System for distributing quantum keys using correction of photon polarization

Abstract

The invention relates to a transmitter (10) of a multiplexed signal (S1) over a transmission channel (50), comprising:-a generator (120) of an initial quantum signal (SQ0), a first reference signal (R01) and a second reference signal (R02),-an encoder (140) having N optical channels (Bn), comprising: a selector (142) for transmitting the initial signal to one of the channels; and a recombiner (148) for generating a multiplexed signal comprising first and second signals (R11, R12) for controlling the first and second polarization coded values (P1, P2), respectively, and a quantum signal (SQ1) coded with coded values determined based on the initial signal. The optical channel comprises two channels (B1, B2), each channel comprising means (144-1, 144-2) for integrating reference signals into the channel, each control signal being determined based on one of the reference signals delivered to the recombiner by the channel in question.

Description

Technical Field

[0001] Generally speaking, the present invention relates to quantum telecommunications, and more specifically, the present invention relates to a transmitter for transmitting a multiplexed signal including a quantum signal, a receiver for receiving a multiplexed signal including a quantum signal, a system including such a transmitter and a receiver, and a related method implemented thereby. Background Art

[0002] The main application of current quantum telecommunications systems is to distribute cryptographic keys (or encryption keys) between two remote telecommunications devices (i.e., two users) via specific quantum protocols using quantum information theory, with the aim of subsequently encrypting the communication between these two devices in a super-secure manner. Such quantum protocols are typically designated by the acronym QKD, which stands for Quantum Key Distribution. The keys obtained via the QKD protocol are secret cryptographic keys with a higher level of security than those obtained using conventional protocols.

[0003] In the field of quantum cryptography, the remote users of a quantum communication system are conventionally named Alice (the transmitting device) and Bob (the receiving device). The QKD protocol includes steps of sending encoded information on quantum particles, receiving these particles, and coordinating the transmitter and the receiver.

[0004] The sending step includes encoding conventional information (0 or 1) on the qubits of quantum particles, which are typically photons. A qubit corresponds to the degree of freedom of a quantum particle and can be the polarization of a photon. The receiving step includes determining the state of the qubit of the received photon to retrieve the encoded conventional information. In the coordination step, the transmitting device and the receiving device communicate to correct potential transmission errors and generate a shared raw key. Thus, these devices (i.e., the transmitter and the receiver) respectively convert the encoded information and the determined information (corresponding to their respective raw keys) into a super-secure key, thereby allowing the confidentiality of their telecommunications exchange to be improved.

[0005] This type of QKD protocol using the polarization of quantum particles as qubits requires encoding (at the transmitting end) and measuring (at the receiving end) the qubits in at least two different and non-orthogonal polarization bases.

[0006] However, the polarization state of a quantum particle undergoes random rotation between its transmission and reception (i.e., during its propagation). These can be due to the birefringence of the various media it passes through, or indeed due 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 a communication (a segment of which passes through outer space) relative to the ground station.

[0007] To avoid such random rotation of the polarization state, some known QKD systems take measures to ensure that photons propagating through the transmission channel only propagate through free space (their polarization then remains stable). However, in some applications, it is necessary to use guided optical transmission as the transmission channel, for example in the case of propagation through a terrestrial network or propagation on a satellite, in order to relax the constraints on the construction of the payload.

[0008] To compensate for (or correct) the random rotation of the polarization state, some known systems use a single polarization reference at the start of the QKD protocol, which allows an initial estimate of the polarization rotation induced during propagation and allows the polarization of the transmitted photons to be aligned with the measurement basis at the receiving end. However, this single reference makes it impossible to correct new polarization rotations after the initial estimation phase. Alternatively, other existing systems instead use a quantum signal source to periodically generate a reference signal in the encoding basis, and the reference signal is thus time-division multiplexed with the qubits, which reduces the bandwidth of the system available for the payload.

[0009] Therefore, there is a need for an improved QKD system that can correct in real time the rotation of the polarization state used to encode and decode qubits. Summary of the Invention

[0010] To this end, a transmitter is provided, the transmitter being configured to transmit a multiplexed signal through a transmission channel. The transmitter includes:

[0011] - a signal generator configured to generate an initial quantum signal, a first reference signal, and a second reference signal,

[0012] - a polarization encoder including a plurality of optical channels numbered N, the polarization encoder further including:

[0013] - an optical selector configured to select one of the optical channels and deliver the generated initial quantum signal to the selected optical channel,

[0014] - an optical recombiner configured to generate the multiplexed signal, the multiplexed signal including a first signal for controlling a first polarization encoding value, a second signal for controlling a second polarization encoding value, and a quantum signal encoded with a polarization encoding value selected from a set of values including at least the first polarization encoding value and the second polarization encoding value, the encoded quantum signal being determined based on the initial quantum signal delivered by the optical channel selected by the optical selector.

[0015] The optical channels include a first optical channel and a second optical channel. The first optical channel includes a first integration unit configured to integrate the first reference signal into the first optical channel. The second optical channel includes a second integration unit configured to integrate the second reference signal into the second optical channel. The first control signal is determined based on the first reference signal delivered by the first optical channel to the optical recombiner, and the second control signal is determined based on the second reference signal delivered by the second optical channel to the optical recombiner.

[0016] In an embodiment, each optical channel of the polarization encoder may be associated with one of the polarization encoding values in the set of values, and at least one of the optical channels may further include an optical element configured to modify the polarization of the optical signal passing through the optical channel according to the associated encoding value.

[0017] In an embodiment, the transmitter may be a guided all-optical device, and the optical channels of the polarization encoder are formed by polarization-maintaining fibers and / or integrated waveguides.

[0018] The present invention also provides a receiver configured to receive a multiplexed signal through a transmission channel. The multiplexed signal includes an encoded quantum signal, a first signal for controlling a first polarization encoding value, and a second signal for controlling a second polarization encoding value. The receiver includes a beam splitter configured to separate the multiplexed signal into two signal components including one component of the first control signal and one component of the second control signal. Each signal component respectively passes through a processing chain associated with a polarization basis composed of at least one polarization state, and one of the signal components further includes the encoded quantum signal.

[0019] Each processing chain includes a correction device configured to determine the polarization state of the integrated control signal of the signal component passing through the chain. The correction device is further configured to modify the polarization of the signal component so as to align the determined polarization state with one of the at least one polarization states in the associated basis. Each processing chain includes a detection module configured to measure the encoded quantum signal in at least one of the at least one polarization states of the associated basis.

[0020] In an embodiment, for each processing chain, the correction device may be configured to demultiplex the signal component so as to select one of the control signal components and route it to a polarization analysis device. The polarization analysis device includes at least one detection unit and is configured to detect the selected component of the integrated control signal in at least one of the at least one polarization states of the associated basis.

[0021] According to certain aspects, in each processing chain, the correction device may further include a processor configured to analyze the determined polarization state and generate a servo control signal applied to a module for correcting the polarization of the signal component.

[0022] In an embodiment, the beam splitter may be a symmetric fiber 50 / 50 optical Y coupler, and wherein the processing chain may be formed of polarization-maintaining fiber and / or single-mode fiber.

[0023] Embodiments of the present invention thus provide a system for distributing quantum encryption keys, the system including a transmitter and a receiver.

[0024] In an embodiment, the multiplexed signal may be a frequency-division multiplexed signal.

[0025] In one embodiment, the absolute value of the wavelength difference between the encoded quantum signal and the first integrated signal and / or the second integrated signal may be greater than or equal to a first minimum wavelength difference value, and wherein 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.

[0026] In addition, the present invention also provides a transmitting method for transmitting a multiplexed signal through a transmission channel, the method including the following steps:

[0027] - Generating an initial quantum signal, a first reference signal, and a second reference signal,

[0028] - Selecting one optical channel from a plurality of N optical channels and delivering the generated initial quantum signal to the selected optical channel, the optical channels including a first optical channel and a second optical channel,

[0029] - Inserting the first reference signal into the first optical channel and inserting the second reference signal into the second optical channel,

[0030] - Forming the multiplexed signal, the multiplexed signal including a first signal for controlling a first polarization encoding value, a second signal for controlling a second polarization encoding value, and a quantum signal encoded with a polarization encoding value selected from a value set including at least the first polarization encoding value and the second polarization encoding value, the encoded quantum signal being determined based on the initial quantum signal delivered by the selected optical channel, the first control signal being determined based on the first reference signal delivered by the first optical channel, and the second control signal being determined based on the second reference signal delivered by the second optical channel.

[0031] The present invention also provides a method for receiving a multiplexed signal via a transmission channel, the multiplexed signal including an encoded quantum signal, a first signal for controlling a first polarization encoding value, and a second signal for controlling a second polarization encoding value, the method including separating the multiplexed signal into two signal components including a first control signal component and a second control signal component, each signal component respectively passing through a processing chain associated with a polarization basis consisting of at least one polarization state, one of the signal components further including the encoded quantum signal.

