QUANTUM COMMUNICATION SYSTEM AND METHOD USING PHOTON PHASE CORRECTION

Abstract

QUANTUM COMMUNICATION SYSTEM AND METHOD USING PHOTON PHASE CORRECTION A quantum communication system (1) is proposed comprising: a transmitter (10) of two multiplexed signals (Sn), each comprising a reference optical signal (Rn) and a quantum signal (Qn), the two quantum signals being entangled; two receivers (30-n) of the multiplexed signals comprising: an interferometer (320) comprising two optical paths (322-i1, 322-i2) dividing the received multiplexed signal into two components (Si-1n, Si-2n) and a modulator (321) arranged on one of the paths configured to apply a phase modulation (n) to one of the components; a processing module (340) performing interferometric measurements of quantum and interferometric reference signals (SQfn, SRfn) from the interferometer, and determining the phase modulation from the measurement of the interferometric reference signal.Each receiver is configured to determine a quantum key shared between the two receivers, from the measurement of the interferometric quantum signal. Figure for the abstract: [Fig.1].

Description

Title of the invention: QUANTUM COMMUNICATION SYSTEM AND METHOD USING PHOTON PHASE CORRECTION technical field

[0001] The present invention relates generally to the field of quantum telecommunications, and in particular to a quantum communication system using a phase correction of photons from entangled photon pairs, as well as an associated method.

[0002] Quantum communication systems use telecommunications devices implementing quantum protocols based on quantum information theory to distribute quantum states from a transmitter to one or more receivers. These quantum states are then used to implement the desired quantum protocol, such as quantum key distribution, or QKD (an acronym for the corresponding English expression 'Quantum Key Dz5trzèutzon'), which establishes a quantum encryption key for the purpose of securely encrypting and decrypting communication signals transmitted between two devices. The resulting quantum secret keys offer a higher level of security than keys obtained using classical cryptography.

[0003] In particular, quantum protocols use quantum entanglement techniques that utilize entangled quantum signals, each comprising quantum particles (called "qubits") belonging to pairs of entangled quantum particles. A communication system using a quantum entanglement technique comprises a transmitting device sharing quantum states with two receiving devices that consume the quantum states to perform the desired quantum application. In the case of a QKD quantum protocol, the receiving devices measure the received quantum states to establish an encryption key that allows them to subsequently exchange encrypted messages. Each receiving device receives the same information (i.e., identical information) and determines the shared encryption key by measuring a random encoding variable of the entangled quantum particles (qubits) emanating from the transmitter.

[0004] The encoding variable of a qubit corresponds to a degree of freedom of the quantum particle, that is, to an intrinsic characteristic of the photon. In quantum entanglement protocols, the encoding variable can be an "energy-time" encoding variable. In this case, the entangled photons are generated from a pump photon in the emitter at the same instant (time base) and have an energy whose The sum is equal to that of the initial pump photon (energy base). Thus, measurements made by receiving devices will successively examine the correlations between the arrival times of the photons and their energy.

[0005] However, known quantum communication systems using quantum entanglement protocols rely on complex techniques for tuning quantum particle measurements to a common energy basis at the receivers, sometimes requiring manual adjustments, and do not allow for efficient and dynamic entanglement sharing (and therefore quantum key sharing). Such solutions have been described, for example, in the article “Operational entanglement-based quantum key distribution over 50 km of field-deployed optical fibers” by Y. Pelet et al., Phys. Rev. Applied 20, 044006, 2023.

[0006] Furthermore, the photon encoding variable can undergo modifications or alterations during photon propagation between the emitter and a receiver, which may be random or difficult to predict. This is particularly the case in quantum communication systems used in the field of space communications, where such modifications / alterations generate a change in the quantum particle measurements in the energy basis at the receivers, which can significantly distort entanglement sharing and impair quantum key sharing.

[0007] There is therefore a need for an improved quantum communication system using entangled photon pairs, capable of correcting in real time the effects of changes in the phase states of qubits. Summary of the invention

[0008] Embodiments of the invention thus provide a quantum communication system comprising:

[0009] - a transmitter configured to emit two distinct multiplexed optical signals, each multiplexed optical signal comprising an initial optical reference signal and an initial quantum signal, the two initial quantum signals being entangled quantum signals comprising photons from entangled photon pairs, and

[0010] - two receivers, each receiver being configured to receive one of the signals multiplexed optical systems, each receiver comprising: • an interferometric module comprising at least two distinct optical paths, the interferometric module being configured to split the received multiplexed optical signal into two intermediate signal components, the interferometric module comprising a phase modulation unit arranged on one of the optical paths and configured to apply phase modulation to only one of the intermediate components, the interferometric module being further configured to generate an interferometric quantum signal and an interferometric optical reference signal from the intermediate signal components, and • a processing module configured to perform interferometric measurement of the interferometric quantum signal and interferometric measurement of the interferometric reference optical signal, the processing module being further configured to determine the phase modulation to be applied, from the interferometric measurement of the interferometric reference optical signal.

[0011] Each receiver is further configured to determine a quantum key shared between the two receivers, from the interferometric measurement of the interferometric quantum signal.

[0012] In embodiments, for each pair of entangled photons from the initial quantum signals, the entangled photons can have a joint quantum state, and the interferometric measurements of the interferometric quantum signals in the receivers can each correspond to the projection of the joint quantum state of the entangled photons onto a common state relative to the interferometric moduli of the receivers.

[0013] Advantageously, the interferometric measurements of the interferometric quantum signals in the receivers can be correlated with each other.

[0014] According to certain embodiments, the common state relating to the interferometric modules of the receivers can correspond to the superposition of a first possible state and a second possible state, the first possible state corresponding to the case where the entangled photons have respectively taken the same optical path among the optical paths of the interferometric module, and the second possible state corresponding to the case where the entangled photons have respectively taken the other same optical path among the optical paths of the interferometric module.

[0015] Interferometric measurements of interferometric quantum signals in the receivers can be anti-correlated with each other.

[0016] The common state relating to the interferometric modules of the receivers may correspond to the superposition of two possible states, the possible states corresponding to cases where the entangled photons have taken different optical paths respectively among the optical paths of the interferometric module.

[0017] According to some embodiments, the two receivers can further be configured to use the shared quantum key to encrypt and / or decrypt a communication message to be transmitted between them.

[0018] In some embodiments, the transmitter may be in motion relative to at least one of the receivers considered, phase modulation being applied to correct phase fluctuations related to a Doppler effect experienced by the multiplexed optical signal between the transmitter and the receiver considered.

[0019] The transmitter can understand:

[0020] - a signal generator configured to generate the initial quantum signals entangled and the initial optical reference signals, and

[0021] - two signal integrators, each signal integrator being configured to generate a multiplexed optical signal comprising one of the initial reference optical signals and one of the initial quantum signals.

[0022] In embodiments, the multiplexed optical signals are frequency-multiplexed signals, for each multiplexed optical signal, the initial reference optical signal and the initial quantum signal being characterized by a frequency difference between a quantum wavelength of the initial quantum signal and a reference wavelength greater than or equal to a predefined minimum wavelength difference value, the signal integrators being wavelength-multiplexing units.

[0023] Advantageously, for each receiver, the processing module may include two signal demultiplexing units, two single-photon detection units, and two auxiliary detection units. Each single-photon detection unit is positioned at a first output of one of the signal demultiplexing units, and each auxiliary detection unit is positioned at a second output of one of the signal demultiplexing units and configured to deliver an electrical signal. The processing module may include a correction unit configured to measure a ratio value of the electrical signals from the auxiliary detection units and to compare the measured ratio value with a predefined signal ratio value.

[0024] The signal demultiplexing units may each include at least one band-rejection filter and / or an "Add / Drop WDM" type filter.

[0025] In embodiments, each receiver can be configured to apply and / or trigger a servo loop between the interferometric module and the processing module by applying an electrical servo signal to control the phase modulation, the electrical servo signal being determined from a servo algorithm implemented to obtain and / or maintain the measured ratio value equal to the predefined ratio value.

[0026] Another object of the invention is a quantum communication method comprising optical signal emission steps implemented by a transmitter and consisting of:

[0027] - generate two initial quantum signals entangled with each other, and two signals initial reference optics,

[0028] - generate and emit two distinct multiplexed optical signals, each signal multiplexed optics comprising an initial optical reference signal and an initial quantum signal.

[0029] The method comprises optical signal reception steps, each implemented by two separate receivers, and consisting of:

[0030] - receive one of the separate multiplexed optical signals,

[0031] - passing the multiplexed optical signal through a receiver interferometer considered,

[0032] - generate an interferometric optical reference signal,

[0033] - perform an interferometric measurement of the reference optical signal interferometric,

[0034] - determine a phase modulation from said interferometric measurement of the interferometric reference optical signal,

[0035] - apply phase modulation to an intermediate component of the optical signal multiplexed received in the interferometer of the receiver in question.

[0036] The method further includes additional steps implemented by two receivers and consisting of determining a quantum key shared between the two receivers, from the interferometric measurement of the interferometric quantum signal.

[0037] In some embodiments, the method may include the transmission of a communication message between the two receivers. The method may further include steps, each carried out by two receivers, consisting of using the shared quantum key to encrypt and / or decrypt the communication message.

