Receiver, quantum key distribution system, and quantum signal detection method

Abstract

A purpose of the present invention is to prevent eavesdropping during quantum key distribution. A synchronization signal reception device (20) receives a synchronization signal (S), which is an optical signal for specifying the detection timing of a quantum signal (Q), and outputs a reference signal (REF) corresponding to the synchronization signal (S). A clock signal generation unit (30) generates a clock signal (CLK) in response to the reference signal (REF). A photon detection device (10) detects a photon of the quantum signal (Q) in response to the clock signal (CLK). The timing of the clock signal varies randomly within a prescribed range.

Description

[Technical field]

[0001] The present invention relates to a receiving device, a quantum key distribution system, and a method for detecting a quantum signal. [Background technology]

[0002] As the Internet becomes more widespread, there is an increasing social need for cryptographic technologies to protect the confidentiality of communications, prevent tampering, authenticate individuals, etc. Against this background, the use of quantum key distribution technology, which is a technology for sharing a secure secret key consisting of random numbers between remote locations and is, in principle, difficult to eavesdrop on, has attracted attention in order to ensure the security of communications (Patent Documents 1 to 3).

[0003] In quantum key distribution, a quantum signal is generally transmitted from a transmitting device to a receiving device, and the quantum signal is detected using multiple photon detectors provided in the receiving device. In this case, if there is variation (bias) in the detection efficiency of the photon detectors, it may be used for eavesdropping. In cryptography, eavesdropping that exploits such information bias is widely known. Therefore, it is known that in order to effectively prevent eavesdropping, it is desirable for the detection efficiency of the photon detectors to be uniform.

[0004] In quantum key distribution, various eavesdropping attempts have been made, including side channel attacks. Among them, a known eavesdropping technique is called a time-shift attack (Non-Patent Document 1), which exploits the vulnerability of the photon detector that detects the quantum signal, allowing an eavesdropper to eavesdrop by manipulating the arrival time of the quantum signal at the photon detector without directly eavesdropping on the quantum signal. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2011-75577 A [Patent Document 2] JP 2019-125961 A [Patent Document 3] JP 2010-166285 A [Non-patent literature]

[0006] [Non-Patent Document 1] Bing Qi, et. Al., “TIME-SHIFT ATTACK IN PRACTICAL QUANTUM CRYPTOSYSTEMS”, 2007, Quantum Information and Computation, vol. 7, pp. 073-082 Summary of the Invention [Problem to be solved by the invention]

[0007] In general, in quantum key distribution, multiple photon detectors are used to detect quantum signals. Ideally, a single photon is desirable for a quantum signal, but in reality, there may be two or more photons. In either case, however, the strength of the quantum signal is weak, so avalanche photodiodes (APDs) are used as photon detectors, which can amplify and output the weak signal they receive.

[0008] When a photon is incident on an APD under a reverse bias exceeding the breakdown voltage, an electron avalanche phenomenon occurs triggered by the electrons generated, which can amplify the signal. Therefore, in quantum key distribution, the reverse bias voltage applied to the APD is increased so as to synchronize with the timing of the photon incident of the quantum signal (for example, Patent Document 1). FIG. 11 shows a schematic diagram of the timing of voltage application to the APD and the time dependency of the photon detection efficiency. When the timing of the reverse bias application to the APD and the timing of the photon incident coincide with each other, the photon detection efficiency of the APD is maximized. However, it is known that when there is a time difference between the timing of the reverse bias application to the APD and the timing of the photon incident, the detection efficiency of the APD decreases as the time difference increases. In other words, the photon detection efficiency of the APD depends on the timing of the photon incident.

[0009] In quantum key distribution, multiple photon detectors (APDs) are used, and in order to ensure high security, it is desirable that the photon detection efficiency at each photon detector is equal. However, there are individual differences in the incidence timing dependency of the photon detection efficiency of a photon detector. FIG. 12 shows an example of the incidence timing dependency of the photon detection efficiency of multiple photon detectors. FIG. 12 shows an example in which 0 and 1 bits in the X-base and 0 and 1 bits in the Z-base are detected by four photon detectors. As shown in FIG. 12, even if photons are incident at the same timing (for example, timing T in FIG. 12), the photon detection efficiency varies among multiple photon detectors.