[0032] The receiving method further includes the following iterative steps:

[0033] - determining the polarization state of the integrated control signal of the signal component passing through the chain, and

[0034] - modifying the polarization of the signal component so as to align the determined polarization state with one of the at least one polarization state of the associated basis.

[0035] The receiving method includes determining the encoded quantum signal in at least one of the at least one polarization state of the associated basis.

[0036] Thus, embodiments of the present invention enable correction of the polarization rotation of qubits transmitted between a transmitter and a receiver of a quantum signal (defined in at least two different and non-orthogonal polarization bases) in order to establish a quantum key.

[0037] In particular, embodiments of the present invention provide a signal transmitter that allows robust integration of a polarization reference signal into a quantum communication signal.

[0038] Such a reference can be generated with any ability independently of the generation of qubits to form an effective solution accessible in terms of hardware complexity. According to embodiments of the present invention, a guiding optical transmitter advantageously has a small volume and weight, as well as optimized footprint and robustness. In addition, frequency division multiplexing such a reference with qubits enables maintaining a high bandwidth for the transmission of payload data (i.e., qubits).

[0039] A receiver according to embodiments of the present invention enables real-time correction of the polarization rotation experienced by qubits before detection. Such a receiver in particular enables independent analysis of qubits and polarization reference signals in order to optimally align the polarization of qubits with the measurement basis of the receiver. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Other features, details, and advantages of the present invention will become apparent upon reading the description given with reference to the accompanying drawings, which are given by way of example.

[0041] Figure 1 Figure 1 is a schematic diagram showing a quantum communication system according to an embodiment of the present invention.

[0042] Figure 2 Figure 2 is a schematic diagram showing a polarization encoder of a transmitter according to an embodiment of the present invention.

[0043] Figure 3 Figure 3 is a schematic diagram showing a polarization encoder of a transmitter according to an embodiment of the present invention.

[0044] Figure 4 Figure 4 is a schematic diagram showing a polarization encoder of a transmitter according to an embodiment of the present invention.

[0045] Figure 5 Figure 5 is a schematic diagram showing an optical selector of a polarization encoder according to an embodiment of the present invention.

[0046] Figure 6 Figure 6 is a schematic diagram showing a signal generator of a transmitter according to an embodiment of the present invention.

[0047] Figure 7 Figure 7 is a schematic diagram showing a receiver of a quantum communication system according to an embodiment of the present invention.

[0048] Figure 8 Figure 8 is a schematic diagram showing a receiver of a quantum communication system according to an embodiment of the present invention.

[0049] Figure 9 Figure 9 is a schematic diagram showing a receiver of a quantum communication system according to an embodiment of the present invention.

[0050] Figure 10 Figure 10 shows a module for detecting an integrated control signal used in a processing chain of a receiver according to an embodiment of the present invention.

[0051] Figure 11 Figure 11 is a schematic diagram showing a module for detecting an integrated control signal used in a processing chain of a receiver according to an embodiment of the present invention.

[0052] Figure 12 Figure 12 is a schematic diagram showing a module for detecting a quantum signal used in a processing chain of a receiver according to an embodiment of the present invention.​​​​​​​​​​​​​​​​​​​​​​​​

[0053] Figure 13 Figure 13 is a flowchart showing a method for transmitting a signal including a quantum signal generated by a transmitter according to an embodiment of the present invention.

[0054] Figure 14 Figure 14 is a flowchart showing a method for transmitting a signal including a quantum signal generated by a receiver according to an embodiment of the present invention.

[0055] Identical or similar elements have been denoted by the same reference numerals in the drawings. For the sake of clarity, the elements shown are not drawn to scale. DETAILED DESCRIPTION

[0056] Figure 1 Schematically shows a quantum communication system 1 including two communication devices 10 and 30 capable of communicating with each other according to an embodiment of the present invention. Both devices include a transmitter 10 (or transmitting device) (also referred to as "Alice") and a receiver 30 (or receiving device) (also referred to as "Bob").

[0057] The quantum communication system 1 can be used, for example, in the field of space technology and includes a transmitter 10 (or vice versa, a receiver 30) installed on a satellite, while the receiver 30 (or vice versa, the transmitter 10) is a terrestrial module (i.e., a module on the ground). As a variant, the system 1 can be used in applications where at least one of the transmitting device 10 and the receiving device 30 is an avionics device. In addition, the system 1 can be used in applications where at least one of the transmitting device 10 and the receiving device 30 is an all-optical guiding device potentially integrated into an optical fiber network on the ground. The transmitting device 10 and / or the receiving device 30 can be stationary or moving relative to the other device (depending on the situation, 30 or 10) with which it communicates.

[0058] The transmitter 10 includes a signal generator 120 and a polarization encoder 140.

[0059] As used herein, an "optical signal" (also simply referred to as a "signal") is generated by one or more pulses of coherent light generated by a light source such as, for example, a laser beam. The laser beam can be characterized in particular by its pulse rate f and by the laser pulse (i.e., the signal), which is defined by the frequency ω of the laser beam, its intensity I, its polarization P, and its phase. The "frequency ω" of the laser beam represents "the optical frequency of the laser pulse multiplied by 2π" and is defined as a function of the wavelength λ of the light beam such that c represents the speed of light.

[0060] ​​​​"Quantum signal" may refer to a pulsed optical signal that on average contains less than one photon per pulse. In the context of the present invention, the transmitted quantum signal may refer to a pulsed optical signal that contains a low number of photons per pulse. The measurement of the quantum signal delivers a photon detection measurement based on the "detection probability" of the photons.

[0061] The transmitter 10 is configured to generate a multiplexed optical signal (also referred to as "multiplexed optical signal" or "multiplexed communication signal") represented as S1 and transmit it via the transmission channel 50. The multiplexed signal S1 includes a quantum signal represented as S Q1 , and the payload information is encoded on the polarization of the constituent pulses of the quantum signal. The polarization of the photons of the encoded quantum signal S Q1 is selected from a set of states (also referred to as "encoding states" or "encoding values") that includes at least a first polarization encoding value (represented as P1) and a second polarization encoding value (represented as P2). The multiplexed signal S1 also includes a first integrated optical signal R (also referred to as "first control signal" R 11 ) for controlling the first polarization encoding value P1 and a second integrated optical signal R 11 (also referred to as "second control signal" R 12 ) for controlling the second polarization encoding value P2. 12 )

[0062] Depending on the application area of the present invention, the transmission channel 50 can be, for example, free space or an optical fiber device for transmitting information, such as a device that uses optical fiber components for communication purposes.

[0063] The receiver 30 is configured to receive the multiplexed signal S1 (i.e., the signal transmitted by the transmitter 10) via the transmission channel 50 and estimate the received control signals, which results in estimated control signals R 31 and R 32 . The receiver 30 is also configured to estimate the received quantum signal based on the estimated control signals R 31 and R 32 , which results in an estimated received quantum signal S Q3 .

[0064] According to one aspect of the present invention, the transmitter 10 and the receiver 30 are configured to establish (i.e., determine) a quantum encryption key using the polarization-encoded quantum signal S Q1 and the estimated received quantum signal S Q3 . Thus, the system 1 can be a system for distributing quantum encryption keys configured to perform QKD within a spatial or terrestrial communication service, with the aim of ensuring the security of some or all of the communications exchanged between the transmitter and the receiver.

[0065] In an embodiment, the quantum communication system 1 may include a plurality of distinct receivers 30. In this case, the transmitter 10 may be configured to, for example, sequentially generate a specific multiplexed optical signal and transmit it, for example, sequentially to at least two of the distinct receivers. Thus, the system 1 may be configured to perform QKD between these distinct receivers.

[0066] Via the polarization encoder 140 (also referred to as "polarization encoding module") using the initial quantum signal represented as S Q0 and using the first reference signal represented as R 01 and the second reference signal represented as R 02 generate the multiplexed signal S1. The initial quantum signal S Q0 and the reference signals R 01 and R 02 are generated by the signal generator 120, as Figure 1 shown.

[0067] Figure 2 、 Figure 3 and Figure 4 schematically show the polarization encoder 140 of the transmitter 10 according to an embodiment of the present invention, and the transmitter 10 is configured to form the multiplexed signal S1.

[0068] The polarization encoder 140 may take the form of an optical instrument (such as, for example, an optical interferometer) having a plurality of polarization encoding optical arms B n (also referred to as "optical channels") with a quantity of N. The index "n" is the index of the nth optical arm of the polarization encoder 140 and is an integer between 1 and N, and the value of N is greater than or equal to 2. Specifically, as in the examples of Figure 2 and Figure 3 , the value of N may be an integer equal to 2, or as in the example of Figure 4 , the value of N may be an integer equal to 4. Thus, the polarization encoder 140 includes at least one first optical arm B1 (i.e., "first optical channel B1") and a second optical arm B2 (i.e., "second optical channel B2").

[0069] The polarization encoder 140 includes an optical selector 142 (also referred to as "optical path selection unit", "optical selector", "optical router" or "optical switch") and an optical recombiner 148 (also referred to as "optical path recombination unit" or "beam recombination unit"), and the optical recombiner 148 is configured to deliver the multiplexed signal S1 transmitted by the transmitter 10. Various optical arms B n extend between the optical selector 142 and the optical recombiner 148.