[0038] The quantum communication system and method according to the embodiments of the invention makes it possible to correct in real time in a receiving device the effects of changes in the phase states of qubits, resulting from entangled photon pairs generated by a transmitting device, and thus to efficiently share entangled quantum states allowing the establishment of quantum encryption keys between devices to perform secure telecommunication.

[0039] The quantum communication system makes it possible, in particular, to compensate for the relative phase induced by motion effects between the devices of the system, such as the Doppler effect, so as to maintain a perfect correlation between distant receiving devices throughout the duration of the quantum key sharing protocol. DESCRIPTION OF FIGURES

[0040] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0041] [Fig.1] [Fig.1] is a diagram representing a quantum communication system, according to embodiments.

[0042] [Fig.2] [Fig.2] is a diagram representing a receiver of a quantum communication system, according to embodiments.

[0043] [Fig.3] [Fig.3] is a diagram representing a transmitter of a quantum communication system, according to embodiments.

[0044] [Fig.4] [Fig.4] is a diagram representing a signal generator of a transmitter of a quantum communication system, according to embodiments.

[0045] [Fig.5] [Fig.5] is a diagram representing an interferometric module of a receiver of a quantum communication system, according to embodiments.

[0046] [Fig.6] [Fig.6] is a diagram representing a processing module of a receiver of a quantum communication system, according to embodiments.

[0047] [Fig.7] [Fig.7] shows diagrams 9(a) and 9(b) representing the results of an experiment emulating the Doppler effect between a transmitter and receivers of a quantum communication system, according to embodiments.

[0048] [Fig.8] [Fig.8] is a flowchart representing a quantum communication process implemented by a transmitter of a quantum communication system, according to embodiments.

[0049] [Fig.9] [Fig.9] is a flowchart representing a quantum communication process implemented by a receiver of a quantum communication system, according to embodiments.

[0050] 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 OF THE INVENTION

[0051] Figure 1 schematically represents a quantum communication system 1, according to embodiments of the invention. The quantum communication system 1 comprises a transmitter 10 (also called the 'transmitter device') and a pair 30 of receivers 30-n (also called the 'receiver devices'), capable of communicating with each other. The subscript 'n' is generally used to designate the two receivers of the pair 30, the first receiver corresponding to the subscript n=1 and the second receiver of system 1 corresponding to the subscript n=2.

[0052] The pair 30 of receptors comprises a first receptor 30-1 (ie n=1) and a second receptor 30-2 (ie n=2).

[0053] 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. In one application case in the domain In a space-based configuration, the transmitter 10 and the receivers 30-n are located remotely. The transmitter 10 may, for example, be mounted on a satellite communications platform, and at least one of the receivers 30-n may be on the ground (for example, integrated into a ground-based device or terminal, such as a ground station). Conversely, at least one receiver 30-n may be mounted on a satellite platform while the transmitter 10 is on the ground (for example, integrated into a ground-based device or terminal, such as a ground station). A satellite platform can be a platform attached to a satellite, which can be any satellite deployed in an orbit above the Earth at a defined altitude. For example, the platform may be in a medium Earth orbit (MEO), a low Earth orbit (LEO), or a highly elliptical orbit (HEO).

[0054] In other application examples, the quantum communication system 1 can be used in avionics systems. In such avionics application examples, at least one of the transmitter 10 and / or receiver 30-n devices can be embedded in an avionics device.

[0055] The transmitter 10 and / or receiver 30-n devices can be guided optical (or all-optical) devices, i.e. comprising optical signal propagation paths made up of optical fibers and / or so-called integrated waveguides, typically used in integrated photonics.

[0056] The quantum communication system 1 can also be used in optical fiber network applications, in which at least one of the transmitter 10 and / or receiver 30-n devices is a device for optical fiber integrated into a ground network.

[0057] According to some embodiments, the transmitter 10 and a receiver 30-n can be fixed spatially relative to each other.

[0058] Alternatively, at least one device (transmitter 10, receiver 30-1, receiver 30-2) of system 1 may be in motion relative to at least one other device of system 1, with which it communicates. By way of non-limiting example, in the application of the invention to the space domain, one of the devices (10, 30-1, 30-2) of system 1 may be mounted on a satellite communication platform that may be geomobile so as to be in relative motion with respect to one of the other devices of system 1, which is arranged in a ground-based station, for example. Such a satellite communication platform may, for example, be arranged in an orbit defined with an orbital period other than 24 hours.

[0059] The quantum communication system 1 can be a quantum key distribution (QKD) system, in which the first receiver 30-1 and the second receiver 30-2 are configured to determine (i.e., establish) a so-called 'quantum key', shared between them. The quantum key distribution(s) can notably be implemented within a space or ground communication service, the aim is to ensure the security, through encryption and / or decryption, of some or all communications exchanged between receivers, for example, communication messages denoted SM, as shown in [Fig. 1]. For example, and without limitation, an SM communication message can be an encrypted message (or encrypted signal) based on the shared quantum key, i.e., a signal previously encoded by the first receiver 30-1 and transmitted to the second receiver 30-2. The second receiver 30-2 can then be configured to decode (or decrypt) the received SM communication message using the shared quantum key.

[0060] The transmitter 10 is configured to generate two separate optical emission signals, comprising a first optical emission signal Si and a second optical emission signal S2.

[0061] An 'optical signal' refers to a continuous wave or results from one or more pulses of light, coherent or incoherent, originating from an optical source, such as a laser beam. The electromagnetic wave (or beam) carrying the optical signal is characterized, in particular, by a given wavelength λ (i.e., relative to a specific optical frequency or frequency band). A frequency band can be a range of optical frequencies, corresponding, for example, without limitation, to wavelengths of 850 nm, or typically between 1500 nm and 1600 nm. The electromagnetic wave can also be characterized by a given phase. A laser beam can further be characterized by a pulse rate, and each pulse can be defined, for example, by its intensity.

[0062] The transmitter 10 is further configured to transmit to each of the receivers 30-n an optical emission signal Si or S2 (also noted Sn).

[0063] Each of the optical emission signals Sn can be transmitted through a transmission channel. The transmission channel can, for example, correspond to a free-space communication channel and / or a fiber-optic (or guided-optics) communication channel suitable for carrying information using, for example, optical fiber elements for communication.

[0064] In embodiments, as shown in [Fig.1], the optical emission signal Si can be transmitted through a first transmission channel from the transmitter 10 to the first receiver 30-1 and the optical emission signal S2 can be transmitted through a second transmission channel from the transmitter 10 to the second receiver 30-2.

[0065] In embodiments, as shown in [Fig. 1], another transmission channel can be implemented between the two 30-n receivers to transfer SM communication messages and / or synchronization or coordination information to generate the shared quantum key.

[0066] The first optical emission signal Si delivered by the emitter 10 comprises a first initial quantum signal Qi and the second optical emission signal S2 delivered by the emitter 10 comprises a second initial quantum signal Q2.

[0067] The expression 'quantum signal' can refer to a low-power continuous optical signal or to a pulsed optical signal having a low number of photons per pulse. The measurement of a quantum signal provides a measurement of the detection of a photon (or 'particle') depending on a 'detection probability' of that photon.

[0068] As used here, a 'quantum signal' corresponds to an optical signal comprising at least one photon that is entangled with another photon from a different 'quantum signal'. These two quantum signals are then called 'entangled quantum signals'. An 'entangled photon pair' comprises these two photons, which form a bound system and exhibit quantum states that are dependent on each other, regardless of the distance separating them. There are correlations between the measurable physical properties of these distinct particles. The entanglement of a photon pair arises from the fact that these photons, each contained within a specific quantum signal, are both generated from the same pump photon.

[0069] Advantageously, the two initial quantum signals Qi and Q2 (generally denoted by Qn) are entangled with each other insofar as each photon of an initial quantum signal originates from an entangled pair of photons generated by the emitter 10 and shared between the two initial quantum signals. It should be noted that the energy of the photons of each entangled pair of the first and second initial quantum signals Qi and Q2, respectively, are correlated with each other.

[0070] Each of the optical emission signals, Si or S2, delivered by the transmitter 10 forms a multiplexed optical signal comprising an initial quantum signal, Qi or Q2, and an additional optical signal called the 'initial reference signal', designated respectively by the notation Ri or R2. A 'reference signal' (generally designated by the reference Rn) is also called the 'phase reference signal', or 'phase control signal'.

[0071] Each receiver 30-n of the pair 30 of receiving devices of system 1 is configured to receive an optical receive signal relative to the optical transmit signal Sn (i.e. a signal Si or S2), transmitted by the transmitter 10.

[0072] In the remainder of this description, for the sake of simplicity and to facilitate understanding of the invention, the received optical signal will be considered to correspond to the transmitted multiplexed optical signal, generally designated by Sn (with Si for n=1 and S2 for n=2). However, those skilled in the art will readily understand that the transmitted optical signal may be different from the received multiplexed optical signal, following the propagation of the optical signal emitted from the transmitter 10 to the receiver 30-n, through the transmission channel.

[0073] As illustrated in [Fig.1], the first receiver 30-n is adapted to receive the first multiplexed optical signal Si transmitted by the transmitter 10, and the second receiver 30-2 is adapted to receive the second multiplexed optical signal S2 transmitted by the transmitter 10.