[0010] Since each of the multiple photon detectors is assigned to each bit of one of the bases of the quantum signal, if there is variation in the incidence timing dependency of the photon detection efficiency of the photon detector, for example, the detection efficiency of each bit of each base may be analyzed, which may result in a situation where eavesdropping is permitted. Therefore, it is required to prevent or suppress the influence of the incidence timing dependency of the photon detection efficiency of the photon detector.

[0011] The present invention has been made in view of the above circumstances, and has an object to prevent eavesdropping in quantum key distribution. [Means for solving the problem]

[0012] A receiving device which is one aspect of the present invention comprises a synchronization signal receiving device which receives a synchronization signal which is an optical signal for specifying the detection timing of a quantum signal and outputs a reference signal corresponding to the synchronization signal, a clock signal generating unit which generates a clock signal corresponding to the reference signal, a photon detecting unit which detects photons of the quantum signal corresponding to the clock signal, and a means for randomly changing the timing of the clock signal within a predetermined range.

[0013] A quantum key distribution system according to one aspect of the present invention includes a transmitting device that transmits a quantum signal, and a receiving device equipped with a photon detection device that detects the quantum signal, the receiving device including a synchronization signal receiving device that receives a synchronization signal, which is an optical signal for defining the detection timing of the quantum signal, and outputs a reference signal corresponding to the synchronization signal, a clock signal generating unit that generates a clock signal corresponding to the reference signal, a photon detecting unit that detects photons of the quantum signal corresponding to the clock signal, and a means for randomly changing the timing of the clock signal within a predetermined range.

[0014] A quantum signal detection method that is one aspect of the present invention includes receiving a synchronization signal, which is an optical signal for specifying the detection timing of the quantum signal, outputting a reference signal corresponding to the synchronization signal, generating a clock signal corresponding to the reference signal, detecting photons of the quantum signal according to the clock signal, and randomly varying the timing of the clock signal within a predetermined range. Effect of the Invention

[0015] According to the present invention, eavesdropping in quantum key distribution can be prevented. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of a quantum key distribution system. [Diagram 2] FIG. 2 is a diagram illustrating a basic configuration of a receiving device according to a first embodiment. [Diagram 3] FIG. 2 is a diagram illustrating a configuration example of a receiving device according to a first embodiment. [Figure 4] FIG. 1 is a diagram illustrating a schematic configuration of a photon detection device according to a first embodiment. [Diagram 5] 1 is a diagram illustrating a schematic configuration of a synchronization signal receiving device according to a first embodiment. [Figure 6] FIG. 2 is a diagram illustrating a schematic configuration of a light receiving section according to the first embodiment. [Figure 7] FIG. 2 is a diagram illustrating a schematic configuration of an optical delay unit according to the first embodiment. [Figure 8] FIG. 11 is a diagram illustrating a configuration of a receiving device according to a second embodiment. [Figure 9] FIG. 11 is a diagram illustrating a schematic configuration of a synchronization signal receiving device according to a second embodiment. [Figure 10] FIG. 2 is a diagram illustrating a schematic configuration of a clock signal generating unit. [Figure 11] 1 is a diagram showing a schematic diagram of the timing of voltage application to an APD and the time dependence of photon detection efficiency. [Figure 12] FIG. 13 is a diagram showing an example of the incidence timing dependency of photon detection efficiency of a plurality of photon detectors. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted as necessary.

[0018] First embodiment A description will be given of a quantum key distribution system 1000 according to the first embodiment. The quantum key distribution system 1000 is configured to which, for example, the BB84 protocol of phase encoding or polarization encoding is applied.

[0019] 1 shows a schematic configuration of a quantum key distribution system 1000. The quantum key distribution system 1000 includes a transmitting device 110, a receiving device 100, and transmission paths 120 and 130. The transmitting device 110 outputs, to the receiving device 100, a quantum signal Q used for providing an encryption key to the receiving device 100 and a synchronization signal S used for controlling the detection timing of the quantum signal Q, via the transmission path 120. The transmitting device 110 and the receiving device 100 can transmit and receive signals including, for example, basis information, test bits, and error correction information, via the transmission path 130, which is a public communication path.

