Quantum key distribution system using photon polarization correction

Abstract

There is provided a transmitter (10) of a multiplexed signal (S1) through a transmission channel (50) comprising: a generator (120) of an initial quantum signal (SQ0), of a first and a second reference signal (R01, R02), an encoder (140) with N optical channels (Bn) comprising a selector (142) for directing the initial signal towards one of the channels, and a recombiner (148) for generating said multiplexed signal comprising a first and a second control signal (R11, R12) of a first and a second polarization encoding value (P1, P2) respectively, and a quantum signal (SQ1) determined from the initial signal encoded on an encoding value. The optical channels comprise two channels (B1, B2) each comprising an integration unit (144-1, 144-2) of a reference signal in the channel, each control signal being determined from one of the reference signals delivered by the channel considered to the recombiner. Figure for the abstract: [Fig.2]

Description

[0001] Title of the invention: Quantum key distribution system using photon polarization correction Technical field

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

[0003] The main application of current quantum telecommunications systems is to use quantum information theory to distribute a cryptographic key (or encryption key) between two remote telecommunications devices (i.e., two users), via specific quantum protocols, with the aim of subsequently encrypting, in an ultra-secure manner, the communications between these two devices. Such quantum protocols are generally designated by the acronym QKD, meaning 'quantum key distribution' in French according to the corresponding Anglo-Saxon expression 'Quantum Key Distribution'. The keys obtained from a QKD protocol are secret cryptographic keys having a higher degree of security than the keys obtained by classical protocols.

[0004] In the field of quantum cryptography, the remote users of a quantum communication system are conventionally called Alice (transmitting device) and Bob (receiving device). A QKD protocol comprises a step of transmitting information encoded on quantum particles, a step of receiving these particles and a step of reconciliation between the transmitter and the receiver.

[0005] The transmission step consists of encoding classical information (0 or 1) on an encoding variable of quantum particles (also called "qubits"), generally corresponding to photons. An encoding variable of a qubit corresponds to a degree of freedom of the quantum particle and can be the polarization of the photon. The reception step consists of determining the state of the received photons according to the encoding variable chosen to recover the encoded classical information. In the reconciliation step, the transmitting device and the receiving device communicate to correct potential transmission errors and generate a shared raw key. These devices, transmitter and receiver, thus respectively transform the encoded information and the determined information (corresponding to their respective raw key), into an ultra-secure key allowing an amplification of confidentiality in their exchanges by telecommunications.

[0006] Such a type of QKD quantum key distribution protocol using the polarization of quantum particles as an encoding variable requires an encoding, on transmission, and a measurement, on reception, of the qubits in at least two different and non-orthogonal polarization bases.

[0007] However, the polarization state of quantum particles between their emission and their reception (i.e. during their propagation) undergoes random rotations. These may be due to the birefringence of the various media crossed or to the movement of the transmitting device relative to the receiving device, such as the movement of a satellite (transmitter or receiver) relative to a ground station in the case of communications via a space segment.

[0008] To avoid such random rotations of polarization state, some known quantum key sharing systems use propagation only in free space in which the polarization of photons is stable, during their propagation through a transmission channel. However, in some applications, it is necessary to use guided optical transmission, as a transmission channel, for example for propagation on a ground network or on board a satellite to relax the construction constraints of the payload.

[0009] To compensate (or correct) random rotations of polarization state, some known systems use, at the start of the QKD protocol, a single polarization reference which makes it possible to initially estimate the polarization rotations induced during propagation and to align the polarization of the transmitted photons with the measurement bases in reception. However, this single reference does not make it possible to correct the new polarization rotations after the initial estimation phase. Alternatively, other existing systems instead use a periodic generation of reference signals of the encoding bases from the quantum signal source and therefore time-multiplexed with the qubits, which reduces the useful throughput of the system.

[0010] There is thus a need for an improved quantum key distribution system capable of correcting in real time the rotations of the polarization states used for the encoding and decoding of qubits. Summary of the invention

[0011] For this purpose, a transmitter is provided configured to transmit a multiplexed signal through a transmission channel. The transmitter comprises:

[0012] - a signal generator configured to generate an initial quantum signal, a first reference signal and a second reference signal,

[0013] - a polarization encoder comprising a plurality of N optical channels, the polarization encoder further comprising:

[0014] - an optical selector configured to select one of the optical paths and to direct the initial quantum signal generated to the selected optical path,

[0015] - an optical recombiner configured to generate the multiplexed signal, the signal multiplexed comprising a first control signal of a first polarization encoding value, a second control signal of a second polarization encoding value, and a quantum signal encoded on a polarization encoding value chosen from a set of values ​​comprising at least the first polarization encoding value and the second polarization encoding value, the encoded quantum signal being determined from the initial quantum signal delivered by the optical channel selected by the optical selector.

[0016] The optical pathways comprise a first optical pathway comprising a first integration unit configured to integrate the first reference signal into the first optical pathway and a second optical pathway comprising a second integration unit configured to integrate the second reference signal into the second optical pathway, the first control signal being determined from the first reference signal delivered by the first optical pathway to the optical recombiner, and the second control signal being determined from the second reference signal delivered by the second optical pathway to the optical recombiner.

[0017] In embodiments, each optical channel of the polarization encoder may be associated with a polarization encoding value of the set of values, and at least one of the optical channels may further comprise an optical element configured to modify the polarization of an optical signal traveling through the optical channel according to the associated encoding value.

[0018] In one embodiment, the transmitter may be an all-optical guided device, with the optical paths of the polarization encoder being formed from polarization-maintaining fibers and / or integrated waveguides.

[0019] The present invention further provides a receiver configured to receive a multiplexed signal through a transmission channel, the multiplexed signal comprising an encoded quantum signal, a first control signal of a first polarization encoding value and a second control signal of a second polarization encoding value. The receiver comprises a beam splitter configured to separate the multiplexed signal into two signal components comprising a component of the first control signal and a component of the second control signal, each signal component respectively passing through a processing chain associated with a polarization base composed of at least one polarization state, one of the signal components further comprising the encoded quantum signal.

[0020] Each processing chain comprises a correction device adapted to determine the polarization state of an integrated control signal of the signal component traveling through the chain, the correction device being further adapted to modify the polarization of the signal component so as to align the determined polarization state with respect to one of the at least one polarization state of the associated base, each processing chain comprising a detection module adapted to measure the encoded quantum signal according to at least one of the at least one polarization state of the associated base.

[0021] In embodiments, for each processing chain, the correction device can be configured to demultiplex the signal component to select one of the control signal components and route it to a polarization analysis device comprising at least one detection unit and adapted to detect the selected integrated control signal component according to one of the at least one polarization state of the associated base.

[0022] According to certain aspects, for each processing chain, the correction device may further comprise a processor configured to analyze the determined polarization state and to generate a servo signal applied to a polarization correction module of the signal component.

[0023] In embodiments, the beam splitter may be a 50 / 50 fiber-coupled Y-symmetric optical coupler, and the processing chains may be formed from polarization-maintaining fibers and / or single-mode optical fibers.

[0024] The embodiments of the invention thus provide a quantum distribution system for encryption keys comprising a transmitter and a receiver.

[0025] In embodiments, the multiplexed signal may be a frequency-multiplexed signal.

[0026] In one embodiment, the absolute value of the wavelength difference between the encoded quantum signal and the first and / or second integrated signal may be greater than or equal to a first minimum wavelength difference value, and the absolute value of the wavelength difference between the first control signal and the second control signal may be greater than or equal to a second minimum wavelength difference value.

[0027] The present invention further provides a transmission method for transmitting a multiplexed signal through a transmission channel, the method comprising the steps of:

[0028] - generate an initial quantum signal, a first reference signal and a second reference signal,

[0029] - selecting an optical channel from a plurality of N optical channels, the channels optics comprising a first optical path and a second optical path, and directing the initial quantum signal generated to the selected optical path,

[0030] - insert the first reference signal into the first optical channel and the second reference signal in the second optical channel,

[0031] - forming the multiplexed signal, the multiplexed signal comprising a first signal of control of a first polarization encoding value, a second control signal of a second polarization encoding value, and a quantum signal encoded on a polarization encoding value chosen from a set of values ​​comprising at least the first polarization encoding value and the second polarization encoding value, the encoded quantum signal being determined from the initial quantum signal delivered by the selected optical channel, the first control signal being determined from the first reference signal delivered by the first optical channel, and the second control signal being determined from the second reference signal delivered by the second optical channel.

[0032] The present invention also provides a method for receiving a multiplexed signal through a transmission channel, the multiplexed signal comprising an encoded quantum signal, a first control signal of a first polarization encoding value and a second control signal of a second polarization encoding value, the method comprising the step of separating the multiplexed signal into two signal components comprising a component of the first control signal and a component of the second control signal, each signal component respectively passing through a processing chain associated with a polarization base composed of at least one polarization state, one of the signal components further comprising the encoded quantum signal.

