JPWO2018159656A1 - Method, apparatus, and program for adjusting timing of single photon detector in quantum key distribution system - Google Patents

Abstract

In order to provide a timing adjustment method, an apparatus, and a recording medium that can be set to the timing at which the detection efficiency of the main pulse is maximized and the adjustment time can be reduced, a weak photon pulse in a quantum key distribution (QKD) system is provided. The method of adjusting the detection timing with a single photon detector for detecting is to scan the gate application timing in a predetermined scan range, record the number of photons detected at each timing, and At least two adjacent peaks are extracted, the error rates of the extracted at least two peaks are evaluated, and the detection timing is set to the lower error rate peak.

Description

  The present invention relates to quantum cryptography, and more particularly, to a method, an apparatus, and a recording medium for adjusting the timing of a single photon detector (SPD) in a quantum key distribution (QKD) system.

  Quantum key distribution (QKD) uses a weak optical signal (eg, 0.1 photons on average) transmitted through a “quantum channel” to communicate between a sender (“Alice”) and a receiver (“Bob”). Including establishing a key between them. The security of key distribution is based on the principle of quantum mechanics, that any quantum system in an uncertain state changes its state when measured. As a result, an eavesdropper ("Eve") trying to intercept or measure an optical signal will manifest itself because it will cause errors in the transmitted signal.

  Non-Patent Document 1 describes a general principle of quantum cryptography. Quantum cryptography is based on the laws of physics to guarantee the security of cryptography, so that ultimate security can be guaranteed without depending on the limit of computer capability. In a quantum key distribution (QKD) system which is currently being studied, one bit of information is encoded into a single photon state and transmitted. This is because photons are more resistant to environmental disturbances than other quantum systems, and at the same time, long-distance encryption key distribution can be expected using existing optical fiber communication technology.

  In a quantum key distribution (QKD) system whose security has been proved theoretically, as described in Non-Patent Document 1, two distinguishable states of a quantum mechanical two-degree-of-freedom system and a conjugate The secret key is securely transmitted using the state (the superimposed state). In other words, in the quantum key distribution protocol disclosed in Non-Patent Document 1, one bit of random number information is loaded for each photon, and a remote transmitter / receiver shares a secret key via an optical fiber. The eavesdropping action disturbs the quantum mechanical state, and the protocol is designed so that the amount of leaked information can be estimated from errors in the data of the authorized sender and receiver.

  A quantum state used for information communication as described above is often called "quantum information". A quantum mechanical two-degree-of-freedom system that carries quantum information is called a "qubit", which is mathematically equivalent to a spin 1/2 system.

Patent Document 1 describes a specific quantum key distribution (QKD) system. Patent Document 1 describes a one-way QKD system. In a one-way QKD system, Alice randomly encrypts the polarization or phase of single photons and Bob randomly measures the polarization or phase of those photons. The one-way QKD system described as prior art in FIG. 1 (FIG. 1) is based on a two-beam Mach-Zehnder interferometer. Alice and Bob can access each part of the interferometer so that they can control the phase of the interferometer. The signal (pulse) transmitted from Alice to Bob is time-divided and follows different paths.
Therefore, the interferometer needs to be dynamically stabilized during transmission to compensate for thermal drift.

  The one-way QKD system indicates a QKD system based on phase coding using coherent weak optical pulses. In this QKD system, an optical interferometer having a structure in which two asymmetric Mach-Zehnder interferometers are connected in series by an optical fiber transmission line is used.

  The QKD system includes a transmitter, a receiver, and an optical fiber transmission line connecting them.

  The transmitter includes a light emitting unit including a weak laser light source, and a modulator including an asymmetric Mach-Zehnder interferometer and a phase modulator. A weak short light pulse generated by a weak laser light source is incident on the asymmetric Mach-Zehnder interferometer, and the output end of the asymmetric Mach-Zehnder interferometer is spatially separated by the long-short optical path difference. Generate (prepare) a weak light pulse. After being modulated by the phase modulator, the two weak light pulses are transmitted to the receiver via the optical fiber transmission line.

  Here, the term coherent means that the relative phase can be clearly defined between two pulses of two weak light pulses by an asymmetric Mach-Zehnder interferometer with clearly defined long and short optical path differences.

  The modulated double weak optical pulse is disturbed during transmission on the optical fiber transmission line, but their relative phase relationship and polarization relationship are preserved.

  The receiver includes a light receiving unit including two photon detectors, and a demodulator including a phase modulator and an asymmetric Mach-Zehnder interferometer. The two weak light pulses phase-modulated by the phase modulator are converted into triple-pulse photon outputs by an asymmetric Mach-Zehnder interferometer, and output to two output ports on the downstream side. The two photon detectors respectively identify and record the presence or absence of a photon contained in the central light pulse of the triple-pulse photon output to two output ports downstream of the asymmetric Mach-Zehnder interferometer. Record on device.

  Here, the central light pulse of the triple pulse light output is called a “main pulse” and is a pulse used for identification. On the other hand, the optical pulses on both sides of the triple-pulse photon output are called "satellite pulses" and are not used for identification.

