This application claims priority to foreign French patent application No. FR 2304460, filed on May 4, 2023, the disclosure of which is incorporated by reference in its entirety.
The invention lies in the technical field of devices enabling quantum key distribution (QKD) or exchange, and more particularly when the key exchange is implemented using a discrete-variable (DV) Prepare & Measure protocol.
The invention relates to a high-performance and compact optical signal transmission device that exhibits high resistance to the risks of electromagnetic radiation measurement attacks.
This exchange/distribution involves:
To establish a key, the transmitter 101 transmits a random string of qubits (string of 0s and 1s) in randomly chosen bases. The receiver 103 measures the qubits of this string on its own bases and reconstructs an incomplete and fragmentary string of bits, following signal degradations related to transmission on the optical link 102 (photons lost on the path, parasitic photons, measurement errors, etc.). What is known as a reconciliation process, carried out on a reconciliation channel 104, makes it possible to generate a secret key known only to the transmitter 101 (Alice) and the receiver 103 (Bob) based on the two random strings: the random string transmitted by Alice and the string received and reconstructed by Bob.
In discrete-variable Prepare and Measure protocols, such as for example the BB84 protocol, the transmitter 101 (Alice) transmits a random string of qubits encoded on multiple possible bases and two possible values (0 and 1). Ideally, the qubits consist of single photons. In embodiments in which the optical source is an attenuated laser, and not a single-photon source, said source generates coherent states that may contain a finite number of photons per pulse with a Poisson distribution. Decoys may be introduced in order to prevent attacks by a hostile third party (called Eve) based on the measurement of additional photons in the case of multi-photon pulses. Reference is then made to the decoy-state BB84 protocol.
The protocol controller 203 of the qubit transmitter 200 is functionally connected to the reconciliation channel by a device that is denoted reconciliation interface (I/F) 205 for the example. The functions of controlling and managing the reconciliation channel and of controlling and running the reconciliation process are divided between the two devices in the example: the protocol controller 203 of the qubit transmitter and the reconciliation interface 205.
The clock 201 clocks the device and defines the transmission rate of the random string of qubits per pulse train.
This random string is provided by a random number generator 202, and more advantageously by a quantum random number generator (QRNG). A QRNG is a generator that uses a quantum phenomenon to generate randomness. It guarantees excellent entropy for the generated random string. Such a generator is based on a microscopic physical phenomenon that generates a statistically random noise signal (for example shot noise of a photodiode). A random number generator typically consists of:
The protocol controller 203 is a digital device (for example a computer, an FPGA (field-programmable gate array), an ASIC (application-specific integrated circuit) or a DSP (digital signal processor)) that drives the photon source in accordance with the chosen protocol. Based on the string of random numbers provided by the QRNG 202, the protocol controller 203 creates a string of instructions or control words intended for the photon emitter 204.
With reference to
The number of bits required to code the instruction or control 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 pulse (qubit or decoy), 1 bit to define the value of the qubit (0 or 1), 1 bit or more to define the encoding base (rectilinear or diagonal polarization, phase, etc.), 1 bit or more to define the amplitude (or intensity) of the decoy pulse (number of photons per pulse), etc. These instructions intended for the photon emitter 204 are developed and encoded by the protocol controller 203 based on the random string from the random number generator 202 and in accordance with the chosen Prepare and Measure protocol.
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 BB84 using six polarization states.
To ensure the security of the generated string of qubits 303 when confronted with an attack, the content of the temporal pulses 304 corresponding to qubits and decoys, their polarization states and the amplitude (or intensity) of the decoys must be completely random, with probability densities defined according to the quantum key exchange protocol that is implemented. The quality of the encryption key that is exchanged depends on the entropy of the random sequence.
In the photon emitter 204, the qubits and decoys (typically their polarization and their amplitude) are encoded using various techniques mostly involving attenuated lasers. This photon “modulator” is driven by the protocol controller 203.
However, the existing solutions based on the architecture shown in
One aim of the invention is therefore to define a compact and competitive optical signal generation device the architecture of which makes it possible to overcome the abovementioned drawbacks of the prior art. The device according to the invention achieves this by integrating, where applicable in one and the same equipment, the functions of photon generator and quantum state modulator, protocol controller, random generation and extraction, and adaptation to the transmission medium (optical fibre or free space).
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 quantum key exchange between a transmitter and a receiver. The device according to the invention comprises, in one and the same equipment:
According to one embodiment of the invention, the optical signal generation device furthermore comprises a device for adapting the optical signal delivered by the quantum state modulator to the transmission medium of the optical signal.
Advantageously, the transmission medium of the optical signal is chosen from among an optical fibre and free space.
Depending on the embodiment of the optical signal generation device according to the invention, the photon source may be:
The ADC transducer is configured to exploit a random quantum phenomenon of the photons of one of the pulse streams generated by the photon source to determine the random binary string.
Depending on the embodiment of the optical signal generation device according to the invention, the random quantum phenomenon exploited by the ADC transducer is chosen from among:
Advantageously, the one or more random sequences generated by the computing means are obtained by applying one-way mathematical functions to the random binary string produced by the ADC transducer.
According to one embodiment of the optical signal generation device according to the invention, the digital computing means are furthermore configured to implement a reconciliation process on the data transmitted to the receiver.