[0070] The optical selector 142 of the polarization encoder 140 is configured to receive the initial quantum signal S Q0and route it to optical arm B n among others. Thus, the polarization encoder 140 can be configured to control the optical selector 142 in response to the command signal S C14 i.e., to select the initial quantum signal S Q0 to be routed to optical arm B n among others. By way of example and not limitation, such a command signal S C14 can be an electrical or radio frequency signal built from N command values, each command value corresponding to a polarization-encoded optical arm B n . Thus, the command signal S C14 can include, for example, a plurality of values randomly selected from predefined command values.

[0071] The polarization encoder 140 further includes 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 B1 of the encoder 140 and is configured to insert (i.e., incorporate) the first reference signal R 01 into the first optical arm B1. 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 R 02 into the second optical arm B2.

[0072] In other words, in response to a particular command signal S C14 , the optical selector 142 can be configured to route to the first optical arm B1. Thus, the first integration unit 144-1 can be configured to multiplex (or combine) the first reference signal R 01 with any signal transmitted by the first optical arm B1 (i.e., the initial quantum signal if the initial quantum signal is routed by the optical selector 142 to the first optical arm B1). If the initial quantum signal S Q0 is not routed to the first optical arm B1, the optical selector 142 can be configured to route the signal S Q0 to the second optical arm B2. Thus, the second signal integration unit 144-2 can be configured to multiplex the second reference signal R 02 with any signal transmitted by the second optical arm B2 (i.e., the quantum signal if the quantum signal is routed by the optical selector 142 to the second optical arm B2 instead of the first optical arm B1).

[0073] In embodiments in which the polarization encoder 140 has N optical arms where N is strictly greater than 2, in response to a particular command signal S C14 , the optical selector 142 can also be configured to route the initial quantum signal S Q0 to an optical arm B n different from the first optical arm B1 and the second optical arm B2.

[0074] For example, for a polarization encoder 140 including two polarization-encoded optical arms B1 and B2, as for example Figure 2 and Figure 3 shown, if the optical selector 142 passes the initial quantum signal S Q0 to the first optical arm B1, the resulting signal output from the first signal integration unit 144-1 is generated by optical multiplexing of the first reference signal R 01 and the initial quantum signal S Q0 , where the initial quantum signal S Q0 is routed to the first optical arm B1, while the resulting signal output from the second signal integration unit 144-2 includes only the second reference signal R 02 . Alternatively, if the optical selector 142 passes the initial quantum signal S Q0 to the second optical arm B2, the resulting signal output from the first signal integration unit 144-1 includes only the first reference signal R 01 , while the resulting signal output from the second signal integration unit 144-2 is generated by optical multiplexing of the second reference signal R 02 and the initial quantum signal S Q0 .

[0075] According to another example, for a polarization encoder 140 including four polarization-encoded optical arms B1, B2, B3, and B4, as Figure 4 shown, if the optical selector 142 passes the initial quantum signal S Q0 to the first optical arm B1 or the second optical arm B2, no signal is transmitted by the third optical arm B3 and the fourth optical arm B4 (i.e., no signal propagates through the third optical arm B3 and the fourth optical arm B4). Further, no signal is transmitted by the third optical arm B3 and the fourth optical arm B4 to the optical recombiner 148. Alternatively, if the optical selector 142 passes the initial quantum signal S Q0 to the third optical arm B3 or the fourth optical arm B4, the resulting signals output from the first signal integration unit 144-1 and the second signal integration unit 144-2 include only the first reference signal R 01 and the second reference signal R 02 , respectively. In this case, the quantum signal determined based on the initial quantum signal S Q0 is delivered by the third optical arm B3 (or by the fourth optical arm B4) to the optical recombiner 148, and depending on the optical channel selected by the optical selector 142, no signal is delivered by the fourth optical arm B4 (or by the third optical arm B3, respectively).

[0076] Each optical arm B n of the polarization encoder 140 is associated with a beam-specific polarization encoding value P n . Thus, the set of possible variables Comprising a plurality of coding variables P of number N that are distinct from each other n Advantageously, the signal S of the optical selector 142 corresponding to one polarized coding optical arm B n Each command value of also corresponds to a polarized coding state P C14 Thus, the command signal S n Can enable at least a part of a so-called raw quantum encryption key to be formed that is to be shared between the transmitter 10 and the receiver 30 in the system 1. The payload information is then transmitted by quantum signals (i.e., quantum particles) that pass through each optical arm of the transmitter Alice and are modulated using the coding state P C14 In such an embodiment, the optical arm B of the polarization encoder 140 n May further include optical elements 146-n (also referred to as "polarization modification units" or "coding units"), which are configured to modify (or code) the polarization of the optical signal passing through the optical arm B

[0077] According to the coding value P associated with the optical arm n To modify (or code) the polarization of the optical signal passing through the optical arm B n According to the coding value P associated with the optical arm n Of the optical signal

[0078] For example, the polarization encoder 140 may include at least one first optical element 146-1 placed on the first optical arm B1 between the output of the first signal integration unit 144-1 and the input of the optical recombinator 148, so as to modify the resulting signal output from the first signal integration unit 144-1 (i.e., corresponding to the signal resulting from multiplexing the first reference signal R 01 And the initial quantum signal S Q0 Or only corresponding to the first reference signal R 01 Of the signal) according to the first polarization coding value P1 associated with the first optical arm B1

[0079] Advantageously, the initial quantum signal S generated by the signal generator 120 Q0 The first reference signal R 01 And the second reference signal R 02 Can initially be characterized by the same polarization P0 at the input of the polarization encoder 140. For example and without limitation, such an initial polarization P0 may be a linear polarization of type H, i.e., horizontal (or alternatively, type V, i.e., vertical). In this case, the polarization coding value P of the optical arm B including the coding unit 146-n n Can correspond to a linear polarization of type V (or type H respectively). Thus, the optical element 146-n can be configured to use, as output from the optical selector 142 and / or as from the optical arm B n Output nThe signal integration unit (144-1 and / or 144-2) on outputs an angle of ±90° rotation of the initial polarization P0 of the delivered optical signal (i.e., applies a rotation of ±90°). Additionally, such a polarization encoding value P n can correspond to 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 the initial polarization P0 of the optical signal passing through the optical arm B n by an angle of ±45° at the output of the optical selector 142 and / or at the output of the signal integration unit (144-1 and / or 144-2) (i.e., applies a rotation of ±45°).

[0080] In an embodiment, the polarization encoder 140 can be a guided all-optical device. As used herein, the term "guided all-optical device" refers to an optical device whose optical signal transmission signal consists of optical fibers and / or so-called integrated waveguides commonly used in integrated photonics devices. In this case, the encoding unit 146-n can include one or more polarization rotation transmission devices configured to apply an angular rotation to the initial polarization of the optical signal delivered as the output from the signal integration unit (144-1 and / or 144-2) or to the initial quantum signal output from the optical selector 142 on the optical arm B n .

[0081] For example and without limitation, such an encoding unit 146-n can take the form of an optical fiber called a "polarization rotation fiber", and in particular corresponds to a polarization-maintaining fiber (PMF) having a stress axis that appropriately modifies (or rotates) the polarization of the signal passing through the fiber to an angle predetermined by the initial polarization of the said signal and the encoding value P n associated with the optical arm B n . Such embodiments are shown in Figure 2 and Figure 4 . In particular, in Figure 2 , the transmission device 144-i1 corresponds to the encoding unit 146-1 of the optical arm B1, while in Figure 4 , the transmission device 148-i1 corresponds to the encoding unit 146-1 of the optical arm B1, and in addition, the transmission device 142-i4 corresponds to the encoding unit 146-4 of the optical arm B4.

[0082] Advantageously, the polarization encoder 140 can include one or more intermediate optical recombiners that allow the structure of the optical instrument to be simplified by combining (or rationalizing) certain optical functions. For example, in Figure 4In this case, the polarization encoder 140 includes intermediate optical recombiners 148-1 and 148-2 arranged upstream of the optical recombiner 148 that delivers the multiplexed signal S1. In this case, the encoding unit 146-n can consist of a plurality of polarization-rotating optical fibers, where the polarization-rotating optical fibers take the form of polarization-maintaining fibers (PMFs), each polarization-maintaining fiber having a stress axis, and the result is set to be associated with the optical arm B n by the encoding value P n predetermined polarization rotation angle. By using Figure 4 the arrangement of this embodiment is illustrated, which uses a combination of the transmitting devices 142-i3 and 148-i1 of the encoding unit 146-3 corresponding to the optical arm B3. Such an intermediate optical recombiner makes it possible to reduce the various encoding values P to be implemented by the transmitter 10 via the combination of the transmitting devices to form the resulting encoding unit n required for encoding the total number of polarization-rotation transmitting devices.