  The following two light pulses contribute to the central light pulse (main pulse) of the triple pulse-like photon output. One of these two light pulses will be referred to as a first light pulse, and the other will be referred to as a second light pulse. The first optical pulse is an optical pulse that has passed the length of the asymmetric Mach-Zehnder interferometer at the transmitter and the short length of the asymmetric Mach-Zehnder interferometer at the receiver. The second optical pulse is an optical pulse that has passed the short length of the asymmetric Mach-Zehnder interferometer at the transmitter and has passed the long length of the asymmetric Mach-Zehnder interferometer at the receiver. Therefore, due to the interference of these two contributions, the intensity ratio of the central light pulse to the two output ports of the asymmetric Mach-Zehnder interferometer of the receiver is the optical delay (relative phase) of the two weak light pulses Sine wave function.

  In the one-way QKD system having such a configuration, by modulating the optical delay (relative phase) to the two weak light pulses, encryption key distribution based on the principle of quantum cryptography can be performed. For this purpose, the optical pulse first performs four phase modulations of {0, π / 2, π, 3π / 2} in the phase modulator of the transmitter. Then, the binary pulse after transmission through the optical fiber transmission line performs binary phase modulation of {0, π / 2} by the phase modulator of the receiver.

  By properly adjusting the optical delay in the asymmetric Mach-Zehnder interferometer of the transmitter and the receiver, the quantum key distribution protocol using non-orthogonal four states proposed in Non-Patent Document 1 is executed, and secure key distribution is performed. It is possible to do.

In such a quantum key distribution (QKD) system, a single-photon detector (SPD) is used as a photon detector for detecting a weak photon pulse.
As a single photon detector (SPD), an avalanche photodiode (APD) is generally used (for example, see Patent Document 2).

  Patent Document 2 discloses a technique for measuring the optical power of an optical pulse train based on the number of photons counted by a counting unit by shifting a drive timing.

  In general, when a sufficiently large reverse bias is applied to a pn or np photodiode, carriers generated by absorption of photons are accelerated by a large electric field in a depletion layer, and electron-hole pairs are formed by impact ionization. appear. In an avalanche photodiode (APD), a photocurrent is amplified in the same manner as a photomultiplier tube due to avalanche occurrence of this phenomenon.

  In single-photon detection, the APD is used under an operating condition called Geiger mode described below. First, a voltage exceeding a breakdown voltage (a reverse bias larger than a breakdown voltage) is applied to an avalanche photodiode (APD). At this time, when there is no thermally excited carrier or carrier generated by absorption of photons, the APD is in an "off" state in which avalanche breakdown does not occur. Here, when photons are input to the APD, avalanche breakdown is caused. Under these operating conditions, the APD can be used as a single-photon detector because avalanche breakdown can be caused by the input of one photon.

  Applying a voltage exceeding the breakdown voltage (a reverse bias greater than the breakdown voltage) is called Gated Geiger mode. More specifically, in order to reduce noise, the APD is set to the Geiger mode only at the photon incidence timing.

  An operation in which the Geiger mode operation is performed intermittently only at a time when photon detection is predicted is referred to as a gate mode operation (gated mode) or a gate operation. The gate operation is realized by performing photon detection periodically.

  Normally, the probability that an electron-hole pair is generated by photon absorption is called quantum efficiency. A photon detector cannot detect a photon unless further avalanche occurs, and the photon detection efficiency (photon detection rate) is obtained by multiplying the quantum efficiency by the avalanche generation rate.

  In order to detect photons using an avalanche photodiode (APD), it is necessary to match the photon incidence timing with the gate application timing so that the gate application timing is optimized. In other words, it is necessary to synchronize the detection of the optical pulse with the predicted pulse arrival time. To that end, in current QKD systems, one or more single photon detectors are gated with a gate signal from a controller.

  However, once the system is set up, the timing fluctuates (drifts) due to various system and environmental factors. This lowers the photon count and consequently lowers the transfer rate of the system and also increases the bit error rate (BER). That is, the performance is lower than the optimum system performance.

  Various methods for solving such a problem have been proposed.

  For example, Patent Literature 3 discloses an “auto calibration method” in which a gate timing is scanned in a certain range to search for a point where the number of detections is maximum.

  Patent Document 4 discloses an “automatic calibration method” in which a gate timing is scanned in a certain range to search for a point where the bit error rate is minimized.

Japanese Patent No. 4095672 JP 2007-124484 A Japanese Patent No. 4663651 JP 2008-538678 A

Bennett, C .; H. and G. Brassard, “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of IEEE International Conference on Computers, Systems, and Signal Processing, Bangalore, India, December 1984, pp. 175-179 (IEEE, New York, 1984)

  However, the methods disclosed in Patent Documents 2 to 4 have the following problems.

  Patent Document 2 merely discloses a technique of measuring the optical power of an optical pulse train based on the number of photons counted by the counting means by shifting the drive timing.

  In the auto-calibration method disclosed in Patent Document 3, this alone is not sufficient depending on the QKD method. Specifically, in the one-way QKD system, as described above, there are unnecessary satellite pulses. In the auto-calibration method disclosed in Patent Document 3, when the pulse repetition frequency (clock) is used at the maximum, the number of detected photons becomes almost the same for the main pulse and the satellite pulse (two overlapping). As a result, the auto-calibration method disclosed in Patent Document 3 has a problem that the timing may be adjusted to the most efficient detection of satellite pulses. The probability is 1/2.