The invention also relates to a payload for a satellite configured to carry out a quantum key exchange mission with a ground station, the payload comprising:
The invention will be better understood and other features, details and advantages will become more clearly apparent on reading the following non-limiting description, and by virtue of the appended figures, which are given by way of example, among which:
Identical references may be used in different figures to designate identical or comparable elements.
Those skilled in the art are familiar with quantum random number generators (QRNG) using the physical properties of random quantum phenomena of pulsed lasers.
The idea of the invention is to combine the functions of quantum random number generator and photon emitter, and advantageously to group these functions and the functionality of protocol controller into a single autonomous equipment. In addition, a random extraction function is added thereto that makes it possible to:
The ADC transducer 503 consists of a transducer that converts a quantum property of the optical pulse into an electrical signal, and an analogue-to-digital converter ADC that converts the electrical signal into a digital value (binary word). The ADC transducer 503 is configured to convert each optical pulse of one of the pulse streams generated by said photon source into a random binary word, exploiting the quantum properties of the pulses to do so, in a manner comparable to what is carried out in quantum random number generators according to the prior art, such as the one shown in
The optical signal generation device according to the invention also comprises digital computing means 504, such as for example an ASIC, an FPGA, a DSP, or a microprocessor, configured to carry out the random sequence generation function 505, and a protocol controller function 506.
The random sequence generation function 505 takes the random binary string delivered by the ADC transducer 503 at input, and uses it to generate one or more random sequences having a probability distribution and rate that are adapted to the key exchange protocol that is implemented. Specifically, the random binary string delivered by the ADC transducer 503 has a rate proportional to the rate of the pulses from the laser 501, and which is 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 has to be increased, and the probability distribution has to be adapted to the chosen transmission protocol. For example, in the case of a decoy-state BB84 protocol implemented by 4-bit control words as described above, one possible implementation is to generate:
When random strings have equal probability densities, such as those for the encoding base and the value of the qubits, they may be grouped within one and the same random sequence the rate of which is increased compared to the bit rate of the random binary string used at input.
The generation, based on a first random sequence (mother sequence), of a new random sequence (daughter sequence) the properties (rate and probability density) of which are adjusted with respect to a given need, is an operation known to those skilled in the art, and may be carried out by applying mathematical functions, such as for example a one-way function such as the hash function, to the daughter random sequences. These mathematical functions ensure that it is impossible for an observer (Eve) to recover the other random sequences based on one of the (daughter) random sequences that may have been reconstructed. Other mathematical functions, such as mixing (random permutation) of the sequences, are also possible.
The choice of the number of sequences, their rates and the respective probability densities depends on the quantum key exchange protocol that is implemented.
The digital computing means 504 are configured to carry out a network controller function 506, which consists in:
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 so as 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 that is provided by the protocol controller 506. The quantum state modulator 502 modifies the stream of optical pulses generated by the pulsed laser 501 into a succession of qubits and decoys, by fixing the average number and quantum state of the photons of each optical pulse in accordance with the control word given by digital computing means 504. To do this, it encodes the qubits and decoys by adjusting the amplitude of the pulses (average number of photons per pulse) and the state (for example the polarization) of the photons in accordance with the control word transmitted by the protocol controller 506.
Advantageously, the optical signal generation device according to the invention comprises a clock 507 that is used to clock the emissions from the photon source 501 and to synchronize the protocol controller function 506 with the pulse stream.
Advantageously again, the optical signal generation device according to the invention comprises a device 508 for adapting to the transmission medium. The transmission medium (optical channel) may be free space or an optical fibre. The device 508 for adapting to the medium controls for example the polarization axes of the transmitted optical signal depending on the medium, by defining the polarization axis in absolute terms, which polarization axis may differ depending on the medium. This adaptation is carried out in collaboration with the receiver to measure distortions in quantum states (for example polarization) that are caused by propagation on the transmission medium. This distortion measurement may be carried out for example via the optical channel before the qubits are transmitted in the case of transmission over fibre or via an optical reference channel dedicated to this function in the case of free-space transmission.
The optical signal generation device according to the invention limits the number of equipments needed to transmit an optical signal enabling quantum key exchange, using one and the same photon source to implement three functions/equipments: the random number generator, the protocol controller and the photon emitter/modulator. These functions/equipments may thus be integrated into a single equipment. The advantages of such an equipment are:
The invention includes a random sequence generator 505, which makes it possible to increase performance in terms of speed (transmission frequency) and entropy (quality of the random) of the random binary string generated by the ADC transducer 503. A quantum key exchange (QKD) link using the invention as a qubit transmitter (Alice) will be more efficient than those from 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 a good quality of the quantum keys that are exchanged.
Ultimately, integrating the random sequence generator and the protocol controller into one and the same device also makes it possible to dispense with the connection between these two units, and therefore to further increase the speed of generation of the random.
The invention relates to a device used as a transmitter of random strings of qubits for a quantum key exchange link. The device may consist of separate equipments (clock 507, photon source 501, modulator 502, ADC transducer 503 and computing means 504), but also on a complete device integrating all of these equipments. It may be used for example:
Using it for quantum key exchange applications in the space sector offers significant advantages:
When the reconciliation channel is a radiofrequency channel, then the modem 902 is an RF modem connected to an antenna.
The satellite quantum key exchange may be implemented in equivalent fashion using the optical ground station as transmitter and the satellite as receiver.
Number | Date | Country | Kind |
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2304460 | May 2023 | FR | national |