Systems and methods for determining encryption keys using phase-coded signals

Abstract

There is provided a transmitter (Alice) configured to transmit an encoded interference signal S2 over a transmission channel, the transmitter comprising a signal generator configured to generate an intermediate quantum signal psi 1gt; and a signal generation module (100) of an intermediate reference signal Sc. The signal generation module (100) is further configured to generate an intermediate quantum signal [psi] 1gt; and a frequency shift [delta] [omega] with the intermediate reference signal Sc. The transmitter further comprises a coding interference module (200) configured to determine a coded interference quantum signal [psi] Agt comprising tracking a substantially similar optical path; and an encoded interference signal S2 that interferes with the reference signal ScA. The encoding interference module (200) comprises an encoding device configured to encode the intermediate quantum signal [psi] 1gt; a phase modulation alpha is applied to only one component.

Description

Technical Field

[0001] The present invention generally relates to the field of quantum communications, and in particular to a system and method for implementing QKD.

[0002] QKD (acronym for quantum key distribution) is a technology that allows the distribution of secret keys between remote users in the context of high-security optical communications (which is also called secret key establishment). The technology uses cryptographic protocols based on the laws of quantum mechanics. Specifically, protocols based on quantum entanglement include P&M-type protocols (P&M stands for prepare and measure). Background Art

[0003] In the cryptographic field, the remote users are usually named Alice (transmitting device) and Bob (receiving device). The P&M QKD protocol using "phase encoding" includes a transmitting step, a receiving step, and a coordination step.

[0004] The first sending step is performed by the transmitter Alice and comprises: preparing a quantum bit encoded in the phase of a pulsed light signal S, which has less than one photon per pulse on average, and then sending the signal S via a quantum transmission channel. Transmitter Alice specifically comprises an optical interferometer consisting of two asymmetric optical arms. Thus, the quantum bit is a quantum signal (i.e., a single photon) modulated with a phase modulation α in one of the arms of the interferometer of transmitter Alice.

[0005] In a second step, performed by receiver Bob, the qubits carried by the received signal are estimated. Receiver Bob also comprises an optical interferometer that is substantially identical to the interferometer of transmitter Alice. Thus, in one of the arms of receiver Bob's interferometer, a phase modulation β (independent of α) is applied to the received quantum signal.

[0006] In the third coordination step, transmitter Alice and receiver Bob communicate the same (ie, α=β) phase modulation via an authenticated common channel.

[0007] In this P&M QKD protocol using phase encoding, any undesired phase fluctuations generated by either interferometer of the QKD system will lead to uncertainty in the measurement probability of a single photon at the receiving end, and therefore to errors in the measured quantum bits and in the establishment of the secret key.

[0008] The possible phase fluctuations are random phase fluctuations whose origin may be thermal or vibrational or caused by any other noise source that affects the quality of the signal propagating through each interferometer. The end result is a phase variation that is unique to each optical interferometer in the system, which will not be exactly the same.

[0009] Other possible phase fluctuations are deterministic phase fluctuations that exist in a system that includes a transmitter Alice and a receiver Bob that move relative to each other. These fluctuations cause phase changes due to changes in the transmission geometry of the signal over time, as described in the article "Interference at the Single Photon Level Along Satellite-Ground Channels" by G. Vallone et al., Phys. Rev. Lett. 116, 253601, 2016.

[0010] To solve the problem of random phase fluctuations, known QKD systems use two different solutions. The first solution consists in passively stabilizing the optical interferometer by thermally insulating it and placing it in a vibration-free environment, as described in the article "Gigahertz decoy quantum key distribution with 1Mbit / s secure key rate" by ARDixon et al., Optics Express, Vol. 16, No. 23, pp. 18790-18797, 2008. This solution, while effective in applications involving controlled environments and terrestrial optical fiber communications, is not suitable for applications involving extreme conditions of use. In particular, in the field of space technology where at least one remote user is a satellite, the solution involves the use of an automatic control loop on the satellite, thereby increasing the cost, weight and power consumption of its payload.

[0011] The second solution consists in time-division multiplexing the quantum signal with a pulsed reference signal containing a large number of photons, as described in the article "10-Mb / s Quantum Key Distribution" by Z. Yuan et al., Journal of Lightwave Technology, Vol. 36, No. 16, 2018. This solution results in a reduction in the throughput of the transmitted quantum bits and requires the use of real-time compensation electronics, which may be difficult to implement. In addition, the effect of the attenuation of the pulse intensity of the reference signal due to changes in the atmospheric transmission level can be interpreted as a reduction in contrast caused by phase fluctuations in the interferometer, and thus leads to incorrect correction of phase fluctuations.

[0012] Therefore, existing QKD systems cannot solve the problem of deterministic phase fluctuations.

[0013]

[0006] Therefore, there is a need for an improved QKD system and method that is particularly capable of correcting for random phase fluctuations caused by the interferometers used to encode and decode the phase, as well as correcting for deterministic phase fluctuations that exist when the transmitter and receiver move relative to each other. Summary of the invention

[0014] The present invention improves this situation by providing a transmitter (Alice) configured to send a coded interference signal S2 through a transmission channel, the transmitter comprising a transmitter configured to generate an intermediate quantum signal |Ψ1> and an intermediate reference signal S c The signal generation module is further configured to generate a signal between the intermediate quantum signal |Ψ1> and the intermediate reference signal S c The transmitter further comprises a coded interferometer module, which is configured to receive the intermediate quantum signal |Ψ1> and the intermediate reference signal S c , and the intermediate quantum signal |Ψ1> and the intermediate reference signal S based on tracking substantially similar optical paths c , to determine the coded interference quantum signal |Ψ A > and the interferometric reference signal S cA The coded interference signal S2 is obtained by the coded interference module, the coded interference module comprising a coding device configured to apply a phase modulation α to only one component of the intermediate quantum signal |Ψ1>.

[0015] In an embodiment, the coded interferometer module may take the form of an asymmetric optical interferometer comprising a first interferometer arm and a second interferometer arm, the first interferometer arm being shorter than the second interferometer arm. Then, the coding device may include at least one frequency splitting unit located on the second interferometer arm, the frequency splitting unit being configured to split the intermediate quantum signal |Ψ1> and the intermediate reference signal S in the frequency domain. c Decoupling.

[0016] Advantageously, the signal generation module may further include a time shift unit configured to shift the intermediate reference signal S in the time domain. c to shift the intermediate quantum signal |Ψ1>.

[0017] The present invention also provides a method configured to receive a coded interference quantum signal |Ψ through a transmission channel A > and interference reference signal S cA A receiver (Bob) comprising a transcoding interferometer module configured to detect the coded interference quantum signal |Ψ based on tracking substantially similar optical paths A > and the interferometric reference signal ScA To generate the transcoded interferometric quantum signal |Ψ B > and the phase modulated interferometric reference signal S cB The transcoding interference module includes a transcoding and phase modulation device, wherein the transcoding and phase modulation device is configured to convert the phase modulation β eff Applied to the coded interferometric quantum signal |Ψ A > only one component and phase modulates Applied to the interferometric reference signal S cA The receiver further comprises a processing module, wherein the processing module is configured to: perform a processing on the transcoded interference quantum signal |Ψ B >at least one measurement of the phase modulated interferometric reference signal S cB At least one measurement of the phase modulation and the phase modulated interference reference signal S cB The at least one interferometric measurement is used to determine the phase modulation β eff .

[0018] In an embodiment, the transcoding interference module may take the form of an asymmetric optical interferometer comprising a first interferometer arm and a second interferometer arm, wherein the first interferometer arm is shorter than the second interferometer arm. Then, the transcoding and phase modulation device may include at least one frequency splitting unit located on the second interferometer arm, wherein the frequency splitting unit is configured to split the coded interference quantum signal |Ψ in the frequency domain A > and the interferometric reference signal S cA Decoupling.

[0019] Advantageously, the transcoded interferometer module may comprise two optical paths at the output of the interferometer, and the processing module may comprise two processing sections, each processing section comprising a frequency splitting unit and configured to perform a processing on the transcoded interferometric quantum signal |Ψ originating from the two optical paths. B > one of the at least one measurements and performing a phase modulated interferometric reference signal S originating from the two optical paths cB A measurement among the at least one measurement.

[0020] Therefore, an embodiment of the present invention provides a system for establishing a quantum encryption key, the system comprising a transmitter (Alice) and a receiver (Bob).