  On the other hand, the automatic calibration method disclosed in Patent Literature 4 estimates that the number of detections is the maximum at the point where the bit error rate is the minimum. In the automatic calibration method disclosed in Patent Literature 4, the satellite pulse is excluded because the bit error rate is high, and the main pulse is selected. However, in order to accurately evaluate the error rate, a large amount of data is required. As a result, the automatic calibration method disclosed in Patent Literature 4 requires about 100 times the number of data points as compared with the case where the number of photons detected is evaluated in the auto calibration method disclosed in Patent Literature 3. I will. Therefore, the automatic calibration method disclosed in Patent Literature 4 has a problem that it takes time to adjust (find an optimum value).

  An object of the present invention is to provide a method, an apparatus, and a recording medium for adjusting the timing of a single photon detector in a quantum key distribution system that solves the above problem.

  In one embodiment of the present invention for solving the above-described problems, in a quantum key distribution (QKD) system, a gate application timing is scanned in a predetermined scan range, and the number of photons detected at each timing is recorded. At least two adjacent peaks in the number of detected photons are extracted, an error rate at the extracted at least two peaks is evaluated, and a double photon obtained by modulating an optical pulse generated by the transmitter is obtained. The timing at which the single-photon detector detects the main pulse at the center of the triplet photon pulse obtained by demodulating the pulse at the receiver that receives the pulse via the optical fiber transmission line is determined by the lower error rate. This is a timing adjustment method for setting the peak at

  According to another embodiment of the present invention, in a quantum key distribution (QKD) system, a gate application timing is scanned in a predetermined scan range, and recording means for recording the number of photons detected at each timing is provided. Extracting means for extracting at least two adjacent peaks in the number of detected photons, evaluating means for evaluating an error rate at the extracted at least two peaks, and modulating an optical pulse generated by the transmitter. The timing at which the single-photon detector detects the main pulse at the center of the three-photon pulse obtained by demodulating the obtained two-photon pulse via the optical fiber transmission line with a receiver, Setting means for setting the peak at the lower error rate.

  Further, in another embodiment of the present invention, in a quantum key distribution (QKD) system, a computer scans a gate application timing within a predetermined scan range and records the number of photons detected at each timing. Extracting at least two adjacent peaks in the recorded number of detected photons, evaluating the error rate at the extracted at least two peaks, and modulating an optical pulse generated by the transmitter. The timing at which the single-photon detector detects the main pulse at the center of the three-photon pulse obtained by demodulating the obtained two-photon pulse via the optical fiber transmission line with a receiver, A step of setting the peak to the lower error rate, and a computer-readable recording medium recording a timing adjustment program for executing the procedure. .

  According to the present invention, it is possible to provide a timing adjustment method, an apparatus, and a recording medium that can be set to a timing at which the detection efficiency of the main pulse is maximized and that the adjustment time can be reduced.

1 is a schematic block diagram showing a one-way quantum key distribution (QKD) system in the related art. FIG. 2 is a diagram illustrating details of a light emitting unit and a modulator in a transmitter in the QKD system illustrated in FIG. 1. FIG. 2 is a diagram illustrating details of a light receiving unit and a demodulator in a receiver in the QKD system illustrated in FIG. 1. 5 is a time chart showing an applied voltage applied to an avalanche photodiode (APD) used as a single photon detector (SPD), and a photon incident timing. 5 is a timing chart showing a relationship between gate application timing and photon detection efficiency. FIG. 4 is a diagram illustrating an example of an optical pulse waveform when a pulse repetition frequency (clock) is used at a maximum. FIG. 2 is a block diagram and a circuit diagram illustrating configurations of a controller including a timing adjustment device according to an embodiment of the present invention and a single photon detector. 4 is a time chart showing an applied voltage applied to an avalanche photodiode (APD) used as a single photon detector (SPD), an incident timing of a photon, and a gate pulse when the applied timing is not optimal. FIG. 8 is a block diagram illustrating a schematic configuration of a timing adjustment device illustrated in FIG. 7. 10 is a flowchart for explaining the operation of the timing adjustment device shown in FIG. FIG. 4 is a diagram illustrating a relationship between a gate application timing and the number of photons detected. 10 is a flowchart for explaining the operation of the timing adjustment device shown in FIG. FIG. 8 is a diagram illustrating an example of a gate detection timing t and a photon detection number N recorded in a memory on the controller illustrated in FIG. 7.

[Related technology]
First, related art will be described with reference to the drawings to facilitate understanding of the present invention.

  FIG. 1 is a diagram showing a one-way QKD system according to the related art. The illustrated one-way QKD system is a QKD system based on phase coding using coherent weak optical pulses. In this QKD system, an optical interferometer having a structure in which two asymmetric Mach-Zehnder interferometers are connected in series by an optical fiber transmission line is used.

  The illustrated QKD system includes a transmitter 10, a receiver 20, and an optical fiber transmission line 30 connecting them.

