Communication system, communication method, and program

Abstract

A transmission device 10 generates a header formed of multiphoton pulses having a light intensity that can be detected by a single-photon detector 31, and sends the header out to a fiber 100 for quantum communication. In a reception device 30, the single-photon detector 31 detects multiphoton pulses, and a synchronization unit 33 synchronizes a clock 11 of the transmission device 10 with a clock 34 of the reception device 30 on the basis of timing of detection of the multiphoton pulses at a predetermined position in the header.

Description

Communication system, communication method, and program

[0001] The present disclosure relates to a communication system, a communication method, and a program.

[0002] Clock synchronization between transmitters and receivers is an important issue in quantum-related technologies such as quantum cryptography and in the construction of quantum networks. However, the optical intensity of synchronization signals currently used in optical communications is approximately 60 dB higher than the weak light at the photon level used in quantum communications (Non-Patent Document 1). When communication signals and synchronization signals are multiplexed and transmitted over the same fiber using wavelength division multiplexing, the receiver requires highly accurate spectroscopy. For this reason, in actual operations, measures such as increasing the wavelength spacing are taken (Non-Patent Document 2).

[0003] Furthermore, when performing time synchronization using GPS or the like, it is necessary to operate at a clock frequency on the order of MHz, since the jitter of the GPS itself makes it possible to synchronize time on a nanosecond scale (Non-Patent Document 3).

[0004] By combining direct fiber links, clock synchronization on the picosecond scale is possible over long distances (Non-Patent Documents 4 and 5). However, the jitter of the optical path itself that carries quantum communication is on the nanosecond scale, so clock frequencies are limited to the MHz order.

[0005] A. Ruiz-Alba et al., “Microwave photonics parallel quantum key distribution”, IEEE Photon. J. 4, 3, 931 (2012).JF Dynes et al., “Ultra-high bandwidth quantum secured data transmission”, Sci. Rep. 6, 35149 (2016).T. Schmitt-Manderbach et al., “Entanglement-based quantum communication over 144km”, Phys. Rev. Lett. 98, 010504 (2007).R. Quan et al., “Implementation of field two-way quantum synchronization of distant clocks across a 7km deployed fiber link”, Opt. Exp. 30, 7, 10269 (2022).M. Alshowkan et al., “Advanced Architectures for High-Performance Quantum Networking”, J. Opt. Commun. Netw. 14, 6, 493 (2022).

[0006] Clock synchronization is necessary to ensure that photons (or faint light called weak coherent light) transmitted via quantum communication are detected at the correct time. Most methods used for time synchronization over long distances perform time synchronization using a network separate from the quantum communication network. Even if there is a perfectly synchronized clock signal source between the transmitter and receiver, deviations in the detection time will occur if the optical path length of the transmission path changes. Jitter caused by temperature fluctuations in the quantum communication network itself cannot be suppressed, and deviations of the order of sub-nanoseconds will occur in the detection timing of the quantum bits shared between the transmitter and receiver. To suppress delays to the picosecond order required for operation on the order of GHz, synchronization must be performed on the same path as the quantum communication path.

[0007] The synchronization signals used in current optical communications have much higher optical intensity than communication signals. Therefore, if the communication signal and synchronization signal are multiplexed and transmitted over the same fiber, the receiving side requires highly accurate optical separation technology, and a separate detector for the clock signal is also required.

[0008] The present disclosure has been made in consideration of the above circumstances, and aims to enable high-speed operation with a simple configuration.

[0009] A communication system according to one aspect of the present disclosure is a communication system in which a transmitting device and a receiving device are connected via a communication fiber, wherein the transmitting device includes a header generation unit that generates a header consisting of a synchronization pulse of an optical intensity detectable by the receiving device and sends it to the fiber, and a first clock that determines the timing for sending a communication signal to the fiber, and the receiving device includes a detection unit that detects the synchronization pulse, a second clock that determines the timing for receiving a communication signal, and a synchronization unit that synchronizes the first clock and the second clock based on the detection timing of a synchronization pulse at a predetermined position in the header.