[0021] A method for transmitting a coded interference signal S2 is also provided, the method being implemented by the transmitter (Alice) and comprising the following steps:

[0022] -Generation frequency is ω q The intermediate quantum signal |Ψ1> and the q -ω c The frequency offset is ω c The intermediate reference signal S c ,

[0023] - applying a phase modulation α to only one component of the intermediate quantum signal |Ψ1> passing through the interferometer, the intermediate quantum signal |Ψ1> and the intermediate reference signal S c Tracking substantially similar optical paths to generate a quantum signal including a coded interference |Ψ A > and the interferometric reference signal S cA The coded interference signal S2,

[0024] - generating a coded interference quantum signal comprising the A > and the interferometric reference signal S cA The coded interference signal S2.

[0025] The sending method further comprises: the intermediate quantum signal |Ψ1> and the intermediate reference signal S c Apply a time delay between steps.

[0026] Also provided is a method for receiving a coded interference quantum signal |Ψ A > and the interferometric reference signal S cA The method is implemented by the receiver (Bob), and the method comprises the following steps:

[0027] - Phase modulation Applied to the interferometric reference signal S cA component, and generates a phase-modulated interference reference signal S cB ,

[0028] - performing a phase-modulated interferometric reference signal S cB At least one measurement of

[0029] -Based on the phase modulation and the phase modulated interference reference signal S cB The measurement of the phase modulation β eff ,

[0030] - The phase modulation β eff Applied to the coded interferometric quantum signal |Ψ A > and generate a transcoded interferometric quantum signal |Ψ B >, the coded interference quantum signal |Ψ A> and the interferometric reference signal S cA Tracing essentially similar optical paths,

[0031] -Perform the transcoded interference quantum signal |Ψ B > Make at least one measurement.

[0032] The system and method for establishing a quantum encryption key according to an embodiment of the present invention makes it possible to correct random phase fluctuations caused by an interferometer used to encode and decode the phase, and to correct deterministic phase fluctuations that exist when a transmitter and a receiver move relative to each other in order to establish a quantum key. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Other characteristics, details and advantages of the invention will become apparent from a reading of the description provided with reference to the accompanying drawings, which are given by way of example.

[0034] [ Figure 1 ] Figure 1 is a schematic diagram showing a system for establishing a quantum encryption key according to an embodiment of the present invention.

[0035] [ Figure 2 ] Figure 2 is a schematic diagram of a signal generating module of a transmitter according to an embodiment of the present invention.

[0036] [ Figure 3 ] Figure 3 is a schematic diagram showing a coded interference module of a transmitter according to an embodiment of the present invention.

[0037] [ Figure 4 ] Figure 4 is a schematic diagram showing a transcoding interference module of a receiver according to an embodiment of the present invention.

[0038] [ Figure 5 ] Figure 5 is a schematic diagram showing a processing module of a receiver according to an embodiment of the present invention.

[0039] [ Figure 6 ] Figure 6 is a schematic diagram of a correction unit of a processing module of a receiver according to an embodiment of the present invention.

[0040] [ Figure 7 ] Figure 7 is a flow chart illustrating a method for establishing a quantum encryption key implemented by a transmitter according to an embodiment of the present invention.

[0041] [ Figure 8 ] Figure 8 is a flow chart illustrating a method implemented by a receiver for establishing a quantum encryption key according to an embodiment of the present invention.

[0042] The same reference numerals are used in the drawings to denote the same or similar elements. For the sake of clarity, the illustrated elements are not to scale. DETAILED DESCRIPTION

[0043] Figure 1 There is schematically shown a system for establishing a quantum cryptographic key 1 comprising two devices 10 and 30 communicating with each other. The two devices comprise: a transmitter 10 (also referred to as 'Alice') and a receiver 30 (also referred to as 'Bob').

[0044] The system 1 for establishing a quantum encryption key can be used, for example, in the field of space technology when the transmitter Alice (or vice versa, the receiver Bob) is installed on a satellite and the receiver Bob (or vice versa, the transmitter Alice) is a ground module. The system 1 for establishing a quantum encryption key can also be used in applications of the present invention in which one or both of the devices Alice and Bob are avionics modules. In an embodiment, the system 1 for establishing a quantum encryption key can also be used in applications of the present invention in which one or both of the devices Alice and Bob are optical fiber modules integrated in a ground network.

[0045] Devices Alice and / or Bob may be stationary, or may be moving relative to another communicating device.

[0046] The transmitter Alice comprises a signal generation module 100 and a coded interferometer module 200. The receiver Bob comprises a transcoded interferometer module 300 and a processing module 400. Advantageously, the transmitter Alice and the receiver Bob are so-called all-fiber devices.

[0047] As used herein, a "signal" or "optical signal" refers to coherent light pulses, such as those obtained from a laser beam. A laser beam can be characterized in particular by its pulse rate f and by the laser pulses (i.e., the signal) defined by their intensity I, their phase, and their frequency ω. The "frequency ω" of a laser beam specifies the "optical frequency of the laser pulses multiplied by 2π" and is defined as a function of the wavelength λ of the beam such that c specifies the speed of light.

[0048] A "quantum signal" is an optical signal that contains, on average, less than one photon per pulse. The measurement of a quantum signal conveys a photon detection measurement that depends on the "probability of detection" of that photon.

[0049] As used herein, "encoding" refers to one or more operations including generating a representation of information according to a particular code. For example and without limitation, one type of encoding may include applying a first phase modulation to a quantum signal.

[0050] The term "transcoding" refers to one or more operations including converting a representation of information according to one particular code into another representation according to a different code. For example and without limitation, one type of transcoding may include applying a second phase modulation to a phase modulated quantum signal.

[0051] Transmitter Alice is configured to generate and send an encoded signal over a transmission channel 50 (also referred to as a "quantum channel").

[0052] Depending on the application of the invention, the transmission channel 50 may be, for example, a free space or fiber optic device for transmitting information. The transmission channel 50 may be observed by a detection device (not shown) called "Eve" that is configured to intercept signals sent by the transmitter Alice.

[0053] The encoding interferometer module 200 of the transmitter Alice and the transcoding interferometer module 300 of the receiver Bob may have substantially the same architecture. An interferometer module includes an interferometer including a first interferometer arm and a second interferometer arm.

[0054] In an embodiment, the first interferometer arm of the encoding interferometer module 200 and the first interferometer arm of the transcoding interferometer module 300 do not include any optical elements capable of modifying the phase of the signals passing through them. In addition, the first interferometer arm of the encoding interferometer module 200 and the first interferometer arm of the transcoding interferometer module 300 may not cause any phase fluctuations (such as causing phase shifts to the signals passing through them) and are referred to as "reference arms". In these embodiments, the second interferometer arm of the encoding interferometer module 200 and the second interferometer arm of the transcoding interferometer module 300 each include one or more optical elements and may be configured to modify the phase of one or more signals passing through them. In addition, the second interferometer arm of the encoding interferometer module 200 may include causing a phase shift (expressed as ) in any signal passing through it. ). In addition, the second interferometer arm of the transcoding interferometer module 300 may include a second interferometer arm that causes a phase shift (expressed as ) phase fluctuation.

[0055] The receiver Bob is configured to receive the coded signal (i.e. the signal sent by the transmitter Alice) via the transmission channel 50 and estimate the received signal (which results in an estimated received signal). The receiver Bob is also configured to determine and perform an analysis of the phase shift in the coded signal and the estimated received signal caused by the interference module included in the system 1. and The differential correction of the phase fluctuations between them is used to establish quantum encryption keys.

[0056] Furthermore, transmitter Alice and receiver Bob are configured to establish (ie, determine) a quantum encryption key using the encoded signal and the estimated received signal.

[0057] The signal generation module 100 of the transmitter Alice is configured to generate an intermediate signal denoted S1 from an initial signal denoted S0 with a frequency of ω0 and an intensity of I0, comprising:

[0058] -Frequency is ω q And the intensity is I q The expression is S q (or use the quantum state annotation |Ψ1>) and

[0059] -Frequency is ω c And the intensity is I c The expression is S c The intermediate reference signal.

[0060] Specifically, the signal generation module 100 of the transmitter Alice is configured to generate a signal between the intermediate quantum signal |Ψ1> and the intermediate reference signal S c The frequency shift between is represented as Δω, which can be defined by the following equation (Equation (01)):

[0061] Δω=ω q -ω c (01)

[0062] The coded interference module 200 of the transmitter Alice is configured to generate a coded interference signal denoted as S2 from the intermediate signal S1, comprising:

[0063] - is represented by |Ψ A > of the coded interferometric quantum signal, and

[0064] -Symbol: S cA The interference reference signal.

[0065] Specifically, the coded interferometer module 200 of the transmitter Alice may include a frequency splitting unit located on the second interferometer arm of the coded interferometer module 200. The components of the intermediate quantum signal |Ψ1> and the intermediate reference signal S c The component of the intermediate quantum signal |Ψ1> passes through the second interferometer arm of the coded interferometer module 200. Then, the frequency splitting unit of the coded interferometer module is configured to separate the component of the intermediate quantum signal |Ψ1> from the intermediate reference signal S by means of a frequency shift Δω. c The coded interferometer module 200 is then configured to apply a phase modulation α to the component of the intermediate quantum signal |Ψ1> passing through the second interferometer arm of the coded interferometer module 200, thereby allowing the quantum encryption key to be encoded.