  The transmitter 10 includes a light emitting unit 11, a modulator 12, and a controller 13. FIG. 2 is a diagram illustrating details of the light emitting unit 11 and the modulator 12 in the transmitter 10. The light emitting unit 11 includes one weak laser light source 112. The modulator 12 includes an asymmetric Mach-Zehnder interferometer 122 and a phase modulator 124. The weak short light pulse generated by the weak laser light source 112 is incident on the asymmetric Mach-Zehnder interferometer 122, so that the output end of the asymmetric Mach-Zehnder interferometer 122 is spatially separated by the long and short optical path difference. Generate (prepare) a coherent dual weak light pulse. The two consecutive weak light pulses are phase-modulated by the phase modulator 124 and then transmitted to the receiver 20 via the optical fiber transmission line 30.

  The controller 13 controls the weak laser light source 112 and the phase modulator 124. Although the phase modulator 124 is provided on the output end side of the asymmetric Mach-Zehnder interferometer 122 in FIG. 2, it may be provided inside the asymmetric Mach-Zehnder interferometer 122.

  Here, the term coherent means that the relative phase can be clearly defined between two pulses of two weak optical pulses by the asymmetric Mach-Zehnder interferometer 122 having a clearly defined long and short optical path difference.

  The modulated two weak light pulses are disturbed during transmission on the optical fiber transmission line 30, but their relative phase relations and polarization plane relations are preserved.

  The receiver 20 has a light receiving unit 21, a demodulator 22, and a controller 23. FIG. 3 is a diagram illustrating details of the light receiving unit 21 and the demodulator 22 in the receiver 20. The light receiving unit 21 includes two photon detectors 212 and 214. The demodulator 22 includes an asymmetric Mach-Zehnder interferometer 222 and a phase modulator 224. The phase modulator 224 modulates the phase of the dual weak light pulse received from the optical fiber transmission line 30. The dual weak optical pulse phase-modulated by the phase modulator 224 is converted into a triple-pulse photon output by the asymmetric Mach-Zehnder interferometer 222 and output to two downstream output ports 222out1 and 222out2. You. The two photon detectors 212 and 214 respectively include the photons included in the central light pulse of the triple pulsed photon output output to the two output ports 222out1 and 222out2 downstream of the asymmetric Mach-Zehnder interferometer 222. Is identified and recorded in a memory (not shown) which is a recording device in the controller 23.

  The controller 23 controls the two photon detectors 212 and 214 and the phase modulator 224. In FIG. 3, the phase modulator 224 is provided on the input end side of the asymmetric Mach-Zehnder interferometer 222, but may be provided inside the asymmetric Mach-Zehnder interferometer 222.

  The controller 13 of the transmitter 10 and the controller 23 of the receiver 20 are connected to each other via a classical communication path 40. The classical communication path 40 may be an optical fiber or an electric line, and may be a normal Internet communication path.

  Here, the central light pulse of the triple pulse light output is called a “main pulse” and is a pulse used for identification. On the other hand, the optical pulses on both sides of the triple-pulse photon output are called "satellite pulses" and are not used for identification.

  The following two light pulses contribute to the central light pulse (main pulse) of the triple pulse-like photon output. One of these two light pulses will be referred to as a first light pulse, and the other will be referred to as a second light pulse. The first optical pulse is an optical pulse that has passed through the length of the asymmetric Mach-Zehnder interferometer 122 at the transmitter 10 and has passed through the length of the asymmetric Mach-Zehnder interferometer 222 at the receiver 20. The second optical pulse is an optical pulse that has passed through the short length of the asymmetric Mach-Zehnder interferometer 122 at the transmitter 10 and has passed through the long length of the asymmetric Mach-Zehnder interferometer 222 at the receiver 20. Therefore, due to the interference of these two contributions, the ratio of the intensity of the central light pulse to the two output ports 222out1, 222out2 of the asymmetric Mach-Zehnder interferometer 222 will be the optical delay (relative phase ) Sinusoidally.

  In the one-way type QKD system shown in FIG. 1, by modulating the optical delay (relative phase) to the two weak light pulses, encryption key distribution based on the principle of quantum cryptography can be performed. For this purpose, the optical pulse first performs four phase modulations of {0, π / 2, π, 3π / 2} in the phase modulator 124 of the transmitter 10. Then, the binary pulse after transmission through the optical fiber transmission line 30 performs binary phase modulation of {0, π / 2} by the phase modulator 224 of the receiver 20.

  Properly adjusting the optical delay in the asymmetric Mach-Zehnder interferometers 122 and 222 to execute the quantum key distribution protocol using non-orthogonal four states proposed in Non-Patent Document 1 and perform secure key distribution Is possible.

  In a quantum key distribution (QKD) system as shown in FIG. 1, a single-photon detector (SPD) is used as the photon detectors 212 and 214 for detecting a weak photon pulse. As described above, an avalanche photodiode (APD) is generally used as the single photon detector (SPD).

  In general, when a sufficiently large reverse bias is applied to a pn or np photodiode, carriers generated by absorption of photons are accelerated by a large electric field in a depletion layer, and electron-hole pairs are formed by impact ionization. appear. In an avalanche photodiode (APD), a photocurrent is amplified in the same manner as a photomultiplier tube due to avalanche occurrence of this phenomenon.

