JP2007036513A - Photon receiver and method for receiving photon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photon receiver capable of discriminating dark noises and capable of corresponding to the change of errors on phases of the driving timing and photon reachable timing of a photon detector, and to provide a method for receiving photons. <P>SOLUTION: An APD 2 is driven by a GGM bias voltage s4, and a plurality of data sets DS are stored in a signal containing the output change of the APD 2. Frequency distributions of signal-change generating timings of the data sets DS are generated, and a time range in which a peak appears is used as photon reachable timing. An output is a photon signal from the APD 2 generating a signal change in timing within the time range, and outputs other than that are decided as dark noises. When the phase difference is changed in the driving timing of the APD 2 and the photon reachable timing, the rate of the change is computed, and the photon receiver can cope with the change by correcting the offset of the phase difference. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photon receiver, and more particularly, to a photon receiver and a photon receiving method intended for noise removal determination and clock synchronization maintenance in a system with a small number of received photons.

  In the photon receiver, an avalanche photodiode (hereinafter referred to as APD) is generally used as an element for detecting a single photon. A reverse bias voltage higher than the breakdown voltage (VBd) is applied to the APD, and the photocurrent generated from one photon is amplified to a sufficiently large signal amplitude by greatly increasing the multiplication factor of the APD. Is a basic photon reception technology.

  As is well known, an APD to which a reverse bias voltage of VBd or higher is applied does not depend on a photon signal or dark noise, but once the breakdown current flows and a multiplication current flows, the breakdown state continues. In general, therefore, a GGM (Gated Geiger Mode) system is employed in which a reverse bias voltage equal to or higher than VBd is applied to the APD in pulses at the timing when the photons reach the APD. By this GGM method, the reverse bias voltage becomes VBd or lower during the time when the photon signal does not reach clearly, and the breakdown state can be stopped.

  However, when a reverse bias of VBd or higher is applied, dark current due to thermal noise generated inside the APD element is also amplified, so that signal current is generated at a probable frequency without receiving photons, and dark noise is generated. It becomes. This dark noise can reduce the probability of occurrence by cooling the APD element to a low temperature. However, in order to accurately receive photons, a reverse bias higher than VBd that causes dark noise should not be applied unnecessarily for a long time. It is. In other words, it is necessary to apply a reverse bias equal to or higher than VBd in accordance with the timing at which the photons arrive.

  Therefore, the photon receiver has not only the problem of dark noise as described above but also the problem of timing synchronization. As described above, in the photon receiver using the GGM method, it is necessary to accurately synchronize the incident timing of the photon to the APD and the phase of the GGM pulse applied to the APD.

  However, in a quantum key distribution (QKD) system using a photon receiver, a single photon state is already required at the time of transmission from the photon transmitter in order to reduce the probability of eavesdropping. In addition, in the process of propagation through the fiber transmission path from the photon transmitter to the photon receiver, the photons are stochastically lost due to the loss of the transmission medium. For this reason, the probability of reaching the photon receiver is very low, and the mark rate is much lower than that of a general optical receiver when viewed as a signal pulse output from the APD. Assuming loss of a practical transmission medium, for example, the mark ratio of an APD output signal can be a very low value of 1/1000 to 1 / 1,000,000. In such a low mark ratio and in the presence of stochastic dark noise, it is impossible to extract and reproduce the synchronous clock from the APD output signal.

  Therefore, a method has been proposed in which a clock signal is transmitted separately from the photon signal and the receiver side synchronizes with the photon signal using the clock signal as a reference clock.

  For example, JP-A-08-505019 (Patent Document 1) discloses a method in which a quantum channel and a public channel are provided on the same optical fiber, and bit synchronization and other system timing calibration are performed using the public channel. Proposed. In the method described in Patent Document 1, the level of the output light of the laser light source is reduced by an attenuator to generate a pulse including at most one photon used in the quantum channel (page 11, line 8 to line 11, FIG. 11). 4). In other words, the clock phase calibration function inside the receiver is realized by switching between the pseudo single photon state and the multi-photon state on the transmitter side.

  In addition, the monitoring system disclosed in US Pat. No. 4,961,644 (Patent Document 2) transmits transmission data and a monitoring signal to the far-distance end by wavelength multiplexing through an optical fiber, and the phase variation of the returned monitoring signal is detected. By analyzing, the effect of wiretapping is monitored by subtracting the effects of fiber expansion and contraction due to temperature changes.

JP-T-08-505019 (see page 11, line 8 to line 11, FIG. 4) US Pat. No. 4,961,644 (see column 3, lines 38-66, FIG. 1)

  As described above, photon reception with weak power has a very low mark rate and probabilistically generates dark noise, so that it is impossible to directly extract and reproduce the synchronous clock from the output signal of the APD. . For this reason, a timing signal for maintaining synchronization as in Patent Document 1 is transmitted separately from the transmission data, or a monitoring signal is transmitted separately as in Patent Document 2, so that the environment such as the temperature expansion and contraction of the transmission line is transmitted. Techniques to deal with changes have been proposed.

  However, with respect to photon reception in a quantum channel, there are the following problems to be solved. These will be described in detail below.

(1) Noise determination In the conventional technique, it is not possible to determine whether a photon signal caused by arrival of a photon or a noise signal caused by thermal noise or the like from only the output of the photon detector APD. This is because in the conventional identification method for identifying a signal by the amplitude of an electric signal, both dark noise and a photon signal appear as an output in which the APD is broken down, so that the difference in amplitude cannot be distinguished.