[0066] The transcoded interference module 300 of the receiver Bob is configured to generate a transcoded interference signal S3 according to the encoded interference signal S2, which includes:

[0067] - is represented by |Ψ B > of the transcoded interferometric quantum signal, and

[0068] -Symbol: S cB A phase-modulated interferometric reference signal.

[0069] Those skilled in the art will appreciate that the transcoding interference module 300 of the receiver Bob is configured as follows:

[0070] -The coded interference quantum signal |Ψ A >projected onto the “first output port” of the interferometer of module 300 is represented by |Ψ B+ > on the transcoded interference quantum state, and projected onto the "second output port" of the interferometer of module 300 as |Ψ B- > on the transcoded interferometric quantum state, and

[0071] -Generate S cB A phase-modulated interferometric reference signal.

[0072] Specifically, the transcoding interferometer module 300 of the receiver Bob may include a frequency splitting unit located on the second interferometer arm of the transcoding interferometer module 300. The coded interferometer quantum signal |Ψ A >component and interference reference signal S cA The component of |Ψ passes through the second interferometer arm of the transcoding interferometer module 300. Then, the frequency splitting unit of the transcoding interferometer module is configured to separate the coded interference quantum signal |Ψ A >component and the intermediate reference signal S cA The transcoding interference module 300 is then configured to separate the phase modulation β eff and phase modulation The coded interference quantum signal |Ψ is applied to the second interferometer arm passing through the transcoding interference module 300. A >component and interference reference signal S cA components, thereby allowing the transcoding of quantum encryption keys.

[0073] The processing module 400 of the receiver Bob is configured as follows:

[0074] - measuring the phase modulated reference signal S delivered by the transcoding interferometer module 300 cB ,

[0075] - Based on the phase modulated reference signal S cB Measurement and phase modulation To determine the phase shift and The differential correction ε (or discrete error signal ε) of the phase fluctuation between

[0076] - Determine one or more phase modulation values ​​β based on the differential correction ε and the phase modulation β eff , and one or more phase modulation values ​​β eff sent to the transcoding interference module 300, and

[0077] - Estimate the transcoded interferometric quantum signal |Ψ B >(i.e., estimate the coded interference quantum signal |Ψ A > to the transcoded interferometric quantum state |Ψ B+ > and |Ψ B- > projection on top).

[0078] It should be noted that in these embodiments, the interference reference signal S cA is not modulated by the coded interference module 200 of the transmitter Alice, but the interference reference signal S cB The component of is phase modulated in the transcoding interferometer module 300 of the receiver Bob. This configuration enables the processing module 400 to determine the phase shift of the interferometer modules of the transmitter Alice and the receiver Bob. and The differential correction of the phase fluctuation between α and β is achieved without compromising the security of the established secret key (i.e. without modifying the phase modulation α), and thus inferring the phase modulation β via fast electronic feedback. eff .

[0079] Similarly, the interferometric quantum signal |Ψ A > and |Ψ B Each component of > is phase modulated in one of the two interferometer arms of the encoding interferometer module 200 and one of the two interferometer arms of the transcoding interferometer module 300. This dual modulation α and β allows the transmitter Alice and the receiver Bob to generate the information needed to establish a secure encryption key.

[0080] Figure 2 The signal generation module 100 of the transmitter Alice according to an embodiment of the present invention is schematically shown.

[0081] The signal generating module 100 includes a laser unit 120 , a beam splitting unit 140 and a beam recombining unit 160 .

[0082] In an embodiment, the laser unit 120 may include a transmissive laser that emits a laser beam having a frequency of ω0 (equal to a wavelength λ0) and represented by L c The coherence length characterization.

[0083] The emission wavelength λ0 of the laser may be in the visible or infrared. For example and without limitation, the laser unit 120 may comprise a DFB laser diode using a Bragg grating (DFB is an acronym for distributed feedback), which allows the emission wavelength λ0 to be selected. The selected emission wavelength λ0 of the laser diode may be equal to 1550 nm. Such a laser diode in particular emits a continuous wave laser beam. Alternatively, the laser unit 120 may comprise a pulsed laser source.

[0084] Therefore, in an embodiment, the laser unit 120 may further include an intensity modulation unit (not shown in the figure), so that laser pulses having a pulse rate f0 and a pulse intensity I0 and a pulse duration τ0 of up to several nanoseconds can be generated from a continuous wave (or pulse) laser beam. The rate f0 of the pulse train may be in the order of a few kilohertz to tens of gigahertz.

[0085] The laser beam may be polarized with any suitable polarization. For example, the laser beam may be linearly polarized.

[0086] Therefore, the laser unit 120 can be configured to generate an initial signal S0 that can be defined by the following equation (Equation (2)):

[0087]

[0088] The beam splitting unit 140 is configured to split the laser beam emitted by the laser unit 120 into two components (e.g., denoted as S and S ) of the initial signal that track two separate optical paths 140-1 and 140-2. 0-1 and S 0-2 ).

[0089] In an embodiment, the beam splitting unit 140 may be a polarization-maintaining optical coupler (e.g., a fiber Y coupler) that is typically placed at the output of the laser unit 120. The beam splitting unit 140 may also be asymmetric so as to split the component of the initial signal consisting of low-intensity pulses (e.g., S 0-1 ) is transmitted to the optical path 140-1, and the component of the initial signal consisting of high-intensity pulses (e.g., S 0-2 ) is transferred to the optical path 140-2. Advantageously, the coupler is a 90 / 10 coupler, ie 90% of the optical power is transferred to the component S of the tracking optical path 140-2 of the initial signal. 0-2and delivers 10% of the optical power to the component S of the tracking optical path 140-1 of the initial signal 0-1 Therefore, the beam splitter 140 can generate two signal components S 0-1 and S 0-2 The signal component S 0-1 and S 0-2 It can be defined by the following equations (Equations (03) and (04)):

[0090]

[0091]

[0092] The signal generation module 100 further comprises a unit 142 configured to generate a component S of the tracking optical path 140-1 from the initial signal. 0-1 Generate a "quantum signal".

[0093] In an embodiment of the present invention, the unit 142 for generating a quantum signal may be an optical attenuator, so that an amplitude I q The average is equal to a quantum signal of less than one photon per pulse. Therefore, the unit 142 for generating a quantum signal is configured to generate a quantum signal S called an intermediate quantum signal q (I q ,ω0), the intermediate quantum signal is also represented by quantum state notation as |Ψ1>.

[0094] Furthermore, the unit 142 for generating a quantum signal may include a component S configured to track the optical path 140-1 of the quantum signal and / or the initial signal. 0-1 Another intensity modulation unit (not shown in the figure) performs additional pulse modulation. For example, such a unit can be used to implement a protocol using quantum decoy states.

[0095] In an embodiment, in particular in the case where the laser unit 120 comprises a continuous wave laser source, the unit 142 for generating a quantum signal may further comprise a phase modification unit (not shown in the figure) configured to modify the phase of each quantum pulse tracing the optical path 140-1. Advantageously, the phase modification unit may be configured to randomize (i.e., make it random) the phase of each of these quantum pulses, such that the phases of two consecutive quantum pulses are independent of each other. It should be noted that phase randomization may make it possible to prevent certain types of quantum key attacks from being performed by a detection device called "Eve" placed on the transmission channel 50, which intercepts a reference signal sent by the transmitter Alice and exploits the phase coherence between the pulses. Such a phase modification unit may be, for example, a phase modulator.

[0096] The signal generation module 100 further comprises a unit 144 configured to generate a component S of the tracking optical path 140-2 from the initial signal. 0-2 Generate a "reference signal".

[0097] In an embodiment of the present invention, the unit 144 for generating a reference signal may be a frequency shift unit configured to form a reference signal having a frequency shifted by Δω=ω0-ω c The component S of the tracking optical path 140-2 relative to the initial signal and the initial signal 0-2 The unit 144 for generating a reference signal then generates a signal S called an intermediate reference signal c (I c ,ω c ), which can be defined by the following equation (Equation (05)):

[0098]

[0099] The unit 144 for generating the reference signal may be, for example, an acousto-optic modulator (AOM) or an acousto-optic frequency shifter (AOFS) or a phase modulator. Thus, the signal generation module 100 may comprise, for example and without limitation, a voltage generator 146 allowing control of the offset Δω. It should be noted that the offset Δω to be applied is selected so as to obtain a voltage at the intermediate reference signal S c spectral decoupling between |Ψ1> and the intermediate quantum signal, so as to prevent a detection device "Eve" placed on the transmission channel 50 and intercepting the reference signal emitted by the transmitter Alice from being able to infer therefrom the phase fluctuations to be corrected and thus derive the emitted quantum bit. According to a non-limiting example, for a selected emission wavelength λ0 equal to 1550 nm, an offset Δω between 1 GHz and 130 GHz may allow to detect the phase fluctuations to be corrected in the intermediate reference signal S c The wavelength of the intermediate quantum signal |Ψ1> changes from a few picometers to 1nm.

