of the 10-n transmitter, from a quantum signal and at least one control signal, delivered by a signal generator 120.

[0084] In embodiments, the 10-n transmitter may include a single signal integrator, denoted 140-1 (or 140), configured to generate the SiOni multiplexed optical signal.

[0085] Advantageously, the 10-n transmitter can include two signal integration modules, denoted 140-1 and 140-2, each configured to generate a multiplexed optical signal. The first integrator 140-1 can be configured to generate the first multiplexed optical signal Sio-ni and the second integrator 140-2 can be configured to generate the second multiplexed optical signal Sio_n2-

[0086] The single (or first) signal integrator 140-1 is configured to generate the (first) multiplexed optical signal SiO ni, from the first quantum signal Qni and (at least) the control signal Rnb as shown in Figures 3 and 4.

[0087] In certain embodiments where a 10-n transmitter is configured to produce the two control signals Rni and Rn2, as shown in [Fig.4], the first (or only) signal integrator 140-1 can be configured to generate the (first) multiplexed optical signal Sio-ni, in addition from the second control signal Rn2.

[0088] In embodiments, the second signal integrator 140-2, shown in [Fig.4], can be configured to generate the second multiplexed optical signal Sio-n 2, from the second quantum signal Qn 2 and the first control signal Rnl and / or the second control signal Rn2.

[0089] Thus, in other words, an integration module of a 10-n emitter, configured to generate a multiplexed optical signal, is adapted to optically multiplex (or optically combine), on the same optical path, a quantum signal with one or more control signals.

[0090] In embodiments where a 10-n transmitter is a guided optical (or all-optical) device, i.e., comprising optical signal transmission channels made up of optical fibers and / or so-called integrated waveguides, typically used in integrated photonics, one or more transmission means The transmitter may consist of polarization maintaining fiber, or PMF (acronym for the Anglo-Saxon expression Polarization Maintaining Fiber) and / or single-mode optical fibers, or SMF (acronym for the Anglo-Saxon expression Single Mode Fiber).

[0091] In embodiments, a multiplexed optical signal generated by a 10-n transmitter of the first set of devices 10 can be frequency multiplexed. In this case, the signal generator 120 of the transmitter can be configured to generate the entangled quantum signals, Qni and Qn2, of wavelength denoted XQ (or of respective wavelengths ni and XQ n 2, and also called 'quantum wavelength(s)'), and at least the first control signal Rni of control wavelength (or 'reference wavelength' denoted ^Rni), the quantum wavelength(s) being distinct from the control wavelength.

[0092] In some embodiments, the signal generator 120 can be further configured to generate the second control signal Rn 2 with a control wavelength denoted ^Rn 2, the wavelengths XQ, ^Rn[ and ÀRn 2 being all three distinct from each other. Figure 5 schematically represents such a signal generator 120 according to some embodiments of the invention.

[0093] Advantageously, the signal generator 120 may include a first laser source 122-0 emitting a laser beam (or 'pump laser') of wavelength λ. The pump wavelength λ of laser emission may be in the visible or infrared range. For example, and without limitation, the first 122-0 laser source could be a DFB laser diode (Distributed Feedback) using a Bragg grating to select the emission wavelength. The chosen emission wavelength of the laser diode could be, for example, 780 nm. Such a laser diode emits, in particular, a continuous laser beam. Alternatively, the first 122-0 laser source could be a pulsed laser unit, i.e., a gain-switched laser.

[0094] The signal generator 120 may also include one or two additional laser sources, denoted 122-1 and 122-2, as shown in Figure 5, configured to emit a laser beam of wavelength XR1 and a laser beam of wavelength XR2, respectively. The laser emission wavelengths XR1 and XR2 may be in the visible or infrared range. For example, and without limitation, the additional laser source(s) 122-1 and 122-2 may be DFB laser diodes or pulsed laser units.

[0095] According to certain embodiments, the signal generation module 120 may further comprise one or more configured intensity modulation units 124 to modulate the intensity of the laser pulses generated at the output of the first 122-0 laser source and form quantum pulses.

[0096] An intensity modulation unit 124 can also be configured to modulate the rate of laser pulses, from the order of a few kilohertz up to a few tens of gigahertz for example, and / or the time width of laser pulses, for example down to a few nanoseconds.

[0097] As shown in [Fig. 5], the signal generation module 120 may further include an entanglement unit 126, configured to receive a single initial optical signal, denoted Sn0, and to deliver the two entangled quantum signals, corresponding to the first and second quantum signals Qn1 and Qn2, comprising pairs of entangled photons. Such an entanglement unit 126 may advantageously be arranged at the output of an intensity modulation unit 124.

[0098] For example, and without limitation, the entanglement unit 126 can be implemented in the form of a Sagnac loop, in which the passage of the initial optical signal Sn0 through a nonlinear crystal (or a microresonator), in particular in two distinct directions, generates a pair of entangled photons in polarization. Such a nonlinear crystal can be a PPLN crystal (or Periodically poled lithium niobate, according to the corresponding Anglo-Saxon expression).

[0099] Advantageously, the quantum wavelengths kQnl and kQn2 (or XQ) of the entangled quantum signals can be determined as a function of the pump wavelength λpompe of the initial optical signal Sn0. In particular, since energy conservation is observed during the generation of a pair of entangled photons, the sum of the frequencies of the entangled photons is equal to the frequency of the initial pump photon. By way of illustration, for a pump wavelength λ of 780 nm, the quantum wavelengths can be approximately 1560 nm, to maintain energy conservation.