[0033] The receiving method further comprises the iterative steps of:

[0034] - determine the polarization state of an integrated control signal of the component of signal running through the chain, and

[0035] - modify the polarization of the signal component so as to align the state of polarization determined with respect to one of at least one polarization state of the associated base.

[0036] The reception method comprises the step of determining the encoded quantum signal according to at least one of the at least one polarization state of the associated base.

[0037] The embodiments of the invention thus make it possible to correct the polarization rotations of the qubits (defined according to at least two different and non-orthogonal polarization bases) transmitted between a transmitter and a receiver of a quantum signal, in order to establish a quantum key.

[0038] In particular, embodiments of the invention provide a signal transmitter for robustly integrating polarization reference signals into a quantum communication signal.

[0039] Such references can be generated at any power, independently of the generation of the qubits, to form an efficient and affordable solution in terms of hardware complexity. The guided optics transmitter, according to the embodiments of the invention, advantageously has a reduced volume and mass compactness, as well as optimized size and robustness. Furthermore, frequency multiplexing of such references to the qubits makes it possible to maintain a high transmission rate of useful information (i.e. the qubits).

[0040] The receiver according to the embodiments of the invention makes it possible to correct in real time the polarization rotations undergone by the qubits before detection. Such a receiver makes it possible in particular to analyze the qubits and the polarization reference signals independently, in order to best align the polarization of the qubits with measurement bases of the receiver. Description of the figures

[0041] Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the appended drawings given by way of example.

[0042] [Fig.l] [Fig.l] is a diagram representing a quantum communication system, according to embodiments of the invention.

[0043] [Fig.2] [Fig.2] is a diagram representing a polarization encoder of a transmitter, according to embodiments of the invention.

[0044] [Fig.3] [Fig.3] is a diagram representing a polarization encoder of a transmitter, according to embodiments of the invention.

[0045] [Fig.4] [Fig.4] is a diagram representing a polarization encoder of a transmitter, according to embodiments of the invention.

[0046] [Fig.5] [Fig.5] is a diagram showing an optical selector of a polarization encoder, according to embodiments of the invention.

[0047] [Fig.6] [Fig.6] is a diagram showing a signal generator of a transmitter, according to embodiments of the invention.

[0048] [Fig.7] [Fig.7] is a diagram representing a receiver of a quantum communication system, according to embodiments of the invention.

[0049] [Fig.8] [Fig.8] is a diagram representing a receiver of a quantum communication system, according to embodiments of the invention.

[0050] [Fig.9] [Fig.9] is a diagram representing a receiver of a quantum communication system, according to embodiments of the invention.

[0051] [Fig. 10] The [Fig. 10] module for detecting an integrated control signal, used in a processing chain of a receiver, according to embodiments of the invention.

[0052] [Fig. 11] [Fig. 11] is a diagram representing a module for detecting an integrated control signal, used in a processing chain of a receiver, according to embodiments of the invention.

[0053] [Fig. 12] [Fig. 12] is a diagram representing a module for detecting a quantum signal, used in a processing chain of a receiver, according to embodiments of the invention.

[0054] [Fig. 13] [Fig. 13] is a flowchart showing a method of transmitting a signal comprising a quantum signal produced by a transmitter, according to embodiments of the invention.

[0055] [Fig. 14] [Fig. 14] is a flowchart of a method of transmitting a signal comprising a quantum signal produced by a receiver, according to embodiments of the invention.

[0056] Identical references are used in the figures to designate identical or similar elements. For reasons of clarity, the elements shown are not to scale. Detailed description

[0057] [Fig.l] schematically represents a quantum communication system 1 comprising two communicating devices 10 and 30 capable of communicating with each other, according to embodiments of the invention. The two devices comprise a transmitter 10 (or transmitter device), also called 'Alice', and a receiver 30 (or receiver device), also called 'Bob'.

[0058] The quantum communication system 1 can be used for example in the space domain and comprise a transmitter 10 (or reciprocally a receiver 30) mounted on board a satellite while the receiver 30 (or reciprocally the transmitter 10) is a terrestrial device (i.e. on the ground). Alternatively, the system 1 can be used in an application where at least one of the transmitter 10 and receiver 30 devices is an avionics device. Furthermore, the system 1 can be used in an application where at least one of the transmitter 10 and receiver 30 devices is an all-optical guided device, potentially integrated into a ground-based fiber network. The transmitter device 10 and / or the receiver device 30 can be fixed or in motion relative to the other device with which it communicates (30 or 10 as the case may be).

[0059] The transmitter 10 comprises a signal generator 120 and a polarization encoder 140.

[0060] As used herein, an 'optical signal' (also referred to simply as a 'signal') results from one or more pulses of coherent light from an optical source, such as for example a laser beam. A laser beam can in particular be characterized by its pulse rate f and by a laser pulse (i.e. the signal) defined by its frequency te, its intensity T, its polarization P and its phase. The 'frequency œ' of the laser beam designates the 'optical frequency of the laser pulse multiplied by 2tt' defined as a function of the wavelength of the beam A, such that X x denotes the speed of light.

[0061] A 'quantum signal' may refer to a pulsed optical signal having on average less than one photon per pulse. For the purposes of this invention, an emitted quantum signal may refer to a pulsed optical signal having a low number of photons per pulse. Measuring a quantum signal provides a measure of detection of a photon dependent on a 'probability of detection' of that photon.

[0062] The transmitter 10 is configured to generate and transmit, through a transmission channel 50, a multiplexed optical signal (also called 'multiplexed optical signal' or 'multiplexed communication signal'), denoted Si. The multiplexed signal Si comprises a quantum signal, denoted SQi, where the useful information is encoded therein on the polarization of the pulses constituting the quantum signal. The polarization of a photon of the encoded quantum signal SQi is chosen from a set n of states (also called 'encoding states' or 'encoding values') comprising at least a first polarization encoding value, denoted Pb and a second polarization encoding value, denoted P2.The multiplexed signal Si also comprises a first integrated optical signal Ru for controlling the first polarization encoding value Pi (also called 'first control signal' Ru) and a second integrated optical signal R12 for controlling the second polarization encoding value P2 (also called 'second control signal' Ri2).

[0063] The transmission channel 50 may be, for example, a free space or a fiber device for transporting information using, for example, fiber optic elements for communication, depending on the field of application of the invention.

[0064] The receiver 30 is configured to receive the multiplexed signal Si from the transmission channel 50, transmitted by the transmitter 10, and to perform an estimation of the received control signals, which provides estimated control signals R3[ and R32. The receiver 30 is further configured to perform an estimation of the received quantum signal, which provides an estimated received quantum signal SQ3, from the estimated control signals R31 and R32.

[0065] According to one aspect of the invention, the transmitter 10 and the receiver 30 are configured to determine (i.e. establish) a quantum encryption key, using the polarization-encoded quantum signal Sqi and the estimated received quantum signal SQ3. The system 1 can thus be a quantum encryption key distribution system configured to carry out quantum distribution of key(s) within a space or terrestrial communication service with the aim of ensuring the security of part or all of the communications exchanged between the transmitter and the receiver.

[0066] In embodiments, the quantum communication system 1 may comprise a plurality of distinct receivers 30. In this case, the transmitter 10 may be configured to generate and transmit to at least two of the distinct receivers, for example sequentially, a specific multiplexed optical signal. The system 1 may thus be configured to carry out a quantum distribution of key(s) between these distinct receivers.

[0067] The multiplexed signal Si is generated, via the polarization encoder 140 (also called 'polarization encoding module'), from an initial quantum signal, denoted SQ0, a first reference signal, denoted ROi, and a second reference signal, denoted Ro2. The initial quantum signal SQ0 and the reference signals R0i and R02 come from the signal generator 120, as shown in [Fig.l].

[0068] Figures 2, 3 and 4 schematically represent the polarization encoder 140 of the transmitter 10 configured to form the multiplexed signal Si, according to embodiments of the invention.

[0069] The polarization encoder 140 can be implemented in the form of an optical instrument, of the optical interferometer type for example, composed of a plurality of N optical arms (also called 'optical channels') for polarization encoding Bn. The index 'n' designates the index of the nth optical arm of the polarization encoder 140 and is an integer between 1 and N, the value of N being greater than or equal to 2. In particular, the value of N can be an integer equal to 2, as in the examples of FIGS. 2 and 3, or an integer equal to 4 as in the example of [Fig. 4]. The polarization encoder 140 thus comprises at least a first optical arm Bi (i.e. 'first optical channel Bi') and a second optical arm B2 (i.e. 'second optical channel B2').

[0070] The polarization encoder 140 comprises an optical selector 142 (also called an 'optical path selection unit', 'optical selector', 'optical router' or 'optical switch') and an optical recombiner 148 (also called an 'optical path recombination unit' or 'beam recombination unit') configured to deliver the multiplexed signal Si transmitted by the transmitter 10. The different optical arms Bn extend between the optical selector 142 and the optical recombiner 148.