[0010] According to the present disclosure, high-speed operation can be achieved with a simple configuration.

[0011] FIG. 1 is a diagram showing an example of the configuration of a quantum communication system. FIG. 2 is a flowchart showing an example of the processing flow of the quantum communication system. FIG. 3 is a diagram showing an example of a header. FIG. 4 is a diagram showing an example of clock correction. FIG. 5 is a diagram showing an example of the configuration of a modified example of a transmitting device. FIG. 6 is a diagram showing an embodiment in which the quantum communication system is applied to quantum entanglement distribution. FIG. 7 is a diagram showing an example of the hardware configuration of each device in the quantum communication system.

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the description of the drawings, the same parts are designated by the same reference numerals and the description thereof will be omitted.

[0013] [Summary of the present disclosure] This disclosure presents a configuration and protocol that enables synchronization and quantum communication using only a single detector required for quantum communication. Quantum communication uses pulses with optical intensity at the single-photon level as communication signals. In this disclosure, multi-photon pulses with higher intensity than the pulses used for quantum communication but at a level that does not destroy the single-photon detector are used as synchronization pulses. This allows for a simple configuration that does not require a synchronization transmitter / receiver. Furthermore, because synchronization is performed using the same optical path as that used for quantum communication, time synchronization that enables operation at GHz can be achieved.

[0014] [Configuration of Quantum Communication System] The configuration of the quantum communication system of this embodiment will be described with reference to Fig. 1. The quantum communication system shown in the figure includes a transmitting device 10 and a receiving device 30. The transmitting device 10 and the receiving device 30 are connected by a fiber 100 for quantum communication. The transmitting device 10 and the receiving device 30 are also connected so as to enable communication by classical communication.

[0015] The transmitting device 10 includes a clock 11, a light source 12, a control unit 13, and an optical modulator 14.

[0016] The clock 11 outputs a clock signal.

[0017] The light source 12 generates light pulses according to the frequency of the clock signal.

[0018] The control unit 13 controls the optical modulator 14 to generate a header for clock synchronization that is composed of multi-photon pulses 110. The control unit 13 controls the optical modulator 14 to encode information to be transmitted into pulses (photons or weak coherent light) 120 for quantum communication.

[0019] In response to instructions from the control unit 13, the optical modulator 14 modulates the optical pulses for clock synchronization into multi-photon pulses 110 of an optical intensity detectable by the single-photon detector 31 of the receiving device 30, and transmits the modulated multi-photon pulses 110 to the fiber 100. Prior to quantum communication, the transmitting device 10 transmits a multi-photon pulse train with varying optical intensity to the receiving device 30, receives notification of information regarding the detected multi-photon pulses, and, based on the notification, specifies an optical intensity that will ensure reliable detection by the single-photon detector 31.

[0020] In response to an instruction from the control unit 13 , the optical modulator 14 modulates the optical pulse for quantum communication to an appropriate intensity using an encoding method, and transmits the pulse 120 for quantum communication to the fiber 100 .

[0021] The receiving device 30 includes a single photon detector 31, a time interval analyzer (TIA) 32, a synchronizer 33, and a clock .

[0022] The single-photon detector 31 detects the multi-photon pulse 110 that constitutes the header and the pulse 120 for quantum communication, and outputs a detection signal.

[0023] The TIA 32 measures and analyzes the time interval of the detected signal.

[0024] The single photon detector 31 and the TIA 32 operate in synchronization with a clock 34 .

[0025] When receiving the header, the synchronization unit 33 identifies the time origin based on the pattern of the time intervals of the detection signal in which the multiphoton pulse 110 is detected, and synchronizes the clock 34 with the clock 11. In other words, the synchronization unit 33 synchronizes the clock 34 with the clock 11 based on the detection position of the multiphoton pulse 110 at a predetermined position in the header.

[0026] During quantum communication, the synchronization unit 33 corrects the clock 34 based on the difference between the detection timing of the detection signal that detects the quantum communication pulse 120 and the timing of the clock 34 .