[0100] In embodiments, the frequency difference between a quantum wavelength kQ and a reference wavelength (kR1 and / or XR2) may be greater than or equal to a first minimum value Sk of wavelength difference, according to the following inequality (01):

[0101] |Xq-Xr1 / R2| >ôà (01)

[0102] Furthermore, in embodiments where the generator 120 comprises two separate laser sources 122-1 and 122-2, the frequency difference between the reference wavelengths (XR1 and / or XR2) of each of the control signals Rn1 and Rn2 may be greater than or equal to a second minimum value 5X' of wavelength difference, according to the following inequality (02):

[0103] |^R1'^R2| >SK (02)

[0104] Advantageously, the first minimum value dÀ and the second minimum value <5X' of wavelength difference can be predefined and equal, for example and without limitation, to 1.6nm and 0.8nm respectively.

[0105] In embodiments where the emitter 10-n is a device comprising means for transmitting a signal in free space, an entanglement unit 126 of the signal generator 120 may include one or more dichroic filters enabling in particular to direct the initial optical signal Sn0 towards the Sagnac loop and / or separate (i.e. filter), on two distinct optical paths, the two photons of each entangled photon pair formed to deliver the two entangled quantum signals Qn i and Qn2.

[0106] In embodiments where a multiplexed signal generated by the 10-n transmitter is frequency multiplexed (i.e. the quantum wavelengths XQ and reference XR1 and / or XR2 are distinct from each other), a signal integrator (140-1 and / or 140-2) of the 10-n transmitter may include one or two Wavelength Division Multiplexing (WDM) units, each unit being adapted to combine a considered quantum signal and one of the control signals (Rn i or Rn 2) on the same optical path into a resulting signal.

[0107] In embodiments, such a signal integrator may alternatively comprise one or two dichroic filters, each filter being adapted to combine a considered quantum signal and one of the control signals (Rn[ or Rn2) on the same optical path into a resulting signal.

[0108] In embodiments, a multiplexed optical signal generated by a 10-n emitter of the first set of devices 10 can be time-multiplexed. In this case, such a multiplexed signal can be a signal comprising a set of two or three temporally distinct pulses, the set being repeated according to a period T, the distinct pulses corresponding respectively to an entangled quantum signal, Qni or Qn2, a first control signal Rn i and / or a second control signal Rn 2.

[0109] Advantageously, the entangled quantum signals and the control signals generated by the signal generator 120 can be impulse signals characterized by a period T identical to the period of the multiplexed signal delivered by the transmitter 10-n.

[0110] In some embodiments, the signal generator 120 can be configured to generate the entangled particle pair (Qn1 and Qn2) and the control signals Rn1 and Rn2 according to a predefined time offset between each pulse. Alternatively (or in addition), a signal integrator (140-1 and / or 140-2) can be configured to apply a predefined time offset between a quantum signal and a control signal in order to obtain time-multiplexed signal pulses.

[0111] It should be noted that in embodiments involving time multiplexing, the entangled particles (Qni and Qn2) of the same pair are not time-shifted.

[0112] The resulting time difference between each of the successive distinct pulses in a multiplexed signal can thus be strictly less than the repetition period T of the resulting signal (or quantum signal), according to the following inequalities (03) and (04):

[0113] |tQ-tR1 / R2| <T(03)

[0114] |tR1-tR2| <T(04)

[0115] In these embodiments where the multiplexed signal generated by a 10-n transmitter is time-multiplexed, the quantum XQ and reference wavelengths (XR1 and / or XR2) can be equal to each other.

[0116] In this case, the first laser source 122-0 and the additional laser source(s) 122-1 and 122-2 may, for example, correspond to a single laser source 122-0, and the signal generator 120 may further include a beam splitter unit (not shown in the figures) configured to provide one or two signal components associated with the control signals Rn i and Rn 2, as well as another signal component associated with the initial optical signal Sn0. Such a beam splitter unit may include one or more optical couplers, symmetrical or asymmetrical, for example, polarization-maintaining. The beam splitter unit may further be an optical selector generating a predefined time offset between each delivered signal component.

[0117] In some embodiments, the beam splitting unit of the signal generator 120 can be arranged at the output of the first laser source 122-0, the resulting control signals Rn i and Rn 2 then corresponding to classical (i.e., non-quantum) light pulse signals. Alternatively, this beam splitting unit can be arranged at the output of an intensity modulation unit 124, the resulting control signals Rn i and Rn 2 then corresponding to low-intensity light signals.

[0118] Figures 6 and 7 schematically represent an intermediate 20-m receiver comprising at least one received multiplexed optical signal processing chain, and a correlation module 260, according to embodiments.

[0119] The correlation module 260 of a 20-m intermediate receiver is configured to perform a correlated measurement of a first received quantum signal Qmi emanating from a first emitter 10-n, with a second received quantum signal Qm 2 from a second emitter 10-h.

[0120] In particular, the correlation module 260 can be implemented in the form of an optical instrument, such as an optical interferometer, to perform a Bell measurement (corresponding to a Bell measurement module). Such a correlation module 260 includes, in particular, a plurality of detection units. Each detection unit can be adapted to measure one or more quantum signals according to a predefined measurement polarization state, for example, in a B26D polarization basis (also called a 'measurement polarization basis' and corresponding, for example, without limitation, to an H / V basis or a D / A basis).

[0121] A processing chain of an intermediate receiver 20-m, denoted Cm, is configured to receive a multiplexed optical signal and deliver a received quantum signal Qm. For example, as shown in Figures 6 and 7, a processing chain Cm can be configured to receive the multiplexed optical signal SiO n or SiO ni) transmitted by a first transmitter 10-n and deliver the first received quantum signal Qmi associated with the signal Qn (or Qni) transmitted by the first transmitter 10-n via the SiO n signal.