  FIG. 4 is a time chart showing an applied voltage applied to an avalanche photodiode (APD) used as a single-photon detector (SPD), and a photon incident timing. 4A is a time chart of the applied voltage, and FIG. 4B is a time chart of the incident timing of the photons.

  In single-photon detection, the APD is used under an operating condition called Geiger mode described below. First, as shown in FIG. 4A, a voltage exceeding a breakdown voltage (a reverse bias larger than a breakdown voltage) is applied to an avalanche photodiode (APD). At this time, when there is no thermally excited carrier or carrier generated by absorption of photons, the APD is in an "off" state in which avalanche breakdown does not occur. Here, when photons are input to the APD, avalanche breakdown is caused. Under these operating conditions, the APD can be used as a single-photon detector because avalanche breakdown can be caused by the input of one photon.

  In this example, the breakdown voltage (breakdown voltage) is about 60V. Applying a voltage exceeding the breakdown voltage (a reverse bias greater than the breakdown voltage) is called Gated Geiger mode. More specifically, in order to reduce noise, the APD is set to the Geiger mode only at the photon incidence timing.

  An operation in which the Geiger mode operation is performed intermittently only at a time when photon detection is predicted is referred to as a gate mode operation (gated mode) or a gate operation. As shown in FIG. 4, the gate operation is realized by performing photon detection periodically.

  FIG. 5 is a timing chart showing the relationship between gate application timing and photon detection efficiency. In FIG. 5, the horizontal axis indicates gate application timing, and the vertical axis indicates photon detection efficiency.

  Normally, the probability that an electron-hole pair is generated by photon absorption is called quantum efficiency. Since photons cannot be detected by the photon detectors 212 and 214 without further avalanche, the photon detection efficiency (photon detection rate) is obtained by multiplying the quantum efficiency by the avalanche generation rate.

  As is apparent from FIG. 5, in order to detect a photon using an avalanche photodiode (APD), it is necessary to match the incident timing of the photon with the application timing of the gate so that the gate application timing is optimized. is there. In other words, it is necessary to synchronize the detection of the optical pulse with the predicted pulse arrival time. Therefore, in the current QKD system, one or more single photon detectors 212 and 214 are gate-controlled by a gate signal from the controller 23.

  However, once the system is set up, the timing fluctuates (drifts) due to various system and environmental factors. This lowers the photon count and consequently lowers the transfer rate of the system and also increases the bit error rate (BER). That is, the performance is lower than the optimum system performance.

  As described above, various methods for solving such a problem have been proposed.

  As described above, Patent Document 3 discloses an “auto calibration method” in which a gate timing is scanned in a certain range to search for a point where the number of detections is maximum.

  Patent Document 4 discloses an “automatic calibration method” in which a gate timing is scanned in a certain range to search for a point where the bit error rate is minimized.

  However, as described above, the methods disclosed in Patent Literature 3 and Patent Literature 4 have the following problems, respectively.

  In the auto-calibration method disclosed in Patent Document 3, this alone is not sufficient depending on the QKD method. More specifically, in the one-way QKD system shown in FIGS. 1 to 3, as described above, unnecessary satellite pulses exist. In the auto-calibration method disclosed in Patent Document 3, when the pulse repetition frequency (clock) is used at the maximum, the number of detected photons is, as shown in FIG. 6, a main pulse and a satellite pulse (two overlapping). Will be almost the same. As a result, in the auto-calibration method disclosed in Patent Literature 3, the timing may be adjusted to a timing at which satellite pulses are detected most efficiently. The probability is 1/2.

  On the other hand, the automatic calibration method disclosed in Patent Document 4 estimates that the number of photon detections is the maximum at the point where the bit error rate is the minimum. In the automatic calibration method disclosed in Patent Literature 4, the satellite pulse is excluded because the bit error rate is high, and the main pulse is selected. However, in order to accurately evaluate the bit error rate, a large amount of data is required. As a result, the automatic calibration method disclosed in Patent Literature 4 requires about 100 times the number of data points as compared with the case where the number of photons detected is evaluated in the auto calibration method disclosed in Patent Literature 3. I will. Therefore, the automatic calibration method disclosed in Patent Literature 4 has a problem that it takes time to adjust (find an optimum value).

[Embodiment]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited by those embodiments, and should be interpreted based on the description in the claims.

  With reference to FIG. 7, the configurations of the controller 23 including the timing adjustment device 60 according to the embodiment of the present invention and the single photon detector 212 will be described.

  FIG. 7 illustrates an example in which the detection timing of the single photon detector 212 in the light receiving unit 21 is adjusted. However, the adjustment of the detection timing of the single photon detector 214 is the same. Its illustration is omitted.