[0027] [Processing Flow in Quantum Communication] An example of the processing flow of the quantum communication system of this embodiment will be described with reference to the flowchart in Figure 2. As a communication flow, first, the transmitting device 10 creates and transmits a header that synchronizes the arrival time of quantum communication between the transmitting and receiving devices using a multi-photon pulse 110 for clock synchronization, thereby synchronizing the clocks. Here, the signal that synchronizes the quantum communication clock between the transmitting and receiving devices is defined as the header. The header does not need to be at the beginning of the communication, and the time for clock synchronization and the time for transmitting quantum communication may be separated. After clock synchronization, quantum communication is performed using pulses for quantum communication. During quantum communication, the receiving device 30 also corrects the clock.

[0028] In step S1, the transmitting device 10 transmits a multi-photon pulse train with varying optical intensity to the receiving device 30. This multi-photon pulse train is used to determine the intensity of the header multi-photon pulse 110. The optical intensity of each multi-photon pulse may be increased gradually or randomly.

[0029] In step S2, the receiving device 30 notifies the transmitting device 10 of the time slots in which the multiphoton pulses were detected with nearly 100% success rate. The transmitting device 10 receives the notification and sets the optical intensity of the header multiphoton pulse 110. The receiving device 30 defines a setting range within which the optical intensity can be monitored.

[0030] In step S3, the transmitting device 10 creates and transmits a header made up of multiphoton pulses 110 at the light intensity set in response to the notification.

[0031] In step S4, the receiving device 30 receives the header and synchronizes the clock 34 based on the header. If the monitored optical intensity has deviated from the set range, the receiving device 30 notifies the transmitting device 10, and the quantum communication system restarts the process from step S1. The method for synchronizing the clock will be described later.

[0032] After clock synchronization, in step S5, the transmitting device 10 and the receiving device 30 start quantum communication via the fiber 100 for quantum communication.

[0033] During quantum communication, in step S6, the quantum communication system determines whether or not the designated time has elapsed, and if the designated time has elapsed, the process returns to step S3 and executes clock synchronization again.

[0034] During quantum communication, in step S7, the receiving device 30 determines whether the clock 34 is out of sync.

[0035] If a deviation of the clock 34 is detected, in step S8 the receiving device 30 corrects the clock 34. The clock correction will be described later.

[0036] The processes from step S3 to step S8 are repeatedly executed until quantum communication is completed.

[0037] [Clock Synchronization] An example of clock synchronization will be described with reference to FIG.

[0038] To determine the time origin of the clock of the receiving device 30, the transmitting device 10 creates a header with characteristics that can be detected by the single-photon detector 31. If the single-photon detector 31 cannot quantitatively determine the light intensity, a header is created that can synchronize the clock using only information about the photon detection time, detection timing, or detection pattern.

[0039] For example, as shown in FIG. 3 , the transmitting device 10 first transmits several cycles of multi-photon pulses 110 over n cycles, and then transmits multi-photon pulses 110 over m cycles. The receiving device 30 synchronizes its clock by setting the time origin at the boundary between the n-cycle multi-photon pulses 110 and the m-cycle multi-photon pulses 110 (e.g., the first multi-photon pulse 110 of the m-cycle). The n-cycle and m-cycle must be longer than the dead time of the single-photon detector 31. The dead time is the time during which a photon cannot be measured after it has been detected once. The header pattern is not limited to the above, and it is sufficient if the multi-photon pulse 110 serving as the time origin can be identified based only on the characteristics of the time at which the multi-photon pulses 110 are detected.

[0040] [Clock Correction] With reference to FIG. 4, an example of clock correction during quantum communication will be described.

[0041] During quantum communication, the detection of the quantum communication pulse 120 is probabilistic, so the detection signal at the detected event is compared with the waveform of the clock being used at that time. For example, if a detection signal is obtained between the rising and falling edges of the clock, the time difference t from the rising edge of the clock to the detection signal is recorded. This result is considered one event. If there is no deviation between the clock and the detection signal, the same time difference t will continue to be recorded for each event. The clock can be corrected according to the change in this time difference, but since each event has fluctuations such as jitter, the time difference changes for each event. The receiving device 30 corrects the (n+1)th clock to correct the deviation according to the change in the difference between the detection signal averaged over n adjacent events and the clock.