[0100] In some embodiments, the unit 144 for generating a reference signal (or the unit 142 for generating a quantum signal) may further include a time shift unit (not shown in the figure) configured to time shift the beam components relative to each other. The component of the tracking optical path 140-1 (i.e., the quantum signal |Ψ1>) may be shifted relative to the beam component of the tracking optical path 140-2 (i.e., the reference signal S c ) is delayed by a time Δτ. The time Δτ can be defined, for example, as the pulse rate f of the light beam. s and / or pulse duration τ s function, so that This time Δτ can also be defined as the realization time τ of the electronic feedbackr function in order to be able to infer the phase modulation β in the receiver Bob eff And correct the phase fluctuation so that Δτ<τ r .

[0101] The beam recombining unit 160 at the end of the two separate optical paths 140-1 and 140-2 allows the two signal components to be recombined on a single optical path into an intermediate signal S1, which is sent to the coded interferometer module 200 and can be defined by the following equation (Equation (06)):

[0102] S1=S c +|Ψ1> (06)

[0103] According to the embodiment used, the frequency reorganization unit 160 for recombining the quantum signal and the reference signal can be, for example, a frequency multiplexer (usually denoted as MUX), the technology of which depends on the intermediate reference signal S c and the frequency shift Δω between the intermediate quantum signal |Ψ1>.

[0104] Figure 3 The coded interference module 200 of the transmitter Alice according to an embodiment of the present invention is schematically shown.

[0105] The coded interferometer module 200 includes a beam splitter 240, which is configured to split the transmitted intermediate signal S1 into two beam components, for example, represented as S 1-1 and S 1-2 Each beam component tracks the first interferometer arm denoted as 240-1 or the second interferometer arm denoted as 240-2. The intermediate signal S1 includes an intermediate reference signal S c and the intermediate quantum signal |Ψ1>, and the component S of the intermediate signal 1-1 and S 1-2 Each of them includes an intermediate reference signal S c A component of c-1 or S c-2 ) and a quantum state (or, more simply, a “component”) of the intermediate quantum signal |Ψ1> (e.g. |Ψ 1-1 > or |Ψ 1-2 >). The beam splitter 240 may be a polarization-maintaining symmetric optical coupler (eg, a 50 / 50Y coupler), which is configured to transmit two components S of the intermediate signal. 1-1 and S 1-2 , these components consist of pulses of equal intensity.

[0106] According to an embodiment, the coded interferometer module 200 may be a Michelson interferometer, such as Figure 3Advantageously, the coded interferometer module 200 may include Faraday mirrors 242 and 248, which are placed at the ends of the interferometer arms 240-1 and 240-2 and are configured to compensate for the component S of the intermediate signal. 1-1 and S 1-2 During its respective travel along one of the two interferometer arms 240-1 or 240-2 it experiences certain polarization changes.

[0107] In addition, if Figure 3 As shown in FIG. 1 , in these embodiments, the beam splitter 240 of the coded interferometer module 200 may be configured to split the beam component S of the intermediate signal into 1-1 and S 1-2 After its respective travel along one of the two interferometer arms 240-1 or 240-2, the beam component S of the intermediate signal is 1-1 and S 1-2 Recombined into the resulting coded interference signal S2.

[0108] Alternatively, the coded interferometer module 200 may be a Mach-Zehnder interferometer (configuration not shown in the figure). In these embodiments, the coded interferometer module 200 includes a beam coupler, which is placed at the ends of the interferometer arms 240-1 and 240-2 and is configured to couple the beam component S of the intermediate signal to the beam component S of the intermediate signal. 1-1 and S 1-2 Recombined into the obtained interference signal S2.

[0109] The first interferometer arm 240-1 does not include any additional optical elements. In addition, the first interferometer arm 240-1 is a phase reference arm, so that the component S of the intermediate signal tracks the first interferometer arm 240-1. 1-1 Does not experience any phase fluctuations.

[0110] In an embodiment, the two interferometer arms 240-1 and 240-2 may have different arm lengths, so that the interferometer includes a short interferometer arm and a long interferometer arm. The length difference between the two interferometer arms is represented as ΔL. The interferometer of the encoding module 200 and the interferometer of the transcoding module 300 (because they have substantially the same architecture) can have the same length difference ΔL. For example, but not by way of limitation, the length difference ΔL may be less than the coherence length L of the laser unit 120. c and is greater than the pulse size L0, which is defined based on the duration of the pulse, L0 = c × τ0, c specifies the speed of light, and τ0 specifies the pulse duration. After recombining into the coded interference signal S2, this length difference ΔL causes the component S in the intermediate signal to be 1-1 and S 1-2(Each component tracks one of the two interferometer arms 240-1 or 240-2). Typically, for an interferometer, this time separation is defined such that the signal components are delayed by a time Δt calculated based on the difference ΔL. In addition, one component output from the interferometer is delayed by a delay, which is called the "interference delay" and is denoted t, such that:

[0111] - the component tracking the short interferometer arm is delayed by an interferometer delay t=0; and

[0112] The component tracking the long interferometer arm is delayed relative to the component tracking the short interferometer arm by an interferometric delay t=Δt.

[0113] In a first variation of the invention, the first interferometer arm 240-1 may be a short interferometer arm. In this variation of the invention, the second interferometer arm 240-2 may be a long interferometer arm, depending on the length difference ΔL between the two arms. Alternatively, in a second variation of the invention, the first interferometer arm 240-1 may be a long interferometer arm, and the second interferometer arm 240-2 may be a short interferometer arm.

[0114] The coded interferometer module 200 further includes an encoding device 240D configured to apply a phase modulation α to only one component of the intermediate quantum signal |Ψ1>. Specifically, the encoding device 240D may be placed (ie, positioned) on the second interferometer arm 240-2.

[0115] The encoding device 240D may include a frequency splitting unit 244 configured to track the component S of the second interferometer arm 240-2 of the intermediate signal. 1-2 Perform frequency decomposition (i.e., choose in the frequency domain or spectral domain) to obtain the following beam components:

[0116] -Intermediate reference signal S c The weight S c-2 , which tracks a path referred to as “transmitter Alice's reference signal path” 244-1, and

[0117] -The quantum state corresponding to the photon of the intermediate quantum signal |Ψ 1-2 >, which traces path 244-2, referred to as "transmitter Alice's quantum signal path."

[0118] Furthermore, the encoding device 240D may include a frequency recombining unit 246 configured to recombines the components S of the tracking signal paths 244-1 and 244-2 in the frequency domain on the second interferometer arm 240-2. c-2 and |Ψ 1-2 >.

[0119] The frequency reorganization unit 246 may be the same as the frequency separation unit 244 of the signal generation module 100. For example, the two units may be based on the intermediate reference signal S c With the intermediate quantum signal S q The frequency shift Δω between the frequencies is achieved using a frequency demultiplexer (denoted as DEMUX).

[0120] like Figure 3 As shown, reference signal path 244-1 of transmitter Alice does not include any additional optical elements.

[0121] Advantageously, the encoding device 240D may include a phase modulation unit 2442 configured to modulate the photons (quantum states |Ψ) of the intermediate quantum signal of the quantum signal path 244-2 of the transmitter Alice according to a parameter α (also referred to as “phase modulation α”). 1-2 >) is modulated. The phase modulation unit 2442 may also be referred to as an "encoding unit" 2442 (because it allows the quantum encryption key to be encoded).

[0122] For example and without limitation, encoding unit 2442 may be an electro-optical modulator composed of an electro-optical crystal to which an electrical signal defined using control unit 2444 is applied. The control unit may be, for example, a voltage generator configured to control parameter α. The photons (quantum states |Ψ) of the intermediate quantum signal passing through unit 2442 are then 1-2 >) is modulated to encode the quantum bit.

