Device for generating an optical signal for quantum key distribution

Abstract

The invention relates to a device for generating an optical signal for quantum key exchange, comprising: a photonic source (501) generating at least one pulse stream, a CAN transducer (503) converting the photons of a pulse stream into a random binary sequence, a quantum state modulator (502) putting the photons of the pulses into a number and a quantum state defined by a control word to generate the optical signal from one of the pulse streams (501), digital calculation means (504): generating at least one random sequence (505) having a given probability distribution and rhythm from the random binary sequence, generating the control word (506) in accordance with the key exchange protocol from said at least one random sequence. The invention also relates to a satellite payload comprising such a device for generating an optical signal. Figure for abstract: Fig. 5

Description

Title of the invention: Device for generating an optical signal for the distribution of quantum keys Technical field

[0001] The invention lies in the technical field of devices enabling the exchange or quantum distribution of keys (QKD for Quantum Key Distribution), and more particularly when the exchange of keys is implemented by a protocol of the Prepare & Measure type (DV for Discrete Variable).

[0002] The invention relates to a device for transmitting an optical signal which is efficient and compact, exhibiting high resistance to the risks of attacks by measuring electromagnetic radiation. Prior art

[0003] [Fig.l] schematically represents the operation of an exchange / distribution of secret keys by quantum optical link (QKD).

[0004] This exchange / distribution involves: - a transmitter 101 (generally called Alice) configured to emit a sequence of qubits on an optical channel 102. In the protocols of the Prepare and Measure type with discrete variable, a qubit corresponds to a photon whose quantum state codes the information 0 or 1 on several possible bases chosen randomly (which corresponds to the Prepare step). The state of the photon can for example be its polarization, its phase, its arrival time, etc.; - a 103 qubit receiver / detector (usually called Bob), which detects photons in a randomly chosen base, and measures their state, therefore their value (0 or 1) (which corresponds to the Measurement step).

[0005] To establish a key, the transmitter 101 transmits a random sequence of qubits (sequence of 0s and 1s) in randomly chosen bases. The receiver 103 measures the qubits of this sequence on its own bases and reconstructs an incomplete and fragmentary sequence of bits, following the degradations of the signal linked to the transmission on the optical link 102 (photons lost on the path, parasitic photons, measurement errors, etc.). A process called reconciliation, carried out on a reconciliation channel 104, makes it possible to generate a secret key known only by the transmitter 101 (Alice) and the receiver 103 (Bob) from the two random sequences: the random sequence transmitted by Alice and the sequence received and reconstructed by Bob.

[0006] For this purpose, we know for example the patent applications EP 3,993,312 A1, WO 2019 / 115984 A1, the article by Huang Donghai et al. “Quantum key distribution over double-layer quantum satellite networks », IEEE Vol.8 2020, ou l’article de Sheng-Kai Liao et al. « Satellite-to-ground quantum key distribution », Cornell university library, 201 OLIN LIBRARY CONRELL UNIVERSITY ITHACA, ny 14853? 3 JUILLET 2017.

[0007] In discrete variable Prepare and Measure protocols, such as the BB84 protocol, the transmitter 101 (Alice) emits a random sequence of qubits encoded on several possible bases and two possible values ​​(0 and 1). Ideally, the qubits are made up of single photons. In embodiments where the optical source is an attenuated laser, and not a single photon source, it generates coherent states that can contain a finite number of photons per pulse with a Poisson probability. Decoys can be introduced to prevent attacks by a hostile third party (called Eve) based on the measurement of additional photons in the case of multi-photon pulses. This is called the BB84 decoy protocol.

[0008] [Fig.2] schematically represents the architecture of a qubit transmitter 200 for a Prepare and Measure protocol with discrete variable, such as for example the BB84 protocol. It comprises: - a clock 201, - equipment configured to generate a random value 202, - equipment configured to perform a protocol controller function 203, and - a source device for “modulated” photons, or photon emitter 204.