[0020] The quantum signal Q is an optical signal transmitted as an optical pulse consisting of a single photon or a number of photons sufficient to cause the quantum effect to occur.

[0021] In this embodiment, the synchronization signal S is an optical signal transmitted by a transmission path 120 used for transmitting the quantum signal Q. The quantum signal Q and the synchronization signal S are, for example, optical signals of different wavelengths, and may be wavelength-multiplexed and transmitted to the receiving device 100.

[0022] 2 shows a schematic diagram of a basic configuration of a receiving device 100 according to the first embodiment. The receiving device 100 includes a photon detecting device 10, a synchronization signal receiving device 20, and a clock signal generating unit 30. A quantum signal Q and a synchronization signal S are input to the receiving device 100, the quantum signal Q is detected by the photon detecting device 10, and the synchronization signal S is received by the synchronization signal receiving device 20.

[0023] The clock signal generating unit 30 is configured as, for example, a phase locked loop (PLL), and generates a clock signal CLK and outputs it to the photon detecting device 10 and the synchronization signal receiving device 20. In this configuration, the clock signal generating unit 30 adjusts the timing of the clock signal CLK in response to a reference signal REF provided by the synchronization signal receiving device 20 and indicating the reception timing of the synchronization signal S. The clock signal CLK is generated as a high-speed clock signal having higher time accuracy than a reverse bias pulse timing signal and an output voltage signal, which will be described later.

[0024] Next, the receiving device 100 will be further described. Fig. 3 shows a schematic configuration example of the receiving device 100 according to the first embodiment. The receiving device 100 is provided with, for example, a wavelength separation unit 40 that separates the quantum signal Q and the synchronization signal S into wavelengths. This allows the quantum signal Q to be detected by the photon detection device 10, and the synchronization signal S to be received by the synchronization signal receiving device 20. The wavelength separation unit 40 can be configured as any wavelength separation means, for example, a WDM coupler.

[0025] Next, the photon detection device 10 will be described. Here, in order to facilitate understanding of the configuration of the photon detection device 10, one avalanche photodiode as a photon detector and a circuit configuration for detecting incident photons using the output current of this one avalanche photodiode will be described. Needless to say, a plurality of circuit configurations shown in the following FIG. 4 are provided in the photon detection device according to the basis used in quantum key distribution. Also, needless to say, a configuration (not shown for simplification) for distributing photons to a plurality of photon detectors (avalanche photodiodes) is provided according to the encoding method (phase encoding, polarization encoding, etc.) applied to the transmitted quantum signal.

[0026] 4 shows a schematic configuration of a photon detector 10 according to the first embodiment. The photon detector 10 according to the first embodiment includes a timing generator 11, a power supply 12, an avalanche photodiode (APD) 13, a current-voltage converter 14, a sampling unit 15, and a signal processor 16.

[0027] The timing generation unit 11 generates a reverse bias pulse timing signal S11 in response to a clock signal CLK, and outputs the signal to the power supply unit 12. The clock signal CLK is supplied from a synchronization signal receiving device 20, as will be described later.

[0028] The power supply unit 12 generates a reverse bias voltage VB1 and applies it in response to a reverse bias pulse timing signal S11 to the APD 13. The reverse bias voltage VB1 includes a DC component equal to or lower than the breakdown voltage of the APD 13 and a reverse bias pulse component equal to or higher than the breakdown voltage.

[0029] When a photon, i.e., a quantum signal Q, is incident on the light receiving surface of the APD 13 at the timing when the reverse bias pulse component is applied to the APD 13, the electrons are multiplied by the avalanche effect of the generated photoelectrons. This causes an output current I1 to be output from the APD 13. Note that even when no photons are incident, dark noise exists, and the output current I1 is detected as a current with a constant value.

[0030] The current-voltage conversion unit 14 converts the output current I1 into an output voltage signal S12, which is a voltage signal, and outputs it to the sampling unit 15. Here, a transimpedance amplifier (hereinafter, TIA) or the like is used as the current-voltage conversion unit 14.