[0123] Advantageously, the parameter α can be selected from a set of two orthogonal bases, such as, for example, [0,π] and [π / 2,3π / 2]. It should be noted that this choice of parameter α and the phase encoding of the quantum bit is a novel alternative to the conventional method of the protocol known as BB84 (as described in the article "Quantum cryptography: Public key distribution and coin tossing" by C. Bennett and G. Brassard in Theoretical Computer Science, 1884, Vol. 560, 1984, pp. 7-11). In the BB84 protocol, the quantum bit is encoded with the polarization of the photon (rather than its phase). Conventional implementations of the BB84 protocol that encodes polarization are limited by system constraints related to error correction in the polarization of the quantum signal. Since the correction is of a mechanical type (it uses, for example, an electric half-wave plate), conventional implementations of the BB84 protocol are limited in terms of the bandwidth of the quantum signal relative to phase encoding schemes that use electro-optical devices (e.g., electro-optical modulators) that allow high throughput. The high throughput obtained with embodiments of the present invention is also greater than the throughput obtained with time-division multiplexing-based coding, which requires more correction of errors caused by fluctuations and therefore requires a reduction in the throughput used to encode and transmit quantum bits. In the application of the present invention in the field of space technology, the number of quantum bits encoded and transmitted is, for example, limited by the passage time of the satellite. Therefore, in this case, the number of quantum bits exchanged during the passage is reduced. In addition, if the fluctuations become too large, the critical correctable error threshold (equal to about 10%) may be exceeded, and then the establishment of the key becomes impossible. Therefore, the high throughput of the phase encoding method has the advantage of increasing the number of quantum bits exchanged in time-limited quantum communications, which makes it possible to establish a secret key between two remote users.

[0124] The control unit 2444 of the encoding unit 2442 of the transmitter Alice can advantageously include a quantum random number generator, which is more secure than the software-based pseudo-random number generators usually used in computers to generate random numbers. Such conventional pseudo-random number generators use deterministic algorithms to produce predictive random number sequences. In contrast, the quantum random number generator makes it possible to randomly obtain the sequence of parameters α to be encoded in the phase of the quantum signal according to unpredictable quantum statistical probabilities, which allows providing a more secure secret key.

[0125] In addition, the component |Ψ of the intermediate quantum signal of the second interferometer arm 240-2 is tracked 1-2 > and the component S of the intermediate reference signal c-2 may experience phase fluctuations. However, these two components |Ψ1-2 > and S c-2 follows a similar optical path, and therefore, upon leaving the interferometer of the coded interferometer module 200, the component |Ψ 1-2 > and S c-2 With respect to tracking the component of the first interferometer arm 240-1 |Ψ 1-1 > and S c-1 With the same phase shift This is not the case in prior art devices where the quantum signal and the reference signal follow different optical paths, e.g. where the reference signal takes a short first interferometer arm and the quantum signal "takes" a long second interferometer arm. It should be noted that the term "take" is used in the case of the quantum signal for simplicity. Specifically, in practice, the quantum signal does not "take" one particular optical path or the other, but is more accurately equivalent to a superposition of two optical paths.

[0126] When leaving the interferometer of the coded interferometer module 200, the obtained interferometric reference signal (denoted as S cA ) can be defined as the intermediate reference signal S by the following equation (Equation (07)): c and phase shift Function:

[0127]

[0128] In particular, the interferometric reference signal S cA It consists of two conventional light pulses of equal intensity, which are separated in time by an amount Δt defined on the basis of the difference ΔL.

[0129] When leaving the interferometer of the coded interferometer module 200, it is represented by |Ψ A The resulting coded interference quantum signal of > can be defined as the phase modulation α and the phase shift by the following equation (Equation (08)): Function:

[0130]

[0131] It should be noted that the quantum state |Ψ A > can be represented by the state |Ψ 1-1 > and |Ψ 1-2 > is represented by the superposition of 1-1 > corresponds to a photon of the intermediate quantum signal that has adopted the first interferometer arm 240-1, and the state |Ψ 1-2 >A photon corresponding to the intermediate quantum signal that has adopted the second interferometer arm 240-2.

[0132] Thus, upon leaving the interferometer of the coded interferometer module 200, the coded interferometer signal S2 includes the interferometer reference signal S cA and the coded interference quantum signal |Ψ A >, and can be defined by the following equation (Equation (09)):

[0133] S2=S cA +|Ψ A > (09)

[0134] like Figure 3 As shown, in an embodiment using a Michelson interferometer, the coded interference signal S2 can then propagate toward the signal generation module 100 on the one hand and toward the receiver Bob via the optical path 240-3 on the other hand. In these embodiments, the coded interference module 200 of the transmitter Alice may also include a signal deflection unit 220, which is configured to deflect (i.e., guide) the coded interference signal S2 to a control unit 222 for controlling the interference signal. For example, the control unit 222 may be a control photodiode that allows the coded interference signal S2 transmitted by the transmitter Alice to the receiver Bob to be controlled. The signal deflection unit 320 may be an optical circulator. The signal deflection unit 220 enables the coded interference signal S2 to be prevented from being transmitted to the signal generation module 100.

[0135] The quantum signals of the coded interference signal S2 and the reference signal may also experience propagation-related phase fluctuations during propagation through the transmission channel 50 between the transmitter Alice and the receiver Bob. Since the propagation through the transmission channel 50 is common to both components of the coded interference signal S2, these propagation-related phase fluctuations have no effect on the result of the quantum interference measurement performed by the receiver Bob.

[0136] Figure 4 The transcoding interference module 300 of the receiver Bob according to an embodiment of the present invention is schematically shown.

[0137] The transcoding interferometer module 300 has an asymmetric optical interferometer architecture similar to the coded interferometer module 200 of the transmitter Alice. Therefore, the transcoding interferometer module 300 includes a beam splitter 340 (e.g., a symmetrical polarization-maintaining fiber 50 / 50 optical Y coupler), which is configured to split the interference signal S2 emitted by the transmitter Alice into a beam represented as S 2-1 and S 2-2 The two beam components of . Component S 2-1 and S 2-2(e.g., they consist of pulses of equal intensity) each tracking the first interferometer arm denoted as 340-1 or the second interferometer arm denoted as 340-2. Since the encoded intermediate signal S2 includes the reference signal and the quantum signal, the intermediate signal S 2-1 and S 2-2 Each of the two includes a component of the interference reference signal (e.g., denoted as S cA-1 or S cA-2 ) and a quantum state (or more simply "component") of the interfering quantum signal (e.g., represented by |Ψ A-1 > or |Ψ A-2 >).

[0138] In one embodiment of the coding interferometer module 200 of the transmitter Alice is a Mach-Zehnder interferometer, the transcoding interferometer module 300 may be a Mach-Zehnder interferometer. In another embodiment of the coding interferometer module 200 is a Michelson interferometer, the transcoding interferometer module 300 may advantageously be a Michelson interferometer including Faraday mirrors 342 and 348, each of which is placed at the end of one of the two interferometer arms 340-1 and 340-2 in order to compensate the beam component S 2-1 and S 2-2 The polarization change experienced.

[0139] Similar to the coded interferometer module 200 of the transmitter Alice, the first interferometer arm 340-1 does not include any additional optical elements. In addition, the first interferometer arm 340-1 is a phase reference arm, so that the component S of the coded interference signal S2 2-1 No phase fluctuation is experienced. The first interferometer arm 340-1 may be a shorter arm, while the second interferometer arm 340-2 is a longer arm, with a length difference ΔL between the two arms. Alternatively, the first interferometer arm 340-1 may be a longer interferometer arm, while the second interferometer arm 340-2 is a shorter interferometer arm.

[0140] The transcoding interference module 300 of the receiver Bob also includes a transcoding and phase modulation device 340D, which is configured to convert the phase modulation β eff Applicable only to coded interferometric quantum signals |Ψ A > a component and phase modulates Applies only to the interferometric reference signal S cA The transcoding and phase modulation device 340D may be placed (ie, positioned) on the second interferometer arm 340-2.

[0141] The transcoding and phase modulation device 340D may include a frequency splitting unit 344 configured to track the component S of the intermediate signal of the second interferometer arm 340-2.2-2 The splitting is performed (i.e., selection is performed in the frequency domain) to obtain the following beam components:

[0142] -Interference reference signal S cA The component S of the reference signal path 344-1 of the tracking receiver Bob cA-2 ,as well as

[0143] - the quantum state corresponding to the photon of the intermediate quantum signal of the quantum signal path 244-2 of the tracking receiver Bob |Ψ A-2 >.

[0144] The transcoding and phase modulation device 340D may include a frequency recombining unit 346 configured to recombines the components S of the tracking signal paths 344-1 and 344-2 in the frequency domain on the second interferometer arm 340-2. cA–2 and |Ψ A-2 >.

[0145] The frequency splitting unit 344 and the frequency recombining unit 346 of the receiver Bob can be respectively the same as the frequency splitting unit 244 and the frequency recombining unit 246 of the transmitter Alice. Therefore, they can adopt the frequency splitting unit 344 and the frequency recombining unit 346 based on the intermediate reference signal S c With the intermediate quantum signal S q The frequency shift Δω between the technology is in the form of a frequency demultiplexer and a multiplexer.