[0071] The optical selector 142 of the polarization encoder 140 is configured to receive the initial quantum signal SQ0 and to direct it (i.e., route it) to one of the optical arms Bn. The polarization encoder 140 can thus be configured to control the optical selector 142, i.e., to control the direction of propagation of the initial quantum signal SQ0 towards one of the optical arms Bn, in response to a control signal SCi4- For example and without limitation, such a control signal SCi4 may be an electrical or radiofrequency signal, constructed from a number N of control values, each control value corresponding to a polarization encoding optical arm Bn. Thus, the control signal Scu may comprise a plurality of values ​​chosen, randomly for example, from the predefined control values.

[0072] The polarization encoder 140 also comprises a first signal integration unit 144-1 and a second signal integration unit 144-2. The first signal integration unit 144-1 is arranged on the first optical arm Bi of the encoder 140 and is configured to insert (i.e., incorporate) the first reference signal ROi into the first optical arm Bb. The second signal integration unit 144-2 is arranged on the second optical arm B2 of the encoder 140 and is configured to insert the second reference signal RO2 into the second optical arm B2.

[0073] In other words, in response to a specific control signal SCi4, the optical selector 142 can be configured to direct to the first optical arm Bp The first integration unit 144-1 can thus be configured to multiplex (or combine) the first reference signal Roi with the possible signal which circulates in the first optical arm Bi (i.e. the initial quantum signal if it has been directed into the first optical arm Bi by the optical selector 142). If the initial quantum signal SQ0 is not directed to the first optical arm B Je optical selector 142 can be configured to direct the signal SQ0 to the second optical arm B2.The second signal integration unit 144-2 can thus be configured to multiplex the second reference signal R02 with the possible signal which circulates in the second optical arm B2 (i.e. the quantum signal if it has been rather directed into the second optical arm B2 by the optical selector 142 and not into the first optical arm Bi).

[0074] In embodiments where the polarization encoder 140 is composed of a number N optical arms strictly greater than 2, in response to a specific control signal SCi4, the optical selector 142 can further be configured to direct the initial quantum signal SQ0 towards an optical arm Bn, which is distinct from the first optical arm Bi and the second optical arm B2.

[0075] For example, for a polarization encoder 140 comprising two polarization encoding optical arms Bi and B2, as shown for example in FIGS. 2 and 3, if the optical selector 142 directs the initial quantum signal SQ0 to the first optical arm Bb the resulting signal at the output of the first signal integration unit 144-1 results from the optical multiplexing of the first reference signal ROi and the initial quantum signal SQ0 which has been routed into the first optical arm Bh while the resulting signal output from the second signal integration unit 144-2 comprises only the second reference signal Ro 2. Alternatively, if the optical selector 142 directs the initial quantum signal SQ0 to the second optical arm B2, the resulting signal output from the first signal integration unit 144-1 comprises only the first reference signal ROi, while the resulting signal output from the second signal integration unit 144-2 results from the optical multiplexing of the second reference signal Ro 2 and the initial quantum signal SQ0.

[0076] According to another example, for a polarization encoder 140 comprising four polarization encoding optical arms Bb B2, B3 and B4, as shown in [Fig.4], if the optical selector 142 directs the initial quantum signal SQ0 towards the first or towards the second optical arm Bi or B2, then no signal circulates (i.e. propagates) on the third and the fourth optical arms B3 and B4. Furthermore, that is, no signal is output from the third and fourth optical arms B3 and B4 to the optical recombiner 148. Alternatively, if the optical selector 142 directs the initial quantum signal SQ0 to the third or fourth optical arms B3 or B4, the resulting signals output from the first signal integration unit 144-1 and the second signal integration unit 144-2 respectively comprise only the first reference signal ROi or the second reference signal R02.In this case, a quantum signal, determined from the initial quantum signal SQ0, is delivered by the third optical arm B3 (or by the fourth optical arm B4) to the optical recombiner 148, and no signal is delivered by the fourth optical arm B4 (or respectively by the third optical arm B3), depending on the optical path selected by the optical selector 142.

[0077] Each optical arm Bn of the polarization encoder 140 is associated with a beam-specific polarization encoding value Pn. The set H of possible variables thus comprises the plurality of N encoding variables Pn which are distinct from each other. Advantageously, each control value of the signal Sci4 of the optical selector 142, corresponding to an optical polarization encoding arm Bn, also corresponds to a polarization encoding state Pn. The control signal SCi4 can thus make it possible to form at least part of a so-called raw quantum encryption key to be shared between the transmitter 10 and the receiver 30 in the system 1. The useful information is then transported by the quantum signal (i.e. the quantum particles) modulated according to the encoding states Pn according to each of the optical arms of the transmitter Alice.

[0078] In such embodiments, an optical arm Bn of the polarization encoder 140 may further comprise an optical element 146-n (also called a 'polarization modification unit' or 'encoding unit') configured to modify (or encode) the polarization of an optical signal which travels through the optical arm Bn according to the encoding value Pn associated with said optical arm.

[0079] For example, the polarization encoder 140 may comprise at least a first optical element 146-1 arranged on the first optical arm Bb between the output of the first signal integration unit 144-1 and the input of the optical recombiner 148, so as to modify the polarization of the resulting signal at the output of the first signal integration unit 144-1 (i.e. the signal corresponding to the multiplexing of the first reference signal ROi and the initial quantum signal SQ0, or only the signal corresponding to the first reference signal ROi), according to the first polarization encoding value Pi associated with the first optical arm Bi.

[0080] Advantageously, the initial quantum signal SQ0, the first reference signal Roi, and the second reference signal R02, from the signal generator 120 may initially be characterized by the same polarization Po at the input of the polarization encoder 140. For example and without limitation, such an initial polarization Po may be a linear polarization, of type H, i.e. “horizontal” (or alternatively of type V, i.e. “vertical”). In this case, a polarization encoding value Pn of an optical arm Bn comprising an encoding unit 146-n may correspond to a linear polarization of type V (or respectively of type H).Thus, an optical element 146-n can be configured to rotate by an angle of ±90° (i.e. apply a rotation of ±90° to) the initial polarization Po of an optical signal delivered at the output of the optical selector 142, and / or at the output of a signal integration unit (144-1 and / or 144-2) on the optical arm Bn. Such an encoding value in polarization Pn can also correspond to a linear polarization of type D, i.e. “diagonal” (or type A, i.e. “anti-diagonal”). In this case, the optical element 146-n can be configured to rotate by an angle of ±45° (i.e. apply a rotation of ±45° to) the initial polarization Po of an optical signal traveling along the optical arm Bn at the output of the optical selector 142, and / or at the output of a signal integration unit (144-1 and / or 144-2).

[0081] In embodiments, the polarization encoder 140 may be a guided all-optical device. As used herein, the term 'guided all-optical device' refers to an optical device whose optical signal transmission paths are made of optical fibers and / or so-called integrated waveguides typically used in integrated photonics. In this case, an encoding unit 146-n may comprise one or more polarization rotation transmission means, adapted to apply an angle rotation to the polarization of an optical signal delivered at the output of a signal integration unit (144-1 and / or 144-2) or to the initial quantum signal at the output of the optical selector 142, on the optical arm Bn.

[0082] For example and without limitation, such a 146-n encoding unit can be implemented in the form of a fiber called a 'polarization rotation fiber' and corresponding in particular to a polarization maintaining fiber, or PMF (acronym for Polarization Maintaining Fiber), and comprising a constraint axis which modifies (or rotates) in an appropriate manner the polarization of the signal, passing through this fiber, according to an angle predetermined by the initial polarization of said signal and the encoding value Pn associated with the optical arm Bn. Such embodiments are illustrated in Figures 2 and 4. In particular, in [Fig.2], the transmission means 144-il corresponds to the encoding unit 146-1 of the optical arm Bb while in [Fig.4], the transmission means 148-i 1 corresponds to the encoding unit 146-1 of the optical arm Bi and furthermore the transmission means 142-i4 corresponds to the encoding unit 146-4 of the optical arm B4.

[0083] Advantageously, the polarization encoder 140 may comprise one or more intermediate optical recombiners making it possible to lighten the structure of the optical instrument by combining (or rationalizing) certain optical functions. For example, in [Fig. 4], the polarization encoder 140 comprises the intermediate optical recombiners 148-1 and 148-2 arranged upstream of the optical recombiner 148 delivering the multiplexed signal Sp. In this case, an encoding unit 146-n may be composed of several polarization rotation fibers, implemented in the form of polarization maintaining fibers PMF each comprising a constraint axis, the resultant of which is adapted to the polarization rotation angle predetermined by the encoding value Pn associated with the optical arm Bn. This embodiment is illustrated by the arrangement of [Fig.4] which uses a combination of the transmission means 142-i3 and 148-i 1 corresponding to the encoding unit 146-3 of the optical arm B3. Such intermediate optical recombiners make it possible to reduce the total number of polarization rotation transmission means for encoding the different Pn encoding values ​​of the transmitter 10 to be implemented, via the combination of transmission means to form resulting encoding units.