[0031] The sampling unit 15 samples the output voltage signal S12 in response to the clock signal CLK, and outputs an output voltage signal S13 having discrete values, i.e., a digital signal, to the signal processing unit 16. The sampling unit 15 may output a binary digital signal, or may output a multi-value digital signal generated by analog-to-digital (AD) conversion. Here, an analog-to-digital converter (ADC) is used as the sampling unit 15.

[0032] The signal processing unit 16 performs predetermined signal processing on the output voltage signal S13, which is a digital signal, in response to the clock signal CLK, and outputs a photon detection signal OUT that indicates the result of photon reception.

[0033] Next, the synchronization signal receiving device 20 will be described. Fig. 5 shows a schematic configuration of the synchronization signal receiving device 20 according to the first embodiment. The synchronization signal receiving device 20 has a light receiving unit 20A that receives a synchronization signal Sd, and an optical delay unit 20B that controls the input timing of the synchronization signal S to the light receiving unit 20A.

[0034] 6 shows a schematic configuration of the light receiving section 20A according to the embodiment 1. The light receiving section 20A includes a power supply section 21, a photodiode 22, a current-voltage conversion section 23, and a reference signal generation section 24.

[0035] The power supply unit 21 generates a reverse bias voltage VB2 and applies it to the photodiode 22. The reverse bias voltage VB2 is output as a DC voltage of, for example, about several volts.

[0036] When a synchronization signal S (in this example, a synchronization signal Sd, described later) is incident on the light receiving surface of the photodiode 22 while the reverse bias voltage VB2 is applied, the photodiode 22 outputs an output current I2 according to the intensity of the incident light. Since the synchronization signal S generally has a stronger optical intensity than the quantum signal Q, the type of the photodiode 22 that receives the synchronization signal S is not particularly limited, and various photodiodes including an APD can be used.

[0037] The current-voltage conversion unit 23 converts the output current I2 into an output voltage signal S21, which is a voltage signal, and outputs it to the reference signal generation unit 24. Here, as the current-voltage conversion unit 23, a transimpedance amplifier (hereinafter, TIA) or the like is used.

[0038] The reference signal generating unit 24 outputs the reference signal REF, which is a digital signal, to the clock signal generating unit 30 in response to the output voltage signal S21. The reference signal generating unit 24 may output a binary digital signal, or may output a multi-level digital signal generated by analog-to-digital (A / D) conversion. The reference signal generating unit 24 may be configured as, for example, a clock data recovery (CDR) circuit.

[0039] Next, the optical delay section 20B will be described. The configuration of the optical delay section 20B according to the first embodiment is shown in FIG. The optical delay section 20B includes an optical delayer 25 and a delay control section .

[0040] The synchronization signal S, which has been wavelength-separated by the wavelength separator 40 , enters the optical delay device 25 of the synchronization signal receiving device 20 .

[0041] The optical delay 25 is configured as an optical delay device that can control the output timing of the synchronization signal S passing therethrough within a predetermined range. Here, the predetermined range may be, for example, ±200 ps. The synchronization signal Sd delayed by the optical delay 25 is output to the photodiode 22.

[0042] The delay control section 26 outputs a delay control signal CON to the optical delay device 25, thereby controlling the optical delay device 25 so that the delay in the optical delay device 25 varies randomly within a predetermined range.

[0043] Next, consider the effect of delaying the synchronization signal S in the optical delayer 25. In this configuration, as described above, the output timing of the synchronization signal Sd from the optical delayer 25 varies randomly within a predetermined range (e.g., ±200 ps). Therefore, the incidence timing of the synchronization signal Sd to the photodiode 22 varies randomly within a predetermined range (e.g., ±200 ps).

[0044] Therefore, when the photodiode 22 receives the synchronization signal Sd, the reference signal generating unit 24 outputs the reference signal REF corresponding to the reception timing of the synchronization signal Sd to the clock signal generating unit 30. The clock signal generating unit 30 adjusts the timing of the clock signal CLK based on the reference signal REF, but as described above, since the incidence timing of the synchronization signal Sd to the photodiode 22 varies randomly, the timing of the clock signal CLK also varies randomly.