[0122] A processing chain Cm is further adapted to determine the polarization state of the control signal(s) from the received multiplexed optical signal. For example, the processing chain Cm can be associated with the first control polarization base BQ ni and adapted to measure in this base the polarization state of the first control signal Rni transmitted by the first transmitter 10-n via the multiplexed optical signal SiO-n. The processing chain Cm can further be associated with the second control polarization base BQ n 2 and adapted to measure in this second base the polarization state of the second control signal Rn 2 transmitted by the first transmitter 10-n via the multiplexed optical signal SiO n.

[0123] A processing chain Cm is further adapted to align a given polarization state with one of the predefined measurement polarization states of the correlation module 260. In some embodiments, the measurement polarization basis B260 may correspond to the first control polarization basis BQn. Alternatively, the measurement polarization basis B260 may correspond to the second control polarization basis Bqn. As used here, the expression 'alignment of a polarization state to a polarization basis' refers to a rotation of the signal's polarization state to correspond to a detection eigenaxis of the basis determined by quantum signal detection equipment.

[0124] Thus, a Cm processing chain of an intermediate 20-m receiver can be adapted to perform a rotation of the polarization state of the entire signal set derived from the received multiplexed optical signal, at the input of the Cm processing chain, to ensure that the particles of the quantum signal Qm at the output of the Cm processing chain are aligned to the measurement polarization basis B26o •

[0125] In embodiments, as shown in [Fig.7], an intermediate receiver 20-m may include two processing chains Cm configured to process either the multiplexed optical signal SiO n transmitted by the first transmitter 10-n, or the multiplexed optical signal Sio h transmitted by the second transmitter 10-h.

[0126] In other embodiments, an intermediate receiver 20-m may comprise a single processing chain Cm configured to process the multiplexed optical signal Si0.n transmitted by the first transmitter 10-n, as shown in [Fig. 6]. In this case, the optical emission signal Sio h transmitted by the second transmitter 10-h is not a multiplexed signal and comprises only a quantum signal Qh, corresponding directly to the second received quantum signal Qm2 from a second transmitter 10-h.

[0127] Advantageously, a processing chain Cm may include a feedback loop between a polarization state correction module 220 and a control signal detection module 240. As shown in [Fig. 8], the correction module 220 may be arranged upstream of the detection module 240.

[0128] A correction module 220 of an intermediate receiver 20-m can be configured to modify the polarization of a signal passing through it, in response to a setpoint signal. The correction module 220 can therefore be configured to receive a multiplexed optical signal (for example Si0.n, or more precisely SiO ni, transmitted by a first transmitter 10-n) and deliver a multiplexed optical signal with modified polarization, denoted Sm.

[0129] The setpoint signal of the correction module 220 can be an electrical or radio frequency signal, for example. Advantageously, a correction module 220 can be a fiber-optic polarization controller comprising, in particular, one or more polarization-rotating fibers whose stress axis(es) (adapted to rotate the signal polarization) are controlled (or adjusted) from the setpoint signal. For example, and without limitation, such a controllable stress axis can be implemented as a coiled fiber component with adjustable geometry, or using a piezoelectric element that induces mechanical stresses on a fiber. Alternatively, a correction module 220 can comprise one or more so-called active delay blades, i.e., whose blade rotation (i.e., of its optical axis) is controlled (or adjusted) from the setpoint signal.The correction module 220 can be, for example and without limitation, a triplet of delay blades. active plates comprising successively a quarter-wave plate, a half-wave plate and a quarter-wave plate.

[0130] In embodiments, the transmission means of an intermediate 20-m receiver, and in particular of a Cm processing chain, can be single-mode optical fibers SMF, and / or advantageously, polarization-maintaining fibers PMF.

[0131] Figures 9 and 10 schematically represent a detection module 240 of a processing chain Cm comprising a signal demultiplexing unit 242 and a DA analysis device for polarizing the control signal, according to embodiments of the invention.

[0132] The signal demultiplexing unit 242 can receive as input the modified polarization multiplexed optical signal Sm and can be configured to separate from this signal, a received quantum signal component Qm relating to the quantum signal from the received multiplexed signal, and respectively one or two received control signal components Rm relating to the control signal or signals from the received multiplexed signal. For example, for a received multiplexed optical signal Si0.n transmitted by a first transmitter 10-n and comprising the quantum signal Qn (or Qni) and the control signal Rn[, the demultiplexing unit 242 can be configured to determine the quantum signal component Qm relating to the quantum signal Qn i and the control signal component Rm relating to the control signal Rn[.

[0133] The demultiplexed quantum component Qm of the signal Sm at the output of unit 242 can then be routed to the correlation module 260 of the intermediate receiver 20-m, while the control component Rm can be routed to a DA analysis device.

[0134] In embodiments where the received multiplexed optical signal Si0.n comprises the quantum signal Qni and the two control signals Rn[ and Rn2, the demultiplexing unit 242 can be configured to separate the quantum component Qm related to the quantum signal Qnb, a first control signal component Rmi related to the first control signal Rn[, and a second control signal component Rm2 related to the second control signal Rn2. In this case, the first control component Rm[ can be routed to a first analysis device, while the second control component Rm2 can be routed to a second analysis device (not shown in the figures).The two analysis devices are configured analogously, each adapted to the characteristics of the control component to be processed, i.e., the control polarization basis (BQni and BQn2) to be used and optionally to the reference wavelength considered. The use of two separate analysis devices. This allows for a better estimation of the polarization rotations (or distortions) undergone by the multiplexed SiO₂ optical signal between transmission and reception on the transmission channel. For example, and without limitation, the second analysis device can be used to confirm the polarization analysis of the control signal determined by the first analysis device.