[0074] Fig. 2 schematically represents a 30-n receiver comprising an interferometric module 320 and a processing module 340, according to embodiments of the invention.

[0075] An interferometric module 320 can be implemented in the form of an optical instrument of the optical interferometer type, and is configured to generate, from the received multiplexed optical signal Sn, an interferometric quantum signal SQfn and an interferometric reference optical signal SRfn.

[0076] The interferometric quantum signal SQfn is thus obtained from the initial quantum signal Qn from the multiplexed optical signal Sn delivered by the emitter 10, and the interferometric reference optical signal SRfn is obtained from the initial reference optical signal Rn from the multiplexed optical signal Sn delivered by the emitter 10.

[0077] The processing module 340 of a 30-n receiver is configured to perform an interferometric measurement of the interferometric quantum signal SQfn and an interferometric measurement of the interferometric reference optical signal SRfn.

[0078] Each of the 30-n receivers is then configured to determine a quantum key shared between them, using the interferometric measurement of the interferometric quantum signal SQfn.

[0079] In particular, the interferometric measurement of the interferometric quantum signal SQfn can provide a quantum information signal L relative to the initial quantum signal Qn, from the multiplexed optical signal Sn delivered by the emitter 10. Thus, the quantum information signal h determined by the first receiver 30-1 and the quantum information signal I2 determined by the second receiver 30-2 are both associated with the entangled photon pairs generated by the emitter 10.

[0080] The interferometric module 320 of each of the receivers 30-n may further comprise a phase modulation unit 321 configured to apply a phase modulation (|)n to an intermediate component of the multiplexed optical signal Sn propagating in the optical interferometer of the interferometric module 320. In particular, an intermediate component of the multiplexed optical signal Sn refers to a portion of the multiplexed optical signal Sn propagating in one of the arms of the interferometric module 320.

[0081] The processing module 340 of each of the 30-n receivers is further configured to determine the phase modulation μ, to be applied from the interferometric measurement of the interferometric reference optical signal SRfn. Such a phase modulation μ>n, determined and applied independently by each of the 30-n receivers of the pair 30 of receivers, makes it possible to compensate for the phase fluctuations experienced by the photons between the generation of the initial quantum signal Qn by the emitter 10 and the interferometric measurement to determine the quantum information signal In from the received multiplexed optical signal Sn. The applied phase modulation μ, corrects the phase shifts experienced independently between the photons of the same pair of entangled photons generated by the emitter 10.

[0082] The value of the phase modulation parameter ¢,, of the phase modulation unit 321 of a receiver 30-n can be applied from a control signal Cn, shown in [Fig.2].

[0083] In particular, the phase modulation ¢,, to be applied can be determined so as to maintain a constant interference pattern of the interferometric measurement of the interferometric reference optical signal SRfn. The applied phase modulation ¢,, allows the interferometer 320 of each of the receivers 30-n separated from the pair 30 to be tuned so that the result of the interferometric measurements of the interferometric quantum signals is correlated, or remains correlated, regardless of the phase fluctuations undergone by the photons received by the receivers 30-n independently, a quantum signal multiplexed to a reference optical signal undergoing the same phase changes from the emitter 10 to a considered receiver 30-n.

[0084] Fig. 3 schematically represents the structure of the emitter 10 used to form the multiplexed optical signals Si and S2, according to embodiments of the invention.

[0085] The transmitter 10 may include a signal generator 120, a first signal integrator 141, and a second signal integrator 142.

[0086] Each multiplexed optical signal, Si and S2, can be generated using a signal integrator, 141 and 142 (also called 'integration module'), from an initial quantum signal, Qi and Q2 respectively, and an initial reference signal, Ri and R2 respectively, delivered by the signal generator 120.

[0087] The first signal integrator 141 can be configured to form the first multiplexed optical signal Si from the first quantum signal Qi and the first reference signal RB and the second signal integrator 142 can be configured to form the second multiplexed optical signal S2 from the second quantum signal Q2 and the second reference signal R2, as shown in [Fig.3].

[0088] Thus, an emitter integration module 10 can be adapted to optically multiplex (or optically combine), on the same optical path, a quantum signal with a reference signal.

[0089] In some embodiments, a multiplexed optical signal generated by the emitter 10 can be a frequency-multiplexed signal. In this case, the signal generator 120 of the emitter 10 can be configured to generate the entangled quantum signals along a quantum wavelength XQ (or along two distinct quantum wavelengths XQ1 and XQ2), and to generate the reference signals along a reference wavelength XR (or along two distinct reference wavelengths XR1 and XR2), the quantum wavelength(s) being distinct from the reference wavelength(s). In particular, the quantum wavelength XQ1 of the first initial quantum signal Qi can be distinct from the reference wavelength XR of the first initial reference signal Rb, and the quantum wavelength XQ2 of the second initial quantum signal Q2 can be distinct from the reference wavelength XR2 of the second initial reference signal R2.

[0090] Advantageously, the signal generator 120 may include a pump laser source 122, a quantum entanglement unit 124 and at least one additional laser source 126, as shown in [Fig.4] by way of non-limiting example.

[0091] The pump laser source 122 can be a source emitting a laser beam (or 'pump laser') Sp of pump wavelength Xp. The laser emission wavelength Xp can, for example, be in the visible or infrared wavelength range. For example, and without limitation, the pump laser source 122 can be a DFB diode (Distributed Fedback diode) using a Bragg grating to select the pump emission wavelength Xp. For example, and without limitation, the wavelength Xp can be 780 nm or 1560 nm. Such a laser diode emits, in particular, a continuous laser beam. The first laser source 122 can also be a pulsed laser unit.A pulsed laser unit can include a laser beam source and be configured to generate pulses from a modulation of the laser beam, implemented by internal modulation within the source. Such pulses can also be generated from a modulator external to the source.

[0092] The quantum entanglement unit 124 can be configured to receive the pump laser signal Sp and to deliver the entangled quantum signals corresponding to the first initial quantum signal Qi and the second initial quantum signal Q2, comprising entangled photon pairs.

[0093] For example, and without limitation, the quantum entanglement unit 124 may comprise a nonlinear medium adapted to generate entangled photon pairs by nonlinear interaction, in response to the passage of the Sp pump laser signal through this nonlinear medium. Such a nonlinear interaction may be, for example, a spontaneous down-parametric conversion or a four-wave mixing conversion. The nonlinear medium may be a resonator or a nonlinear crystal, such as, for example, a PPLN crystal (or 'Periodically poled lithium niobate' according to the corresponding Anglo-Saxon expression).

[0094] According to certain embodiments, the signal generator 120 may further comprise one or more intensity modulation units (not shown in the figures) positioned at the output of the pump laser source 122, or of the quantum entanglement unit 124, to form quantum pulses. An intensity modulation unit may be configured to modulate the intensity of laser pulses, and optionally the laser pulse rate from the order of a few kilohertz up to a few tens of gigahertz, for example, and / or the time width of the laser pulses, for example, down to a few nanoseconds.

[0095] Advantageously, a quantum wavelength (Xq, or XQ1 and XQ2) of the entangled quantum signals can be determined as a function of the pump wavelength Xp. In particular, given that 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 Xp of 780 nm, the quantum wavelength XQ of the two quantum signals Q1 and Q2 can be approximately 1560 nm to maintain energy conservation during signal conversion.

[0096] In embodiments where the emitter 10 is a device comprising means for signal propagation in free space, the entanglement unit 124 of the signal generator 120 may include one or more dichroic filters allowing to separate (i.e. filter) on two distinct optical paths, the two photons of each of the entangled photon pairs formed so as to deliver the two entangled quantum signals Qi and Q2.

[0097] In some embodiments, the signal generator 120 may include a single additional laser source 126, as shown in [Fig. 4], configured to deliver an initial optical reference signal Ro. In this case, the signal generator 120 may further include a signal separation unit 128 to form the two reference signals, Ri and R2.

[0098] In other embodiments, the signal generator 120 may include two additional laser sources 126-1 and 126-2 (not shown in the figures) configured to deliver a reference optical signal, Ri and R2 respectively.

[0099] A reference wavelength (XR, or XR1 and AR2) may be in the visible or infrared wavelength range. For example, and without limitation, an additional laser source may be a DFB laser diode or a pulsed laser unit.

[0100] In embodiments, the frequency difference between a quantum wavelength (XQ, or XQ1 and ÀQ2) and a reference wavelength (ÀR, or ÀR1 and ÀR2) may be greater than or equal to a minimum value ÔX of wavelength difference, defined according to the following inequality (01):

[0101] |Xq-Xr| > 6X (01)

[0102] Advantageously, the minimum value 6X of wavelength difference can be predefined. For example, and without limitations, the minimum value 0X of wavelength difference can be between 0.8 nm and 2 nm.

[0103] In embodiments where a multiplexed signal generated by the emitter 10 is frequency-multiplexed, an integration module of the emitter 10 may include a wavelength division multiplexing (WDM) unit adapted to combine a quantum signal and a reference signal along the same optical path into a resulting multiplexed signal. Such an integration module may alternatively include a filter or a dichroic mirror adapted to combine a quantum signal and a reference signal along the same optical path into a resulting multiplexed signal.