  First, the circuit configuration of the single photon detector 212 will be described. As described above, the single photon detector 212 includes the avalanche photodiode APD. A voltage smaller than the breakdown voltage (breakdown voltage) is applied to the cathode of the avalanche photodiode APD as a bias voltage via the load resistor R1. Here, the breakdown voltage (breakdown voltage) is, for example, about 60V. The anode of the avalanche photodiode APD is grounded via the resistor R2. The controller 23 described later is connected to the cathode of the avalanche photodiode APD via the capacitor C1. In other words, the controller 23 applies a gate pulse to the cathode of the avalanche photodiode APD via the capacitor C1. As a result, a voltage exceeding the breakdown voltage (a reverse bias larger than the breakdown voltage) is applied to the valanche photodiode APD.

  The anode of the avalanche photodiode APD is connected to the output terminal of the single photon detector 212 via the capacitor C2 and the amplifier A1. The output terminal of the single photon detector 212 is connected to the controller 23.

  Next, the configuration of the controller 23 will be described with reference to FIG. The controller 23 can be realized by a computer using a combination of hardware and software. In the illustrated example, the controller 23 includes a photon counter 51, a timing control circuit 52, a bias application circuit 53, a memory 54, a program memory 55, and a timing adjustment device 60 according to the present embodiment.

  The timing control circuit 52 can freely shift the phase of the clock signal CLK from 0 to 2π by 2π / n on the time axis by the timing shift control from the timing adjustment device 60. In the example shown, n is equal to 32. Here, the cycle T of the clock signal CLK substantially coincides with the cycle of the incident photon pulse. However, it is not known at which phase the photon pulse arrives. In this example, it is assumed that the laser light source 112 of the transmitter 10 emits photon pulses at a frequency of 1.25 GHz. In this case, the cycle T of the clock signal CLK is equal to 800 psec. As described above, since n is equal to 32, the timing control circuit 52 can shift the clock signal CLK by 25 p seconds.

  If the application timing of the phase-shifted clock signal (gate pulse) is the same as the incident timing of the photon as shown in FIGS. 8A and 8B, the single-photon detector 212 converts the photon into efficiency. It can be detected well. However, actually, as shown in FIGS. 8C and 8D, the timing of applying the gate pulse may not be optimal. When a single photon detector 212 detects a photon, the number of detected photons is counted by the photon counter 51. The photon counter 51 stores the counted number of detected photons in the memory 54 under the control of the timing adjustment device 60.

  The timing adjustment device 60 controls the timing control circuit 52 so that the phase of the clock signal CLK is sequentially shifted from 0 to 2π by 2π / 32 as described later. Thereby, the bias application circuit 53 applies the gate pulse to the single photon detector 212 at the application timing corresponding to each clock phase. The timing adjusting device 60 stores the clock phase (gate application timing) at that time and the number of photons detected by the photon counter 51 in the memory 54. A time interval corresponding to the cycle T of the clock signal CLK is called a time slot. In this case, shifting the phase of the clock signal CLK from 0 to 2π in increments of 2π / 32 is equivalent to shifting the application timing of the gate pulse by 25 p seconds over the entire range of the time slot T. In this example, the time slot T is equal to the scan range.

  The timing adjustment device 60 receives information on the encryption key from the controller 13 of the transmitter 10 via the classical communication path 40. The timing adjustment device 60 evaluates the quantum bit error rate before error correction based on the information of the encryption key, as described later.

  The timing adjustment device 60 can be realized by a processor such as a CPU (central processing unit) or an arithmetic processing device. That is, the timing adjustment device 60 can be realized as various means by operating hardware such as a CPU based on the timing adjustment program stored in the program memory 55. Further, the timing adjustment program is read into the program memory 55 via a cable, wirelessly, or via the recording medium itself, and operates the hardware of the timing adjustment device 60. In addition, examples of the recording medium include an optical disk, a magnetic disk, a semiconductor memory device, and a hard disk.

  FIG. 9 is a block diagram showing a schematic configuration of the timing adjustment device 60 shown in FIG. The timing adjustment device 60 includes a recording circuit unit 62, an extraction circuit unit 64, an evaluation circuit unit 66, and a setting circuit unit 68.

  The general operation of the timing adjusting device 60 shown in FIG. 9 will be described with reference to FIGS. FIG. 10 is a flowchart for explaining the operation of the timing adjustment device 60. FIG. 11 is a diagram illustrating the relationship between the gate application timing and the number of detected photons.

  The recording circuit unit 62 scans the gate application timing in a predetermined scan range T and records the number of photons detected at each timing (step S101). Specifically, as shown in FIG. 11, the recording circuit unit 62 first sets the gate application timing t to 0, and counts the photon counter 51 for a predetermined period (for example, for 1 second). The number of detected photons is recorded in the memory 54. After that, the recording circuit unit 62 controls the timing control circuit 52 so as to shift the clock signal CLK by 25 psec, and repeats the above operation until the gate application timing t becomes the scan range T equal to 800 psec.

  The extraction circuit unit 64 extracts two adjacent peaks in the recorded number of detected photons (step S102). Specifically, in FIG. 11, the number of photons N1 detected at the gate application timing t1 and the number N2 of photons detected at the gate application timing t2 are extracted.

  The evaluation circuit unit 66 evaluates the error rates at the extracted two peaks (step S103). Specifically, the evaluation circuit unit 66 evaluates the quantum bit error rate before error correction based on the encryption key information received from the controller 13 of the transmitter 10. Here, one of the two peaks is a peak due to the main pulse, and the other is a peak due to the satellite pulse. For unnecessary satellite pulses, the qubit error rate increases at about 50%.