[0084] In the example illustrated in [Fig.4], each optical arm Bb B2, B3 or B4 is formed from the optical selector 142 to the optical recombiner 148, the optical arms Bi and B3, and the optical arms B2 and B4, respectively having common optical paths 148-il and 148-i2. By way of illustration, the transmission means 142-i3 and 142-i4, shown in [Fig.4], which extend between the optical selector 142 and the input of the intermediate optical recombiners 148-1 and 148-2, without intermediate elements, can be configured to rotate by an angle of +90° the initial polarization Po of the quantum signal traveling respectively through the optical arms B3 and B4. In addition, the transmission means 148-il, arranged between the output of the first intermediate optical recombiner 148-1 and the input of the optical recombiner 148, can be configured to rotate by an angle of +45° the polarization of the resulting signal traveling along the optical path common to the optical arms Bi and B3 (i.e. the signal comprising at least the first reference signal Roi). In this example, the polarization encoding value Pi of the optical arm Bi can correspond to a polarization rotation of +45°, the polarization encoding value P3 of the optical arm B3 can correspond to a polarization rotation of -45°, and the polarization encoding value P4 of the optical arm B4 can correspond to a polarization rotation of 90°.

[0085] Alternatively, in embodiments where the polarization encoder 140 is a device comprising at least one free-space signal transmission means, an integration unit (144-1; 144-2) may be implemented from one or more dichroic filters. Furthermore, an encoding unit 146-n may be implemented from one or more polarization-rotating thin plates (or delay plate), such as a half-wave plate and / or a quarter-wave plate, as shown in [Fig. 3].

[0086] In embodiments, a polarization encoding value Pn, associated with the optical arm Bn, may correspond directly to the initial polarization Po. In this case, such an optical arm of the polarization encoder 140 may be arranged so as not to modify the polarization of the optical signal(s) passing through it (i.e. polarization rotation of 0°). The optical arm Bn of the polarization encoder 140 may thus comprise one or more polarization-maintaining transmission means, and adapted to transmit the optical signal(s) of the resulting signal at the output of the signal integration unit 144-n to the input of the optical recombiner 148, or the initial quantum signal at the output of the optical selector 142 to the input of the optical recombiner 148. Such transmission means may be, for example, a polarization-maintaining fiber PMF, if the polarization encoder 140 is a guided all-optical device.

[0087] By way of illustration, as shown in Figures 2, 3 and 4, the second polarization encoding value P2 of the signal resulting from the second optical arm B2, at the input of the optical recombiner 148 (i.e. the multiplexing of the second reference signal Ro 2 and the initial quantum signal SQ0, or only the second reference signal Ro 2) can be characterized by the initial polarization Po, i.e. for example and without limitation the initial linear polarization of type H (or of type V). In this case, the transmission means 144-i2 and 148-i2, arranged between the signal integration unit 144-n and the input of the optical recombiner 148, may be polarization-maintaining fibers PMF.

[0088] In embodiments, the optical recombiner 148 and optionally the intermediate optical recombiner(s) (148-1, 148-2) of the polarization encoder 140 may also be optical couplers (for example, fibered Y-couplers) adapted to combine resulting signals from optical arms Bn of the encoder 140. Such optical couplers may in particular be polarization-maintaining couplers. In certain embodiments, the optical recombiner 148 may be the telescope of the satellite in which the transmitter 10 is mounted on board.

[0089] The first reference signal ROi, modified in polarization by passing through the first optical arm Bi (i.e. the polarization modification unit 146-1), is encoded on the first encoding value Pi to form, at the input of the optical recombiner 148, the first control signal Ru of the first polarization encoding value Pb. Equivalently, the second reference signal R02, modified or not in polarization by passing through the second optical arm B2, is then said to be “encoded” on the second encoding value P2 to form, at the input of the optical recombiner 148, the second control signal Ri 2 of the second polarization encoding value P2. The initial quantum signal SQ0, modified or not in polarization by crossing any one of the optical arms Bn of the polarization encoder 140 is then said to be “encoded” on the encoding value Pn to form, at the input of the optical recombiner 148, the encoded quantum signal SQi.

[0090] In embodiments where the polarization encoder 140 is an all-optical guided device, the polarization encoder 140 may further comprise a plurality of optical fibers adapted to transmit the optical signal(s) between the different units of the encoder (and in particular the optical arms Bn). Some or all of these optical fibers may in particular be PMF polarization-maintaining fibers. Advantageously, the transmission means 142-il and 142-i2 between the optical selector 142 and the signal integration units 144-1 and 144-2, shown in FIGS. 2, 3 and 4, may be PMF polarization-maintaining fibers. The transmission means 144-i 1 and 146-il of the optical arm Bb shown in FIGS. 3 and 4, may also be PMF polarization-maintaining fibers.

[0091] In certain embodiments, the transmission means 140-i0, 140-il and 140-i2 for inputting the optical signals from the signal generator 120 into the polarization encoder 140, shown in FIGS. 2, 3 and 4, may be single-mode optical fibers or SMF (acronym for Single Mode Fiber) and / or polarization-maintaining fibers PMF. The means of 148-iO transmission of the multiplexed signal output If in the polarization encoder 140 can be a single-mode optical fiber SMF.

[0092] [Fig.5] schematically represents an optical selector 142 of a polarization encoder 140 comprising four optical arms Bb B2, B3 and B4, according to embodiments of the invention. In this case, the optical selector 142 may comprise a set of intermediate optical selectors (142-0, 142-1 and 142-2) configured to receive the initial quantum signal SQ0 and to direct it towards a specific optical path. Each intermediate optical selector may be controlled individually by a control sub-signal (SCi4 o, SCi4 i and Scu - 2) defined for example from the control signal SCi4 for controlling the optical selector 142.

[0093] It is noted that an encoder 140 can be configured to polarization encode the initial quantum signal SQ0 on two different and non-orthogonal polarization bases, as illustrated in [Fig.4], comprising:

[0094] - a first base corresponding in particular to the encoding values ​​in polarization Pi and P3 of the two optical arms Bi and B3 (formed by a first intermediate optical selector 142-1 and linked by a first intermediate optical recombiner 148-1), such as for example the diagonal base (D / A), and

[0095] - a second base corresponding in particular to the encoding values ​​in polarization P2 and P4 of the two optical arms B2 and B4 (formed by a second intermediate optical selector 142-2 and linked by a second intermediate optical recombiner 148-2), such as for example the rectilinear base (H / V).

[0096] Such an encoder therefore makes it possible to generate four distinct encoding states, H, V, D and A, and to use these four states to apply the quantum key distribution protocol called BB84 (as described in the article “Quantum cryptography: Public key distribution and coin tossing” by C. Bennett and G. Brassard, 1884, Theoretical Computer Science, vol. 560, 1984, pp. 7-11).

[0097] Furthermore, it should be noted that an encoder 140 can be configured to polarization encode the initial quantum signal SQ0 only on two different polarization states, as illustrated in FIGS. 2 and 3. In this embodiment, the encoder 140 comprises only two optical encoding arms, each of these states being able to be assimilated to an encoding basis called a 'simplified basis', for simplification. The two simplified bases can advantageously be non-orthogonal. In this case, a first simplified basis can for example correspond to the polarization encoding value Pi of the optical arm Bb and correspond for example to a linear polarization of type D (or A), while a second simplified basis can for example correspond to the encoding value P2 of the optical arm B2 and correspond for example to a linear polarization of type H (or V).

[0098] Advantageously, the first control signal Ru of the first polarization encoding value Pb can correspond to the control signal of the first polarization encoding base (for example the D / A base or other simplified base). Similarly, the second control signal R[2 of the second polarization encoding value P2, can correspond to the control signal of the second polarization encoding base (for example the H / V base or other simplified base).

[0099] In embodiments, the multiplexed signal Si can be frequency multiplexed. In this case, the signal generator 120 (also called 'signal generation module') of the transmitter 10 can be configured to generate an initial quantum signal SQ0 of wavelength denoted λQ, a first reference signal Ro of wavelength denoted XR1 and a second reference signal Ro2 of wavelength denoted XR2, these three wavelengths being distinct from each other. [Fig.6] schematically represents such a signal generator 120, according to embodiments of the invention.

[0100] Advantageously, the signal generator 120 may comprise a first laser source 122-0 emitting a laser beam of wavelength (equivalent to a frequency tûQ). The laser emission wavelength aq (also called 'quantum wavelength') may be in the visible or infrared range. For example and in a non-limiting manner, the first laser source 122-0 may be a DFB laser diode (acronym for the corresponding English expression "Distributed Feedback") using a Bragg grating making it possible to choose the emission wavelength XQ. The chosen emission wavelength of the laser diode may be equal to, for example, 1550nm. Such a laser diode emits in particular a continuous laser beam. Alternatively, the first laser source 122-0 may be a pulsed laser unit, i.e. with switched gain (or gain-switched according to the English expression).