[0104] In some embodiments, a multiplexed optical signal generated by the emitter 10 can be a time-multiplexed signal. In this case, such a signal can be a signal comprising a set of two temporally distinct pulses, the set being repeated according to a period T. The two distinct pulses of the set correspond respectively to a pulse of the entangled quantum signal (Qi or Q2) and to a pulse of the reference signal (Ri or R2).

[0105] Advantageously, the entangled quantum signals and the reference optical signals can be impulse signals characterized by a period T identical to the period of the set of two pulses of the multiplexed optical signals delivered by the emitter 10.

[0106] In embodiments, the signal generator 120 can be configured to generate the entangled particle pairs forming the initial quantum signals Qi and Q2, and the initial optical reference signals Ri and R2, according to a predefined time offset between each pulse.

[0107] According to some embodiments, an integration module (141 and / or 142) of the emitter 10 can be configured to apply a predefined time offset between the pulses of a quantum signal and a reference optical signal respectively so as to obtain time-multiplexed signals.

[0108] It should be noted that in the embodiments of the invention (involving or not time multiplexing), the entangled particles of the same pair of photons (Qi and Q2) are not time-shifted at the generation of the multiplexed signals.

[0109] The resulting time difference (tç -tR) between each of the successive distinct pulses of a set of pulses (i.e. quantum and reference) in a multiplexed signal can thus be strictly less than the repetition period T of the pulse set, according to the following inequality (02):

[0110] |tQ-tR| <T(02)

[0111] In embodiments where a multiplexed optical signal generated by the emitter 10 is time-multiplexed, a quantum wavelength (ÀQ, or ÀQ1 and XQ2) and a reference wavelength (XR, or XR1 and XR2) may be different from each other.

[0112] Alternatively, in embodiments where a multiplexed optical signal generated by the emitter 10 is time-multiplexed, a quantum wavelength (XQ, or XQ1 and XQ2) and a reference wavelength (XR, or XR1 and XR2) may be equal. In this case, the first laser source 122 and the additional laser source(s) 124 (or XQ1 and XR2) may, for example, correspond to a single laser source. The signal generator 120 may then further comprise at least one beam splitter unit (not shown in the diagrams) configured to provide a signal component Ro and / or to form the reference optical signals Ri and R2, as well as a signal component associated with the pump optical signal Sp. Such a beam splitter unit may comprise one or more optical couplers.The beam splitting unit can also be an optical selector generating a predefined time offset between each delivered signal component.

[0113] In some embodiments, the beam splitting unit of the signal generator 120 can be arranged at the output of the single laser source, the resulting initial optical reference signals Ri and R2 then corresponding to so-called 'classical' (i.e., non-quantum) light pulse signals. Alternatively, this beam splitting unit of the signal generator 120 can be arranged at the output of an intensity modulation unit, the resulting initial optical reference signals Ri and R2 then corresponding to low-intensity light pulse signals.

[0114] Fig. 5 schematically represents an interferometric module 320 of a receiver 30-n, according to embodiments of the invention.

[0115] An interferometric module 320 (also called an 'interferometer') may include a beam splitter unit 322 and two interferometric arms 322-11 and 322-12. The beam splitter unit 322 is configured to split the received multiplexed optical signal Sn into two intermediate beam components, denoted for example S1'n and S12n. In the interferometric module 320, a first intermediate beam component S' travels only a first interferometric arm (i.e., the first intermediate beam component S' travels only one single arm which is the first interferometric arm) 322-il and a second intermediate beam component S12„ travels only a second interferometric arm 322-i2 (i.e., the second intermediate beam component S12n travels only one single arm which is the second interferometric arm), as shown in [Fig.5].The received multiplexed optical signal Sn comprises a quantum signal derived from the initial quantum signal Qn and a reference signal derived from the initial reference signal Rn. Each of the intermediate beam components S1 and S12n of the multiplexed optical signal Sn can comprise: . - a component of the reference signal, called the 'intermediate component of the reference signal', denoted SR1 'n or SR1 2n, and - a quantum state of the quantum signal (also called 'intermediate component' of the quantum signal), denoted SQ1 'n or SQ1 2n.

[0116] Thus, the first intermediate beam component S1, comprising a first component of the reference signal SR1 'n and a first "component" of the quantum signal SQ1 'n, travels along the first interferometric arm 322-11, corresponding to a single optical path. The second intermediate beam component S1 2n, comprising a second component of the reference signal SR1 2n and a second "component" of the quantum signal SQ1 2n, travels along the second interferometric arm 322-12, also corresponding to a single optical path.

[0117] The beam splitter unit 322 can be a symmetrical optical coupler (for example, a 50 / 50 type fiber Y), configured to provide two intermediate beam components of the multiplexed optical signal, each composed of a component of the reference signal of equal intensity.

[0118] It should be noted that the term "component" is used here in relation to the quantum signal, for the sake of simplification. Those skilled in the art will readily understand that, in In practice, the quantum signal (i.e. photon) does not "take" one specific optical path or the other in the optical interferometer, or "split" in terms of intensity via a beam splitter, and the resulting quantum signal from the interferometer is equivalent to a quantum superposition of the two optical paths.

[0119] According to some embodiments, the interferometric module 320 can be a Michelson-type interferometer, as illustrated in [Fig. 5]. Advantageously, such an interferometric module can comprise two mirrors 323-1 and 323-2, each disposed at the end of one of the two interferometric arms, 322-11 or 322-12. The mirrors 322-1 and 322-2 can, for example, be Faraday mirrors configured to reflect the intermediate beam components S1 and S12 of the received multiplexed optical signal Sn. In this case, the beam splitter unit 322 can be configured to recombine the intermediate beam components S1 and S12 into the resulting interferometric signal, following their passage along one of the two interferometric arms 322-11 or 322-12.

[0120] Alternatively, the interferometric module 320 may be a Mach-Zehnder type interferometer (configuration not shown in the figures). In these embodiments, the interferometric module 320 may include an additional beam coupler (not shown in the figures) disposed at the end of the interferometric arms 322-il or 322-i2, and configured to recombine the intermediate beam components S1'n and S12n into a resulting interferometric signal.

[0121] The first 322-il interferometric arm does not include any additional optical elements. The first 322-il interferometric arm can be considered as the phase reference arm, which is configured such that the first intermediate beam component S1 traversing the first 322-il interferometric arm does not undergo any phase-induced modification related to any passage through an optical element.

[0122] In some embodiments, the two interferometric arms 322-il and 322-i2 may have different arm lengths, the interferometer then comprising a short interferometric arm and a long interferometric arm. The difference in length between the two interferometric arms is denoted AL. For example, and without limitation, this difference in length AL may be less than or equal to the coherence length of the laser unit 122, and greater than the temporal width of the reference signal pulse, for example.

[0123] In a first embodiment, the first interferometric arm 322-il may be the short interferometric arm while the second interferometric arm 322-i2 is the long interferometric arm, the two arms having a length difference AL. Alternatively, in a second embodiment, the first Interferometric arm 322-i 1 may be the long interferometric arm, while the second interferometric arm 322-i2 may be the short interferometric arm.

[0124] The phase modulation unit 321 of the interferometric module 320 can be positioned on the second interferometric arm 322-i2. The phase modulation unit 321 is therefore unique and positioned on the same and unique optical path 322-i2. The phase modulation unit 321 can be configured to modulate, according to a phase modulation parameter 4>n, the phase of the second intermediate beam component S12n traversing the optical path of the second interferometric arm 322-i2. In other words, the 321 phase modulation unit can be configured to modulate, according to a single phase modulation parameter, the phase of the intermediate component of the reference signal SR12n, and the phase of the photon (i.e. quantum state or intermediate component of the quantum signal) SQ12n of the second intermediate component of the beam S12n, both traversing the optical path of the second interferometric arm 322-i2.

[0125] For example and without limitation, the phase modulation unit 321 of a receiver 30-n can be an electro-optical modulator consisting of electro-optical crystals to which an electrical signal Cn is applied in order to control (or command) the phase modulation parameter ¢,,.

[0126] In some embodiments, the phase modulation unit 321 of a receiver 30-n can be a piezoelectric component, for example positioned on the optical fiber forming the second interferometric arm 322-i2, and configured to modify the length difference AL between the two interferometric arms of the module 320 in a very precise and controlled manner. This control of the length difference AL allows the phase modulation parameter (])n to be controlled. Such a piezoelectric component can be driven, for example, by the electrical control signal Cn.

[0127] Upon recombination of the intermediate beam components S1 or S12n, the resulting interferometric signal may include the interferometric quantum signal SQfn and the interferometric reference optical signal SRfn (generated by the optical interferometer).

[0128] For example and without limitations, the interferometric reference optical signal SRfn can be defined as a function of the received reference signal from the initial reference signal Rn and the phase modulation (|)n.

[0129] In some embodiments, the resulting interferometric signal Sfn can propagate along two distinct optical paths, 322-i3 and 322-i4, from the output of the optical interferometer 320, as illustrated in [Fig. 5]. In particular, in [Fig. 5], a first component Sf of the resulting interferometer signal travels the first optical output path of interferometer 322-i3 and a second component Sf 2n of the resulting interferometer signal travels through a second optical output path of interferometer 322-i4.

[0130] In embodiments using a "Michelson" type interferometer, the resulting interferometric signal Sfn can be obtained from the recombination of intermediate components by the beam splitter unit 322 and the first optical output path of interferometer 322-i3 can correspond to the input return path of the received multiplexed optical signal Sn, as shown in [Fig.5].