[0009] The protocol controller 203 of the qubit emitter 200 is functionally connected to the reconciliation channel by a device denoted Reconciliation Interface (I / F) 205 for the example. The functions of controlling and managing the reconciliation channel, controlling and running the reconciliation process are distributed between the two devices of the example: the protocol controller 203 of the qubit emitter and the reconciliation interface 205.

[0010] Clock 201 clocks the device and defines the transmission rate of the random sequence of qubits per pulse train.

[0011] This random sequence is provided by a random number generator 202, and more advantageously by a quantum random number generator (QRNG). A QRNG is a generator using a quantum phenomenon to generate randomness. It guarantees excellent entropy to the generated randomness sequence. Such a generator is based on a microscopic physical phenomenon that generates a statistically random noise signal (for example, the shot noise of a photodiode). A random number generator typically consists of: - a transducer to convert the physical phenomenon into an electrical signal random, - possibly an amplifier to amplify random fluctuations to a measurable level, and - an analog-to-digital converter (ADC) to convert the signal into a number (binary sequence of 0s and 1s).

[0012] The protocol controller 203 is a digital device (for example a computer, an FPGA (Field Programmable Gate Array), an ASIC (Application-Specific In-tegrated Circuit) or a DSP (Digital Signal Processor) which controls the photonic source in accordance with the chosen protocol. From the sequence of random numbers provided by the QRNG 202, the protocol controller 203 creates a sequence of instructions or control words intended for the photon emitter 204.

[0013] [Fig. 3] illustrates the constitution of an optical signal used for a quantum key exchange. For example, in the case of a polarization-encoded BB84 decoy protocol, the optical signal consists of a succession of time pulses 304, these pulses are either qubit pulses or decoy pulses.

[0014] With reference to [Fig. 3], at each time pulse 304 of the pulse train generated by the clock, the photon emitter or source 204 emits, according to the instructions or control words provided by the protocol controller: - either a time pulse containing an average number of photons equal to a predefined value of the protocol and whose polarization state defines the binary value. This pulse is defined as a qubit. For the BB84 or BB84 decoy protocol, the photon of a qubit has four possible polarization states 301: vertical (I), horizontal (-), diagonal ( / ) and anti-diagonal (\). The value of the polarization state is defined by the control word corresponding to the pulse; - either a pulse containing an average number of photons (or intensity) defined by the corresponding control word. This pulse is defined as a decoy 302. The polarization state of the photons of the decoy pulse takes one of the four possible polarization states of the photons of the qubit pulses according to the control word.

[0015] The number of bits required to encode the instruction or command word of the protocol controller 203 for the photon source 204 is at least 4 bits per pulse. For example, 1 bit or more to define the type of the pulse (qubit or decoy), 1 bit to define the value of the qubit (0 or 1), 1 bit or more to define the encoding basis (rectilinear or diagonal polarization, phase, ...), 1 bit or more to define the amplitude (or intensity) of the decoy pulse (number of photons per pulse), etc. These ins- Instructions for the photon emitter 204 are developed and encoded by the protocol controller 203 from the random sequence from the random number generator 202 and in accordance with the chosen prepare and measure protocol.

[0016] Other quantum key exchange protocols are known, such as for example the B92 protocol, which is a simplified version of the BB84 protocol using two polarization states (the horizontal polarization state (-) of the rectilinear base and +45° ( / ) of the diagonal base), or the SSP protocol (Six-State Protocol), which is also a modified version of the BB84 using six polarization states.

[0017] To ensure the security of the generated qubit sequence 303 against an attack, the content of the time pulses 304 corresponding to the qubits and the decoys, their polarization states and the amplitude (or intensity) of the decoys must be completely random, with probability densities defined according to the implemented quantum key exchange protocol. The quality of the exchanged encryption key depends on the entropy of the random sequence.

[0018] In the photon emitter 204, the encoding of the qubits and the decoys (typically their polarization and their amplitude) is carried out by various techniques involving for the most part attenuated lasers. The control of this photon “modulator” is done by the protocol controller 203.