[0101] The signal generator 120 may also comprise two other additional laser sources 122-1 and 122-2, as shown in FIG. 6, configured to respectively emit a laser beam of wavelength XR1 (equivalent to a frequency œR[) and a laser beam of wavelength ^R2 (equivalent to a frequency œR2). The laser emission wavelengths XR1 and ^R2 (also called 'reference wavelengths') may be in the visible or infrared range. For example and in a non-limiting manner, the additional laser sources 122-1 and 122-2 may be DFB laser diodes or pulsed laser units.

[0102] In embodiments, the frequency difference between the quantum wavelength XQ and a reference wavelength (XR1 and / or kR2) may be greater than or equal to a first minimum wavelength difference value XX, according to the following inequality (01):

[0103] |Xq-XR]7R2| ^^X (01)

[0104] Furthermore, the frequency difference between the reference wavelengths (XR1 and / or XR2) of each of the reference signals Ro i and R02 may be greater than or equal to a second minimum wavelength difference value ÔX', according to the following inequality (02):

[0105] |XR1-XR2| >5X' (02)

[0106] Advantageously, the first minimum value 5X and the second minimum value ÔX' of wavelength difference may be predefined and equal, for example and without limitation, to 1.6nm and 0.8nm respectively.

[0107] According to certain embodiments, the signal generation module 120 may further comprise one or more intensity modulation units 124 configured to modulate the intensity of the laser pulses generated at the output of the first laser source 122-0 and form quantum pulses. Such a unit may be used to implement a secure quantum key distribution protocol applying decoy states (or Decoy States QKD according to the corresponding English expression).

[0108] The intensity modulation unit 124 may also be configured to modulate the rate of the laser pulses, 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 up to a few nanoseconds.

[0109] In embodiments where the first laser source 122-0 is continuous, the signal generator 120 may comprise a phase modification unit 126 configured to modify the phase of each of the quantum pulses. Advantageously, the phase modification unit may be configured to randomize (i.e. randomize) the phase of each of these quantum pulses, so that the phases of two consecutive quantum pulses are independent of each other.

[0110] Phase randomization via the use of a pulsed laser unit and / or a phase modification unit makes it possible to protect against certain quantum key interception attacks that can be carried out by a spy device, conventionally called 'Eve', placed on the transmission channel 50, which seeks to intercept the multiplexed signal Si (and therefore the quantum signal encoded in polarization Sqi), transmitted by the transmitter 10 'Alice' and taking into account the phase coherence between quantum pulses.

[0111] In the embodiments where the multiplexed signal Si is frequency multiplexed (i.e. the quantum wavelengths XQ and reference and ^R2 are distinct from each other) the signal integration units 144-1 and 144-2 of the polarization encoder 140 may be wavelength division multiplexing or WDM units (meaning Wavelength Division Multiplexing) adapted to combine the initial quantum signal SQ0 directed and one of the reference signals ROi or R02 on the same optical path into a resulting signal.

[0112] In embodiments, the multiplexed signal Si may be time-multiplexed. In this case, the multiplexed signal Si may be a signal comprising a set of three temporally distinct pulses, the set being repeated according to a period T, the three pulses corresponding respectively to the polarization-encoded quantum signal Sqi, to the first control signal Ru and to the second control signal R[2.

[0113] Advantageously, the initial quantum signal SQ0 and the reference signals Roi and R02 generated by the signal generator 120 can be pulse signals characterized by a period T identical to the period of the multiplexed signal Sp

[0114] In embodiments, the signal generator 120 may be configured to generate the initial quantum signal SQ0 and the reference signals Roi and R02 according to a predefined time shift between each pulse of the signals. Alternatively (or in addition), the signal integration units 144-1 and 144-2 of the polarization encoder 140 may be configured to apply a predefined time shift so as to obtain time-multiplexed signal pulses.

[0115] The resulting time difference between each of the successive distinct pulses can thus be strictly less than the repetition period T of the resulting multiplexed signal Si (or of the initial quantum signal SQ0), according to the following inequalities (03 and (04):

[0116] |tQ-tR1 / R2| <T(03)

[0117] |tR1-tR2| <T(04)

[0118] In these embodiments where the multiplexed signal Si is time-multiplexed, the quantum wavelengths XQ and reference wavelengths (7R1 and / or ^) may be equal to each other.

[0119] In this case, the additional laser sources 122-1 and 122-2 may for example be assimilated to the first laser source 122-0 and the signal generator 120 may further comprise a beam splitting unit (not shown in the figures) configured to provide two signal components associated with the reference signals ROi and R02, as well as another signal component associated with the initial quantum signal SQ0. Such a beam splitting unit may comprise one or several optical couplers, symmetrical or asymmetrical, for example polarization-maintaining. The beam splitting unit can also be an optical selector generating a predefined time shift between each delivered signal component.

[0120] In embodiments, the beam splitting unit of the signal generator 120 may be arranged at the output of the first laser source 122-0, the resulting reference signals ROi and R02 then corresponding to conventional (i.e. non-quantum) light pulse signals. Alternatively, this beam splitting unit may be arranged at the output of one of the additional quantum signal generation units (124, 126), the resulting reference signals Roi and R02 then corresponding to low light intensity signals and / or quantum signals.

[0121] Figures 7, 8 and 9 schematically represent the receiver 30, according to embodiments. In these embodiments, the receiver comprises a beam splitter 320 and two processing chains Ci and C2.

[0122] The beam splitter 320 (also called 'beam splitting unit') is configured to separate the multiplexed signal Si transmitted by the transmitter 10 into two signal components, denoted S2i and S22, each signal component thus obtained passing respectively through one of the two processing chains Ci or C2. In the remainder of the description and in the figures, the index 'x' is the index associated with one of the two processing chains of the receiver 30 and can be an integer equal to 1 or 2. The two processing chains Ci or C2 are thus generally designated by the notation Cx.

[0123] In embodiments, the beam splitter 320 may be a symmetrical optical coupler (for example, a 50 / 50 type fiber Y) arranged for example at the input of the receiver 30. Such an optical coupler may in particular be a polarization-maintaining coupler. The beam splitter 320 may thus be configured to provide two signal components S2i and S22 of the multiplexed signal Si, of pulses of equal intensity, each composed of 50% of the optical power of the first control signal Ru of the first encoding value in polarization Pb and 50% of the optical power of the second control signal R12 of the second encoding value in polarization P2.

[0124] Furthermore, the signal Sqi being a quantum signal, the beam splitter 320 is configured to direct (or route) the polarization-encoded quantum signal Sqi from the multiplexed signal Si to one of the two processing chains Cx (i.e. Ci or C2) of the receiver 30.

[0125] Each processing chain Cx comprises a detection module (generally denoted 380-x such as the detection modules 380-1 or 380-2) adapted to measure the quantum signal SQi according to at least one polarization state defined in a predefined polarization base (a 'polarization encoding base' in the transmitter 10, or a 'polarization decoding base' in the receiver 30). Each processing chain Cx also comprises a correction device, generally denoted Dx (such as Di or D2 in [Fig.7]), adapted to determine the polarization state of the control signal associated with the polarization base of the chain considered. The correction device Dx is further capable of correcting the polarization state of the signals making up the component S2x (and in particular of the quantum signal SQi) passing through the processing chain Cx, according to the determined polarization state of the control signal, so as to align this corrected polarization state with a polarization state (in particular Px) of the detection polarization base of the quantum signal predefined via the detection module 380-x.

[0126] As used herein, the term 'alignment of a polarization state to a polarization basis' refers to a rotation of the polarization state of a signal to correspond to a proper axis of detection of the basis determined by quantum signal detection equipment.

[0127] By way of illustration, the processing chain Ci can be associated with the processing of the signals according to the basis determined by the polarization Pi (or by the polarizations Pi and P3, defined for example and without limitation on the diagonal basis D / A) and the processing chain C2 can be associated with the processing of the signals according to the basis determined by the polarization P2 (or by the polarizations P2 and P4, defined for example and without limitation on the rectilinear basis H / V).

[0128] Thus, for each processing chain Cx, the correction device Dx can comprise a control loop between a polarization state correction module and a 360-x detection module of a control signal R[x.

[0129] A correction module of the receiver 30 may be configured to modify the polarization of a signal passing through it, in response to a setpoint signal. Such a setpoint signal may be an electrical or radiofrequency signal for example. Advantageously, a correction module of the receiver 30 may be a fiber polarization controller comprising in particular one or more polarization rotation fibers whose constraint axis(es) (adapted for rotating the polarization of the signal) are controlled (or adjusted) from the setpoint signal. For example and without limitation, such a controllable constraint axis may be implemented in the form of a wound fiber component of adjustable geometry, or using a piezoelectric element inducing mechanical constraints on a fiber. Alternatively, a correction module may comprise one or more so-called active delay plates, that is to say whose blade rotation (i.e. its optical axis) is controlled (or adjusted) from the setpoint signal.