[0135] In embodiments involving time-division multiplexing where the received multiplexed optical signal Si0.n comprises the quantum signal Q„i and the two control signals Rni and Rn2, the first and second control signal components Rm[ and Rm2 demultiplexed can be routed to a single analysis device, configured to alternately analyze these components as a function of the time difference between successive distinct pulses associated with the control signals.

[0136] The demultiplexing unit 242 of a detection module 240 may in particular include one or more demultiplexing elements determined according to the type of multiplexing of the signal by the transmitter considered, i.e. frequency and / or time.

[0137] In embodiments where the received multiplexed signal is frequency-multiplexed, the demultiplexing unit 242 may include at least one filter configured to separate the quantum component Qm from a control component Rm. For example, and without limitation, such a filter may be a band-rejection filter such as an FBG (Fiber Bragg Grating) filter or a filter called "Add / Drop WDM". The filter may be chosen from the predetermined frequency difference between the quantum wavelength λ and a reference wavelength (XR1 and / or λR2), defined, for example, by equation (01).

[0138] In embodiments, the demultiplexing unit 242 may further include a filter configured to separate control components from each other. Such a filter may be chosen from the predetermined frequency difference between the reference wavelengths XR1 and ^R2 of each of the control signals, and defined for example by equation (02).

[0139] The transmission means 240-i and 242-i at the output of the demultiplexing unit 242 to the correlation module 280 and the DA analysis device respectively, as well as transmission means included in the unit 242 (not shown in the figures), may be single-mode SMF optical fibers. Advantageously, these transmission means may be polarization-maintaining PMF fibers.

[0140] A DA analysis device for polarizing the control signal of a processing chain Cm can be configured to detect the control component Rm to be processed, based on a predefined polarization basis, so as to provide an estimated control signal.

[0141] It should be noted that at the output of a 10-n transmitter, a control signal (Rn i and / or Rn 2 ) is characterized by its well-defined polarization state. During the propagation of a multiplexed signal between the 10-n transmitter and an intermediate 20-m receiver, the polarization state of the control signal in question may have undergone random rotations such that the polarization state of the received and detected control component Rm, relative to the control signal in question, may be different from the initially defined polarization state.

[0142] Thus, the DA analysis device for control signal polarization may include at least one detection unit configured to detect the control signal in question, in particular according to a predefined polarization, so as to provide the estimate of the received control signal (for example Rmni, or Rmn 2 respectively).

[0143] In embodiments, a detection unit of the DA analysis device can be adapted to detect conventional light pulse signals. For example, and without limitation, such a unit can be a photodiode configured to deliver a photocurrent, depending on the measurement of the received control signal component associated with the Cm processing chain.

[0144] Alternatively, a detection unit of the DA analysis device may be a single-photon detection unit. Such a unit may consist of a detection surface configured to detect the "presence" of single photons at its detection surface (i.e., by photon / surface interaction). This detection of the presence of single photons is defined according to a given quantum detection efficiency. For example, and without limitation, the single-photon detection unit may be an avalanche photodiode or APD (acronym for Avalanche Photodiode Detectof) or a superconducting nanowire single-photon detector or SNSPD (acronym for Superconducting Nanowire Single Photon Detectof).In particular, the single-photon detection unit may include an internal amplification mechanism configured to deliver a voltage when a photon is detected.

[0145] In certain embodiments, as shown in [Fig. 9], the DA analysis device may comprise a polarizer 244-A and a single detection unit 246 of the control component Rm associated with the control signal under consideration, for example Rn[ (or Rn 2). The polarizer 244-A (also called a 'polarizing filter') may be arranged to transmit to the detection unit 246 only the optical signals defined in a predefined polarization state. The detection unit 246 is thus configured to detect the light energy relative to the control component Rm defined solely according to the considered polarization state (i.e., the predefined polarization base processed by the DA analysis device), and to provide an estimate of the received control signal, for example, Rmni (or Rmn2, respectively). According to this configuration, the value of the control signal detected (or measured) by the detection unit 246 is maximum if the polarization state of the received control signal component is equal to the predefined polarization state. Conversely, the value of the detected control signal may be minimum if the polarization state of the received control signal component is orthogonal to the predefined polarization state.

[0146] Advantageously, the DA analysis device may include a polarizing detection unit directly combining (i.e. combining) the functionalities of the polarizer 244-A and the detection unit 246.

[0147] In embodiments, as shown in [Fig. 10], the DA analysis device may comprise a polarized beam splitter 244-B, preceded by two separate detection units 246-1 and 246-2. The polarized beam splitter 244-B (also called a 'polarizing splitter') may be adapted to provide two polarized signal subcomponents relative to the control component Rm under consideration. Each subcomponent may propagate over a transmission means (244-11 or 244-12) from the splitter 244-B to one of the two detection units (246-1 or 246-2), which is then defined only in one of the two polarization states of the predefined polarization base processed by the DA analysis device.Each detection unit (246-1 and 246-2) is thus configured to detect the light energy relative to one of the two polarized subcomponents of the control component Rm to provide an estimate of the received control signal. For example, and without limitation, according to this configuration, the value relative to the estimated control signal, measured by the first detection unit 246-1, can be maximum, and the value measured by the second detection unit 246-2 can be minimum, if the polarization state of the control component Rm is equal to the initial polarization state of the control signal emitted by the transmitter.Conversely, the value relative to the estimated control signal, measured by the first detection unit 246-1, may be minimum and the value measured by the second detection unit 246-2 may be maximum, if the polarization state of the control component Rm is orthogonal to the initial polarization state of the considered control signal emitted by the transmitter.