[0146] Different from the encoding device 240D of the transmitter Alice, the transcoding and phase modulation device 340D of the receiver Bob may include a phase modulation unit 3446, which is configured to, according to the parameter The interference reference signal S cA The component S propagating along the reference signal path 344-1 of the receiver Bob cA-2 The phase is modulated.

[0147] Phase modulation parameters of the phase modulation unit 3446 It can be controlled by a control unit 3448, which can be, for example, a voltage generator configured to send a radio frequency signal that allows the generation of a sinusoidal phase modulation. The radio frequency signal can have a zero phase and depends on the frequency ω m . It can be expressed as follows (Equation (10)) This phase modulation is defined as the parameter corresponding to the modulation depth Function:

[0148]

[0149] Similar to the encoding device 240D of the transmitter Alice, the transcoding and phase modulation device 340D of the receiver Bob may include a phase modulation unit 3442, which is configured to, according to the parameter β eff The photon (quantum state |Ψ) of the intermediate quantum signal of the quantum signal path 344-2 of the tracking receiver Bob A-2 >) is modulated. For example and without limitation, phase modulation unit 3442 can be similar to phase modulation unit 2442. Phase modulation unit 3442 can also be referred to as transcoding unit 3442 (because it allows the encoded information of the quantum encryption key to be transcoded).

[0150] Tracking component |Ψ of the interference quantum signal of the second interferometer arm 340-2 A-2 > and the interference reference signal component S cA-2 However, the two components follow similar optical paths, so when leaving the interferometer of the transcoding interferometer module 300, the component |Ψ A-2 > and S cA-2 With the same phase shift

[0151] When leaving the interferometer of the transcoding interferometer module 300, it is represented by S cB The in-phase interference reference signal of can be defined as the interference reference signal S by the following equation (Equation (11)): cA and phase shift and Function:

[0152]

[0153] In particular, the interferometric reference signal S cA It consists of three conventional light pulses separated in time by the amount Δt (i.e., the delays of the interference pulses are equal to 0, Δt, and 2×Δt), which is defined based on the length difference ΔL of the interferometers of the transmitter Alice and the receiver Bob.

[0154] In an embodiment, the processing module 400 is configured to, based on the interference reference signal S at the output of the transcoding interference module 300 cA The phase shift in the interferometric modules 200 and 300 is determined by measuring the predefined intensity fluctuations in and The intensity fluctuations correspond in particular to interference between photons of the reference signal, which consist of conventional light pulses with an interference delay Δt, and therefore have been employed:

[0155] On the one hand, - the first interferometer arm 240-1 of the transmitter Alice, and then the second interferometer arm 340-2 of the receiver Bob, and

[0156] On the other hand, - the second interferometer arm 240-2 of the transmitter Alice, and then the first interferometer arm 340-1 of the receiver Bob.

[0157] According to an embodiment of the present invention, the phase modulation parameter β of the transcoding unit 3442 is eff It can be directly controlled by the processing module 400 of the receiver Bob, such as Figure 4 In particular, then, the parameter β eff can be defined based on the phase modulation parameter β obtained in a similar manner to the parameter α, but including the phase shift in the interferometric modules 200 and 300 and The differential correction of the phase fluctuation between the two is based on the interferometric reference signal S cB is defined by at least one measurement.

[0158] To simplify the problem, when leaving the interferometer of the transcoding interferometer module 300, it is represented by |Ψ B The resulting transcoded interferometric quantum signal of can be defined as the coded interferometric quantum signal |Ψ A >, phase modulation β eff and phase shift function.

[0159] Therefore, upon leaving the interferometer of the transcoding interferometer module 300, the transcoded interferometer signal S3 includes the interferometer reference signal S cB and the transcoded interferometric quantum signal |Ψ B >, and can be defined by the following equation (Equation (12)):

[0160] S3=S cB +|Ψ B > (12)

[0161] like Figure 4As shown in FIG. 5 , in an embodiment using a Michelson interferometer, the transcoded interference signal S3 may propagate toward the transmitter Alice on the one hand and toward the processing module 400 (which corresponds to the first output port of the interferometer of the module 300) along the optical path 340-3 on the other hand. In such an embodiment, the transcoded interference module 300 may further include a signal deflection unit 320, which is configured to deflect (i.e., guide) the transcoded interference signal S3 to the processing module 400 along the optical path 340-4 (which corresponds to the second output port of the interferometer of the module 300). For example, the signal deflection unit 320 may be an optical circulator. The signal deflection unit 320 enables the transcoded interference signal S3 to be prevented from being transmitted to the transmission channel 50.

[0162] Figure 5 The processing module 400 of the receiver Bob according to an embodiment of the present invention is schematically shown.

[0163] The processing module 400 comprises two processing sections, each associated with one of the two optical paths 340 - 3 and 340 - 4 obtained at the output of the interferometric module 300 of the receiver Bob.

[0164] Each processing section 420 (or 440) includes a frequency splitting unit 422 (or 442) configured to split the phase modulated interference reference signal S cB The resulting transcoded interference quantum signal |Ψ B > split (i.e., selected in the frequency domain) so that they track one of the two measurement arms 422-1 or 422-2 (or 442-1 or 442-2) associated with processing portion 420 (or 440). For example and without limitation, measurement arm 422-1 of processing portion 420 and measurement arm 442-1 of processing portion 440 can be quantum measurement arms, while measurement arm 422-2 of processing portion 420 and measurement arm 442-2 of processing portion 440 can be reference measurement arms. Thus, the transcoded interferometric quantum signal |Ψ B > Tracking quantum measurement arm 422-1 (or 442-1), and phase modulated interferometric reference signal S cB Tracking reference measurement arm 422-2 (or 442-2). Frequency splitting units 422 and 442 may be based on the intermediate reference signal S c and the intermediate quantum signal S q The frequency shift Δω between the frequencies is determined by the frequency demultiplexer.

[0165] Each processing portion 420 (or 440) includes a single photon detector 4222 (or 4422) located on a quantum measurement arm 422-1 (or 442-1). The single photon detector may include a detection surface and is configured to detect the "presence" of a single photon at its detection surface (i.e., via a photon / surface interaction). This detection of the presence of a single photon is defined according to a given quantum detection efficiency. For example and without limitation, the single photon detector 4222 (or 4422) may be an avalanche photodiode (APD), or even a superconducting nanowire single photon detector (SNSPD). In particular, the single photon detector 4222 (or 4422) may include an internal amplification mechanism configured to deliver a voltage when a photon is detected.

[0166] Each photon detector 4222 and 4422 can be configured to detect the transcoded interferometric quantum signal |Ψ B > perform what is known as “interferometry”. In other words, each photon detector 4222 and 4422 is able to detect the quantum state of the photon based on the quantum state of the photon. A >The associated quantum state is projected onto the transcoded interferometric quantum signal |Ψ B >The interference results between photons are measured on the associated quantum states.

[0167] Those skilled in the art will appreciate that the processing module 400 is configured to select the transcoded interferometric quantum signal |Ψ in the time domain. B >, then the encoded interference quantum signal |Ψ A > Cast as follows:

[0168] - In the receiving module 4222, it is projected on the B+ > in the quantum state, so that:

[0169]

[0170] - In the receiving module 4422, it is projected on the B- > in the quantum state, so that:

[0171]

[0172] Quantum state |Ψ B+ > and |Ψ B- > is also called a "transcoded quantum state". As shown in equations (13) and (14), the quantum state |Ψ B+ > and |Ψ B- > can be represented by the state |Ψ A-1 > and |Ψ A-2 > is represented by the superposition of A-1> corresponds to a photon of the coded interference quantum signal that has adopted the first interferometer arm 340-1, and the state |Ψ A-2 >A photon corresponding to the coded interference quantum signal that has adopted the second interferometer arm 340-2.