[0042] [Modification] A modification of the transmission device 10 will be described with reference to FIG.

[0043] In the transmitting device 10 of FIG. 1, the same light source 12 is used to generate the optical pulse for clock synchronization and the optical pulse for quantum communication. However, in the modified example of FIG. 5, a light source 15 for quantum communication and a light source 16 for a synchronization signal are provided, and the multi-photon pulse 110 for clock synchronization and the pulse 120 for quantum communication are multiplexed in time.

[0044] The transmitting device 10 shown in FIG. 5 includes a clock 11 , a quantum communication light source 15 , a synchronization signal light source 16 , and a multiplexing unit 17 .

[0045] The clock 11 manages the scheduling of the quantum communication light source 15 and the synchronization signal light source 16, and supplies clock signals to the quantum communication light source 15 and the synchronization signal light source 16 at the timing of sending out the quantum communication pulse 120 and the synchronization signal multi-photon pulse 110.

[0046] The quantum communication light source 15 emits pulses 120 for quantum communication in response to a clock signal.

[0047] The synchronization signal light source 16 emits a synchronization signal multi-photon pulse 110 in response to a clock signal.

[0048] In this modification, the quantum communication light source 15 and the synchronization signal light source 16 each output pulses with optical intensities suitable for quantum communication and synchronization, so there is no need to modulate and control the optical pulses output from the light sources.

[0049] The multiplexing unit 17 temporally multiplexes the quantum communication pulse 120 and the synchronization signal multi-photon pulse 110 and sends the multiplexed pulse to the quantum communication fiber 100. The quantum communication pulse 120 and the synchronization signal multi-photon pulse 110 may be combined using polarization and transmitted after polarization multiplexing. Alternatively, a multi-core fiber may be used for the fiber 100, and the quantum communication pulse 120 and the synchronization signal multi-photon pulse 110 may be multiplexed using different cores.

[0050] [Example of quantum entanglement delivery] An example in which the quantum communication system of this embodiment is applied to quantum entanglement delivery to two locations will be described with reference to Figure 6. The quantum communication system shown in the figure includes a transmitting device 10 and receiving devices 30A and 30B. The transmitting device 10 and receiving device 30A are connected by a quantum communication fiber 100A, and the transmitting device 10 and receiving device 30B are connected by a quantum communication fiber 100B. The transmitting device 10 and receiving devices 30A and 30B are also connected so that communication via classical communication is possible.

[0051] The transmitting device 10 has a configuration in which a quantum entanglement generator 18 and optical modulators 19A and 19B are added to the transmitting device 10 shown in Fig. 1. The receiving devices 30A and 30B have the same configuration as the receiving device 30 shown in Fig. 1, and therefore a description thereof will be omitted here.

[0052] The quantum entanglement generator 18 has a function of generating photon pairs in a quantum entangled state, and generates photon pairs when the input light intensity is appropriate. In the case of photon pair generation by down-conversion, multi-photon pulse pairs can be generated by inputting high-intensity light, and therefore, header transmission and quantum communication can be performed by controlling the optical modulator 14 in the upstream stage of the quantum entanglement generator 18.

[0053] The optical modulators 19A and 19B adjust the optical intensity of the pulses to be sent in accordance with the state of the fibers 100A and 100B.

[0054] The clocks are synchronized between the transmitting device 10 and the receiving device 30A, and between the transmitting device 10 and the receiving device 30B. This makes it possible to synchronize the time of photons detected between the respective locations.