[0131] Advantageously, a receiver 30-n may include a signal deflection unit 324, arranged on the first optical output path of the interferometer 322-i3, and configured to deflect (or redirect) the first component Sr of the resulting interferometer signal to the processing module 340, and in particular to a first signal detection chain 342 in the processing module 340, along an optical path 324-i. For example, the signal deflection unit 324 may be an optical circulator. Such an optical circulator makes it possible, in particular, to prevent the return transfer of the first component Sf of the resulting interferometer signal to the transmission channel from which the received multiplexed optical signal Sn originates.

[0132] The receiver 30-n may further include a second signal detection chain 344 at the end of the second optical output path of the interferometer 322-i4, for example in the processing module 340.

[0133] Fig. 6 schematically represents a processing module 340 of a receiver 30-n comprising the two signal detection chains 342 and 344, according to embodiments.

[0134] For each receiver 30-n, each of the signal detection chains 342 and 344 is associated with one of the two optical paths 324-i and 322-i4 obtained at the output of the interference module 320.

[0135] As shown in [Fig. 6], each processing chain (342 or 344) may include a signal demultiplexing unit (3422 or 3442, respectively) configured to receive as input a component (Sf or Sf2n, respectively) of the resulting interferometer signal Sfn, and to separate from this component, a portion (SQf'n or SQf2n, respectively) of the interferometric quantum signal SQfn and a portion (SRf or SRf2n, respectively) of the interferometric optical reference signal SRfn. The portions of the interferometric quantum signal SQfn may then travel respectively through a measurement arm, referred to as the 'quantum measurement arm', of the processing chains, while the portions of the interferometric optical reference signal SRfn traverse a measurement arm called the 'reference measurement arm' of the processing chains.

[0136] The signal demultiplexing units 3422 and 3442 may in particular include one or more demultiplexing elements determined according to the type of signal multiplexing by the transmitter 10, which may be, for example, frequency and / or time multiplexing.

[0137] In embodiments where the received multiplexed optical signal Sn is frequency-multiplexed, the signal demultiplexing units 3422 and 3442 may each include at least one filter configured to separate the quantum signal component (SQ n or SQ n) from the reference signal component (SR n or SRf 2n). For example, and without limitation, such a filter may be a band-rejection filter such as a Fiber Bragg Grating (FBG) filter or an Add / Drop WDM filter. The filter may be chosen from the predetermined frequency difference between the quantum wavelength (XQ or ΔQrj) and the reference wavelength (XR or XRn), defined, for example, according to formula (01).

[0138] Advantageously, the processing chains 342 and 344 of a 30-n receiver can each include a single-photon detection unit, 3424 and 3444, arranged at a first output of the signal demultiplexing unit (i.e. of the quantum measuring arm), and configured to detect a portion of the interferometric quantum signal SQfn relative to the quantum signal from the received multiplexed optical signal Sn.

[0139] According to certain embodiments, for a given processing chain, the single photon detection unit (3424 or 3444) and the auxiliary detection unit (3426 or 3446) can each include a frequency filter (not shown in the figures) associated respectively with the quantum (ÀQ or ÀQn) and reference (ÀR or ARn) wavelengths so as to optimize the detection of equivalent signals.

[0140] In embodiments, the single-photon detection units 3424 and 3444 may each comprise 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 associated with a given level of quantum detection efficiency. For example, and without limitation, the single-photon detection units 3424 and 3444 may be avalanche photodiodes (or APDs) and / or superconducting nanowire single-photon detectors (SNSPDs). In particular, the units Single photon detection 3424 and 3444 can each include an internal amplification mechanism configured to deliver a voltage, in response to the detection of a photon.

[0141] Advantageously, the processing chains 342 and 344 of a 30-n receiver can further comprise each an auxiliary detection unit, 3426 and 3446 arranged at a second output of the signal demultiplexing unit (i.e. of the reference measuring arm), and configured to detect a part of the interferometric reference signal SRfn relative to the reference signal from the received multiplexed optical signal Sn.

[0142] In some embodiments, the auxiliary detection units 3426 and 3446 can be configured to detect "classical" (i.e., non-quantum) light pulse signals. For example, and without limitation, the auxiliary detection units 3426 and 3446 can be photodiodes configured to each deliver a photocurrent (denoted, for example, I342 and I344 in [Fig. 6]), allowing the measurement of the estimated interferometric reference signal SRfn.

[0143] The "interference figure" relating to the interferometric measurement of the interferometric reference optical signal SRfn can thus designate the distribution of the light powers of the interferometric reference signal SRfn detected between the two outputs of the interference module 320, i.e. between the two auxiliary detection units 3426 and 3446.

[0144] Advantageously, this interference pattern (called 'reference interference pattern'), relating to the interferometric measurement of the interferometric reference optical signal SRfn can correspond to the ratio Pm between the intensities of the electrical signals, I342 and I344, provided each by one of the auxiliary detection units 3426 and 3446.

[0145] It should be noted that this reference interference figure is in fact related to the term “COs( <p( t)) * ou « tan(<p( t)) », de sorte que le terme « (p( t) » correspond au déphasage interférométrique entre les composantes intermédiaires des signaux circulant dans chacun des bras interférométriques du module d’interférence 320. En particulier, le terme « (p ( t ) » peut correspondre au déphasage relatif entre une composante intermédiaire (notamment S12n) circulant sur le bras interférométrique long par rapport à la composante intermédiaire circulant sur le bras interférométrique court, dont la différence de longueur est AL. Le terme « (p ( t ) » peut ainsi être formulé selon l’équation (03) suivante :

[0146] <p(t) = lpn+ <p(t) +<Mt)(03) U -U

[0147] In equation (03), the term 0(t) corresponds to a time variation of the phase due to the perturbations experienced by the signal between its emission by the transmitter 10 and its reception by the receiver 30-n. For example, and without limitation, the term 0(t) can be equal to the term 6k(t) AL, in which the term 6k(t) can correspond to the frequency-dependent shift of the signal's wave vector due to a Doppler effect experienced by the signal transmitted from the transmitter 10.

[0148] Furthermore, in equation (03), the term<Pn(t) peut correspondre à une variation temporelle de la phase due à la modulation de phase 4> n applied to the intermediate beam component S12n traversing the optical path of the second interferometric arm 322-i2 of the interferometric module 320.

[0149] In equation (03), the term 0Q can correspond to a constant component of the phase. This value of the constant component 0O can, for example and without limitation, be equal to 0, n, or n 12. (pQ

[0150] Advantageously, the interference module 320 and the processing module 340 are adapted to apply (or drive) the phase modulation (j)n, so that the time variation of the resulting phase 0n(t) compensates for the time variation of the phase due to the disturbances experienced by the signal 0^(t), as defined in a simplified way by the following equation (04):

[0151] <t>a(t) = -4>p(t) (04)

[0152] In this case, the interference module 320 and the processing module 340 are adapted to obtain a constant reference interference figure such that the interferometric phase shift (p(t) is equal to the constant component of the phase "0O".

[0153] As shown in [Fig.6], a processing module 340 of a receiver 30-n may further include a correction unit 346, according to embodiments of the invention.

[0154] For each of the 30-n receivers, the correction unit 346 of the processing module 340 can be configured to determine a correction for the phase fluctuations experienced by the multiplexed optical signal Sn from its emission by the transmitter 10 to its analysis by the receiver 30-n in question. Such a correction corresponds to determining a value of the phase modulation 4>n to be applied to the second intermediate beam component S12n, traveling along the optical path of the second interferometric arm 322-i2 of the interferometric module 320, based on the measurement (or estimation) of the result of the reference interference pattern from the different interferometric arms traversed by the reference signal, and therefore from the measurements carried out by the auxiliary detection units 3426 and 3446.

[0155] Advantageously, the correction unit 346 can be configured to analyze the estimated signals from the auxiliary detection units 3426 and 3446, corresponding to the components of the estimated interferometric reference signal (for example I342 and I344), and to generate a control signal Cn (or control signal), corresponding to a phase modulation setpoint signal 4>n to be delivered to the phase modulation unit 321 of the interferometric module 320.

[0156] In some embodiments, the correction unit 346 can be configured to determine (for example, by performing a calculation) the ratio Pm (or Pm(n)) between the electrical signals, I342 and I344, determined by the auxiliary detection units 3426 and 3446. The correction unit 346 can further be configured to perform a comparison of the determined ratio value Pm with a reference ratio value Po (or P0(n)) between the electrical signals, which can be predefined or selected. This reference ratio value Po between electrical signals can be variable or fixed beforehand, for example, according to given parameters relating to the position of the devices (10, 30-n) in the system 1.The determined control signal Cn then corresponds to a phase modulation setpoint ¢, allowing a value of the ratio Pm between the electrical signals I342 and I344 equal to the predefined reference ratio value Po, that is to say, such that the value of the ratio Pm between the electrical signals I342 and I344 remains constant.

[0157] In some embodiments, the determined control signal Cn may, for example, be a signal providing an instruction for a given value of the phase modulation setpoint 4>n. Alternatively, the determined control signal Cn may, for example, be a signal providing an instruction to decrease or increase the value of the phase modulation 4>n to be applied, depending on the comparison of the value of the ratio Pm with the predefined reference value of the ratio Po. In all cases, the control signal Cn is adapted to maintain the term "(p(t)" equal to the constant component " " of the setpoint of the phase.