[0083] In Figure 4 the example shown, each optical arm B1, B2, B3, or B4 extends from the optical selector 142 to the optical recombiner 148, and the optical arms B1 and B3 and the optical arms B2 and B4 have common optical paths 148-i1 and 148-i2, respectively. By way of illustration, Figure 4 the transmitting devices 142-i3 and 142-i4 shown extending between the optical selector 142 and the inputs of the intermediate optical recombiners 148-1 and 148-2 without intermediate elements can be configured to rotate the initial polarization P0 of the quantum signals passing through the optical arms B3 and B4 by an angle of +90°, respectively. In addition, the transmitting device 148-i1 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 the polarization of the resulting signal (i.e., at least including the first reference signal R 01 of the signal) following the optical path common to the optical arms B1 and B3 by an angle of +45°. In this example, the polarization encoding value P1 of the optical arm B1 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°.

[0084] Alternatively, in an embodiment where the polarization encoder 140 is a device including at least one device for transmitting signals through free space, an integrated unit (144-1; 144-2) based on one or more dichroic filters can be employed. In addition, an encoding unit 146-n based on one or more thin polarization rotation wave plates (or retarders) (such as half-wave plates and / or quarter-wave plates) can be employed, as Figure 3 shown.

[0085] In an embodiment, associated with the optical arm B nThe associated polarization encoding value P n can directly correspond to the initial polarization P0. In this case, such an optical arm of the polarization encoder 140 can be arranged such that the polarization of one or more optical signals passing through it is not modified (i.e., a polarization rotation of 0°). Thus, the optical arm B of the polarization encoder 140 n can include one or more polarization-maintaining transmitting devices configured to transmit one or more optical signals of the resulting signal output from the signal integration unit 144-n to the input of the optical recombinator 148, or to transmit the initial quantum signal output from the optical selector 142 to the input of the optical recombinator 148. If the polarization encoder 140 is a guided all-optical device, such a transmitting device can be, for example, a PMF.

[0086] By way of illustration, as Figure 2 、 Figure 3 and Figure 4 shown, the second polarization encoding value P2 of the signal obtained at the input of the optical recombinator 148 from the second optical arm B2 (i.e., the multiplexing of the second reference signal R 02 and the initial quantum signal S Q0 , or only the second reference signal R 02 ) can be characterized by the initial polarization P0, i.e., for example, and without limitation, an initial linear polarization of type H (or type V). In this case, the transmitting devices 144-i2 and 148-i2 arranged between the signal integration unit 144-n and the input of the optical recombinator 148 can be PMFs.

[0087] In an embodiment, the optical recombinator 148 and optionally one or more intermediate optical recombinators (148-1, 148-2) of the polarization encoder 140 can also be optical couplers (such as fiber-optic Y-couplers) configured to combine the resulting signals delivered by the optical arm B of the encoder 140 n . In particular, such an optical coupler can be a polarization-maintaining coupler. In certain embodiments, the optical recombinator 148 can be the telescope of a satellite equipped with a transmitter 10.

[0088] Encoding the first polarization-modified reference signal R 01 passing through the first optical arm B1 (i.e., the polarization modification unit 146-1) with the first encoding value P1 to form a first signal R 11 for controlling the first polarization encoding value P1 at the input of the optical recombinator 148. Equivalently, then, it is said that the second reference signal R 02 passing through the second optical arm B2 (whether polarization-modified or not) is "encoded" with the second encoding value P2 to form a second signal R 12 for controlling the second polarization encoding value P2 at the input of the optical recombinator 148.。Then, it is said that the use of the coding value P n encodes any one of the initial quantum signals S n in the optical arm B Q0 of the polarization encoder 140 (whether polarization - modified or non - polarization - modified) to form an encoded quantum signal S Q1 at the input of the optical recombinator 148.

[0089] In an embodiment where the polarization encoder 140 is a guided all - optical device, the polarization encoder 140 may further include a plurality of optical fibers configured to send one or more optical signals between respective units of the encoder (and in particular, the optical arm B n ). In particular, some or all of these optical fibers may be polarization - maintaining fibers (PMF). Advantageously, Figure 2 、 Figure 3 and Figure 4 the transmission devices 142 - i1 and 142 - i2 (between the optical selector 142 and the signal integration units 144 - 1 and 144 - 2) shown may be polarization - maintaining fibers (PMF). Figure 3 and Figure 4 The transmission devices 144 - i1 and 146 - i1 of the optical arm B1 shown in and may also be PMF.

[0090] In some embodiments, the input transmission devices 140 - i0, 140 - i1, and 140 - i2 for sending the optical signals generated by the signal generator 120 to the polarization encoder 140 shown in Figure 2 、 Figure 3 and Figure 4 may be single - mode fibers (SMF) and / or PMF. The output transmission device 148 - i0 for sending the multiplexed signal S1 from the polarization encoder 140 may be a single - mode fiber (SMF).

[0091] Figure 5 Schematically shows an optical selector 142 of a polarization encoder 140 including four optical arms B1, B2, B3, and B4 according to an embodiment of the present invention. In this case, the optical selector 142 may include a set of intermediate optical selectors (142 - 0, 142 - 1, and 142 - 2) configured to receive the initial quantum signal S Q0 and pass it to a specific optical path. Each intermediate optical selector may be controlled individually by a sub - command signal (S C14 、S C14-0 、S C14-1 and S C14-2 ) defined, for example, based on a command signal S

[0092] ​It should be noted that the encoder 140 can be configured to perform polarization encoding on an initial quantum signal S on two different and non-orthogonal polarization bases Q0 as Figure 4 shown, namely:

[0093] - A first basis, which particularly corresponds to the polarization encoding values P1 and P3 of two optical arms B1 and B3 (formed by the first intermediate optical selector 142-1 and joined by the first intermediate optical recombinator 148-1), such as, for example, the diagonal basis (D / A), and

[0094] - A second basis, which particularly corresponds to the polarization encoding values P2 and P4 of two optical arms B2 and B4 (formed by the second intermediate optical selector 142-2 and joined by the second intermediate optical recombinator 148-2), such as, for example, the linear basis (H / V).

[0095] Thus, such an encoder enables the generation of four distinct encoded states H, V, D, and A, and these four states are used to apply the QKD protocol called BB84 (as described in the article "Quantum cryptography: Public key distribution and coin tossing" by C. Bennett and G. Brassard in 1984, Theoretical Computer Science, Volume 560, Pages 7-11, 1984).

[0096] Furthermore, it should be noted that the encoder 140 can be configured to perform polarization encoding on the initial quantum signal S only on two different polarization states Q0 as Figure 2 and Figure 3 shown. In this embodiment, the encoder 140 includes only two encoding optical arms, and for simplicity, each of its states can be similar to an encoding basis, called a "simplified basis". Advantageously, these two simplified bases can be non-orthogonal. In this case, the first simplified basis can, for example, correspond to the polarization encoding value P1 of the optical arm B1 and, for example, correspond to a linear polarization of type D (or A), while the second simplified basis can, for example, correspond to the encoding value P2 of the optical arm B2 and, for example, correspond to a linear polarization of type H (or V).

[0097] Advantageously, a first control signal R for the first polarization encoding value P1 11 can correspond to the control signal of a first polarization encoding basis (e.g., the basis D / A or another simplified basis). Similarly, a second control signal R for the second polarization encoding value P2 12 can correspond to the control signal of a second polarization encoding basis (e.g., the basis H / V or another simplified basis).

[0098] In an embodiment, the multiplexed signal S1 can be frequency-division multiplexed. In this case, the signal generator 120 (also referred to as a "signal generation module") of the transmitter 10 can be configured to generate an initial quantum signal S of a wavelength represented as λ Q 、a first reference signal R of a wavelength represented as λ Q0 、and a second reference signal R of a wavelength represented as λ R1 , where these three wavelengths are different from each other. 01 and a second reference signal R of a wavelength represented as λ R2 , where these three wavelengths are different from each other. 02 Figure 6 schematically shows such a signal generator 120 according to an embodiment of the present invention. Figure 6 Figure 6 schematically shows such a signal generator 120 according to an embodiment of the present invention.

[0099] Advantageously, the signal generator 120 can include a first laser source 122-0 that emits a laser beam with a wavelength of λ Q (equivalent to a frequency ω Q ). The emission wavelength λ Q of the laser (also referred to as the "quantum wavelength") can be in the visible light or infrared range. For example and without limitation, the first laser source 122-0 can be a DFB laser diode (DFB is the abbreviation of distributed feedback) using a Bragg grating, thereby allowing the selection of the emission wavelength λ Q . The selected emission wavelength λ Q of the laser diode can be equal to, for example, 1550 nm. In particular, such a laser diode emits a continuous-wave laser beam. Alternatively, the first laser source 122-0 can be a pulsed laser unit, i.e., a gain-switched laser unit.