[0055] [Effects of the Present Embodiment] As described above, the transmitting device 10 generates a header composed of multi-photon pulses with an optical intensity detectable by the single-photon detector 31 and transmits it to the quantum communication fiber 100. In the receiving device 30, the single-photon detector 31 detects the multi-photon pulses, and the synchronization unit 33 synchronizes the clock 11 of the transmitting device 10 with the clock 34 of the receiving device 30 based on the detection timing of the multi-photon pulse at a predetermined position in the header. This allows both quantum communication and clock synchronization to be performed by the single-photon detector 31, enabling operation with a simpler configuration. Furthermore, in this embodiment, clock synchronization and correction are performed via the quantum communication fiber 100, eliminating the need for links between the clocks 11 and 34 associated with each device, and enabling operation at GHz when the jitter of the clock signal source itself is suppressed to the order of picoseconds.

[0056] Each device in the quantum communication system described above can be, for example, a general-purpose computer system including a central processing unit (CPU) 901, a memory 902, a storage 903, a communication device 904, an input device 905, and an output device 906, as shown in Fig. 7. In this computer system, each device in the quantum communication system is realized by the CPU 901 executing a predetermined program loaded onto the memory 902. This program can be recorded on a computer-readable non-transitory recording medium such as a magnetic disk, an optical disk, or a semiconductor memory, or can be distributed via a network.

[0057] REFERENCE SIGNS LIST 10 Transmitter 11 Clock 12 Light source 13 Control unit 14 Optical modulator 15 Light source for quantum communication 16 Light source for synchronization signal 17 Multiplexer 18 Quantum entanglement generator 19A, 19B Optical modulator 30, 30A, 30B Receiver 31 Single photon detector 32 Time Interval Analyzer (TIA) 33 Synchronizer 34 Clock 100, 100A, 100B Fiber

Claims

1. A communications system in which a transmitting device and a receiving device are connected via a communications fiber, wherein the transmitting device comprises: a header generating unit that generates a header composed of a synchronization pulse of an optical intensity detectable by the receiving device and sends it to the fiber; and a first clock that determines the timing of sending a communication signal from the transmitting device to the fiber; and the receiving device comprises: a detecting unit that detects the synchronization pulse; a second clock that determines the timing of receiving a communication signal; and a synchronizing unit that synchronizes the first clock and the second clock based on the detection timing of a synchronization pulse at a predetermined position in the header.

2. A communication system according to claim 1, wherein the receiving device is provided with a correction unit that detects a deviation in the second clock by averaging multiple detection timings of a communication signal transmitted through the fiber, and corrects the deviation in the second clock.

3. A communication system according to claim 1, wherein the transmitting device comprises: a light source which outputs optical pulses in accordance with the first clock; and an optical modulator which modulates the optical intensity of the optical pulses into the optical intensity of a communication signal and the optical intensity of a synchronization pulse for a header.

4. A communication system as claimed in claim 1, wherein the transmitting device comprises: a first light source that outputs a communication signal in accordance with the first clock; a second light source that outputs a synchronization pulse for a header in accordance with the first clock; and a multiplexing section that multiplexes the communication signal and the header.

5. A communications system as claimed in claim 1, wherein the transmitting device sends out a plurality of synchronization pulses of varying optical intensity onto the fiber, the receiving device notifies the transmitting device of information regarding synchronization pulses which have been detected among the plurality of synchronization pulses, and the transmitting device generates the header with a synchronization pulse of an optical intensity which can be detected by the receiving device based on the notification.

6. A communication method in a communication system in which a transmitting device and a receiving device are connected via a communication fiber, wherein the transmitting device has a first clock that determines the timing of sending a communication signal to the fiber, and the receiving device has a second clock that determines the timing of receiving a communication signal, the transmitting device generates a header composed of a synchronization pulse of an optical intensity detectable by the receiving device and sends it to the fiber, and the receiving device detects the synchronization pulse and synchronizes the first clock and the second clock based on the detection timing of the synchronization pulse at a predetermined position in the header.

7. A communication method according to claim 6, wherein the receiving device detects a deviation in the second clock by averaging multiple detection timings of the communication signal transmitted through the fiber, and corrects the deviation in the second clock.

8. A program for causing a computer to operate as each part of a transmitting device or a receiving device included in a communication system according to any one of claims 1 to 5.

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