[0019] Existing solutions based on the architecture shown in [Fig.2] however have drawbacks: - the throughput (i.e. the operating frequency) of the random number generator 202 must be at least four times higher than the transmission throughput of the sequence of qubits (corresponding to the frequency of the pulse train). Indeed, the protocol controller 203 must provide a control word to the photon transmitter 204 with a length of at least 4 bits. The constitution of the sequence of time pulses (qubits, decoys) must be random and result from the sequence of random numbers provided by the generator 202. The current state of the art of quantum random number generators (QNRGs) reports operating frequencies of a few tens to a few hundreds of Megahertz, which constitutes a limit to the future increase in the throughput of qubit sequence transmitters; - the architecture of [Fig.2] designates several distinct devices: the QNRG 202, the protocol controller 203, the photon source 204. This architecture has a negative impact on the acquisition cost, the duration of development, supply, integration and testing of the qubit transmitter; - an electrical connection is required between the QRNG 202 and the controller protocol 203. This connection can be easily spied on by remotely measuring the electromagnetic radiation it generates, which constitutes a security breach.

[0020] An object of the invention is therefore to define a device for generating a compact and competitive optical signal whose architecture makes it possible to overcome the defects of the prior art stated above. The device according to the invention achieves this by integrating, where appropriate in a single piece of equipment, the functions of generator and modulator of quantum state of photons, protocol controller, generation and extraction of randomness, and adaptation to the transmission medium (optical fiber or free space). Summary of the invention

[0021] To this end, the present invention describes a device for generating an optical signal in the form of a succession of optical pulses, a quantum state of which codes binary information for the quantum exchange of keys between a transmitter and a receiver. The device according to the invention comprises: a photonic source configured to generate at least one pulse stream comprising one or more photons, a device called a CAN transducer configured to convert the photons of one of the pulse streams generated by the photonic source into a random binary sequence, a quantum state modulator configured to generate the optical signal from one of the pulse streams generated by the photonic source by adjusting the number and quantum state of the photon(s) of the pulses, the number and quantum state of the photon(s) of the pulses being defined by a control word, digital computing means configured for: generating at least one random sequence having a given probability distribution and rate, from the random binary sequence produced by the CAN transducer, generating the control word transmitted to the quantum state modulator in accordance with a given key exchange protocol, from the random sequence(s).

[0022] According to one embodiment of the invention, the device for generating an optical signal further comprises a device for adapting the optical signal delivered by the quantum state modulator to the optical signal transmission medium.

[0023] Advantageously, the optical signal transmission medium is chosen from an optical fiber and free space.

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[0029] According to the embodiment of the device for generating an optical signal according to the invention, the photonic source may be: - a pulsed laser configured to generate a stream of optical pulses, associated with a power divider configured to generate at least two streams of pulses from the stream of pulses of the pulsed laser; - a pulsed laser configured to generate two pulse streams, - a single photon source configured to generate a pulse stream each comprising a photon, associated with a splitter plate configured to divide the pulse stream into at least two distinct pulse streams, or - a single photon source configured to generate two single photon streams. The CAN transducer is configured to exploit a random quantum phenomenon of the photons of one of the pulse streams generated by the photonic source to determine the random binary sequence. According to the embodiment of the device for generating an optical signal according to the invention, the random quantum phenomenon exploited by the CAN transducer is chosen from: - the initial phase of the impulses, - the amplitude of the pulses, - the time of appearance of the impulses, - the speckle patterns of the pulses. Advantageously, the random sequence(s) generated by the calculation means are obtained by applying mathematical functions to the random binary sequence produced by the CAN transducer. According to one embodiment of the device for generating an optical signal according to the invention, the digital calculation means are further configured to implement a process of reconciliation of the data transmitted to the receiver. The invention also relates to a payload for a satellite configured to perform a quantum key exchange mission with a ground station, the payload comprising: - a device for generating an optical signal according to one embodiment of the invention, - an optical terminal connected to an output of the device for generating an optical signal, - an optical modem connected to said optical terminal or a radio frequency modem connected to a radio antenna, - a mission controller configured to supervise the sequencing of quantum key exchange operations, and - a polarization synchronization and measurement device configured to ensure synchronization between the optical signal generation device and the ground station, and to measure distortions suffered by the optical signal between the optical signal generation device and the ground station. Brief description of the drawings

[0030] The invention will be better understood and other characteristics, details and advantages will appear more clearly on reading the following description, given without limitation, and thanks to the appended figures, given by way of example.