[0148] For example, and without limitation, a DA device for polarization analysis of a received Rm control component associated with the H / V diagonal basis may include a 244-B separation unit configured to provide a first subcomponent having a linear H-type polarization propagating over the transmission medium 244-il, and a second sub-component having a linear polarization of type V propagating on the transmission medium 244-i2. In this example, the two corresponding detection units 246-1 and 246-2 are therefore configured to detect respectively the sub-component relating to the linear polarization of type H of the control component Rm and the sub-component relating to the linear polarization of type V of the control component Rm.

[0149] In some embodiments, the detection unit(s) (246, or 246-1 and 246-2) of a DA analysis device can be adapted to the reference wavelength XRx (i.e. AR1 or AR2) of the control component Rm to be detected.

[0150] The intermediate receiver 20-m may further include one or more processors (also called 'central processing units') or CPUs (acronym for the Anglo-Saxon expression Central Processing Unit).

[0151] In some embodiments, each DA analysis device of the receiver 20-m may include a specific processor, generally denoted 248, configured to analyze the electrical signal(s) from the sensing unit(s) (246, or 246-1 and 246-2) corresponding to the estimated control signal associated with the DA analysis device. A processor 248 may be configured to generate a feedback signal, denoted Sc, corresponding to a polarization correction setpoint signal to be delivered to the correction module 220 associated with the processing chain Cm.

[0152] In certain embodiments, the intermediate receiver 20-m may comprise a single processor 248, configured to analyze all the electrical signals from the sensing units of the analysis devices of the receiver 20-m. In this case, the processor 248 may be configured to generate a servo signal Sc specific to each correction module 220 of a processing chain Cm.

[0153] A feedback loop of a processing chain Cm (i.e., a bias correction loop generating a feedback signal) can be implemented continuously or intermittently. A processor 248 can thus be configured to control the feedback loop(s) of the intermediate receiver 20-m. In particular, a feedback loop can be activated periodically and / or after evaluating the bias state of one or both of the estimated control signals with respect to one or more associated bias base(s). Furthermore, a feedback loop can be implemented until the bias state of one or both of the estimated control signals is aligned with a chosen associated (or reference) bias state and / or within a chosen bias base.

[0154] In embodiments, a processor 248 of the intermediate receiver 20-m can be configured to determine for a specific correction device D a polarization state difference value <5p between the polarization state of the considered estimated control signal and the polarization state of the polarization base associated with the correction device D. The processor 248 can further be configured to evaluate whether this polarization state difference value <5p is strictly greater than (or greater than or equal to) a predefined reference difference value <5pref.

[0155] In particular, a servo loop can be activated if a determined polarization state difference value dp is greater than or equal to the reference difference value <5pref.

[0156] Advantageously, a control loop can be implemented so as to optimize (i.e. maximize or minimize) the detection of the component of the control signal(s) according to the associated polarization state(s).

[0157] For example, and without limitation, a feedback signal for a feedback loop can be generated from a differentiable optimization algorithm, such as a gradient descent algorithm, so as to search (by increment or iteration) for an optimal point of an objective function associated in particular with the determined polarization state difference value of a correction device D of the receiver 20-m. If an optimal point is found, the feedback loop can be stopped.

[0158] The control loop can also be stopped, for example and without limitation, if a determined polarization state difference value ôp is evaluated as strictly less than (or less than or equal to) the reference difference value dp ref*

[0159] Thus, in some embodiments, a feedback signal Sc relative to the setpoint signal of a correction module 220 can be generated to control this module 220 and, in particular, to rotate the polarization of the received multiplexed signal until the value of the estimated control signal, measured by the detection unit 240 (for example, via the first detection unit 246-1), is optimal, that is, until the polarization state of the control component Rm is equal to the initial polarization state of the control signal considered, emitted by the transmitter, or alternatively orthogonal to it. The modification of the polarization of the received multiplexed signal via the feedback signal Sc thus induces a modification of the polarization state of the demultiplexed quantum signal Qm of the signal Sm, at the output of the unit 242, and routed to the correlation module 260 of the receiver 20-m.

[0160] Furthermore, the detection units of the correlation module 260 of a 20-m intermediate receiver can be single-photon detection units. The detection of quantum components, Qmi and Qm2, allows to provide at least one entanglement information signal Imk to be provided respectively to a final receiver 30-k of the third set of devices 30 of system 1.

[0161] In some embodiments, a correlation module 260 of an intermediate receiver 20-m may include at its input one or two additional demultiplexing units (not shown in the figures). Each additional demultiplexing unit of the correlation module 260 is associated with one of the quantum components, Qmi or Qm2, and may be equivalent to the signal demultiplexing unit 242 of the detection module 240 of a processing chain Cm, and configured to transmit the quantum component in question to the Bell measurement module, for example. The residual component(s) of the control signals from the demultiplexing unit are then directed to a beam absorber.

[0162] The additional demultiplexing unit of the correlation module 260 may in particular include a demultiplexing element determined according to the type of multiplexing of the signal Sm. For example, for frequency multiplexing, the additional demultiplexing unit may be a spectral filter configured to separate a quantum signal Qm from the two residual components of the integrated signals, such as an FBG filter or an "Add / Drop WDM" filter, and chosen from the predetermined frequency difference between the quantum wavelength AQ and the reference wavelengths (kR1 and / or AR2).

[0163] Such an additional demultiplexing unit in the correlation module 260 makes it possible in particular to increase the filtering capacity of the control signal(s) of the multiplexed signal(s) received at the input of the intermediate receiver 20-m, in order to improve the quantum correlation measurement performed by the module 260.

[0164] Figure 11 schematically represents a 30-kΩ final receiver comprising a received multiplexed optical signal processing chain and a 360 quantum photon analysis module, according to embodiments. In this case, the 30-kΩ final receiver can be configured to receive an optical transmission signal being a multiplexed SiO₂ signal transmitted by a 10⁻ⁿ transmitter of the first set of devices 10. Such a signal can, for example, be the multiplexed Si₀ₙ₂ signal comprising a quantum signal Qₙ₂ and a first control signal Rₙ₋₁ and / or a second control signal Rₙ₂.