[0045] Furthermore, in the photon detection device 10, a reverse bias pulse timing signal S11 is applied at a timing based on the clock signal CLK to the APD 13 which detects photons of the quantum signal Q. Since the timing of the clock signal CLK varies randomly, the application timing of the reverse bias pulse timing signal S11 to the APD 13 also varies randomly.

[0046] Naturally, the quantum signal Q is originally received in synchronization with the timing indicated by the synchronization signal S that is not delayed. In contrast, in this embodiment, a delay is applied to the synchronization signal S, thereby randomly varying the timing of the clock signal CLK that defines the reception timing of the quantum signal Q. As a result, the incidence timing of the quantum signal Q to the APD 13 and the application timing of the reverse bias voltage VB1 to the APD 13 are different timings. Moreover, the difference between the timings of the two also varies each time the synchronization signal S is received.

[0047] This causes variation in the photon detection sensitivity of the APD 13, and the detection efficiency of the quantum signal Q also varies randomly.

[0048] As described above, according to this configuration, even if an eavesdropper knows the incidence timing dependency of the photon detection sensitivity of the APD 13, the photon detection sensitivity of the APD 13 varies randomly, so that eavesdropping can be effectively prevented.

[0049] In particular, according to this configuration, it is possible to vary the timing of photon detection in the photon detector, thereby making it possible to effectively prevent eavesdropping by time-shift attacks that exploit the dependency of the timing of photon incidence on the photon detector.

[0050] Embodiment 2 A photon detection device according to a second embodiment will be described. In the first embodiment, the variation in the reception timing of the quantum signal Q (i.e., the timing of the clock signal CLK) is realized by providing a delay to the synchronization signal S, but in the present embodiment, a configuration will be described in which the variation in the reception timing of the quantum signal Q is realized by a clock generation process.

[0051] 8 is a schematic diagram showing a configuration of a receiving device 200 according to the second embodiment. The receiving device has a configuration in which the synchronization signal receiving device 20 and the clock signal generating unit 30 of the receiving device 100 are replaced with a synchronization signal receiving device 50 and a clock signal generating unit 60, respectively.

[0052] The synchronizing signal receiving device 50 will now be described. Fig. 9 shows a schematic configuration of the synchronizing signal receiving device 50 according to the second embodiment. The synchronizing signal receiving device 50 has a configuration in which the optical delay unit 20B of the synchronizing signal receiving device 20 is removed.

[0053] Next, a description will be given of the clock signal generating section 60. The configuration of the clock signal generating section 60 is shown in FIG.

[0054] The clock signal generating unit 60 is configured as a phase-locked loop circuit, and includes a phase comparator 61 , a loop filter 62 , a noise generator 63 , a VCO (Voltage-Controlled Oscillator) 64 , and a frequency divider 65 .

[0055] The phase comparator 61 compares the frequency and phase of the input reference signal REF with the frequency and phase of the feedback signal FB from the frequency divider 65, and outputs an error pulse signal Err indicating the error detected by the comparison to the loop filter 62.

[0056] The loop filter 62 removes the frequency ripple component of the error pulse signal Err and converts it into a DC voltage signal Vr.

[0057] The noise generator 63 generates and outputs a noise signal Vn, for example, wideband white noise. The noise signal Vn is superimposed on the DC voltage signal Vr, and the DC voltage signal Vrn obtained as a result of the superposition is input to the VCO 64.

[0058] The VCO 64 is configured as a circuit that outputs a clock signal CLK, which is a pulse signal having a frequency corresponding to the DC voltage signal Vrn. Here, the clock signal CLK is a signal having a frequency N times (N is an integer equal to or greater than 2) that of the reference signal REF.

[0059] The DC voltage signal Vrn is a signal in which a noise signal Vn is superimposed on the DC voltage signal Vr, so that the timing of the rise and fall of the clock signal CLK output by the VCO 64 varies randomly due to the influence of the noise signal Vn.