[0173] The “detection probability” (denoted as P) of the single photon detector 4222 (or 4422) + (or P - )) can be defined by the following equation (Equation (15)):

[0174] P ± =|<Ψ B± |Ψ A >| 2 (15)

[0175] According to equation (15), the transcoded interferometric quantum signal generated by the transcoded interferometric module 300 can be decomposed into four defined temporal pattern bases such that:

[0176] - The first time mode (corresponding to the state |Ψ A-1 Ψ 1-1 >) corresponds to a photon that has taken the first interferometer arm 240-1 of the transmitter Alice and then taken the first interferometer arm 340-1 of the receiver Bob. In this case, the interference delay of the photon is zero (t=0), and by convention the detection probability of a photon with zero phase does not depend on the phase modulations α and β eff ;

[0177] - The second time mode (corresponding to the state |Ψ A-2 Ψ 1-2 >) corresponds to having employed the second interferometer arm 240-2 of the transmitter Alice (including phase modulation α) and then the second interferometer arm 340-2 of the receiver Bob (including phase modulation β) eff ) photon. In this case, the interference delay is 2×Δt. Although the phase of the photon is equal to α+β eff , but the probability of detecting a photon is proportional to the square modulus of the electric field and is therefore not affected by the two phase modulation parameters α and β eff The impact of

[0178] - The third time mode (corresponding to the state |Ψ A-1 Ψ 1-2 >) corresponds to a photon that has taken the second interferometer arm 240-2 of the transmitter Alice (including the phase modulation α) and then taken the first interferometer arm 340-1 of the receiver Bob. In this case, the interference delay of the photon is Δt, and the photon acquires the phase α and experiences the induced phase shift associated specifically with the interferometer arm 240-2 of the coded interferometer module 200 Phase fluctuations of

[0179] - The fourth time mode (corresponding to the state |Ψ A-2 Ψ 1-1 >) corresponds to having employed the first interferometer arm 240-1 of the transmitter Alice and then the second interferometer arm 340-2 of the receiver Bob (including the phase modulation β eff ) photons. In this case, the interference delay of the photons is Δt, and the photons acquire a phase β eff and undergoes an induced phase shift associated with the interferometer arm 340-2 of the transcoding interferometer module 300 phase fluctuation.

[0180] In particular, only photons with the same interference delay (i.e. t=Δt) can interfere with each other. Therefore, the result of this interference corresponds to the detection probability of unit 4222 (or 4422) for photons received with delay Δt (i.e., the third and fourth time modes). The "detection probability" P + and P - It can be defined by the following equation (Equation (16)):

[0181]

[0182] It should be noted that the + or - sign is randomly (via configuration) assigned to the detection probability P in beam splitter 340 + or P - , is then set according to the path of the photon on one of the two optical paths 340-3 or 340-4 (and then 422-1 or 442-1).

[0183] The processing module 400 may also include a module for storing the detection probability P ± The value of unit 460.

[0184] The system 1 for establishing a quantum encryption key may further include a display device (not shown in the figure) configured to generate a display of the stored probabilities on a human-machine interface.

[0185] Each processing section 420 (or 440) may further include an auxiliary detector 4226 (or 4426) located on another measurement arm, namely, the reference measurement arm 422-2 (or 442-2). The auxiliary detector 4226 (or 4426) may be configured to detect a conventional (i.e., non-quantum) optical pulse signal. For example and without limitation, the auxiliary detector 4226 (or 4426) may be a detector configured to detect a phase modulated interference reference signal S cB A photodiode that delivers photocurrent for measurement.

[0186] Each auxiliary detector 4226 and 4426 can be configured to detect the phase modulated interferometric reference signal S cB A measurement known as “interferometry” is performed. In other words, each auxiliary detector 4226 and 4426 is able to measure the interference results between various possible interferometer arms of the reference signal passing through the interferometer modules 200 and 300 .

[0187] Each measuring arm 422-1 and 422-2, 442-1 and 442-2 of the processing module 400 may also include an electronic filtering unit (represented as 4224, 4228, 4424 and 4428, respectively) and is located after the single photon detector or after the auxiliary detector. These filtering units can be configured to generate a rectangular signal and are configured to extract a central time component equivalent to the interference delay Δt from the detection probability (i.e., the measured voltage) or from the photocurrent measured by the detectors 4222, 4226, 4422 and 4426.

[0188] In some embodiments, the auxiliary detector 4226 (or 4426) and the single photon detector 4222 (or 4422) of the processing portion 420 (or 440) may each include a single photon detector 4222 (or 4422) having a frequency ω c and ω q Associated frequency filters (not shown in the figure) are provided to optimize the detection of identical signals (ie, reference signal and quantum signal).

[0189] As defined by the detection probability P ± As indicated by equation (18), the contrast quality of the interference of a single photon is affected by the phase shift associated with the interference modules 200 and 300. and Specifically, these parameters lead to a significant reduction in the detection probability between the two single-photon detectors 4222 and 4422, which can be achieved via phase modulation β eff to correct for the probability of detection according to equation (17):

[0190]

[0191] Therefore, the system 1 for establishing a quantum encryption key according to an embodiment of the present invention can be configured to modulate the phase determined by feedback based on the measurement of the reference signal. eff Applied to quantum signals to correct for phase shifts and In one embodiment, the processing module 400 may further include a correction unit 480, which is configured to determine the phase shift of the interference modules 200 and 300. and The differential correction of phase fluctuations between the two signals can be achieved without compromising the security of the established secret key.

[0192] The auxiliary detectors of the processing parts 420 and 440 are advantageously configured to measure the interference results between various possible interferometer arms of the reference signal passing through the interferometer modules 200 and 300, including the phase-modulated interference reference signal S of three light pulses of various intensities separated in time by said amount Δt. cB Only the central time component Δt is filtered by the filtering units 4228 and 4428, which filter the two photocurrents (e.g., denoted as I ) measured during each of the interferometric measurements of the reference signals tracking the two optical paths 340-3 and 340-4 at the output of the interferometric module 300 of the receiver Bob. 420 and I 440 ) is passed to the correction unit 480.

[0193] Figure 6 The correction unit 480 of the processing module 400 according to an embodiment of the present invention is schematically shown.

[0194] The correction unit 480 may be an electronic module.

[0195] In an embodiment, the correction unit 480 may include a subtraction unit 482 configured to convert the two photocurrents I 420 and I 440 Subtract δ = |I 420 -I 440 |, its delivery and item Proportional electronic signal δ.

[0196] The correction unit 480 may further include a multiplication unit 484 configured to multiply the electronic signal δ and the sinusoidal modulation signal (which depends on the control unit 3448 of the interference module 300 of the receiver Bob) multiplied The correction unit 480 may include a low pass filter 486 having a ratio ω m The lower cutoff frequency is configured according to the product δ m An electronic signal ε, called a discrete error signal, is determined. The discrete error signal ε is thus independent of the phase modulation And can be defined by the following equation (Equation (18)):

[0197]

[0198] In equation (20) of the discrete error signal ε, component J1 corresponds to a Bessel function of order 1. The discrete error signal ε can therefore correspond to “in phase shift and The differential correction ε of the phase fluctuation between .

[0199] The correction unit 480 may finally comprise an adder 488 configured to add the discrete error signal ε and the phase modulation β defined by the control unit 4882, which delivers the modulation β eff , which allows the phase shift and be cancelled out (either analogically or digitally).

[0200] Advantageously, the parameter β may be obtained in a similar manner as the parameter α defined by the control unit 2444. For example, the parameter β may be selected, for example, from a set of two orthogonal bases such as [0,π] and [π / 2,3π / 2], according to a quantum random number generator.

[0201] Therefore, the coincidence measured on the single-photon detector and the probability P defined by equation (17) ± is uniquely given by α and β. Advantageously, according to an embodiment of the present invention, unlike the prior art, the correction unit 480 uses electronic components to feedback modulate β eff , which enables faster and continuous electronic compensation (or feedback).

[0202] Figure 7 is a flow chart illustrating a method for establishing a quantum encryption key (ie, a method for sending a coded interference signal S2) implemented by a transmitter Alice according to an embodiment of the present invention.

[0203] In step 700, an intermediate signal S1 is generated by the module 100 of the transmitter Alice. The intermediate signal S1 comprises an intermediate quantum signal S q (denoted as |Ψ1>) and a frequency-modulated intermediate reference signal S with a frequency shift Δω relative to the quantum signal |Ψ1> c .

[0204] In step 702, the intermediate quantum signal |Ψ1> and the intermediate reference signal S c The signal |Ψ1> and the signal S are thus c Each is split into two interference components, which then pass through one of the two interferometer arms of the coded interferometer module 200 .

[0205] In step 704, the coded interferometry module 200 of the transmitter Alice applies a phase modulation α to one of the two interference components of the quantum signal |Ψ1> in the interferometer.

[0206] In step 706, a coded interference signal S2 is generated at the output of the interferometer by the coded interference module 200 of the transmitter Alice. The coded interference signal S2 includes a phase-modulated coded interference quantum signal |Ψ A > and the (frequency modulated) interferometric reference signal S cA .

[0207] In step 708, the coded interference signal S2 is then transmitted by transmitter Alice to receiver Bob.

[0208] Figure 8 is a flow chart showing a method for establishing a quantum encryption key (ie, a method for receiving a coded interference signal S2) implemented by a receiver Bob according to an embodiment of the present invention.

[0209] In step 800, the coded interferometric quantum signal |Ψ A > and the interferometric reference signal S cA The signal |Ψ passes through the interferometer (having two interferometer arms) of the transcoding interferometer module 300 of the receiver Bob. A > and signal S cA Each is split into two interference components, which then pass through one of the two interferometer arms of the transcoding interferometer module 300 .