[0158] For example and without limitation, the correction unit 346 may include one or more electrical units for modifying an electrical signal, or one or more processors (also called 'central processing units') or CPUs (acronym for the Anglo-Saxon expression 'Central Processing Unif'), configured to implement a phase modulation control algorithm ¢,, by generating the control signal Cn from the interferometric measurement of the signal SRfn.

[0159] Such a control algorithm may, for example, be a differentiable optimization algorithm using an incremental or iterative search method. Such a control algorithm may also use a predefined calibration or lookup table, for example.

[0160] A feedback loop between the interferometric module 320 and the processing module 340 (i.e., a phase correction loop generating a feedback signal) can be implemented continuously or intermittently (or discontinuously). In particular, a feedback loop can be activated periodically and / or after evaluation of the detection ratio of the reference signal components, with respect to one or more predefined or associated base ratios. Furthermore, a feedback loop can be implemented until the determined ratio Pm and the reference ratio Po match, and / or to maintain a value of the determined ratio Pm equal to the value of the reference ratio Po.

[0161] For example, and without limitation, the correction unit 346 can be configured to determine the difference between the determined ratio Pm and the reference ratio Po. The correction unit 346 can further be configured to evaluate whether this difference value between the ratios Pm and Po is strictly greater than, or alternatively greater than or equal to, a threshold difference between the ratios Pm and Po, and depending on the evaluation of this condition, implement or not the feedback loop between the interferometric module 320 and the processing module 340.

[0162] In embodiments, the reference value of the ratio Po between electrical signals can be between 0 and 1. For example, and without limitation, the reference value of the ratio Pm between electrical signals can be equal to 0.5. By way of non-limiting example, if the determined ratio Pm is different from the reference value Po of the ratio (for example Pm = 0.8), the servo loop between the interferometric module 320 and the processing module 340 can be triggered.

[0163] The single photon detection units 3424 and 3444 of the processing chains 342 and 344 of a 30-n receiver can measure the interferometric quantum signal SQfn relative to the quantum signal from the received multiplexed optical signal Sn.

[0164] For each 30-n receiver, the measured interferometric quantum signal SQfn can correspond in particular to the detection of at least one photon on one of the detectors 3424 and 3444 of the receiver in question. It can be noted that the path of a detected photon in the interferometric module 320 and the processing module 340 can be defined according to different possibilities (i.e., according to several use cases): The first case corresponds to the detection of a photon that has traveled along the first 322-il interferometric arm of the 30-n receiver, and then having been redirected to the first optical return path 322-i3 to be detected via the first processing chain 342; - a second case corresponds to the detection of a photon having taken the first interferometric arm 322-i 1 of the receiver 30-n, then having been redirected on the second optical return path 322-i4 to be detected via the second processing chain 344; - a third case corresponds to the detection of a photon having taken the second interferometric arm 322-i2 of the receiver 30-n, then having been redirected on the first optical return path 322-i3 to be detected via the first processing chain 342; - a fourth case corresponds to the detection of a photon having taken the second interferometric arm 322-i2 of the receiver 30-n, then having been redirected on the second optical return path 322-i4 to be detected via the second processing chain 344.

[0165] In other words, the interferometric module 320 and the processing module 340 of each receiver 30-n make it possible to provide a "probability of detection" of photons by the single photon detection units 3424 and 3444 (each configured to deliver in particular a voltage, for example when a photon is detected).

[0166] For example, and without limitation, for each of the 30-n receptors in the pair 30 of receptors, the processing chain 342 can be configured to detect the "probability P+" and the processing chain 344 can be configured to detect the "probability P" as shown in [Fig. 6]. Those skilled in the art will readily understand that the notation '+' or '-' used as a subscript, in association with a probability P, is used arbitrarily, so as to distinguish the probability values.

[0167] Thus, in a 30-n receiver, the single-photon detection unit 3424 can measure the arrival of photons at the detector in question (P+), and therefore their presence on the optical path 324-i leading to the detector (i.e., to the associated quantum measuring arm). Similarly, the single-photon detection unit 3444 can measure the arrival of photons at the detector in question (P), and therefore their presence on the optical path 322-i4 leading to the detector (i.e., to the associated quantum measuring arm).

[0168] The "energy-time" entanglement of photons of the same pairs from the emitter 10, as well as the adjustment of each of the receivers 30-n, including the implementation of a feedback control between the interferometric module 320 and the processing module 340 of each of the receivers 30-n, makes it possible to obtain a coincidence (i.e., a correlation or alternatively an anti-correlation) between the detection of the signal quantum interferometric SQfi in receiver 30-1 and detection of quantum interferometric signal SQf2 in receiver 30-2.

[0169] For example, and not limitingly, in a first variant where the detection of interferometric quantum signals in the 30-n receivers is correlated, when at a given time tx the single photon detection unit 3424, in the first receiver 30-1, measures a pb photon then at the same time tx, the single photon detection unit 3424 in the second receiver 30-2 measures a p2 photon then entangled with the pi photon, the pi and p2 photons being formed beforehand according to the particle pair (pb p2) in the emitter 10. Thus, at time tx, the 3444 units in the receivers 30-1 and 30-1 do not detect any photon.

[0170] A person skilled in the art will readily understand that the same reasoning can be applied to a correlated detection on the 3444 units.

[0171] In a second variant where the detection of interferometric quantum signals in the 30-n receivers is anti-correlated, when at a given time ty, the single-photon detection unit 3424 in the first receiver 30-1 measures a pb photon, then at the same time ty the single-photon detection unit 3444, in the second receiver 30-2, measures a p2 photon, then entangled with the pi photon, according to the particle pair (pb p2) formed in the emitter 10. Thus, at time ty, the 3444 unit in the first receiver 30-1 and the 3424 unit in the second receiver 30-2 do not detect any photon.

[0172] The person skilled in the art will readily understand that similar features can be implemented, according to an inverse anti-correlated detection on unit 3444, in the first receiver 30-1, and unit 3424, in the second receiver 30-2.

[0173] In other words, when the interferometers of the 30-n receivers are properly tuned, photons of the same pairs will always be detected on the same quantum measuring arm of each of the 30-n receivers. For example, and without limitation, in the first variant where the detection (i.e., the measurement) of the interferometric quantum signals in the 30-n receivers is correlated, the two photons of pairs can always be detected on the same side. That is to say, in a sequence of pairs formed by the emitter 10: for a first pair of photons in the sequence, the two photons can be detected on the same unit of the 30-n receivers (on unit 3424 or alternatively on unit 3444); for a second pair of photons in the sequence, the two photons can be detected on the same unit of the 30-n receptors (on unit 3424 or alternatively on unit 3444), and so on.

[0174] The interferometric measurement of the interferometric quantum signal, whether measured on the first receiver 30-1 or on the second receiver 30-2, corresponds in both cases to the projection of the "joint quantum state"

[0175]

[0176]

[0177]

[0178]

[0179]

[0180]

[0181]

[0182] » of the two particles (pb p2) of the same pair on a common state | y / } relative to the pair of receptors 30-1 and 30-2 (i.e. pair 30). The common state | y / ) can refer to the superposition of possible states relating to photons each taking a possible path of the interferometric modules 320 of the receiver pair 30-1 and 30-2. In particular, the common state |j of the two interferometric modules 320 of the receivers 30-1 and 30-2 can refer to the superposition of a first possible state and a second possible state. In embodiments, the first possible state may correspond to the case where the two particles (pb p2) have each taken the first interferometric arm 322-il of the 30-n receptors, and the second possible state may correspond to the case where the two particles (pb p2) have each taken the second interferometric arm 322-i2 of the 30-n receptors. For example, the first possible state can be denoted |S-|S7, with the notation "s" used to indicate that the particles took the short path. Similarly, the second possible state can be denoted j, with the notation "1" indicating that the particles took the long path. For example, and without limitation, the common state |yr) onto which the joint quantum state can be projected for each of the receptors 30-1 and 30-2 can be defined according to the following equation (05): . -?2> (05) I —- In equation (05), the terms t) and correspond to the interferometric phase shift defined according to equation (03) for the 30-1 receptor and the 30-2 receptor respectively. In other embodiments, the first possible state may correspond to the case where the pi particle of the entangled pair (pb p2) has taken the first interferometric arm 322-i1 (for example, in receptor 30-1) and the p2 particle has taken the second interferometric arm 322-i2 (for example, in receptor 30-2). In this case, the second possible state may correspond to the case where the pi particle of the entangled pair (pb p2) has taken the second interferometric arm 322-i2 (in receptor 30-1) and the p2 particle has taken the first interferometric arm 322-i1 (in receptor 30-2). These possible states thus correspond to cases where the photons of an entangled photon pair take different optical paths among the possible 322-il or 322-i2 optical paths of the 320 interferometric module of the 30-n receivers.