[0100] The signal generator 120 can also include two other additional laser sources 122-1 and 122-2, as Figure 6 shown, which are configured to emit laser beams with wavelengths of λ R1 (equivalent to a frequency ω R1 ) and λ R2 (equivalent to a frequency ω R2 ) respectively. The wavelengths λ R1 and λ R2 of the laser emissions (also referred to as the "reference wavelengths") can be in the visible light or infrared range. For example and without limitation, the additional laser sources 122-1 and 122-2 can be DFB laser diodes or gain-switched laser units.

[0101] In an embodiment, according to the following inequality (01), the frequency difference between the quantum wavelength λ Q and the reference wavelengths (λ R1 and / or λ R2 ) can be greater than or equal to a first minimum wavelength difference δλ,

[0102] |λQ -λ R1 / R2 |≥δλ (01)

[0103] In addition, according to the following inequality (02), for each of the reference signals R 01 and R 02 the frequency difference between the reference wavelengths (λ R1 and / or λ R2 ) can be greater than or equal to a second minimum wavelength difference δλ':

[0104] |λ R1 -λ R2 |≥δλ' (02)

[0105] Advantageously, the first minimum wavelength difference δλ and the second minimum wavelength difference δλ' can be predefined and, for example and without limitation, be equal to 1.6 nm and 0.8 nm, respectively.

[0106] According to some embodiments, the signal generation module 120 may further include one or more intensity modulation units 124 configured to modulate the intensity of the laser pulses output by the first laser source 122-0 and form quantum pulses. Such units can be used to implement a secure decoy state QKD protocol.

[0107] The intensity modulation unit 124 may also be configured to modulate the rate of the laser pulses (which will be on the order of, for example, several kilohertz to several tens of gigahertz) and / or the time width of the laser pulses (which will be, for example, up to several nanoseconds).

[0108] In embodiments in which the first laser source 122-0 is a continuous wave, the signal generator 120 may include 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 the phase of each of these quantum pulses (i.e., make it random) such that the phases of two consecutive quantum pulses are independent of each other.

[0109] Phase randomization via the use of the pulsed laser unit and / or the phase modification unit enables defense against certain attacks based on quantum key interception that can be performed by a spy device (commonly referred to as "Eve") placed on the transmission channel 50 and seeking to intercept the multiplexed signal S1 transmitted by the transmitter 10 "Alice" (and thus the polarization encoded quantum signal S Q1 ) and taking into account the phase coherence between the quantum pulses.

[0110] In cases where the multiplexed signal S1 is frequency division multiplexed (i.e., when the quantum wavelength λ Q as well as the reference wavelengths λ R1 and λR2 In embodiments where they are different from each other), the signal integration units 144-1 and 144-2 of the polarization encoder 140 can be WDM units (WDM stands for wavelength division multiplexing), and the WDM units are configured to combine the initial quantum signal S transmitted on a given optical path Q0 and the reference signal R 01 or R 02 into a resulting signal.

[0111] In an embodiment, the multiplexed signal S1 can be time-division multiplexed. In this case, the multiplexed signal S1 can be a signal including a set of three pulses that are distinguishable in time, and the set repeats with a period T. The three pulses respectively correspond to the polarization-encoded quantum signal S Q1 、the first control signal R 11 and the second control signal R 12 .

[0112] Advantageously, the initial quantum signal S generated by the signal generator 120 Q0 as well as the reference signal R 01 and R 02 can be pulse signals characterized by a period T that is the same as the period of the multiplexed signal S1.

[0113] In an embodiment, the signal generator 120 can be configured to generate the initial quantum signal S with a predefined time offset between each pulse of the signal Q0 as well as the reference signal R 01 and R 02 . Alternatively (or in addition), the signal integration units 144-1 and 144-2 of the polarization encoder 140 can be configured to apply a predefined time offset in order to obtain time-division multiplexed signal pulses.

[0114] Therefore, according to the following inequalities (03) and (04), the resulting time difference between each of the consecutive distinguishable pulses can be strictly less than the repetition period T of the resulting multiplexed signal S1 (or the initial quantum signal S Q0 )):

[0115] |t Q -t R1 / R2 |<T (03)

[0116] |t R1 -t R2 |<T (04)

[0117] In these embodiments where the multiplexed signal S1 is time-division multiplexed, the quantum wavelength λ Q as well as the reference wavelength λ R1 and / or λ R2 can be equal to each other.

[0118] In this case, the additional laser sources 122-1 and 122-2 can be assimilated, for example, into the first laser source 122-0, and the signal generator 120 can also include a beam splitting unit (not shown in the figure), which is configured to deliver two signal components associated with the reference signals R 01 and R 02 and another signal component associated with the initial quantum signal S Q0 Such a beam splitting unit can include one or more symmetric or asymmetric optical couplers, such as polarization-maintaining optical couplers. The beam splitting unit can also be an optical selector that generates a predefined time offset between each of the delivered signal components.

[0119] In an embodiment, the beam splitting unit of the signal generator 120 can be placed at the output of the first laser source 122-0, and the resulting reference signals R 01 and R 02 then correspond to conventional optical pulses (i.e., non-quantum signals). Alternatively, the beam splitting unit can be placed at the output of one of the additional quantum signal generation units (124, 126), and the resulting reference signals R 01 and R 02 then correspond to signals of low optical intensity and / or quantum signals.

[0120] Figure 7 , Figure 8 and Figure 9 Schematically illustrate a receiver 30 according to an embodiment. In these embodiments, the receiver includes a beam splitter 320 and two processing chains C1 and C2.

[0121] The beam splitter 320 (also referred to as a "beam splitting unit") is configured to separate the multiplexed signal S1 transmitted by the transmitter 10 into two signal components denoted as S 21 and S 22 , and each of the signal components thus obtained passes through one of the two processing chains C1 and C2 respectively. In the remainder of the specification and the drawings, the index "x" is an index associated with one of the two processing chains of the receiver 30 and can be an integer equal to 1 or 2. Thus, the two processing chains C1 or C2 are generally designated by the symbol C x specified.

[0122] In an embodiment, the beam splitter 320 can be, for example, a symmetric optical coupler (e.g., a 50 / 50 fiber-optic Y-coupler) placed at the input of the receiver 30. In particular, such an optical coupler can be a polarization-maintaining coupler. The beam splitter 320 can thus be configured to deliver two signal components S 21 and S 22, each signal component consists of 50% of the optical power of a first control signal R with a first polarization encoding value P1 and 50% of the optical power of a second control signal R with a second polarization encoding value P2. 11 and 50% of the optical power of a second control signal R with a second polarization encoding value P2. 12 compose.

[0123] In addition, the signal S Q1 is a quantum signal, and the beam splitter 320 is configured to transfer (or route) the polarization-encoded quantum signal S obtained from the multiplexed signal S1 Q1 to one of the two processing chains C x in the receiver 30 (i.e., C1 or C2).

[0124] Each processing chain C x includes a detection module (usually denoted as 380-x, such as detection module 380-1 or 380-2), which is configured to measure the quantum signal S in at least one polarization state defined in a predefined polarization basis (the "polarization encoding basis" in the transmitter 10, or the "polarization decoding basis" in the receiver 30). Q1 Each processing chain C x also includes a correction device usually denoted as D x (such as Figure 7 D1 or D2 in), which is configured to determine the polarization state of the control signal associated with the polarization basis of the chain in question. The correction device D x is also capable of correcting the polarization state of the component S passing through the processing chain C x according to the determined polarization state of the control signal, and in particular, for the quantum signal S 2x ), so as to align the corrected polarization state with the polarization state of the detection polarization basis of the quantum signal predefined by the detection module 380-x (in particular, P Q1 ). x ) alignment.

[0125] As used herein, the expression "aligning the polarization state with the polarization basis" refers to the rotation of the polarization state of the signal such that it corresponds to a specific detection axis of the basis determined by a piece of equipment used to detect the quantum signal.

[0126] By way of illustration, processing chain C1 can be associated with signal processing in a basis determined by polarization P1 (or by polarizations P1 and P3, for example and without limitation, defined in the diagonal basis D / A), and processing chain C2 can be associated with signal processing in a basis determined by polarization P2 (or by polarizations P2 and P4, for example and without limitation, defined in the linear H / V basis).

[0127] Therefore, for each processing chain C x , the correction device D xmay include a servo control loop between a module for correcting the polarization state and a module for detecting the control signal R 1x of module 360-x.

[0128] The correction module of receiver 30 may be configured to modify the polarization of the signal passing through it in response to a setpoint signal. For example, such a setpoint signal may be an electrical signal or a radio frequency signal. Advantageously, the correction module of receiver 30 may be an optical fiber polarization controller, particularly including one or more polarization rotation optical fibers, and one or more stress axes thereof (which are configured to rotate the polarization of the signal) are controlled (or adjusted) based on the setpoint signal. For example and without limitation, such a controllable stress axis may take the form of a wound optical fiber component with an adjustable geometry, or a piezoelectric element that induces mechanical stress in the optical fiber. Alternatively, the correction module may include one or more so-called active waveplates, i.e., waveplates whose waveplate rotation (i.e., its optical axis) is controlled (or adjusted) according to the setpoint signal.