[0130] In embodiments, the receiver 30 may comprise two separate correction modules, 340-1 and 340-2 (and generally denoted 340-x), each module being associated with the correction of the polarization of the signals passing through it. In particular, a correction module 340-x may be arranged to correct the polarization of the signals constituting the component S2x associated with the correction device Dx (according to the specific polarization state Px for example). Alternatively, the receiver 30 may comprise a single correction module 340-0 arranged to simultaneously correct the polarization of the signals composing the component S2i of the correction device Di and the polarization of the signals composing the component S2 2 of the correction device D2 (according to the polarization states Pi and P2 respectively for example).

[0131] For each processing chain Cx, a correction module (340-x or 340-0) can be arranged upstream of the detection module 360-x, and the detection module 360-x can be arranged downstream of the beam splitter 320.

[0132] In embodiments, the two correction modules 340-1 and 340-2 of the two processing chains of the receiver can be positioned downstream of the beam splitter 320, as shown in [Fig.7].

[0133] In some embodiments, one of the two correction modules 340-1 (or 340-2) of the corresponding correction device Di (or respectively D2) can be positioned upstream of the beam splitter 320, while the other correction module 340-2 (or respectively 340-1) can be positioned downstream of the beam splitter 320, as shown in [Fig.8].

[0134] In embodiments where the receiver 30 comprises a single correction module 340-0, associated with the two correction devices Di and D2, the module can be positioned upstream of the beam splitter 320, as shown in [Fig.9]. In this case, the correction module 340-0 can be a triplet of active delay plates successively comprising a quarter-wave plate, a half-wave plate and a quarter-wave plate. In this case also, the beam splitter 320 can further comprise a so-called passive delay plate at the output of the coupler, the plate being positioned on one of the optical paths coming from the coupler and transporting one of the two signal components S2i or S22, determined from the multiplexed signal Si.

[0135] The transmission means 320-ix at the output of the beam splitter 320 and the transmission means 340-ix at the output of the polarization correction units 340-x may be SMF single-mode optical fibers. Advantageously, these transmission means may be PMF polarization-maintaining fibers.

[0136] Figures 10 and 11 schematically represent a detection module 360-x of a control signal Ri x comprising a signal demultiplexing unit 362-x and a device DAX for analyzing polarization of the integrated control signal Ri x, according to embodiments of the invention.

[0137] The signal demultiplexing unit 362-x (ie 362-1 or 362-2) receives as input the signal component S2x from the multiplexed signal Si at the output of the correction module 340-x and / or the beam splitter 320. The demultiplexing unit 362-x can be configured to separate from the signal S2x, the quantum signal Sqi, the component denoted Ri i of the first control signal Ri i and the component denoted R2[ of the second control signal R[2. The quantum signal Sqi demultiplexed from the signal S2x at the output of the unit 362-x is then routed to the detection module 380-x of the processing chain Cx. One of the two components of the control signal, denoted R2x (R2[ or R22 ), is then processed by the chain Cx, while the other component of the control signal (R22 or respectively R2[) is not used (for example and without limitation, such a component can then be directed towards a beam absorber 362-0 as shown in Figures 10 and 11).

[0138] The signal demultiplexing unit 362-x may in particular comprise one or more demultiplexing elements determined according to the type of multiplexing of the signal Si, i.e. frequency and / or time.

[0139] In embodiments where the multiplexed signal Si is frequency multiplexed, the signal demultiplexing unit 362-x may comprise a first filter Fi configured to separate the quantum signal Sqi from the two integrated control signal components (R21 and R22), and a second filter F2 configured to separate the two integrated control signal components (R2i and R22) from each other. For example and without limitation, such filters may be band rejection filters such as FBG filters (acronym for Fiber Bragg Grating) or a filter called “Add / Drop WDM”. The first filter Fi may be chosen from the predetermined frequency difference between the quantum wavelength XQ and the reference wavelengths (XR1 and / or XR2), and defined for example by equation (01).Similarly, the second filter F2 can be chosen from the predetermined frequency difference between the reference wavelengths (\u and / or 7R2) of each of the reference signals Ro i and R02, and defined for example by equation (02).

[0140] The transmission means 360-ix and 362-ix at the output of the signal demultiplexing unit 362-x to the detection module 380-x and the analysis device DAX respectively, as well as the transmission means included in the unit 362-x (not shown in the figures), may be single-mode optical fibers SMF. Advantageously, these transmission means can be polarization maintaining fibers PMF.

[0141] A DAX (i.e. DAi or DA2) analysis device for polarizing the control signal Rix can be configured to detect the control signal component R2x to be processed by the chain Cx, according to a predefined polarization basis, so as to provide the estimated control signal R3x (i.e., R3[ or R32).

[0142] It should be noted that at the output of the transmitter 10, the control signal Ru is generated in the transmitter 10 according to a well-defined polarization state Pn. During the propagation of the signal between the transmitter 10 and the receiver 30, the polarization state of the control signal R[x may have undergone random rotations so that the polarization state of the control signal component R2x, relating to the control signal Ru and detected by the receiver 30, may be different from the initially defined polarization state Pn.

[0143] Thus, the DAX analysis device for polarization of the control signal Ru can comprise at least one detection unit configured to detect signals, in particular according to a predefined polarization, so as to provide the estimate of the received control signal R3x (i.e., RM or R32).

[0144] In embodiments, a detection unit of the DAX analysis device may be adapted to detect conventional light pulse signals. For example and without limitation, such a unit may be a photodiode configured to deliver a photocurrent, depending on the measurement of the received control signal component R2x associated with the processing chain Cx.

[0145] Alternatively, a detection unit of the DAX analysis device may be a single photon detection unit. Such a unit may be composed of a detection surface configured to detect the “presence” of single photons at its detection surface (i.e. by photon / surface interaction). This detection of the presence of single photons is defined according to a given detection quantum efficiency. For example and without limitation, the single photon detection unit may be an avalanche photodiode or APD (acronym for the corresponding English expression Avalanche Photodiode Detectof) or a superconducting nanowire single photon detector or SNSPD (acronym for the corresponding English expression Superconducting Nanowire Single Photon Detectof).In particular, the single photon detection unit may include an internal amplification mechanism configured to output a voltage when a photon is detected.

[0146] In some embodiments, the DAX analysis device may include a polarizer 364A-X and a single detection unit 366-x of the control signal Rix, as shown in [Fig. 10]. The polarizer 364A-X (also called 'polarizing filter') can be arranged to transmit to the detection unit 366-x only the optical signals defined in the polarization state Px. The associated detection unit 366-x is thus configured to detect the light energy relating to the integrated control signal component R2x defined according to the polarization state Px only to provide the estimate of the received control signal R3x. According to this configuration, the value of the control signal R3x detected (or measured) by the detection unit 366-x is maximum if the polarization state of the control signal component R2x is equal to the polarization state Px of the control signal R[x. Conversely, the value of the control signal R3x is minimal if the polarization state of the control signal component R2x is orthogonal to the polarization state Px of the control signal Rr.

[0147] Advantageously, the DAX analysis device can comprise a polarizing detection unit directly grouping (i.e. combining) the functionalities of the polarizer 364A-X and the detection unit 366-x.

[0148] In embodiments, the DAX analysis device may comprise a polarized beam separation unit 364B-X, preceded by two detection units 366-xl and 366-x2, as shown in [Fig. 11]. The polarized beam separation unit 364B-X (also called 'polarizing separator') is adapted to provide two polarized signal sub-components relating to the control signal component R2x, each sub-component propagating on a transmission means (364-ixl or 364-ix2) at the output of the separation unit 364B-X to one of the detection units (366-xl or 366-x2) and is defined only in one of the two predefined polarization states of the polarization base, processed by the processing chain Cx and including in particular the polarization state Px.Each detection unit (366-xl and 366-x2) is thus configured to detect the light energy relating to one of the two polarized sub-components to provide the estimated control signal R3x. For example and without limitation, according to this configuration, the value relating to the estimated control signal R3x, measured by the first detection unit 366-xl, may be maximum and the value relating to the integrated signal R3x, measured by the second detection unit 366-x2, may be minimum, if the polarization state of the control signal component R2x is equal to the polarization state Px of the control signal Rlx.Conversely, the value relative to the estimated control signal R3x, measured by the detection unit 366-x, may be minimal and the value relative to the estimated control signal R3x, measured by the second detection unit 366-x2, may be maximal, if the polarization state of the control signal component R2x is orthogonal to the polarization state Px of the control signal Rlx.

[0149] For example, for the processing chain Ci, the device DAi for analyzing the polarization of the component of the received control signal R2i associated with the diagonal base (D / A) may comprise a separation unit 364B-1 configured to provide a first sub-component having a linear polarization Pb of diagonal type D propagating on the transmission means 364-ill, and a second sub-component having a linear polarization P3, of anti-diagonal type A propagating on the transmission means 364-il2. In this example, the two corresponding detection units 366-11 and 366-12 are therefore configured to respectively detect the sub-component relating to the linear polarization Pi of the received control signal R2i and the sub-component relating to the linear polarization P3 of the received control signal R21.