[0031] [Fig-1] [Fig.l] schematically represents the operation of an exchange / of a distribution of secret keys by quantum optical link (QKD).

[0032] [Fig.2] [Fig.2] schematically represents the architecture of a qubit transmitter for a state-of-the-art discrete variable Prepare and Measure protocol.

[0033] [Fig.3] [Fig.3] illustrates the constitution of an optical signal according to the state of the art, used for quantum key exchange.

[0034] [Fig.4] [Fig.4] schematically represents the architecture of a type of generator of quantum random numbers, as known from the state of the art.

[0035] [Fig.5] [Fig.5] represents a device for generating an optical signal according to a embodiment of the invention.

[0036] [Fig.6] [Fig.6] represents a device for generating an optical signal according to a another embodiment of the invention.

[0037] [Fig.7] [Fig.7] represents a device for generating an optical signal according to a another embodiment of the invention.

[0038] [Fig.8] [Fig.8] illustrates the components of a quantum exchange mission of satellite keys.

[0039] [Fig.9] [Fig.9] illustrates an example of a payload carried on board a satellite

[0040] Identical references may be used in different figures. when they designate identical or comparable elements. Description of the embodiments

[0041] Those skilled in the art are aware of quantum random number generators (QRNGs) using the physical properties of quantum random phenomena of pulsed lasers. [Fig. 4] shows such a quantum random number generator. It comprises a pulsed laser 402 configured to generate a pulse stream clocked by a clock 401. The pulse stream is converted into a random binary sequence by a transducer 403 and an analog-to-digital converter (ADC) 404 using the quantum random properties of the pulses, e.g. for example the initial phase of the pulses, their amplitude or intensity, their time of appearance (position in the pulse), the speckle patterns, etc. The transducer 403 converts the chosen random quantum physical property into an electrical signal, amplifies it and formats it. The analog to digital converter (ADC) 404 converts the electrical signal generated and amplified by the transducer into a digital value (an 8-bit binary word for example). Due to the eminently random nature of the physical properties of quantum phenomena, the sequence of digital values ​​generated is random with very good entropy.

[0042] The idea of ​​the invention consists in combining the functions of quantum random number generator and photon emitter, and advantageously in grouping these functions and the functionality of protocol controller in a single autonomous equipment. In addition, a random extraction function is added thereto, allowing: - to increase the throughput of the qubits, - to increase the entropy of the random sequence of qubits, and - to provide the protocol controller with random sequences of bits with probability densities adapted to the protocol.

[0043] [Fig. 5] represents a device for generating an optical signal according to an embodiment of the invention. It comprises a photonic source 501, here a pulsed laser emitting a series of pulses. This series of pulses is transmitted on the one hand to a quantum state modulator 502, and on the other hand to a CAN transducer 503. The division of the pulse flow can be done for example using an optical divider, such as a semi-reflecting mirror, which will sample and redirect a part of the main optical flow.

[0044] The CAN transducer 503 consists of a transducer that converts a quantum property of the optical pulse into an electrical signal, and an analog / digital converter CAN that converts the electrical signal into a digital value (binary word). The CAN transducer 503 is configured to convert each optical pulse of one of the pulse streams generated by said photonic source into a random binary word, by exploiting for this the quantum properties of the pulses, in a manner comparable to what is done in quantum random number generators according to the state of the art such as that shown in [Fig. 4]. The succession of random binary words constitutes a random binary sequence that feeds a random sequence generation function 505.

[0045] The device for generating an optical signal according to the invention also comprises digital calculation means 504, such as for example an ASIC, an FPGA, a DSP, or a microprocessor, configured to carry out the function 505 of generating random sequences, and a protocol controller function 506.