[0129] In an embodiment, receiver 30 may include two independent correction modules 340-1 and 340-2 (generally denoted as 340-x), each module being associated with the correction of the polarization of the signal passing through it. In particular, correction module 340-x may be arranged to correct the polarization of the component signal S x associated with correction device D 2x (e.g., depending on a specific polarization state P x ). Alternatively, receiver 30 may include a single correction module 340-0, and correction module 340-0 is arranged to simultaneously correct the polarization of the component signal S of correction device D1 21 and the polarization of the component signal S of correction device D2 22 (e.g., depending on the respective polarization states P1 and P2).

[0130] For each processing chain C x , the correction module (340-x or 340-0) may be arranged upstream of detection module 360-x, and detection module 360-x may be arranged downstream of beam splitter 320.

[0131] In an embodiment, the two correction modules 340-1 and 340-2 of the two processing chains of the receiver may be located downstream of beam splitter 320, as Figure 7 shown.

[0132] In certain embodiments, one of the two correction modules 340-1 (or 340-2) of the corresponding correction device D1 (or correspondingly D2) may be located upstream of beam splitter 320, while the other correction module 340-2 (or correspondingly 340-1) may be located downstream of beam splitter 320, asFigure 8 as shown

[0133] In an embodiment in which the receiver 30 includes a single calibration module 340-0 associated with two calibration devices D1 and D2, the module may be located upstream of the beam splitter 320, as Figure 9 shown. In this case, the calibration module 340-0 may be a triple active wave plate, which sequentially includes a quarter-wave plate, a half-wave plate, and a quarter-wave plate. Also in this case, the beam splitter 320 may further include a so-called passive wave plate at the output of the coupler, which is located on one of the optical channels that conveys one of the two signal components S 21 or S 22 obtained from the multiplexed signal S1 in one of the optical channels of the coupler.

[0134] The transmitting devices 320-ix at the output of the beam splitter 320 and the transmitting devices 340-ix at the output of the polarization calibration unit 340-x may be SMF. Advantageously, these transmitting devices may be PMF.

[0135] Figure 10 and Figure 11 schematically shows a detection module 360-x for detecting the control signal R 1x and a polarization analysis device DA for analyzing the polarization of the integrated control signal R 1x , the detection module 360-x includes a signal demultiplexing unit 362-x. x

[0136] The signal demultiplexing unit 362-x (i.e., 362-1 or 362-2) receives the signal component S obtained from the multiplexed signal S1 at the output of the calibration module 340-x and / or the beam splitter 320 2x as an input. The demultiplexing unit 362-x may be configured to separate the quantum signal S 2x from the signal S Q1 , the representation of the first control signal R 11 as R 11 and the representation of the second control signal R 12 as R 21 . Then the quantum signal S demultiplexed from the signal S 2x at the output of the unit 362-x is routed to the detection module 380-x of the processing chain C Q1 . One of the two components of the control signal (represented as R x (R 2x (R 21 or R 22 )) is then processed by the chain C x , while the other component of the control signal (correspondingly R 22 or R21 ) Not in use (e.g., and without limitation, such components can then be passed to a beam absorber 362-0 as shown in Figure 10 and Figure 11 ).

[0137] In particular, the signal demultiplexing unit 362-x can include one or more demultiplexing elements determined according to the multiplexing type used for multiplexing the signal S1 (i.e., depending on whether this is a matter of frequency division multiplexing and / or time division multiplexing).

[0138] In an embodiment in which the multiplexed signal S1 is frequency division multiplexed, the signal demultiplexing unit 362-x can include a first filter F1 configured to separate the quantum signal S 21 and R 22 from two integrated control signal components (R Q1 ), and a second filter F2 configured to separate the two integrated control signal components (R 21 and R 22 ) from each other. For example, and without limitation, such filters can be band-stop filters, such as FBG filters (FBG stands for fiber Bragg grating) or WDM add-drop filters. The first filter F1 can be selected according to a predetermined frequency difference between the quantum wavelength λ Q and a reference wavelength (λ R1 and / or λ R2 ), which predetermined frequency difference is defined, for example, by equation (01). Similarly, the second filter F2 can be selected according to a predetermined frequency difference between the reference wavelengths (λ 01 and R 02 ) of each of the reference signals R R1 and / or λ R2 ), which predetermined frequency difference is defined, for example, by equation (02).

[0139] Transmission devices 360-ix and 362-ix that respectively transmit to the detection module 380-x and the analysis device DA x at the output of the signal demultiplexing unit 362-x and the transmission devices (not shown in the figure) included in the unit 362-x can be SMF. Advantageously, these transmission devices can be PMF.

[0140] A device DA 1x for analyzing the polarization of the control signal R x (i.e., DA1 or DA2) can be configured to detect, in a predetermined polarization basis, the control signal component R x to be processed by the chain C 2x in order to deliver an estimated control signal R 3x (i.e., R 31 or R 32 ).

[0141] It should be noted that at the output of the transmitter 10, the polarization state P is well-defined in the transmitter 10 n Generate the control signal R 1x During the signal propagation between the transmitter 10 and the receiver 30, the polarization state of the control signal R 1x May have undergone a random rotation, and thus the control signal component R detected by the receiver 30 2x With respect to the control signal R 1x The polarization state may be different from the initially defined polarization state P n .

[0142] Therefore, the device DA for analyzing the polarization of the control signal R 1x May include at least one detection unit configured to detect a signal (in particular, a signal with a predefined polarization) in order to deliver an estimate of the received control signal R x (i.e., R 3x Or R 31 Or R 32 ).

[0143] In an embodiment, the detection unit of the analysis device DA x May be configured to detect a conventional optical pulse. For example and without limitation, such a unit may be a photodiode configured to deliver a photocurrent based on a measurement of the received control signal component R associated with the processing chain C x Associated with the received control signal component R 2x .

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

[0145] In certain embodiments, the analysis device DA x May include a polarizer 364A-x and a single detection unit 366-x for detecting the control signal R 1x As shown in Figure 10 Shown. The polarizer 364A-x (also referred to as a "polarization filter") may be arranged to send only the polarization state P to the detection unit 366-x xThe optical signal defined in. Therefore, the associated detection unit 366-x is configured to detect only the radiation energy related to the polarization state P x of the integrated control signal component R defined in 2x to deliver the received control signal R 3x estimation. In this configuration, if the polarization state of the control signal component R 2x is equal to the polarization state of the control signal R 1x polarization state P x , then the value of the control signal R detected (or measured) by the detection unit 366-x 3x is the maximum. On the contrary, if the polarization state of the control signal component R 2x is orthogonal to the polarization state of the control signal R 1x polarization state P x , then the value of the control signal R 3x is the minimum.

[0146] Advantageously, the analysis device DA x may include a polarization detection unit having both the functions of the polarizer 364A-x and the detection unit 366-x (i.e., combining the functions of the polarizer 364A-x and the detection unit 366-x).

[0147] In an embodiment, the analysis device DA x may include a polarization beam splitter 364B-x, followed by two detection units 366-x1 and 366-x2, as Figure 11 shown. The polarization beam splitter 364B-x (also referred to as a "polarization separator") is configured to deliver two polarization signal sub-components of the control signal component R 2x , each sub-component propagating on one of the transmission devices (364-ix1 or 364-ix2) at the output from the separation unit 364B-x to one of the detection units (366-x1 or 366-x2), and defined only in one of the two predefined polarization states of the polarization basis processed by the processing chain C x , and particularly including the polarization state P x . Each detection unit (366-x1 and 366-x2) is thus configured to detect the radiation energy related to one of the two polarization sub-components to deliver the estimated control signal R 3x . For example and without limitation, in this configuration, if the polarization state of the control signal component R 2x is equal to the polarization state of the control signal R 1x polarization state P x , then the value related to the estimated control signal R measured by the first detection unit 366-x1 3x may be the maximum, and the integrated signal R measured by the second detection unit 366-x2 3xThe relevant value can be the minimum. Conversely, if the polarization state of the control signal component R 2x is orthogonal to the polarization state P 1x of the control signal R x then the value related to the estimated control signal R 3x measured by the detection unit 366-x can be the minimum, and the value related to the estimated control signal R 3x measured by the second detection unit 366-x2 can be the maximum.

[0148] For example, for the processing chain C1, the device DA1 for analyzing the polarization of the received control signal component R 21 associated with the diagonal basis (D / A) may include a separation unit 364B-1 configured to deliver a first sub-component having a linear polarization P1 of type D (i.e., diagonal polarization) and a second sub-component having a linear polarization P3 of type A (i.e., anti-diagonal polarization), the first sub-component propagating on the transmitting device 364-i11 and the second sub-component propagating on the transmitting device 364-i12. In this example, two corresponding detection units 366-11 and 366-12 are thus respectively configured to detect the sub-component related to the linear polarization P1 of the received control signal component R 21 and the sub-component related to the linear polarization P3 of the received control signal component R 21 .

[0149] In some embodiments, one or more detection units (366-x or 366-x1 and 366-x2) of the processing chain C x may be configured to the reference wavelength λ 2x of the control signal component R Rx to be detected (i.e., λ R1 or λ R2 ).