  The setting circuit unit 68 sets the detection timing at the lower error rate peak (step S104). That is, since the error rate of the main pulse is lower than that of the satellite pulse, the setting circuit 68 sets the detection timing to the timing corresponding to the main pulse.

  Next, the operation of the timing adjustment device 60 shown in FIG. 9 will be described in more detail with reference to FIGS. FIG. 12 is a flowchart for explaining the operation of the timing adjustment device 60. FIG. 13 is a diagram showing an example of the gate application timing t and the photon detection number N recorded in the memory 54 of FIG.

  At the time of evaluation, which will be described later, the timing adjustment device 60 of the controller 23 of the receiver 20 receives the encryption key information from the controller 13 of the transmitter 10 via the classical communication path 40. The timing adjustment device 60 stores the information of the encryption key in the memory 54.

  First, the recording circuit unit 62 sets the gate application timing t to 0 seconds (Step S201). As a result, the timing control circuit 52 supplies the clock signal CLK to the bias application circuit 53 without shifting the phase of the clock signal CLK (that is, while keeping the phase at 0). The recording circuit unit 62 measures the number N of photon detections counted by the photon counter 51 during a period of 1 second with the gate application timing t being 0 second. In this example, as shown in FIG. 13, the number N of photons detected at the gate application timing t = 0 seconds was 29,654. The recording circuit unit 62 stores the gate application timing t of “0” and the detected photon number N of “29,654” in the memory 54 (step S202).

  Next, the recording circuit unit 62 determines whether or not the gate application timing t is larger than a preset scan range T (Step S203). In this example, the scan range T is equal to the period (time slot) T of the clock signal CLK, and is 800 psec. Here, since the gate application timing t is 0 second, the recording circuit unit 62 increments the gate application timing t by d (step S204), and returns to step S201. Here, d is a preset increment value of the gate application timing, and is 25 psec in this example.

  The recording circuit unit 62 repeats the operations of steps S201 to S204 until the gate application timing t becomes larger than the scan range T equal to 800 psec (t> T).

  FIG. 13 shows an example of the gate application timing t and the number of detected photons N stored in the memory 54 by the recording circuit unit 62 by normalizing the increment value d to “1”. Since the gate application timing t moves (shifts) from “0” to “32”, the total number of detected photons N stored in the memory 54 is 33.

  Next, the extraction circuit unit 64 extracts two peaks (t1, N1) and (t2, N2) from the gate application timing t and the photon detection number N stored in the memory 54 (step S205). In the present example, as shown in FIG. 13, the extraction circuit unit 64 sets the gate application timing t to “9” and the photon detection number N at that time, “101, 245”, to the first It is extracted as the gate application timing t1 and the first detected number of photons N1. In addition, the extraction circuit unit 64 determines that the gate application timing t is “25” and the number N of photons detected at that time is “99,854”. Extract as number N2.

  Note that the first gate application timing t1 of “9” is equal to 225 psec, and the second gate application timing t2 of “25” is equal to 625 psec.

  The evaluation circuit unit 66 first sets the gate application timing t to 225 psec, which is the first gate application timing t1, and based on the encryption key information, sets the first quantum bit error rate before error correction. E1 is measured, and the first quantum bit error rate E1 is stored in the memory 54 (step S206).

  Similarly, the evaluation circuit unit 66 sets the gate application timing t to 625 psec, which is the second gate application timing t2, and based on the encryption key information, the second quantum bit error before error correction. The rate E2 is measured, and the second quantum bit error rate E2 is stored in the memory 54 (step S207).

  The setting circuit unit 68 compares the first quantum bit error rate E1 with the second quantum bit error rate E2 (step S208).

  When the first quantum bit error rate E1 is smaller than the second quantum bit error rate E2 (YES in step S208), the setting circuit unit 68 sets the first gate application timing t1 having the first quantum bit error rate E1. Is set as the gate application timing to be detected (step S209). Conversely, when the first quantum bit error rate E1 is equal to or greater than the second quantum bit error rate E2 (NO in step S208), the setting circuit unit 68 sets the second quantum bit error rate E2 to the second The gate application timing t2 is set as the gate application timing to be detected (Step S210).

  As described above, it is possible to adjust the detection timing of the single photon detector 212. After the adjustment of the single photon detector 212, the timing of detection by the single photon detector 214 can be adjusted by selecting the single photon detector 214 with a switch (not shown) and performing the same operation. It becomes. As described above, the detection timings of the single photon detectors 212 and 214 can be adjusted at different times, but the detection timings of the single photon detectors 212 and 214 are adjusted in parallel (time division). Of course, it may be adjusted.

  As described above, in the present embodiment, the detection timing is set by searching for two peaks of the number of detected photons and evaluating the error rate for each of the two peaks, so that the following effects are obtained.

  The first effect is that, unlike the auto-calibration method disclosed in Patent Literature 3, it is possible to set the timing at which the detection efficiency of the main pulse is maximized instead of unnecessary satellite pulses.