[0046] The random sequence generation function 505 takes as input the random binary sequence delivered by the CAN transducer 503, and uses it to generate one or more random sequences having a probability distribution and a rate adapted to the implemented key exchange protocol. Indeed, the random binary sequence delivered by the CAN transducer 503 has a rate proportional to the rate of the pulses of the laser 501, potentially insufficient to generate the control words used by the quantum state modulator 502 to define the content and characteristics of the optical pulses (qubit, decoy). This rate must be increased and the probability distribution adapted to the chosen transmission protocol. For example, in the case of a BB84 decoy protocol implemented by 4-bit control words as described previously, a possible implementation consists of generating: a pulse-rate sequence, having a first probability density to define whether the pulse is a signal pulse or a decoy pulse, a pulse-rate sequence, having a second probability density to define the amplitude of the decoy pulses, a sequence, at the rate of the pulses, having a third probability density for the encoding basis of the qubits, a sequence, at the rate of the pulses, having a probability density equal to the third probability density for the value of the qubits, etc.

[0047] When random sequences have equal probability densities, such as those for the encoding base and the value of the qubits, they can be grouped within the same random sequence whose rate is increased compared to the binary rate of the random binary sequence used as input.

[0048] The generation, from a first random sequence (mother sequence), of a new random sequence (daughter sequence) whose properties (rhythm and probability density) are adjusted with respect to a given need is an operation known to those skilled in the art, and can be done by applying mathematical functions, such as for example a one-way function such as hashing, on the daughter random sequences. These mathematical functions ensure that it is impossible for an observer (Eve) to find the other random sequences from one of the random sequences (daughter) that would have been reconstituted. Other mathematical functions, such as mixing (random permutation) of the sequences are also possible.

[0049] The choice of the number of sequences, their rhythms and the respective probability densities depends on the implemented quantum key exchange protocol.

[0050] The digital calculation means 504 are configured to perform a function of network controller 506, which consists of: - use the random sequence(s) generated by the generator 505 to form, for each pulse, control instructions for the quantum state modulator 502. These instructions therefore define the qubits and decoys which will be transmitted over the optical link in accordance with the implemented QKD protocol, - possibly, exchange with the receiver through a radiofrequency (RF) or optical reconciliation channel, in order to determine a secret key, - when the device for generating an optical signal is compatible with several QKD protocols, adjust the properties of the random sequence generation function 505 according to the chosen protocol.

[0051] The quantum state modulator 502 has two actions on the optical pulses delivered by the pulsed laser 501: it attenuates the intensity of the optical pulse to fix the average number of photons per pulse and it fixes the quantum state of the photons of the pulse. The intensity of the pulse (average number of photons) and the quantum state of the photons are defined for each pulse by a control word provided by the protocol controller 506. The quantum state modulator 502 modifies the flow of optical pulses generated by the pulsed laser 501 into a succession of qubits and decoys, by fixing the average number and the quantum state of the photons of each optical pulse in accordance with the control word given by digital calculation means 504.To do this, it encodes the qubits and decoys by adjusting the pulse amplitude (average number of photons per pulse) and the state (e.g. polarization) of the photons according to the command word transmitted by the protocol controller 506.

[0052] Advantageously, the device for generating an optical signal according to the invention comprises a clock 507 used to clock the emissions from the photonic source 501, and to synchronize the protocol controller function 506 with the flow of pulses.

[0053] Advantageously still, the device for generating an optical signal according to the invention comprises a device for adaptation to the transmission medium 508. The transmission medium (optical channel) can be free space, or an optical fiber. The device for adaptation to the medium 508 controls for example the polarization axes of the optical signal emitted according to the medium, by defining in an absolute manner the polarization axis, which can differ according to the medium. This adaptation is done in collaboration with the receiver to measure the distortions of the quantum states (for example the polarization) caused by the propagation on the transmission medium. This measurement of the distortions can be done for example via the optical channel before transmission of the qubits in the case of a transmission on fiber or via a reference optical channel. dedicated to this function in the case of free space transmission.