[0150] The receiver 30 may also include one or more processors (also referred to as "system units") or CPUs (acronym for Central Processing Unit).

[0151] In an embodiment, each analysis device DA x may include a specific processor generally denoted as 368-x (i.e., 368-1 and 368-2) configured to analyze one or more electrical signals delivered by one or more detection units (366-x or 366-x1 and 366-x2) and corresponding to the estimated control signal R 3x . The processor 368-x may be configured to generate a servo control signal denoted as S C36-x corresponding to the one to be delivered to the processing chain DA xThe polarization correction setpoint signal of the associated correction module 340-x.

[0152] In some embodiments, the receiver 30 may include a single processor 368 configured to analyze all electrical signals delivered by the detection units of the analysis devices DA1 and DA2, where these electrical signals correspond to the estimated control signals R 31 and R 32 . The processor 368 may be configured to generate one or more servo control signals S C36 or S C36-x . The servo control signal S C36 generated by the single processor 368 may correspond to, for example, the polarization correction setpoint signal to be delivered to a single correction module 340-0.

[0153] The servo control loop (i.e., the polarization correction loop that generates the servo control signal) associated with one or two correction devices may be implemented continuously or intermittently. Thus, the processor (368-x or 368) may be configured to control one or more servo control loops of the receiver 30. In particular, the servo control loop may be activated periodically and / or after evaluating the polarization state of one or both of the control signals for the polarization state estimates with respect to one or more associated polarization bases. Additionally, the servo control loop may be implemented until the polarization state of one or two of the estimated control signals is aligned with the associated selected (or reference) polarization state.

[0154] In an embodiment, the processor of the receiver 30 may be configured to determine the value δP x of the polarization state difference between the polarization state of the estimated control signal R 3x and the polarization state P x of the associated polarization basis for a particular correction device D. The processor may also be configured to evaluate whether this value δP x of the polarization state difference is strictly greater than (or greater than or equal to) a predefined reference difference δP x . ref

[0155] In particular, if the determined value δP x of the polarization state difference is greater than or equal to the reference difference δP ref , then the servo control loop may be activated.

[0156] Advantageously, the servo control loop may be implemented to optimize (i.e., maximize or minimize) the detection of the components of the control signal according to the associated polarization state.

[0157] For example and without limitation, a differentiable optimization algorithm (such as a gradient descent algorithm) can be used to generate a servo control signal for a servo control loop in order to (incrementally or iteratively) search for an optimum point of an objective function that is particularly associated with one or more values of the polarization state difference of a correction device D x or two correction devices of a receiver. If the optimum point is found, the servo control loop can be stopped.

[0158] For example and without limitation, if the determined value δP of the polarization state difference x is strictly less than (or less than or equal to) a reference difference value δP ref , the servo control loop can also be stopped.

[0159] Thus, in an embodiment, a servo control signal S related to a setpoint signal of a correction module 340-x can be generated C36-x in order to control the module 340-x and, in particular, the polarization of a rotational signal component S 2x until the value of an estimated control signal R measured by a detection unit 366-x (or a first detection unit 366-x1) 3x is optimal, i.e., the polarization state of the control signal component R 2x is then equal to or orthogonal to the polarization state P 1x of the control signal R x . The modification of the polarization of the signal component S via the servo control signal S C36-x thus causes a modification of the polarization state of the quantum signal S 2x output by the unit 362-x and routed to the detection module 380-x of the processing chain C x (demultiplexed from the signal S Q1 ). 2x

[0160] In some embodiments, a servo control signal S related to a setpoint signal to be delivered to a single correction module 340-0 can be generated C36 in order to control the module 340-0 and, in particular, the polarization of two signal components S 21 and S 22 until the two values of the estimated control signals R measured by the detection units 366-1 and 366-2 (or the first detection units 366-11 and 366-21) 31 and R 32 are optimal, i.e., the polarization states of the control signal components R 21 and R 22 are respectively equal to (or orthogonal to) the polarization states P1 and P2 of the control signals R 11 and R 12 .

[0161] For each processing chain C x , the module 380-x for detecting a quantum signal includes at least one single-photon detector. The polarization-encoded quantum signal S is detected by the set of photon detection units of the processing chains (i.e., C1 and C2) of the receiver 30 Q1 such that an estimated received quantum signal S can be delivered Q3 , and thus an estimate of how the quantum signal is polarization-encoded using a predefined encoding state is delivered.

[0162] In some embodiments, for example, in a set where only two possible polarization states P1 or P2 are utilized (i.e., to simplify the polarization basis definition) for the quantum signal S Q1 being encoded, the module 380-x for detecting the quantum signal of the processing chain C x may include a single single-photon detector 386-x configured to detect the quantum signal S defined in said polarization state P x Q1 .

[0163] In other embodiments, such as in a case where four possible polarization states P1, P2, P3 or P4 are utilized (i.e., defined in polarization bases D / A and H / V, for example) in a set for the quantum signal S Q1 being encoded, the module 380-x for detecting the quantum signal may include a switching unit 384-x preceded by two single-photon detectors 386-x1 and 386-x2, as Figure 12 shown. The switching unit 384-x may be assimilated to the polarization beam splitter 364B-x of the analysis device DA x . Thus, the switching unit 384-x may be configured to route (i.e., switch) the quantum signal S demultiplexed from the signal S to one of the two single-photon detectors (386-x1 or 386-x2) according to the polarization state of the quantum signal. Thus, each single-photon detector (386-x1 and 386-x2) is arranged to detect the presence of a single photon defined in a predefined polarization state of the polarization basis processed by the processing chain C 2x Q1 (in particular, including the polarization state P x x ).

[0164] ​​By way of illustration, with respect to processing chain C1, the module 380-1 for detecting a quantum signal associated, for example and without limitation, with the diagonal basis D / A may include a switching unit 384-1 arranged to transmit a quantum signal S having a linear polarization P1 of diagonal type D via a transmitting device 384-i11 Q1 to a first single-photon detector 386-11, or to transmit a quantum signal S having a linear polarization P3 of anti-diagonal type A via a transmitting device 384-i12 Q1 to a second single-photon detector 386-12. Equivalently, with respect to processing chain C2, the module 380-2 for detecting a quantum signal associated, for example, with the diagonal basis H / V may include a switching unit 384-2 arranged to transmit a quantum signal S having a linear polarization P2 of horizontal type H via a transmitting device 384-i21 Q1 to a single-photon detector 386-21, or to transmit a quantum signal S having a linear polarization P4 of vertical type V via a transmitting device 384-i22 Q1 to a single-photon detector 386-22.

[0165] In an embodiment, the module 380-x for detecting a quantum signal may include, at its input, an additional demultiplexing unit 382-x, which is similar to the signal demultiplexing unit 362-x of the detection module 360-x and is configured to transmit the quantum signal S Q1 to the single-photon detector 386-x or the switching unit 384-x, as Figure 12 shown. Then, the residual component of the control signal is transmitted to the beam absorber 382-0, as Figure 12 shown.

[0166] In particular, the additional demultiplexing unit 382-x of the module 380-x may include a demultiplexing element determined according to the multiplexing type used for multiplexing the signal S1. For example, in the case of frequency-division multiplexing, the additional demultiplexing unit 382-x may be a spectral filter configured to separate the quantum signal S Q1 (i.e., a filter such as an FBG filter or a WDM add-drop filter) from two residual components of the integrated signal, the filter being selected according to a predetermined frequency difference between the quantum wavelength λ Q and a reference wavelength (λ R1 and / or λ R2 ).

[0167] In particular, such an additional demultiplexing unit 382-x makes it possible to increase the control signals R 11 and R 12The filtering capacity to improve the quantum measurement for detecting quantum signals performed by module 380-x.

[0168] In an embodiment, for example, in which the control signals R 11 and R 12 are quantum signals and the multiplexed signal S1 can be time-division multiplexed, the calibration device D x may include a module for correcting the polarization state and a module 380-x for detecting quantum signals. In this case, the module 380-x for detecting quantum signals may be configured to detect the quantum signal S Q1 and the control signal R x associated with the calibration device D 1x . Such a module 380-x may then include a switching unit 384-x corresponding to a polarization beam splitter, at least one single-photon detector 386-x, and a processor (equivalent to system unit 368-x or 368), which is configured to generate one or more servo control signals corresponding to the polarization correction setpoint signals to be delivered to one or more associated calibration modules.

[0169] Figure 13 Shows a method for transmitting the multiplexed signal S1 implemented by the transmitter 10 according to an embodiment of the present invention.

[0170] The transmission method includes a preparatory step 1020 of generating an initial quantum signal S Q0 and a first reference signal R 01 and a second reference signal R 02 .

[0171] In step 1042, the initial quantum signal S Q0 is delivered to one of the optical arms B n in the transmitter 10.

[0172] In step 1044 (equivalent to two separate sub-steps 1044-1 and 1044-2), the first reference signal R 01 is inserted into the first optical arm B1 of the optical arms in the transmitter 10, and the second reference signal R 02 is inserted into the second optical arm B2 of the optical arms in the transmitter 10.