[0150] In certain embodiments, the detection unit(s) (366-x, or 366-xl and 366-x2) of the processing chain Cx may be adapted to the reference wavelength XR x (i.e. XR1 or XR2) of the control signal component R2x to be detected.

[0151] The receiver 30 may further comprise one or more processors (also called 'central computing units') or CPU (acronym for the English expression Central Processing Unit).

[0152] In embodiments, each DAX analysis device may comprise a specific processor, generally denoted 368-x (i.e. 368-1 and 368-2) configured to analyze the electrical signal(s) from the detection unit(s) (366-x, or 366-xl and 366-x2) and corresponding to the estimated control signal R3x. A processor 368-x may be configured to generate a servo signal, denoted SC36_x, corresponding to a polarization correction setpoint signal to be delivered to the correction module 340-x associated with the DAX processing chain.

[0153] In certain embodiments, the receiver 30 may comprise a single processor 368, configured to analyze all of the electrical signals from the detection units of the analysis devices DAi and DA2, and corresponding to the estimated control signals R31 and R32. The processor 368 may be configured to generate one or more servo signals, SC36 or SC36 x- A servo signal SC36 generated by the single processor 368 may correspond, for example, to the polarization correction setpoint signal to be delivered to the single correction module 340-0.

[0154] A servo loop associated with one or both correction devices (i.e. a polarization correction loop generating a servo signal) can be implemented continuously or intermittently. A processor (368-x or 368) can thus be configured to control the servo loop(s) of the receiver 30. In particular, a servo loop can be activated periodically and / or after evaluation of the polarization state of one or both signals. estimated control signals with respect to one or more basic polarization states of associated polarization. Furthermore, a feedback loop can be implemented until the polarization state of one or both estimated control signals is aligned with the associated chosen (or reference) polarization state.

[0155] In embodiments, a processor of the receiver 30 may be configured to determine, for a specific correction device Dx, a polarization state difference value 5px between the polarization state of the estimated control signal R3x and the polarization state Px of the associated polarization base. The processor may further be configured to evaluate whether this polarization state difference value dpx is strictly greater than (or greater than or equal to) a predefined reference difference value ôprc[.

[0156] In particular, a control loop can be activated if a determined polarization state difference value 5px is greater than or equal to the reference difference value <5Pref.

[0157] Advantageously, a control loop can be implemented so as to optimize (i.e. maximize or minimize) the detection of the component of the control signal(s) according to the associated polarization state(s).

[0158] For example and without limitation, a servo signal of a servo loop can be generated from a differentiable optimization algorithm, such as a gradient descent algorithm, so as to search (by incrementation or iteration) for an optimal point of an objective function associated in particular with the determined polarization state difference value(s) of a Dx correction device or of the two correction devices of the receiver. If an optimal point is found, the servo loop can be stopped.

[0159] The control loop can also be stopped, for example and without limitation, if a determined polarization state difference value dpx is strictly less than (or less than or equal to) the reference difference value <5pref.

[0160] Thus, in embodiments, a control signal SC36 x relating to the setpoint signal of a correction module 340-x can be generated to control said module 340-x and in particular to rotate the polarization of the signal component S2x until the value of the estimated control signal R3x, measured by the detection unit 366-x (or the first detection unit 366-xl), is optimal, that is to say that the polarization state of the control signal component R2x is then equal to the polarization state Px of the control signal Rix or orthogonal to it. The modification of the polarization of the signal component S2x via the control signal SC36 x thus induces a modification of the polarization state of the signal quantum SQi demultiplexed from the S2x signal, at the output of the 362-x unit, and routed to the 380-x detection module of the Cx processing chain.

[0161] In certain embodiments, a control signal SC36 relating to a setpoint signal to be delivered to the single correction module 340-0, can be generated to control said module 340-0 and in particular to rotate the polarization of the two signal components S2i and S22 until the two values ​​of the estimated control signals R31 and R3 2, measured by detection units 366-1 and 366-2 (or the first detection units 366-11 and 366-21), are optimal, that is to say that the polarization states of the control signal components R2[ and R22 are respectively equal to the polarization states Pi and P2 of the control signals Ri iet Rn (or orthogonal to them).

[0162] For each processing chain Cx, the quantum signal detection module 380-x comprises at least one single photon detection unit. The detection of the polarization-encoded quantum signal SQi on all the photon detection units of the processing chains (i.e. Ci and C2) of the receiver 30 makes it possible to provide the estimated received quantum signal SQ3 and thus an estimation of the polarization encoding of the quantum signal on the predetermined encoding states.

[0163] In certain embodiments, such as for example in modes where the quantum signal SQi is encoded on a set n of only two possible polarization states, Pi or P2 (i.e. defined according to a simplified polarization basis), the quantum signal detection module 380-x of a processing chain Cx may comprise a single single photon detection unit 386-x configured to detect the quantum signal SQi defined according to said polarization state Px.

[0164] In other embodiments, such as in cases where the quantum signal Sqi is encoded on a set n of four possible polarization states, Pb P2, P3 or P2 (i.e. defined according to the polarization bases D / A and H / V for example), the quantum signal detection module 380-x may comprise a switching unit 384-x, preceded by two single photon detection units 386-xl and 386-x2, as shown in [Fig. 12]. The switching unit 384-x may be likened to the polarized beam separation unit 364B-X of the analysis device DAx. The 384-x switching unit can therefore be adapted to direct (i.e. route or switch) the demultiplexed SQI quantum signal from the S2x signal to one of the two single photon detection units (386-xl or 386-x2) depending on the polarization state of the quantum signal.Each single photon detection unit (386-xl and 386-x2) is thus configured to detect the presence of single photons defined in one of the predefined polarization states of the polarization base, processed by the Cx processing chain and including in particular the Px polarization state.

[0165] By way of illustration, for the processing chain Ci, the quantum signal detection module 380-1 associated, for example and without limitation, with the diagonal base D / A, may comprise the switching unit 384-1 configured to direct the quantum signal Sqi having a linear polarization Pb of diagonal type D, towards a first single photon detection unit 386-11 via the transmission means 384-il 1, or to direct the quantum signal Sqi having a linear polarization P3, of antidiagonal type A, towards a second single photon detection unit 386-12 via the transmission means 384-il2.Equivalently, for the processing chain C2, the quantum signal detection module 380-2 associated, for example with the diagonal base H / V, may comprise the switching unit 384-2 configured to direct the quantum signal Sqi having a linear polarization P2, of diagonal type H, towards the single photon detection unit 386-21 via the transmission means 384-i21, or to direct the quantum signal Sqi having a linear polarization P4, of anti-diagonal type V, towards the single photon detection unit 386-22 via the transmission means 384-i22.

[0166] In embodiments, the quantum signal detection module 380-x may include at its input an additional demultiplexing unit 382-x, comparable to the signal demultiplexing unit 362-x of the detection module 360-x, and configured to transmit the quantum signal Sqi to the single photon detection unit 386-x, or to the switching unit 384-x, as shown in [Fig. 12]. The residual components of the control signals are then directed to a beam absorber 382-0 as shown in [Fig. 12].

[0167] The additional demultiplexing unit 382-x of the module 380-x may in particular comprise a demultiplexing element determined as a function of the type of multiplexing of the signal Sp. For example, for frequency multiplexing, the additional demultiplexing unit 382-x may be a spectral filter configured to separate the quantum signal Sqi from the two residual components of the integrated signals, such as an FBG filter or an “Add / Drop WDM” filter, and chosen from the predetermined frequency difference between the quantum wavelength λQ and the reference wavelengths (XR1 and / or XR2).

[0168] Such an additional demultiplexing unit 382-x makes it possible in particular to increase the filtering capacity of the control signals Ru and Rn in order to improve the quantum measurement carried out by the quantum signal detection module 380-x.

[0169] In embodiments, for example in modes where the control signals Rn and Rn are quantum signals and where the multiplexed signal Si can be time multiplexed, a correction device Dx can comprise a polarization state correction module and a quantum signal detection module 380- x. In this case, the quantum signal detection module 380-x may be configured to detect the quantum signal SQi, as well as the control signal R[x associated with the correction device Dx. Such a module 380-x may then comprise a switching unit 384-x corresponding to a polarized beam splitting unit, at least one single photon detection unit 386-x and a processor (similar to a central computing unit 368-x or 368) configured to generate the servo signal(s) corresponding to the polarization correction setpoint signals to be delivered to the associated correction module(s).

[0170] [Fig. 13] represents the method of transmitting a multiplexed signal Si implemented by the transmitter 10, according to embodiments of the invention.

[0171] The emission method comprises a preliminary step 1020 of generating an initial quantum signal SQ0, as well as a first reference signal ROi, and a second reference signal R02.

[0172] In step 1042, the initial quantum signal SQ0 is directed towards one of the optical arms Bn of the transmitter 10.