[0054] [Fig. 6] represents a device for generating an optical signal according to another embodiment of the invention. In this embodiment, the photonic source 601 is a source of single photons known to those skilled in the art. The device then comprises a photon separation device 601, such as for example a splitter plate. A portion of the photons is taken to power the quantum state modulator 502, and the other portion is transmitted to the CAN transducer 503. With the exception of the photon source, the operation of the device is identical to that described in [Fig. 5].

[0055] [Fig.7] represents a device for generating an optical signal according to another embodiment of the invention, in which the photon source 701 is configured to generate two photon streams, a first stream towards the quantum state modulator 502 and a second stream towards the CAN transducer 503. Two pulsed lasers connected in parallel or two single photon sources constitute examples of a photon source with two outputs.

[0056] The device for generating an optical signal according to the invention limits the number of devices required for the emission of an optical signal allowing the quantum exchange of keys, by using the same photon source to carry out three functions / devices: the random number generator, the protocol controller and the photon transmitter / modulator. These functions / devices can thus be integrated into a single device.The advantages of such equipment are: - a reduction in the mass, power consumption and volume of the equipment, linked in particular to the integration of the random sequence generator in the same equipment as the protocol controller and the photon emitter, - a reduction in the total cost, - an increase in robustness against attacks by analysis of electromagnetic radiation on the communication links between equipment, since all the functions can be implemented within the same equipment, thus increasing compatibility with TEMPEST standards preventing electromagnetic leaks.

[0057] The invention includes a random sequence generator 505, which allows an increase in the performance in terms of throughput (transmission frequency) and entropy (quality of the randomness) of the random binary sequence generated by the CAN transducer 503. A quantum key exchange (QKD) link using the invention as a qubit transmitter (Alice) will be more efficient than those of the prior art, and will allow more keys to be exchanged. In addition, the random sequence generator makes it possible to offer very good entropy of the transmitted data, and therefore contributes to good quality quantum keys exchanged.

[0058] Finally, the integration of the random sequence generator and the protocol in the same device also makes it possible to eliminate the connection between these two units, and therefore to further increase the randomness generation rate.

[0059] The invention relates to a device serving as a transmitter of random sequences of qubits for a quantum key exchange link. The device may consist of separate equipment (clock 507, photon source 501, modulator 502, CAN transducer 503 and calculation means 504), but also on a complete device integrating all of this equipment. It may be used for example: - for a quantum key exchange link on optical fiber, - embarked on a satellite to create a quantum exchange link of keys between a satellite (Alice) and an optical ground station (Bob).

[0060] Its use for quantum key exchange applications in the space domain offers significant advantages: - simplification of the satellite payload offering a competitive mass, consumption and volume budget, - reduction of the cost of development, assembly, integration and testing (AIT) of the payload, - reduction of electromagnetic leaks revealing random numbers on different payload signals.

[0061] [Fig.8] illustrates the components of a satellite quantum key exchange mission. There are two stations, a satellite station 801 (Alice) and an optical ground station 802 (Bob), with: - a quantum optical link 803, produced by carrying, on board the satellite, a device for transmitting an optical signal according to the invention (qubit transmitter) and an optical terminal (telescope), - a “classic” 804 link, optical or RF, to ensure data reconciliation, which requires an optical modem and an optical terminal on board the satellite for an optical link, or an RF modem and an antenna for an RF link. - an optical service link 805, allowing: • synchronization between the qubit transmitter (the device for transmitting an optical signal according to the invention) and the qubit receiver of the station, • the measurement of polarization distortions caused by propagation in free space and the crossing of the atmosphere of the optical signal. This measurement makes it possible to act on the media adaptation 508 of the device of the invention.