[0060] The frequency divider 65 divides the frequency of the clock signal CLK by 1 / N to obtain a feedback signal FB, which is output to the phase comparator 61 .

[0061] In this configuration, the timing of incidence of the quantum signal Q to the APD 13 is different from the timing of application of the reverse bias voltage VB1 to the APD 13, which is generated in response to the clock signal CLK. This causes variation in the photon detection sensitivity of the APD 13, and the detection efficiency of the quantum signal Q also varies randomly.

[0062] As described above, according to this configuration, as in embodiment 1, even if an eavesdropper knows the incidence timing dependency of the photon detection sensitivity of APD 13, the photon detection sensitivity of APD 13 varies randomly, so that eavesdropping can be effectively prevented.

[0063] Furthermore, according to this configuration, the timing of photon detection in the photon detector can be varied, so that, as in embodiment 1, eavesdropping by time-shift attacks that exploit the dependency of the timing of photon incidence on the photon detector can be effectively prevented.

[0064] Other embodiments The present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit and scope of the present invention. For example, in the above-described embodiment, an avalanche photodiode is used as the single-photon detector, but a single-photon detector having a different configuration may be used as long as the single-photon detector has a photon detection efficiency that depends on the timing of incidence of a photon.

[0065] Although the present invention has been described above with reference to the embodiment, the present invention is not limited to the above. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the invention.

[0066] This application claims priority based on Japanese Patent Application No. 2021-13985, filed on January 29, 2021, the disclosure of which is incorporated herein in its entirety. [Explanation of symbols]

[0067] 10 Photon detection device 11, 21 Timing generation section 12, 21 Power supply section 13 APD 14, 23 Current-to-voltage converter 15 Sampling section 16 Signal Processing Section 20, 50 Synchronous signal receiving device 20A light receiving part 20B Optical delay section 22 Photodiode 24 Reference voltage generator 25 Optical Delay 26 Delay control section 30, 60 Clock signal generator 40 Wavelength separation section 61 Phase comparator 62 Loop Filter 63 Noise Generator 64 VCO 65 divider 100, 200 Receiver 110 Transmitting device 120, 130 Transmission line 1000 Quantum Key Distribution System CLK Clock signal CON Delay control signal Err Error pulse signal FB Feedback signal I1, I2 Output current OUT Photon detection signal Q Quantum Signal REF Reference signal REF S, Sd Sync signal S11 Reverse bias pulse timing signal S12, S13, S21 output voltage signal VB1, VB2 reverse bias voltage Vn Noise signal Vr, Vrn DC voltage signal

Claims

1. a synchronization signal receiving device that receives a synchronization signal, which is an optical signal for defining a detection timing of the quantum signal, and outputs a reference signal corresponding to the synchronization signal; a noise generating unit that superimposes a noise signal on a DC voltage signal corresponding to the reference signal, and a VCO that outputs a clock signal whose timing varies randomly within a predetermined range in response to the DC voltage signal on which the noise signal is superimposed; a photon detection unit that detects photons of the quantum signal in response to the clock signal, Receiving device.

2. A transmitting device for transmitting a quantum signal; a receiving device including a photon detection device for detecting the quantum signal; The receiving device includes: a synchronization signal receiving device that receives a synchronization signal, which is an optical signal for defining a detection timing of the quantum signal, and outputs a reference signal corresponding to the synchronization signal; a noise generating unit that superimposes a noise signal on a DC voltage signal corresponding to the reference signal, and a VCO that outputs a clock signal whose timing varies randomly within a predetermined range in response to the DC voltage signal on which the noise signal is superimposed; a photon detection unit that detects photons of the quantum signal in response to the clock signal, Quantum key distribution system.

3. receiving a synchronization signal, which is an optical signal for defining a detection timing of the quantum signal, and outputting a reference signal corresponding to the synchronization signal; a noise signal is superimposed on a DC voltage signal corresponding to the reference signal, and a clock signal whose timing varies randomly within a predetermined range is output in response to the DC voltage signal on which the noise signal is superimposed; detecting photons of the quantum signal in response to the clock signal; A method for detecting quantum signals.

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