  A second effect is that the number of time-consuming error rate evaluations can be reduced as compared with the automatic calibration method disclosed in Patent Document 4, so that the adjustment time can be shortened.

  If the above embodiment is described in another expression, a processor that operates as a timing adjustment device is configured based on a timing adjustment program developed in a memory, a recording circuit unit 62, an extraction circuit unit 64, an evaluation circuit unit 66, and a setting circuit. It can be realized by operating as the circuit unit 68.

  As described above, the embodiments of the present invention have been described with reference to the drawings. However, those skilled in the art can use other similar embodiments, and appropriately change the form without departing from the present invention. It should be noted that changes or additions can be made.

  For example, in the above embodiment, two adjacent peaks in the recorded number of detected photons are extracted, but it is needless to say that three or more adjacent peaks may be extracted. That is, in the present invention, at least two adjacent peaks in the recorded number of detected photons may be extracted. In this case, the error rates at at least two extracted peaks are evaluated.

  The present invention has been described above using the above-described embodiment as a typical example. However, the invention is not limited to the embodiments described above. That is, the present invention can apply various aspects that can be understood by those skilled in the art within the scope of the present invention.

  This application claims priority based on Japanese Patent Application No. 2017-040134 filed on March 3, 2017, the disclosure of which is incorporated herein in its entirety.

10: Transmitter 11: Light emitting unit 112: Laser light source 12: Modulator 122: Asymmetric Mach-Zehnder interferometer 124: Phase modulator 13: Controller 20: Receiver 21: Light receiving unit 212: Single photon detector 214: Single One-photon detector 22: Demodulator 222: Asymmetric Mach-Zehnder interferometer 222out1: Output port 222out2: Output port 224: Phase modulator 23: Controller 30: Optical fiber transmission path 40: Classic communication path 51: Photon counter 52: Timing Control circuit 53: Bias application circuit 54: Memory 55: Program memory 60: Timing adjustment device 62: Recording circuit unit 64: Extraction circuit unit 66: Evaluation circuit unit 68: Setting circuit unit

Claims (10)

In a quantum key distribution (QKD) system,
Scan the gate application timing in a predetermined scan range, record the number of photons detected at each timing,
Extracting at least two adjacent peaks in the recorded number of detected photons;
Evaluating the error rate at the extracted at least two peaks,
A main pulse at the center of a triplet photon pulse obtained by demodulating by a receiver that receives two photon pulses obtained by modulating a photon pulse generated by a transmitter through an optical fiber transmission line. Set the timing of detecting a pulse with a single photon detector to the lower peak of the error rate,
Timing adjustment method.   The timing adjustment method according to claim 1, wherein the predetermined scan range is equal to or longer than a period of the photon pulse generated by the transmitter.   The timing adjustment method according to claim 1, wherein the error rate is a quantum bit error rate. In a quantum key distribution (QKD) system,
Recording means for scanning the gate application timing in a predetermined scan range and recording the number of photons detected at each timing;
Extracting means for extracting at least two adjacent peaks in the recorded number of detected photons;
Evaluation means for evaluating an error rate at the extracted at least two peaks;
A main pulse at the center of a triplet photon pulse obtained by demodulating by a receiver that receives two photon pulses obtained by modulating a photon pulse generated by a transmitter through an optical fiber transmission line. Setting means for setting the timing at which the pulse is detected by the single-photon detector to the lower peak of the error rate;
Timing adjustment device provided with.   The timing adjustment device according to claim 4, wherein the predetermined scan range is equal to or longer than a period of the photon pulse generated by the transmitter.   The timing adjustment device according to claim 4, wherein the error rate is a quantum bit error rate. A quantum key distribution (QKD) system having a transmitter, a receiver, and an optical fiber transmission line connecting between the transmitter and the receiver,
The transmitter is
A laser light source that generates a photon pulse at a predetermined cycle,
A modulator for modulating the photon pulse to generate a series of photon pulses;
With
The optical fiber transmission line transmits the double photon pulse from the transmitter to the receiver,
The receiver,
A demodulator that demodulates the double photon pulse and generates a triple photon pulse;
Light receiving means including at least one single photon detector for receiving the triple photon pulse;
A controller for controlling the light receiving means,
With
The controller comprises the timing adjustment device according to any one of claims 4 to 6,
Quantum key distribution (QKD) system. In a quantum key distribution (QKD) system,
Scanning the gate application timing within a predetermined scan range and recording the number of photons detected at each timing;
Extracting at least two adjacent peaks in the recorded number of detected photons;
Evaluating the error rate at the extracted at least two peaks;
A main pulse at the center of a triplet photon pulse obtained by demodulating by a receiver that receives two photon pulses obtained by modulating a photon pulse generated by a transmitter through an optical fiber transmission line. A step of setting the timing of detecting a pulse with a single photon detector to the lower peak of the error rate;
And a computer-readable recording medium recording a timing adjustment program for executing the program.   The computer-readable recording medium according to claim 8, wherein the predetermined scan range is equal to or longer than a period of the photon pulse generated by the transmitter.   10. The computer-readable recording medium according to claim 8, wherein the error rate is a quantum bit error rate.

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