[0062] [Fig. 9] illustrates an example of a payload carried on board a satellite, in the case where the reconciliation link is provided by an optical link. The payload 900 comprises a device for generating an optical signal according to the invention 901, an optical modem 902 making it possible to transmit the reconciliation data exchanged between the device 901 and the receiver, both connected to an optical terminal 903, or telescope. The payload also comprises a synchronization and polarization measurement device 905 ensuring the synchronization of the qubit receiver of the ground station with the device of the invention and the measurement of the distortions of the polarization of the optical channel.The payload is completed by a mission controller 904, configured to ensure the establishment, maintenance and closure of links (quantum and classical), the sequencing of operations, and the management of the payload, including the command and control of the signal generation device 901.

[0063] When the reconciliation channel is a radio frequency channel, then the modem 902 is an RF modem, connected to an antenna.

[0064] Quantum key exchange via satellite can be implemented equivalently using the optical ground station as the transmitter and the satellite as the receiver.

Claims

Claims

1. Device for generating an optical signal in the form of a succession of optical pulses (303) of which a quantum state codes binary information for the quantum exchange of keys between a transmitter (101) and a receiver (103), the device comprising, in the same equipment: - a photonic source (501, 601, 701) configured to generate at least one pulse stream comprising one or more photons, - a quantum state modulator (502) configured to generate said optical signal from one of the pulse streams generated by said photonic source (501, 601, 701) by adjusting the number and quantum state of the photon(s) of the pulses, said number and quantum state of the photon(s) of the pulses being defined by a control word, the device being characterized in that it further comprises,in said equipment: - a device called CAN transducer (503) configured to convert the photons of one of the pulse streams generated by said photonic source into a random binary sequence, - digital calculation means (504) configured to: • generate at least one random sequence (505) having a given probability distribution and rhythm, from the random binary sequence produced by said CAN transducer, • generate the control word (506) transmitted to said quantum state modulator (502) in accordance with a given key exchange protocol, from said at least one random sequence.,

2. Device for generating an optical signal according to claim 1, further comprising a device (508) for adapting the optical signal delivered by the quantum state modulator (502) to the transmission medium of said optical signal.

3. Device for generating an optical signal according to claim 2, wherein the optical signal transmission medium is selected from an optical fiber and free space.

4. Device for generating an optical signal according to one of the preceding claims, wherein the photonic source is one of: - a pulsed laser (501) configured to generate a stream of optical pulses, associated with a power divider configured to generate at least two streams of pulses from said stream of pulses of the pulsed laser, - a pulsed laser (701) configured to generate two streams of pulses, - a single photon source (601) configured to generate a stream of pulses each comprising a photon, associated with a splitter plate (602) configured to divide said stream of pulses into at least two distinct streams of pulses, - a single photon source (701) configured to generate two streams of single photons.

5. Device for generating an optical signal according to one of the preceding claims, in which the CAN transducer (503) is configured to exploit a random quantum phenomenon of the photons of one of the pulse streams generated by the photonic source to determine said random binary sequence.

6. Device for generating an optical signal according to the preceding claim, in which said random quantum phenomenon exploited by the CAN transducer (503) is chosen from: - an initial phase of the pulses, - an amplitude of the pulses, - an instant of appearance of the pulses, - a speckle pattern of the pulses.

7. Device for generating an optical signal according to one of the preceding claims, in which said at least one random sequence (505) generated by the calculation means (504) is obtained by applying one-way mathematical functions to the random binary sequence produced by the CAN transducer (503).

8. Device for generating an optical signal according to one of the claims-

9. preceding indications, wherein the digital calculation means (504) are further configured to implement a process of reconciliation of the data transmitted to the receiver. Payload (900) for a satellite (801) configured to carry out a quantum key exchange mission with a ground station (802), the payload being characterized in that it comprises: - a device (901) for generating an optical signal according to one of the preceding claims, - an optical terminal (903) connected to an output of the device (901) for generating an optical signal, - an optical modem (902) connected to said optical terminal (903) or a radiofrequency modem connected to a radio antenna, - a mission controller (904) configured to supervise the sequencing of quantum key exchange operations, and - a polarization synchronization and measurement device (905) configured to ensure synchronization between the device (901) for generating an optical signal and the ground station, and to measure distortions undergone by the optical signal between the device (901) for generating an optical signal and the ground station.

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