[0165] A processing chain of a 30-k final receiver, denoted Ck, can be configured to receive a multiplexed optical signal and deliver a received quantum signal Qk relative to the quantum signal from the multiplexed signal Si0_n. The 360 ​​quantum photon analysis module of a 30-k final receiver can be adapted to measure the quantum signal received, according to at least one polarization state defined in a polarization basis.

[0166] The Ck processing chain can be adapted to determine the polarization state of the control signal(s) from the multiplexed optical signal received by the final 30-k receiver. The Ck processing chain can be further adapted to align this determined polarization state with one of the predefined measurement polarization states of the 360 ​​analysis module. Thus, the Ck processing chain can be adapted to perform a rotation of the polarization state of all the signals from the received multiplexed optical signal, at the input of the Ck processing chain, to ensure that the particles of the received quantum signal Ck at the output of the Ck processing chain are properly aligned in the polarization basis associated with the 360 ​​analysis module. The Ck processing chain can thus be equivalent (understand similar units) to a Cm processing chain of an intermediate 20-m receiver, as illustrated in [Fig. 8].

[0167] In particular, the Ck processing chain of a 30-k final receiver may include a feedback loop between a polarization state correction module arranged upstream of a control signal detection module.

[0168] The correction module of the processing chain Ck (equivalent to the correction module 220 of a processing chain Cm of a 20-m receiver) can be configured to modify the polarization of a signal passing through it, in response to a setpoint signal. The correction module is therefore configured to receive a multiplexed optical signal SiO n received by the final receiver 30-k (for example Si0.n 2 transmitted by a first transmitter 10-n) and deliver a multiplexed optical signal with modified polarization, denoted Sk for example.

[0169] The detection module of the processing chain Ck (equivalent to the correction module 240 of a processing chain Cm of a 20-m receiver and as illustrated in Figures 9 and 10) may include a signal demultiplexing unit and at least one control signal polarization analysis device. The demultiplexing unit may be configured to separate the modified-polarization multiplexed optical signal into at least one received quantum signal component Qk, which is then routed to the quantum photon analysis module 360. A control signal polarization analysis device in the processing chain Ck may be configured to process a received control signal component from the modified-polarization multiplexed optical signal, in particular to generate a feedback signal corresponding to the polarization correction setpoint signal to be delivered to the correction module of the processing chain Ck.

[0170] In embodiments where a 30-k final receiver is configured to receive an optical emission signal comprising only a quantum signal (i.e., not multiplexed), the optical emission signal Si0.n then corresponds directly to the received quantum signal Qk to be processed by the 360 ​​analysis module.

[0171] The 360 ​​analysis module of a 30-k final receiver may include at least one single-photon detection unit. Detection of the received quantum signal Qk makes it possible to provide the estimated received quantum signal SQk and thus an estimate of the polarization state of the signal with respect to one or more polarization states defined in a polarization basis.

[0172] In some embodiments, the analysis module 360 ​​may include a single single-photon detection unit 366 configured to detect the received quantum signal Qk defined according to a predetermined polarization state.

[0173] In other embodiments, as shown in [Fig. 12], the analysis module 360 ​​may comprise a switching unit 364, preceded by two single-photon detection units 366-1 and 366-2. The switching unit 364 may be equivalent to the polarized beam splitter 244-B of a DA analysis device included in an intermediate receiver 20-m. The switching unit 364 may therefore be adapted to direct (i.e., route or switch) the demultiplexed quantum signal Qk from the multiplexed SiO₂ signal received at the input of the final receiver 30-k to one of the two single-photon detection units (366-1 or 366-2) depending on the polarization state of the quantum signal. Each single photon detection unit (366-1 and 366-2) is thus configured to detect the presence of single photons defined according to one of the polarization states of the polarization base, processed by the analysis module 360 ​​(i.e. the final 30-k receiver considered)

[0174] By way of illustration, for an analysis module 360 ​​associated, for example and without limitation, with the H / V basis, may include the switching unit 364 configured to direct the received quantum signal Qk having a horizontal linear polarization H, to the first single photon detection unit 366-1 via the transmission means 364-11, or to direct the received quantum signal Qk having a vertical polarization V, to the second single photon detection unit 366-2 via the transmission means 364-12.

[0175] In embodiments where the SiO₂ emission optical signal received by the final receiver 30-k is a multiplexed signal, the analysis module 360 ​​may include at its input an additional demultiplexing unit 362, equivalent to a signal demultiplexing unit 242 of a processing chain Cm, or to an additional demultiplexing unit of the correlation module 260, included in an intermediate receiver 20-m. The additional demultiplexing unit 362 may thus be configured to transmit the received quantum signal Qk to the single-photon detection unit 366, or to the switching unit 364, as shown in the [Fig. 12]. The residual components of the control signals are then directed towards a beam absorber 362-0 as shown in [Fig. 12].

[0176] Furthermore, the analysis module 360 ​​of a 30-k final receiver may include a processor 368 configured to analyze the electrical signal(s) from the sensing unit(s) (366, or 366-1 and 366-2) corresponding to the estimated received quantum signal SQk. The processor 368 of a 30-k final receiver may be configured to determine a quantum encryption key, i.e., a key shared with the other by the 30-q final receiver, from the estimated received quantum signal SQk and the entanglement information signal Imk received by the 30-k final receiver.

[0177] The determination of a quantum encryption key can be carried out from a quantum key distribution protocol, such as for example a protocol comparable to the protocol named BBM92 (as described in the article “Quantum cryptography without Bell's theorem” by C. Bennett, G. Brassard and D. Mermin, 1992, Physical Review Letters 68 (5), p. 557-559).