[0183]

[0184]

[0185]

[0186]

[0187]

[0188]

[0189]

[0190]

[0191]

[0192]

[0193] For example, the first possible state can be denoted |S^} and the second possible state can be denoted 11^2 ) • In embodiments, the common state | y / ) onto which the joint quantum state can be projected for each of the receptors 30-1 and 30-2 can be defined according to the following equation (06): . . (°6) 1^) = ——g Thus, when the control signals Cn, defined independently for each of the 30-n receivers, are configured to maintain the terms (^(t) and (p^(t)) equal to a constant component " " of the predefined phase setpoint (independently for each of the 30-n receptors), then the term " in equation (05) remains constant. The detection of entangled photons in the pair of receptors 30-1 and 30-2 is then said to be "stable over time". In the first variant where the detection of interferometric quantum signals in the 30-n receptors is correlated, the correlated detection probability P++ on the 3424 units of the 30-n receptors, or the correlated detection probability P- on the 3444 units can be defined according to the following equation (07): P++=P- = l (l + cosf^æ+^Ct))) (07) In the second variant where the detection of interferometric quantum signals in the 30-n receivers is anti-correlated, the correlated detection probability P+- or the correlated detection probability P^. can be defined according to the following equation (08): P^=P- = j (l-COS^tj+^t))) (08) In equations (07) and (08), the terms ^^(t) and t) are equal to a constant component « » of the setpoint (i.e., a constant « » defined independently for each of the 30-n receptors), so that the term "cos^Jt) + <p2(t) ) » reste constant. A 30-n receiver can include an analysis unit 348 of the interferometric measurement of the SQfn signal, i.e., detection probability values ​​obtained by the processing chains 342 and 344, as shown in [Fig.6]. Advantageously, an analysis unit 348 can include a storage element for the obtained detection probability values ​​P+ and P. The analysis unit 348 can further be configured to record, for each detection probability value stored, the measurement time relative to the moment when the quantum particles were detected respectively.

[0194] For example and without limitation, the value associated with a P+ detection on a 3424 unit of the 30-n receivers may be equal to a bit value of 1, and similarly, the value associated with a P detection on a 3444 unit of the 30-n receivers may be equal to a bit value of 0.

[0195] Between a time T0 and TF, the analysis units 348 of each of the receivers 30-n can be configured to record a sequence of entangled photon detection values, forming for each receiver 30-n a vector composed of a number #n of bits of 0s and 1s. The number #i of bits obtained for the receiver 30-1 can, for example, be equal to or different from the number #2 of bits obtained for the receiver 30-2. These vectors correspond to the so-called "raw keys" obtained by each of the receivers.

[0196] The analysis unit 348 may further include a time synchronization or coordination device between the receivers 30-n. Thus, the receivers 30-n can be configured to communicate with each other the different measurement times relating to each bit of the raw keys obtained. In addition, the analysis unit 348 of each of the receivers 30-n can be configured to apply one or more raw key algorithmic processing to generate the final key shared between the two receivers 30-n.

[0197] The analysis unit 348 can thus be configured to determine a quantum key using a set of stored probability values, each associated with a given measurement time.

[0198] In embodiments, a receiver 30-n (or an analysis unit 348) may further include a display device (not shown in the figures) configured to generate a display of the probability values ​​stored on a human-machine interface.

[0199] The quality and / or accuracy of the interferometric measurement of the SQfn signal, i.e. the measurement of the detection probabilities P+ and P at the level of the single photon detection units 3424 and 3444 of the processing chains, may be affected by the phase fluctuations undergone by the multiplexed optical signal Sn from its emission by the emitter 10 to its analysis by the receiver 30-n considered.

[0200] Indeed, such phase fluctuations can transform the interference pattern of the quantum signal initially transmitted by the emitter. In particular, in embodiments where the emitter 10 of system 1 is in motion relative to a receiver 30-n or relative to two receivers 30-n, the transmission of the multiplexed optical signal Sn transmitted from the emitter 10 to a receiver 30-n undergoes an effect Doppler (also called Doppler-Fizeau effect) involving a transformation of the relative phase between quantum states in the two arms of the 30-n receiver optical interferometer, thus inducing a transformed interference pattern.

[0201] Like all phase fluctuations, the reference signal of the multiplexed optical signal Sn undergoes the same Doppler shift as the qubits of the multiplexed optical signal Sn.

[0202] Furthermore, for the initial quantum signals Qi and Q2, composed of photons from entangled photon pairs, each receiver 30-n is normally assumed to generate the same interference pattern (i.e., the same probabilities P+ and P measured at the same times) of the quantum signal relative to the same pair of entangled photons. Since the relative phase changes occur independently between the different receivers 30-n, the interference pattern observed by the first receiver 30-1 may differ from the interference pattern observed by the second receiver 30-2. In other words, for a system 1, such phase changes induce a phase shift between the phase of the first initial quantum signal Qi and the phase of the second initial quantum signal Q2, altering the measurement of entanglement between the photons.

[0203] Figure 7 shows the empirical results of a Doppler effect emulation experiment between a transmitter 10, corresponding to a satellite, and receivers 30-n, corresponding to ground stations. In particular, Figure 7 illustrates the impact of the Doppler effect on the correlations between the first receiver 30-1 and the second receiver 30-2, without correction for phase fluctuations. Graph [a] in Figure 7 illustrates the variation (in gray) of the distance, in kilometers, between a transmitter 10 and a receiver 30-n, and the evolution (in black) of the phase of the signal transmitted between this transmitter 10 and this receiver 30-n over time. Graph [b] in [Fig.7] illustrates the evolution of the correlations of the estimated quantum signal interference patterns, not corrected by phase modulations, between the first receiver 30-1 and the second receiver 30-2 over time.The value 1 corresponds to a perfect correlation of the interference patterns of the 30-n receptors (i.e., the same probability measured on both 30-n receptors), while the value 0 corresponds to an anti-correlation of the interference patterns (i.e., different probabilities measured on the two receptors).

[0204] Each receiver 30-n of the pair 30 of receivers, according to the embodiments of the invention, is thus capable of correcting the phase fluctuations experienced by the multiplexed optical signal Sn considered, and thus obtaining a constant interference pattern of the interferometric measurement of the interferometric reference optical signal SRfn, which makes it possible to obtain an exact interference pattern of the interferometric measurement of the interferometric quantum signal SQfn with respect to the multiplexed optical signal Sn emitted by the emitter 10, that is to say, to obtain identical interference patterns or perfectly correlated from the interferometric measurement of the interferometric quantum signal SQfn between all 30-n receptors of the 30 pair.

[0205] In some embodiments, the variation in the distance between a transmitter 10 and a receiver 30-n can be predictable, for example, as a function of the satellite's path relative to a ground station (i.e., whether it is approaching or moving away from the ground in its orbit). In this case, the evolution of the phase of the signal transmitted between this transmitter 10 and this receiver 30-n over time can be predicted from this variation in distance. Advantageously, the processing module 340 of a receiver 30-n can be configured to determine (or simulate) a phase evolution of the signal as a function of time from the variation in the theoretical transmitter-receiver distance. Thus, the processing module 340 of a receiver 30-n can be configured to implement a servo algorithm for the phase modulation (|)n by generating the servo signal Cn from this phase evolution of the signal as a function of time (i.e.variation of the transmitter-receiver distance) and of the interferometric measurement of the SRfn signal. .

[0206] Fig. 8 is a flowchart representing the quantum transmission process implemented by the emitter 10 to emit optical signals, according to embodiments of the invention.

[0207] At step 1010, two initial quantum signals Qi and Q2 are generated by a signal generator 120, such that the initial quantum signals Qi and Q2 each comprise photons from pairs of photons entangled with each other.

[0208] At step 1020, two optical reference signals Ri and R2 can be generated by a signal generator 120.

[0209] At step 1030, the first initial quantum signal Qi and the first reference signal Ri are multiplexed to form the first multiplexed optical signal Si to be emitted.

[0210] At step 1040, the initial second quantum signal Q2 and the second reference signal R2 are multiplexed to form the second multiplexed optical signal S2 to be emitted.

[0211] At step 1050, the first multiplexed optical signal Si is transmitted from the transmitter 10 to the first receiver 30-1 of the set 30 of receivers.

[0212] At step 1060, the second multiplexed optical signal S2 is transmitted from transmitter 10 to the second receiver 30-2 of the set 30 of receivers.

[0213] A person skilled in the art will readily understand that certain steps in the emission process can be carried out simultaneously, in parallel, sequentially, independently or not, and / or in a different order.

[0214] Fig. 9 is a flowchart representing the quantum reception process implemented by each of the 30-n receivers to receive and process the quantum signals emitted by the transmitter 10, according to embodiments of the invention.

[0215] At step 3010, each of the receivers 30-n of the pair 30 of receivers receives a multiplexed optical signal Sn, in response to the transmission of quantum signals by the emitter 10, a multiplexed optical signal comprising a reference signal and a quantum signal.

[0216] In step 3020, in each of the 30-n receivers, the multiplexed optical signal Sn passes through an interferometer (having two interferometric arms) of the interferometric module 320 of the receiver considered. The multiplexed optical signal Sn then splits into two intermediate components (or interferometric components) which then pass through one of the interferometric arms of the interferometric module 320.

[0217] At step 3030, the passage through the reference signal part of the multiplexed optical signal Sn in the interferometer provides an interferometric reference optical signal SRfn at the output of the interferometric module 320.

[0218] At step 3040, at least one interferometric measurement of the SRfn interferometric reference optical signal is performed by the processing module 340 for each of the 30-n receivers.