[0173] In one of the insertion sub-steps 1044-1 or 1044-2, according to the routing of the initial quantum signal S Q0 to one of the optical arms B n in step 1042, the first reference signal R 01 or the second reference signal R 02 can be multiplexed with the initial quantum signal S Q0 .

[0174] In step 1046, a polarization modification is applied to the optical signal passing through the first optical arm B1 (i.e., the first reference signal R Q0 and the initial quantum signal S 01 as a result of multiplexing, or only the first reference signal R Q0 ) according to the routing of the initial quantum signal S 01 ), which results in encoding using the first polarization encoding value P1.

[0175] In step 1048, the resulting signals delivered by the optical arm B n of the transmitter 10 are recombined to form a multiplexed signal S1, which includes:

[0176] - A first control signal R 11 for controlling the first polarization encoding value P1, which is 01 determined based on the first reference signal R

[0177] delivered by the optical arm B1, 12 - A second control signal R 02 determined based on the second reference signal R 12 and delivered by the second optical arm B2, the second signal R

[0178] corresponding to the control signal for controlling the second polarization encoding value P2 associated with the second optical arm B2, and - A quantum signal S Q1 encoded using the polarization encoding values defined in the set of values including the first polarization encoding value P1 and the second polarization encoding value P2.

[0179] In step 1050, the multiplexed signal S1 is transmitted through the transmission channel 50.

[0180] Figure 14 FIG. shows a method for receiving the multiplexed signal S1 implemented by the receiver 30 according to an embodiment of the present invention.

[0181] The receiving method includes a preparatory step 3000 of receiving the multiplexed signal S1 transmitted through the transmission channel 50.

[0182] In step 3020, the multiplexed signal S1 is separated into two signal components S 21 and S 22 (or S 2x ), each signal component including one component of the first control signal R 11 and one component of the second control signal R 12 , and each signal component S 2x is respectively propagated to the processing chain C xOne of them. In addition, the encoded quantum signal S included in the multiplexed signal S1 Q1 is transmitted to the processing chain C x One of them.

[0183] The receiving method further includes: for each processing chain C associated with a predefined polarization basis (composed of at least the polarization state P x ), a servo control loop between steps 3040 and 3060; step 3060 corresponds to determining the control signal R of the signal component S passing through the chain x , and step 3040 corresponds to modifying the polarization of the signal component S 2x . When the polarization state determined in step 3060 aligns with one of the polarization states of the basis of the chain C 1x , the servo control loop between steps 3040 and 3060 is stopped. 2x x

[0184] In step 3080, the polarization state of the encoded quantum signal S of the signal component S passing through one of the processing chains C x is determined. 2x Q1

[0185] Those skilled in the art will readily understand that Figure 13 and Figure 14 certain steps of the sending and receiving methods can be performed simultaneously, sequentially, independently or not independently and / or in a different order, for example, in the order defined by the transmitter and the receiver respectively.

[0186] The quantum system according to an embodiment of the present invention or a subsystem of the system (transmitter and receiver) and the above method can be implemented in various ways by hardware or a combination of hardware and software, and in particular, in the form of program code that can be distributed as a program product in various forms. The program code can be distributed using a computer-readable medium, which can include a computer-readable storage medium and a communication medium. The methods described in this specification can be particularly implemented in the form of computer program instructions executable by one or more processors in a computer-based computing device. These computer program instructions can also be stored on a computer-readable medium.

[0187] The present invention is not limited to the embodiments described above by way of non-limiting examples. The present invention includes any variant of the embodiments conceivable by those skilled in the art.

Claims

1. A transmitter (10), configured to transmit a multiplexed signal (S1) via a transmission channel (50), characterized in that: The transmitter (10) comprises: - a signal generator (120), the signal generator (120) being configured to generate an initial quantum signal (S Q0 ), the first reference signal (R 01 ) and the second reference signal (R 02 ), - a polarization encoder (140), the polarization encoder (140) comprising a plurality of optical channels (B n ), the polarization encoder (140) further comprising: ○ An optical selector (142) configured to select the optical channel (B n ) and the generated initial quantum signal (S Q0 ) is transmitted to the selected optical channel (B n ), ○ An optical recombiner (148) configured to generate the multiplexed signal (S1), the multiplexed signal including a first signal (R) for controlling a first polarization coding value (P1) 11 ), a second signal (R ) for controlling the second polarization coding value (P2) 12 ), and using a value set including at least the first polarization encoding value (P1) and the second polarization encoding value (P2) The quantum signal (S Q1 ), the encoded quantum signal (S Q1 ) is based on the initial quantum signal (S) delivered by the optical channel selected by the optical selector (142) Q0 ) to determine, And it is characterized in that the optical channel (B n ) comprises a first optical channel (B1) and a second optical channel (B2), wherein the first optical channel (B1) comprises a 01 ) is integrated into the first optical channel (B1) of the first integrated unit (144-1), the second optical channel (B2) includes a second reference signal (R 02 ) is integrated into a second integrated unit (144-2) in the second optical channel (B2), the first control signal (R 11 ) is based on the first reference signal (R) delivered by the first optical channel (B1) to the optical recombiner (148) 01 ) is determined, and the second control signal (R 12 ) is based on the second reference signal (R) delivered to the optical recombiner (148) by the second optical channel (B2) 02 ) to determine.

2. The transmitter (10) according to claim 1, wherein: Each optical channel (B n ) and the value set A polarization encoding value (P n ) is associated, and wherein the optical channel (B n ) further comprises an optical element (146-n) configured to generate an optical signal according to an associated coding value (P n ) to modify the optical channel (B n )’s polarization.

3. The transmitter (10) according to any one of claims 1 and 2, wherein: The transmitter (10) is a guided all-optical device, and the optical channel (B n ) is formed by polarization maintaining fiber (PMF) and / or integrated waveguide.

4. A receiver (30), the receiver (30) being configured to receive a multiplexed signal (S1) through a transmission channel (50), the multiplexed signal (S1) comprising an encoded quantum signal (S Q1 ), a first signal (R ) for controlling the first polarization coding value (P1) 11 ) and a second signal (R ) for controlling a second polarization encoding value (P2) 12 ), It is characterized in that The receiver (30) comprises a beam splitter (320) configured to separate the multiplexed signal (S1) into two signals each comprising the first control signal (R 11 ) and the second control signal (R 12 ) of two signal components (S 21 and S 22 ), each signal component (S 21 ; S 22 ) respectively through a processing chain (C1; C2) associated with a polarization basis consisting of at least one polarization state (P1; P2), one of the signal components (S 21 or S 22 ) also includes the encoded quantum signal (S Q1 ), and characterized in that each processing chain (C1; C2) comprises a correction device (D1; D2) configured to determine the signal component (S 21 , S 22 ) of the integrated control signal (R 11 ; R 12 ) of the polarization state, the correction device (D1; D2) is further configured to modify the signal component (S 21 , S 22 ) in order to align the determined polarization state relative to one of the at least one polarization state in the associated basis, each processing chain (C1; C2) comprising a detection module (380-1; 380-2) configured to measure the encoded quantum signal (S) in at least one of the at least one polarization state of the associated basis Q1 ).

5. The receiver (30) according to claim 4, wherein: For each processing chain (C1; C2), the correction device (D1; D2) is configured to correct the signal component (S 21 , S 22 ) is demultiplexed to select the control signal component (R 11 ; R 12 ) and routes it to a polarization analysis device (DA1; DA2), the polarization analysis device (DA1; DA2) comprising at least one detection unit (366-x) and configured to detect the integrated control signal (R) in one of the at least one polarization state of the associated basis 11 ; R 12 )'s selected component.

6. The receiver (30) according to any one of claims 4 and 5, wherein: In each processing chain (C1; C2), the correction device (D1; D2) further comprises a processor (368-1; 368-2) configured to analyze the determined polarization state and generate a signal to be applied to correct the signal component (S 21 , S 22 ) of the polarized module (340-1; 340-2) C36 ).

7. The receiver (30) according to any one of claims 4 to 6, wherein: The beam splitter (320) is a symmetrical fiber 50 / 50 optical Y-coupler, and wherein the processing chain (C1; C2) is formed by polarization maintaining fiber (PMF) and / or single mode fiber (SMF).

8. A system (1) for distributing quantum cryptographic keys, the system (1) comprising a transmitter (10) according to any one of claims 1 to 3 and a receiver (30) according to any one of claims 4 to 7.

9. The system (1) according to claim 8, wherein: The multiplexed signal (S1) is a frequency division multiplexed signal.

10. The system (1) according to claim 9, wherein: The encoded quantum signal (S Q1 ) and the first integrated signal and / or the second integrated signal (R 11 and / or R 12 ) is greater than or equal to the first minimum wavelength difference (δλ), and wherein the first control signal (R 11 ) and the second control signal (R 12 ) is greater than or equal to the second minimum wavelength difference (δλ′).

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