[0210] In step 802, the transcoding interference module 300 of the receiver Bob modulates the phase Applied to the interferometric reference signal S cA This delivers a phase-modulated interferometric reference signal S at the output of the interferometric module 300. cB .

[0211] In step 804, the processing module 400 of the receiver Bob processes the phase modulated interference reference signal S cB at least one interference measurement of.

[0212] In step 806, based on phase modulation and the phase modulated interferometric reference signal S cB The interferometric measurement is used to perform the operation of determining the discrete error signal ε, which allows the phase shift between the interferometric modules 200 and 300 to be determined. and The phase fluctuation between them is differentially corrected.

[0213] In step 808, determining the phase modulation β is performed based on the discrete error signal ε and the phase modulation β. eff (Consider the phase shift and The operation of differential correction of phase fluctuation between.

[0214] In step 810, the transcoding interference module 300 of the receiver Bob modulates the phase eff Application to coded interferometric quantum signals |Ψ A > one of the two components of this, which delivers the transcoded interferometric quantum signal |Ψ at the output of the interferometric module 300 B >(i.e., transmit signal |Ψ A > to the transcoded quantum state |Ψ B+ > and |Ψ B- > projection on top).

[0215] In step 812, the processing module 400 of the receiver Bob performs a processing on the transcoded interference quantum signal |Ψ at the output of the interference module 300. B >at least one interferometric measurement (i.e., of the coded interferometric quantum signal |Ψ A >To state|Ψ B+ > and |Ψ B- > to derive the quantum key from it.

[0216] In step 700, the method for establishing a quantum encryption key may further include sub-step 701, in which the intermediate quantum signal S q Apply a time delay relative to the intermediate reference signal S c It is delayed to generate an intermediate signal S1.

[0217] Therefore, embodiments of the present invention enable real-time measurement and compensation of phase shifts caused by interferometric modules 200 and 300. and And thus enables the signal-to-noise ratio of the generated and encoded quantum components to be increased. This results in maximum capabilities in terms of modulation speed. Furthermore, the system 1 for establishing a quantum cryptographic key has the advantage of not being affected by intensity fluctuations of the emitted laser pulses, in particular with respect to the reference signal.

[0218] Those skilled in the art will appreciate that the system 1 for establishing a quantum encryption key or a subsystem of the system 1 for establishing a quantum encryption key according to an embodiment of the present invention can be implemented in various ways by hardware, software, or a combination of hardware and software, and in particular in the form of various forms of program codes that can be distributed as program products. The program code can be distributed using a computer-readable medium, which may include a computer-readable storage medium and a communication medium. The methods described in this specification can be implemented in particular in the form of computer program instructions that can be executed by one or more processors in a computer-based computing device. These computer program instructions can also be stored in a computer-readable medium.

[0219] As a non-limiting example, the present invention is not limited to the above-described embodiments. It encompasses any variant of the embodiments that a person skilled in the art may conceive. In particular, a person skilled in the art will readily appreciate that the present invention is not limited to the various modules of the transmitter Alice and the receiver Bob given by way of non-limiting examples.

Claims

1. A transmitter, configured to transmit a coded interference signal S2 via a transmission channel (50), characterized in that The transmitter comprises: - a signal generation module (100) configured to generate an intermediate quantum signal |Ψ1> and an intermediate reference signal S c The signal generation module (100) is further configured to generate a signal between the intermediate quantum signal |Ψ1> and the intermediate reference signal S c The frequency shift between Δω; - a coded interferometer module (200), which is configured to: receive the intermediate quantum signal |Ψ1> and the intermediate reference signal S c , and the intermediate quantum signal |Ψ1> and the intermediate reference signal S based on tracking substantially similar optical paths c To determine the coded interferometric quantum signal |Ψ A > and the interferometric reference signal S cA The coded interference module (200) comprises a coding device (240D) configured to apply a phase modulation α to only one component of the intermediate quantum signal |Ψ1>.

2. A transmitter according to the preceding claim, wherein: The coding interference module (200) is in the form of an asymmetric optical interferometer, the asymmetric optical interferometer comprising a first interferometer arm (240-1) and a second interferometer arm (240-2), the first interferometer arm (240-1) being shorter than the second interferometer arm (240-2), and wherein the coding device (240D) comprises at least one frequency splitting unit (244) located on the second interferometer arm (240-2), the frequency splitting unit (244) being configured to split the intermediate quantum signal |Ψ1> and the intermediate reference signal S in the frequency domain. c Decoupling.

3. The transmitter according to any one of claims 1 and 2, wherein: The signal generation module (100) further comprises a time shift unit, wherein the time shift unit is configured to shift the intermediate reference signal S in the time domain. c Shift the intermediate quantum signal |Ψ1>.

4. A receiver configured to receive a coded interference quantum signal |Ψ via a transmission channel (50) A > and the interferometric reference signal S cA , characterized in that, The receiver comprises: - a transcoded interferometer module (300) configured to generate a transcoded interferometer quantum signal based on tracking substantially similar optical paths |Ψ A > and the interferometric reference signal S cA To generate the transcoded interferometric quantum signal |Ψ B > and the phase modulated interferometric reference signal S cB The transcoding interference module (300) includes a transcoding and phase modulation device (340D), wherein the transcoding and phase modulation device (340D) is configured to phase modulate β eff Applied to the coded interferometric quantum signal |Ψ A > only one component, and the phase modulation Applied to the interferometric reference signal S cA Only one component of - a processing module (400) configured to perform processing on the transcoded interferometric quantum signal |Ψ B >at least one measurement of the phase modulated interferometric reference signal S cB The processing module (400) is further configured to measure at least one of the phase modulation and the phase modulated interference reference signal S cB The at least one measurement of the phase modulation β eff .

5. The receiver according to claim 4, wherein: The transcoding interference module (300) is in the form of an asymmetric optical interferometer, the asymmetric optical interferometer comprising a first interferometer arm (340-1) and a second interferometer arm (340-2), the first interferometer arm (340-1) being shorter than the second interferometer arm (340-2), and wherein the transcoding and phase modulation device (340D) comprises at least one frequency splitting unit (344) located on the second interferometer arm (340-2), the frequency splitting unit (244) being configured to split the coded interference quantum signal |Ψ in the frequency domain A > and the interferometric reference signal S cA Decoupling.

6. A receiver according to any one of claims 4 and 5, wherein: The transcoded interferometer module (300) comprises two optical paths (340-3, 340-4) at the output of the interferometer, and wherein the processing module (400) comprises two processing sections (420, 440), each of which comprises a frequency splitting unit (422, 442) and is configured to perform a processing on the transcoded interferometric quantum signal |Ψ originating from the two optical paths (340-3 and 340-4). B > one of the at least one measurements and performing a phase modulated interferometric reference signal S originating from the two optical paths (340-3 and 340-4) cB A measurement among the at least one measurement.

7. A system (1) for establishing a quantum cryptographic key, the system comprising a transmitter according to one of claims 1 to 3 and a receiver according to one of claims 4 to 6.

8. A method for transmitting a coded interference signal S2, the method being implemented by a transmitter according to claims 1-3, the method comprising the following steps: -Generate (700) frequency ω q The intermediate quantum signal |Ψ1> and the frequency ω c and has a value equal to ω q -ω c The frequency shifted intermediate reference signal S c ; - applying (704) a phase modulation α to only one component of the intermediate quantum signal |Ψ1> passing through the interferometer, the intermediate quantum signal |Ψ1> and the intermediate reference signal S c Tracking substantially similar optical paths to generate a quantum signal including a coded interference |Ψ A > and the interferometric reference signal S cA The coded interference signal S2; - generating (706) a coded interference quantum signal including said A > and the interferometric reference signal S cA The coded interference signal S2.

9. The method for transmitting a coded interference signal S2 according to claim 8, wherein: The method further comprises: comparing the intermediate quantum signal |Ψ1> with the intermediate reference signal S c A step of applying (701) a time delay between.

10. A method for receiving a coded interferometric quantum signal |Ψ A > and the interferometric reference signal S cA A method, which is implemented by a receiver according to claims 4-6, comprising the following steps: - Phase modulation Apply (802) to the interference reference signal S cA component and generates a phase-modulated interference reference signal S cB ; - performing (804) a phase modulated interference reference signal S cB at least one measurement of -Based on the phase modulation and the phase modulated interference reference signal S cB to determine (808) a phase modulation βeff; - The phase modulation β eff Apply (810) to the coded interferometric quantum signal |Ψ A > and generate a transcoded interferometric quantum signal |Ψ B >, the coded interference quantum signal |Ψ A > and the interferometric reference signal S cA Tracing substantially similar optical paths; - performing (812) processing of the transcoded interferometric quantum signal |Ψ B >at least one measurement.

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