[0219] In step 3050, an operation to determine a phase modulation value ¢, allowing to correct phase fluctuations suffered by the multiplexed optical signal Sn since its emission by the transmitter 10, is carried out from the interferometric measurement of the signal SRfn.

[0220] In step 3060, the phase modulation 0n is applied by the interferometric module 320 of the considered receiver 30-n to an intermediate component of the multiplexed optical signal Sn passing through one of the interferometric arms of the interferometric module 320, which provides in step 3070 an interferometric quantum signal SQfn at the output of the interferometric module 320

[0221] At step 3080, at least one interferometric measurement of the interferometric quantum signal SQfn is performed by the processing module 340 for each of the 30-n receivers.

[0222] At step 3090, for each 30-n receiver considered, a quantum encryption key shared by the set 30 of receivers (i.e. the same quantum key), is deduced from the interferometric measurement of the interferometric quantum signal SQfn.

[0223] A person skilled in the art will readily understand that certain steps in the receiving process can be carried out simultaneously, in parallel, sequentially, independently or not, and / or in a different order.

[0224] In particular, in embodiments where the signals are frequency multiplexed (i.e. relative to wavelength multiplexing), steps 3030 and 3070 for example, of output of the interferometric quantum signal SQfn and an interferometric reference signal SRfn of the interferometric module 320, can be carried out simultaneously.

[0225] The devices, components, or subcomponents of the quantum communication system 1, as well as the methods described according to the embodiments, can be implemented in various ways by hardware, software, or a combination of hardware and software implemented 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. Certain steps of the methods described in the present invention can be at least partially implemented in a computer device or system, and in particular in the form of computer program instructions executable by one or more processors, in a computer device or system.These computer program instructions can also be stored without computer-readable media.

[0226] The invention is not limited to the embodiments described above by way of non-limiting example. It encompasses all the variant embodiments that could be envisaged by a person skilled in the art. < / t>

Claims

1. Demands Quantum communication system (1) comprising: an emitter (10) configured to emit two distinct multiplexed optical signals (Sn), each multiplexed optical signal (Sn) comprising an initial reference optical signal (Rn) and an initial quantum signal (Qn), the two initial quantum signals (Qn) being entangled quantum signals comprising photons from entangled photon pairs, and two receivers (30-n), each receiver (30-n) being configured to receive one of said multiplexed optical signals (Sn), each receiver comprising: • an interferometric module (320) comprising at least two separate optical paths (322-11 and 322-12), the interferometric module (320) being configured to divide said received multiplexed optical signal (Sn) into two intermediate signal components (S1'n and S12n), said interferometric module (320) comprising a phase modulation unit (321) arranged on one of said optical paths (322-12) and configured to apply phase modulation ( <j)n) à uniquement une desdites composantes intermédiaires (S12n), ledit module interférométrique (320) étant en outre configuré pour générer un signal quantique interférométrique (SQfn) et un signal optique de référence interférométrique (SRfn) à partir desdites composantes intermédiaires de signal (Si-1n et Si 2n), et • a processing module (340) configured to perform the interferometric measurement of said interferometric quantum signal (SQfn) and the interferometric measurement of the interferometric reference optical signal (SRfn), said processing module (340) being further configured to determine the phase modulation (4>n) to be applied, from said measurement interferometric of the interferometric reference optical signal (SRfn), each receiver (30-n) being further configured to determine a quantum key shared between the two receivers (30-n), from said interferometric measurement of the interferometric quantum signal (SQfn).

2. System (1), according to claim 1, wherein for each pair of entangled photons arising from said initial quantum signals (Qn) said entangled photons have a joint quantum state, and wherein said interferometric measurements of said interferometric quantum signals (SQfi and SQf2) in said receivers (30-n) each correspond to the projection of said joint quantum state of said entangled photons onto a common state ( | y / ) ) relative to said interferometric modules (320) of said receivers (30-n).

3. System (1), according to any one of claims 1 to 2, wherein said interferometric measurements of said interferometric quantum signals (SQfi and SQf2) in said receivers (30-n) are correlated with each other.

4. System (1), according to claim 3, wherein said common state ( | y / ) ) relating to said interferometric modules (320) of said receivers (30-n) corresponds to the superposition of a first possible state and a second possible state, said first possible state ( | S]S2 ) ) corresponding to the case where said entangled photons have respectively taken the same optical path among said optical paths (322-il) of said interferometric module (320), and said second possible state ( 1) ) corresponding to the case where said entangled photons have respectively taken the other same optical path among said optical paths (322-i2) of said interferometric module (320).

5. System (1), according to any one of claims 1 to 2, wherein said interferometric measurements of said interferometric quantum signals (SQfi and SQf2) in said receiver (30-n) are anti-correlated with each other.

6. System (1), according to claim 5, wherein said common state ( | 1 / / ) ) relating to said interferometric modules (320) of said receivers (30-n) corresponds to the superposition of two states possible, the said possible states ( | S^l^) | iiS2) ) corresponding to cases where the said entangled photons have taken different optical paths respectively among the said optical paths (322-il, 322-i2) of the said interferometric module (320).

7. System (1), according to any one of claims 1 to 6, wherein the two receivers (30-n) are further configured to use the shared quantum key to encrypt and / or decrypt a communication message (SM) to be transmitted between them.

8. System (1), according to any one of claims 1 to 7, wherein said transmitter (10) is in motion relative to at least one of said receivers (30-n) considered, said phase modulation ((K) being applied to correct phase fluctuations related to a Doppler effect experienced by said multiplexed optical signal (Sn) between said transmitter (10) and said receiver (30-n) considered.

9. System (1), according to any one of claims 1 to 8, wherein said emitter (10) comprises: - a signal generator (120) configured to generate said entangled initial quantum signals (Qn) and said initial optical reference signals (Rn), and - two signal integrators (141), each signal integrator (141) being configured to generate a multiplexed optical signal (Sn) comprising one of said initial optical reference signals (Rn) and one of said initial quantum signals (Qn).

10. System (1), according to claim 9, wherein said multiplexed optical signals are frequency-multiplexed signals, for each multiplexed optical signal (Sn), said initial reference optical signal (Rn) and said initial quantum signal (Qn) being characterized by a frequency difference between a quantum wavelength (XQ, or XQ1; XQ2) of said initial quantum signal (Qn) and a reference wavelength (XR, or XR1; greater than or equal to a minimum value (6X) of predefined wavelength difference, said signal integrators (141 and 142) being wavelength-multiplexing units.

11. System (1), according to any one of claims 1 to 10, wherein for each receiver (30-n), said processing module (340) comprises two signal demultiplexing units (3422 and 3442), two single-photon detection units (3424 and 3444) and two auxiliary detection units (3426 and 3446), each single-photon detection unit (3424; 3444) being positioned at a first output of one of said signal demultiplexing units (3422; 3442), each auxiliary detection unit (3426; 3446) being positioned at a second output of one of said signal demultiplexing units (3422; 3442) and configured to deliver an electrical signal (I342; I344), and wherein said processing module (340) comprises a correction unit (346) configured to perform a measurement of a ratio value (Pm) of the electrical signals (I342 and I344) of said auxiliary detection units (3426 and 3446) and to perform a comparison of said value of the measured ratio (Pm) relative to a predefined signal ratio value (Po).

12. System (1), according to claims 10 and 11, wherein said signal demultiplexing units (3422 and 3442) each comprise at least one band-reject filter and / or an Add / Drop WDM type filter.

13. System (1), according to any one of claims 11 to 12, wherein each receiver (30-n) is configured to apply and / or trigger a servo loop between said interferometric module (320) and said processing module (340) by applying an electrical servo signal (Cn) to control said phase modulation (4>n), said electrical servo signal (Cn) being determined from a servo algorithm implemented to obtain and / or maintain said measured ratio value (Pm) equal to the predefined ratio value (Po).

14. A quantum communication method comprising optical signal emission steps implemented by a transmitter (10) and consisting of: - generate two initial quantum signals (Qn) entangled with each other, and two initial optical reference signals (Rn), - generate and emit two distinct multiplexed optical signals (Sn), each multiplexed optical signal (Sn) comprising an initial reference optical signal (Rn) and an initial quantum signal (Qn), said method comprising optical signal reception steps implemented each by two separate (30-n) receivers and consisting of: - receive one of said distinct multiplexed optical signals (Sn), - pass said multiplexed optical signal (Sn) through an interferometer of the receiver (30-n) considered, - generate an interferometric reference optical signal (SRfn), - perform an interferometric measurement of said interferometric reference optical signal (SRfn), - determine a phase modulation (4>n) from said interferometric measurement of the interferometric reference optical signal (SRfn), - apply said phase modulation (¢,,) to an intermediate component (S12n) of the multiplexed optical signal (Sn) received in the interferometer of said receiver (30-n) considered, said method further comprising additional steps carried out each by two receivers (30-n) and consisting of determining a quantum key shared between the two receivers (30-n), from said interferometric measurement of the interferometric quantum signal (SQfn).

15. A quantum communication method according to claim 14, wherein said method comprises the transmission of a communication message (SM) between the two receivers (30-n), and wherein said method further comprises steps carried out each by two receivers (30-n) and consisting of using the quantum key shared to encrypt and / or decrypt the communication message (SM).