[0178] In embodiments such as, for example, when the control signals are quantum signals and the multiplexed signal received by the final 30-k receiver is a time-division multiplexed signal, the processing chain Ck may consist of a polarization state correction module and at least part of the analysis module 360. In this case, the module 360, configured to detect the received quantum signal Qk, may be further configured to detect one or more of the control signals from the multiplexed optical signal received by the final 30-k receiver. In this case, the processor 368 may also be configured to generate a feedback signal corresponding to a polarization correction setpoint signal to be delivered to the correction module of the processing chain Ck.

[0179] The [Fig. 13] represents the optical signal emission method implemented by a 10-n transmitter, according to embodiments of the invention.

[0180] The optical signal emission method includes a preliminary step 1020 of generating two entangled quantum signals Qn[ and Qn2, as well as at least one polarization control optical signal Rni.

[0181] At step 1040, the optical polarization control signal Rn[ is inserted onto the optical path carrying the first entangled quantum signal Qn[ so as to generate a multiplexed optical signal Sio_ni-

[0182] At step 1060, the multiplexed optical signal SiO ni and an optical emission signal comprising the second quantum signal Qn2, are transmitted through a transmission channel 50.

[0183] Figure 14 represents the intermediate optical signal reception method implemented by an intermediate receiver 20-n, according to embodiments of the invention.

[0184] The receiving method includes a preliminary step 2020 of receiving a multiplexed SiO n (or SiO ni) optical signal and an optical transmission signal, each comprising an entangled quantum signal emitted independently, respectively by two separate transmitters, and transmitted through a transmission channel 50.

[0185] At step 2040, the multiplexed optical signal SiO ni is directed to a processing chain Cm, associated with a predefined polarization base.

[0186] The intermediate optical signal reception method further includes, for the processing chain Cm, a feedback loop between steps 2042 and 2044. Step 2044 corresponds to determining the polarization state of a component of the received multiplexed optical signal traveling through the chain and relative to a control signal Rni emitted by the transmitter 10-n, and step 2042 corresponds to modifying the polarization of the received multiplexed optical signal according to the determined polarization state. The feedback loop between steps 2042 and 2044 is stopped when the polarization state determined in step 2044 is aligned with one of the polarization states of the predefined basis for the processing chain Cm.

[0187] At step 2060, an interferometric measurement is performed to project onto an entangled polarization state (relative to a measurement polarization basis) the particles associated respectively with the quantum component of the received multiplexed optical signal relative to the entangled quantum signal Qni, and with the entangled quantum signal from the received emission optical signal.

[0188] At step 2080, at least one information signal 1^ is generated from entanglement information of determined polarization states of received quantum signals, and then transmitted through a transmission channel 50.

[0189] Figure 14 represents the final method for receiving optical signals put into work by a final receiver 30-n, according to embodiments of the invention.

[0190] In embodiments, the final optical signal reception process may include a preliminary step 3020 of receiving a multiplexed optical signal Sio-n (or Sio-n 2) comprising an entangled quantum signal emitted by a transmitter, as well as an Imk information signal of quantum signal polarization state entanglement, transmitted through a transmission channel 50.

[0191] At step 3040, the multiplexed optical signal Si0 n2 can be directed to a processing chain Ck, associated with a predefined biasing basis.

[0192] The final optical signal reception process may further include, for the Ck processing chain, a servo loop between steps 3042 and 3044. Step 3044 involves determining the polarization state of a component of the received multiplexed optical signal traveling through the chain and relative to a control signal Rni emitted by the transmitter 10-n, and step 3042 involves modifying the polarization of the received multiplexed optical signal according to the determined polarization state. The feedback loop between steps 3042 and 3044 is stopped when the polarization state determined in step 3044 is aligned with one of the polarization states of the predefined basis for the processing chain Ck.

[0193] At step 3060, the polarization state of the quantum component of the received multiplexed optical signal relative to the entangled quantum signal Qn 2 can be determined.

[0194] At step 3080, a quantum encryption key, shared with other final receiver 30-q, can be determined from the polarization state of the quantum component of the received multiplexed optical signal relative to the entangled quantum signal Qn2 and the entanglement information of the polarization states of the quantum signals from the information signal 1^.

[0195] A person skilled in the art will readily understand that certain steps in the transmission and reception processes can be carried out respectively simultaneously, sequentially, independently or not, and / or in a different order, for example in an order defined by a given transmitter and receiver.

[0196] The quantum system or subsystems of the system (emitters and receivers), as well as the methods described above, according to embodiments of the invention, can be implemented in various ways by hardware, or a combination of hardware and software, in particular in the form of program code that can be distributed as a program product in various forms. The program code can be distributed using computer-readable media, which may include computer-readable storage media and communication media. The methods described herein can, in particular, be implemented in the form of computer program instructions executable by one or more processors in a computer system. These computer program instructions can also be stored in computer-readable media.

[0197] The invention is not limited to the embodiments described above by way of non-limiting example. It encompasses all embodiment variations that can be envisaged by a person skilled in the art. In particular, a person skilled in the art will understand that the invention is not limited to the various modules of the emitters and receivers of the quantum system described by way of non-limiting example.

Claims

1.