[0173] In step 1044 (equivalent to two distinct sub-steps 1044-1 and 1044-2), the first reference signal Roi is inserted into a first optical arm B i and the second reference signal R02 is inserted into a second optical arm B2 among the optical arms of the transmitter 10.

[0174] In one of the insertion sub-steps 1044-1 or 1044-2, the first reference signal Roi or the second reference signal R02 may be multiplexed with the initial quantum signal SQ0, depending on the routing of the initial quantum signal SQ0 in one of the optical arms Bn carried out in step 1042.

[0175] In step 1046, a polarization modification is applied to the optical signal passing through the first optical arm Bi (i.e. the multiplexing of the first reference signal ROi and the initial quantum signal SQ0, or only the first reference signal ROi, depending on the routing of the initial quantum signal SQ0), which generates an encoding according to a first polarization encoding value Ph

[0176] In step 1048, all of the resulting signals from the optical arms Bn of the transmitter 10 are recombined to form the multiplexed signal Si comprising:

[0177] - a first control signal Ru of the first polarization encoding value Pi determined from the first reference signal ROi and coming from the optical arm Bh

[0178] - a second control signal R[2 determined from the second signal of reference R02 and coming from the second optical arm B2, the second signal R[2 corresponding to a control signal of a second encoding value in polarization P2 associated with the second optical arm B2, and

[0179] - a quantum signal SQi encoded on a defined polarization encoding value from a set of values ​​H including the first polarization encoding value Pi and the second polarization encoding value P2.

[0180] In step 1050, the multiplexed signal Si is transmitted through a transmission channel 50.

[0181] [Fig.14] represents the method of receiving a multiplexed signal Si carried out by the receiver 30, according to embodiments of the invention.

[0182] The reception method comprises a preliminary step 3000 of receiving a multiplexed signal Si transmitted through a transmission channel 50.

[0183] In step 3020, the multiplexed signal Si is separated into two signal components S2i and S22 (or S2 x) each comprising a component of the first control signal Ru and a component of the second control signal Rn, each signal component S2 x propagating respectively towards one of the processing chains Cx of the receiver 30. Furthermore, an encoded quantum signal SQi included in the multiplexed signal Si is directed towards one of the processing chains Cx.

[0184] The reception method further comprises, for each processing chain Cx associated with a predefined polarization base, composed of at least the polarization state Px, a control loop between steps 3040 and 3060; step 3060 corresponds to the determination of the polarization state of a control signal R[x of the signal component S2x traveling through the chain, and step 3040 corresponds to the modification of the polarization of the signal component S2x. The control loop between steps 3040 and 3060 is stopped when the polarization state determined in step 3060 is aligned with respect to one of the polarization states of the base of the chain Cx.

[0185] In step 3080, the polarization state of the encoded quantum signal Sqi of the signal component S2x traveling through one of the processing chains Cx is determined.

[0186] Those skilled in the art will easily understand that certain steps of the transmission and reception methods of figures 13 and 14 can be carried out respectively simultaneously, sequentially, independently or not, and / or in a different order, for example in an order defined by the transmitter and the receiver respectively.

[0187] The quantum system or subsystems of the system (transmitter and receiver), as well as the methods described above, according to the embodiments of the invention, may be implemented in various ways by hardware, or a combination of hardware and software, including in the form of program code that may be distributed as a program product, in various forms. The program code may be distributed using computer-readable media, which may include computer-readable storage media. computer and communication media. The methods described in the present description may be implemented in particular in the form of computer program instructions executable by one or more processors in a computer computing device. These computer program instructions may also be stored in a computer-readable medium.

[0188] The invention is not limited to the embodiments described above as a non-limiting example. It encompasses all the variant embodiments that may be envisaged by those skilled in the art. In particular, those skilled in the art will understand that the invention is not limited to the different modules of the transmitter and receiver of the quantum system described as a non-limiting example.

Claims

Claims

1. Transmitter (10) configured to transmit a multiplexed signal (Si) through a transmission channel (50), characterized in that said transmitter (10) comprises: a signal generator (120) configured to generate an initial quantum signal (SQ0), a first reference signal (Roi) and a second reference signal (R02), a polarization encoder (140) comprising a plurality of N optical channels (Bn), the polarization encoder (140) further comprising: • an optical selector (142) configured to select one of said optical channels (Bn) and to direct the initial quantum signal (SQ0) generated towards the selected optical channel (Bn), • an optical recombiner (148) configured to generate said multiplexed signal (Si), the multiplexed signal comprising a first signal (Rn) for controlling a first polarization encoding value (Pi), a second signal (R[2) for controlling a second polarization encoding value (P2), and a quantum signal (Sqi) encoded on a polarization encoding value chosen from a set of values ​​(H) comprising at least said first polarization encoding value (Pi) and said second polarization encoding value (P2), said encoded quantum signal (Sqi) being determined from said initial quantum signal (SQ0) delivered by the optical channel selected by said optical selector (142), and in that the optical paths (Bn) comprise a first optical path (Bi) comprising a first integration unit (144-1) configured to integrate said first reference signal (Roi) into the first optical path (BJ and a second optical path (B2) comprising a second integration unit (144-2) configured to integrate the second reference signal (R02) into the second optical channel (B2), said first control signal (Ru) being determined from the first reference signal (Roi) delivered by the first optical channel (Bi) to the optical recombiner (148), and said second control signal (Rn) being determined from said second reference signal (R02) delivered by the second optical channel (B2) to the optical recombiner (148).

2. Transmitter (10), according to claim 1, wherein each optical channel (Bn) of said polarization encoder (140) is associated with a polarization encoding value (Pn) of said set of values ​​(fl), and wherein at least one of said optical channels (Bn) further comprises an optical element (146-n) configured to modify the polarization of an optical signal traveling along said optical channel (Bn) according to the associated encoding value (Pn).

3. Transmitter (10), according to one of claims 1 or 2, wherein said transmitter (10) is an all-optical guided device, said optical paths (Bn) of said polarization encoder (140) being formed from polarization maintaining fibers (PMF) and / or integrated waveguides.

4. Receiver (30) configured to receive a multiplexed signal (Si) through a transmission channel (50), said multiplexed signal (Si) comprising an encoded quantum signal (SQi), a first signal (Ru) for controlling a first polarization encoding value (Pi) and a second signal (Rn) for controlling a second polarization encoding value (P2), characterized in that said receiver (30) comprises a beam splitter (320) configured to separate said multiplexed signal (Si) into two signal components (S2i and S22) comprising a component of said first control signal (Ru) and a component of said second control signal (Ri2), each signal component (S2i; S22) respectively passing through a processing chain (Ci; C2) associated with a polarization base composed of at least one polarization state (Pi;P2), one of said signal components (S2i or S22) further comprising said encoded quantum signal (SQi), and in that each processing chain (Ci; C2) comprises a correction device (Di; D2) adapted to determine the polarization state of an integrated control signal (Ru; Rn) of said signal component (S2i, S22) traveling through said chain, the device; correction (Di; D2) being further adapted to modify the polarization of said signal component (S2i, S22) so as to align said determined polarization state with respect to one of said at least one polarization state of said associated base, each processing chain (Ci; C2) comprising a detection module (380-1; 380-2) adapted to measure said encoded quantum signal (Sqi) according to at least one of said at least one polarization state of said associated base.

5. Receiver (30), according to claim 4, wherein, for each processing chain (Ci; C2), said correction device (Di; D2) is configured to demultiplex said signal component (S2b S22) to select one of said control signal components (Ru; R[2) and route it to a polarization analysis device (DAi; DA2) comprising at least one detection unit (366-x) and adapted to detect said selected integrated control signal component (Rn; R[2) according to one of said at least one polarization state of said associated base.

6. Receiver (30), according to one of claims 4 or 5, wherein, for each processing chain (Ci; C2), said correction device (Di; D2) further comprises a processor (368-1; 368-2) configured to analyze said determined polarization state and to generate a servo signal (SC3e) applied to a correction module (340-1; 340-2) for polarization of said signal component (S2b S22).

7. Receiver (30), according to one of claims 4 to 6, wherein said beam splitter (320) is a symmetrical Y-fiber optical coupler of the 50 / 50 type, and wherein said processing chains (Ci; C2) are formed from polarization maintaining fibers (PMF) and / or single-mode optical fibers (SMF).

8. System (1) for quantum distribution of encryption keys comprising a transmitter (10) defined according to one of claims 1 to 3 and a receiver (30) defined according to one of claims 4 to 7.

9. System (1), according to claim 8, wherein said multiplexed signal (Si) is a frequency multiplexed signal.

10. System (1), according to claim 9, and wherein the absolute value of the wavelength difference between said encoded quantum signal (Sqi) and said first and / or said second integrated signal (Ru and / or Rn) is greater than or equal to a first minimum wavelength difference value (SX), and wherein the absolute value of the wavelength difference between said first control signal (Ru) and said second control signal (R[2) is greater than or equal to a second minimum wavelength difference value (ÔX').

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