2. Demands Receiver (20-m) configured to receive a multiplexed optical signal (SiO n) and an optical signal (Sio-h) transmitted independently through a transmission channel (50), said multiplexed optical signal (SiO-n) comprising a first quantum signal (Qn), said optical signal (Sio h) comprising a second quantum signal (Qh), said multiplexed optical signal (SiO.n) further comprising at least one polarization state control signal (Rni), said receiver (20-m) comprising a processing chain (Cm) associated with a polarization base composed of at least one polarization state and adapted to determine the polarization state of said at least one polarization state control signal (Rnl) and to modify the polarization of said multiplexed optical signal (Sio-n) so as to align said determined polarization state with respect to one of said at least one polarization state of said associated base,said receiver (20-m) further comprising a correlation module (260) adapted to perform a correlation measurement between said first quantum signal (Q„i) from said multiplexed optical signal (SiO n) with modified polarization and said second quantum signal (Qh), said correlation module (260) being associated with said polarization basis, said correlation module (260) further being adapted to generate at least one information signal (Imk) from said correlation measurement, said information signal (I^) comprising entanglement information of the polarization states of said first and second quantum signals (Qn, Qh). Receiver (20-m), according to claim 1, wherein said processing chain (Cm) comprises a control signal detection module (240) and an analysis device, the control signal detection module (240) being configured to demultiplex said at least one control signal (Rnl) and said first quantum signal (Qn) from said multiplexed optical signal (SiO n), said detection module (240) being further configured to route said demultiplexed control signal (R„i) to the polarization analysis device (DA), the polarization analysis device (DA) comprising at least one detection unit (246) adapted to detect said control signal (Rnl) according to one of said at least one polarization state of said associated base.

3. Receiver (20-m), according to claim 2, wherein said detection module (240) further comprises a processor (248) configured to analyze said determined polarization state and to generate a servo signal (Sc) applied to a polarization correction module (220) of said multiplexed optical signal (SiO n).

4. Receiver (20-m), according to any one of claims 1 to 3, wherein said receiver (20-m) is formed from polarization-maintaining fibers (PMF) and / or single-mode optical fibers (SMF).

5. Transmitter (10-n) configured to emit optical signals comprising: - a signal generator (120) configured to generate a first quantum signal (Qni), a second quantum signal (Qn2), and a polarization state control signal (Rni), said first quantum signal (Qni) and said second quantum signal (Qn2) being entangled quantum signals, - a signal integrator (140) configured to generate a multiplexed optical signal (Sio-ni), the multiplexed signal comprising said first control signal (Rnl) and said first quantum signal (Q„i), said transmitter (10-n) being configured to transmit through a transmission channel (50), said multiplexed optical signal (SiO ni) and an optical signal (Sio-n2) comprising said second quantum signal (Qn2).

6. Quantum communication system (1) comprising a plurality of transmitters (10-n) according to claim 5, and at least one receiver (20-m) according to any one of claims 1 to 4.

7. System (1), according to claim 6, wherein said plurality of emitters comprises at least a first emitter and a second emitter, and said system (1) further comprises a plurality of auxiliary receivers comprising a first auxiliary receiver (30-1) configured to receive an optical signal (SiO n) comprising a quantum signal (Qn) transmitted by the first emitter (10-n), and a second auxiliary receiver (30-2) configured to receive an optical signal (SiO_h) comprising a quantum signal (Qh) transmitted by the second emitter (10-h), each auxiliary receiver (30-1; 30-2) being associated with a measurement polarization basis composed of at least one polarization state and adapted to measure said associated quantum signal (Qn; Qh) according to at least one of said at least one polarization state of said associated measurement polarization basis, and wherein each auxiliary receiver (30-1, 30-2) is configured to receive an entanglement information signal (Imi; Im2 or Ip2) comprising entanglement information of polarization states of quantum signals transmitted by said at least one receiver (20-m; 20-p), each auxiliary receiver (30-1; 30-2) being configured to determine a shared quantum encryption key from said measurement of said associated quantum signal (Qn; Qh) and said entanglement information of polarization states of quantum signals.

8. System (1), according to claim 7, wherein, for one or both auxiliary receivers (30-1, 30-2), said optical signal (Si0.n; Sio-h) received by said auxiliary receiver (30-1; 30-2) is a multiplexed optical signal further comprising a polarization state control signal (Rnl), said auxiliary receiver(s) (30-1; 30-2) comprising a processing chain (Ck) associated with said measurement polarization base and adapted to determine the polarization state of said polarization state control signal (Rni) and to modify the polarization of said multiplexed optical signal (Si0.n; Sio-h) so as to align said determined polarization state with respect to at least one of said polarization states of said associated measurement polarization base.

9. System (1), according to any one of claims 6 to 8, wherein said multiplexed signals are frequency-multiplexed signals.

10. System (1), according to claim 9, wherein the absolute value of the wavelength difference between the quantum wavelength (XQ) of a quantum signal and the reference wavelength (kR) of a control signal is greater than or equal to a minimum wavelength difference value (ÔX).

11. A method for determining at least one information signal in response to the reception of a multiplexed optical signal (Si0.n) and an optical signal (Si0_h) transmitted independently through a transmission channel (50), said multiplexed optical signal (Si0_n) comprising a first quantum signal (Qn), said optical signal (SiO-h) comprising a second quantum signal (Qh), said multiplexed optical signal (SiO-n) further comprising at least one polarization state control signal (Rnl), said method comprising a processing phase (Cm), associated with a polarization basis composed of at least one polarization state, for determining the polarization state of said at least one polarization state control signal (Rni) and for modifying the polarization of said multiplexed optical signal (SiO-n) so as to align said determined polarization state with respect to one of said at least one polarization state of said associated basis, the method further comprising a correlation step comprising a correlation measurement between said first quantum signal (Qni) from said modified polarization multiplexed optical signal (SiO-n) and said second quantum signal (Qh), said correlation measurement being associated with said polarization basis,said correlation step (260) further comprising the generation of said at least one information signal (1^) from said correlation measurement, said information signal (1^) comprising entanglement information of the polarization states of said first and second quantum signals (Qn, Qh).

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