  In particular, indium gallium arsenide-based APD (InGaAs-APD) is used as a photon detector because it has a sensitivity in the 1.55 μm wavelength band where the transmission loss of the fiber is minimized. Widening increases the probability of dark noise. Therefore, if dark noise cannot be determined quickly and accurately, the S / N ratio of the received photon signal is further deteriorated, and the final key generation rate is reduced in the quantum key distribution system.

(2) Detection of clock phase error The conventional technique cannot clearly detect the phase error between the arrival timing of photons and the reference clock used for synchronization of photon reception. There are various causes of the phase error. For example, if the parallel clock is wavelength-multiplexed, the change in the chromatic dispersion of the fiber due to environmental temperature changes, and if the parallel clock is transmitted through a different fiber without wavelength multiplexing, differences in temperature characteristics and local temperature differences between the fibers will cause phase errors. Cause. Another possible cause is the temperature change characteristic of the delay of the electric circuit inside the apparatus.

  One reason why such a phase error cannot be clearly detected is that a clock extraction method of a normal optical receiver circuit cannot be used. The other is that the mark rate and reception rate of the APD output signal are very low, and dark noise occurs in a phase that is unrelated to the photon signal. It is impossible to determine whether it is a decrease.

  When such a phase error occurs, the arrival timing of the photon signal to the APD does not coincide with the application timing of the reverse bias pulse to the APD, and photon reception fails. If the phase difference widens and photon reception becomes impossible, it is necessary to interrupt photon communication and execute the reference clock phase matching procedure again, and the quantum key generation rate of the quantum key distribution system decreases.

(3) Calibration of clock phase error The conventional technique cannot execute phase synchronization and photon detection simultaneously. For this reason, when the phase difference widens and photon reception becomes impossible, the transmitter side switches the photon wavelength signal to the multiphoton state, and the receiver side cancels the APD Geiger mode and sets to normal bias reception. Must. That is, photon reception is interrupted for a long time during this phase synchronization process.

  In order to switch from a single photon to a multi-photon state on the transmitter side, at present, it is common to use a mechanical variable optical attenuator, and most of them require several hundreds of milliseconds to several seconds for switching attenuation. . The time required for this switching is very long compared to the photon transmission clock frequency. For example, in a quantum communication system that operates at a clock frequency of 50 MHz, a communication time of about 100 Mbits is stopped during one phase synchronization using a mechanical variable attenuator. In the quantum key distribution system, quantum key generation stops during this period, resulting in a decrease in quantum key generation capability.

(4) Approach In order to solve the above-mentioned problem, the present inventors have a timing where a photon signal arrives within a limited time range, whereas dark noise occurs in a distribution different from that of a photon signal. Focused on that.

  For example, a photon signal can only reach a time width of a pulse width (for example, several hundreds psec) emitted by a transmission laser light source, whereas dark noise is spread over the entire time width of a GGM pulse (for example, several nsec). It is distributed across. At the same time, it can be said that the frequency of dark noise among the signals detected with the GGM pulse width is considerably less (1 / several to 1/1000) with respect to the frequency of the photon signal. Therefore, when the time width for the arrival of photons is sufficiently narrower than the time width of the GGM pulse, it is sufficiently clear that a time distribution in which the generation frequency of the output signal of the photon detector is totaled in the time width of the GGM pulse is created. There is a peak. By using the relative timing (phase difference) of this peak area with respect to the time width of the GGM pulse as the timing at which the arrival of photons can be expected, it is possible to provide a photon receiver and a photon receiving method that solve the above problems. .

  On the other hand, when the photon arrival timing changes with time, a sufficiently clear peak does not appear even if a frequency distribution is created by summing up the frequency of occurrence of the output signal of the photon detector with the time width of the GGM pulse. There is. This is because, as time changes, the generation of relatively frequent photon signals is gradually shifted and the peaks are dispersed.

  However, the main cause of such a phase change is a delay variation due to a temperature change of a fiber or an electronic component, and does not become a sudden change. The signal processing time scale discussed here can be approximated as a linear change. Similarly, a system having a highly accurate clock source without separately sending clocks between transmission and reception, and even if the frequency difference between the clock sources is very small, it may be approximated as a linear change. . For example, if the amount of change in the phase difference for each cycle of the clock signals from the two clock sources can be regarded as being very small relative to the GGM pulse width of the photodetector, it can be approximated to a linear change.

  Therefore, when the peaks of the frequency distribution are dispersed, the phase difference is assumed to change linearly at a certain rate with respect to time, and the rate of change is obtained. Then, by determining whether or not the generation of the output signal of the photon detector matches the timing according to the obtained change rate, it is possible to identify whether or not it is a photon signal.

  In addition, by gradually shifting the timing of the driving pulse applied to the photon detector in the direction in which the change in the timing phase is canceled based on the obtained change rate, the arrival of photons with respect to the time width of the driving pulse The timing phase difference is maintained at a constant value, and the drive pulse can be made to follow the arrival timing of the photon.

  In summary, the inventors have focused on the following two characteristics relating to photon detection.

  The first characteristic is that the occurrence frequency of dark noise is relatively low compared to the frequency of receiving photon signals. This is because the probability of occurrence of dark noise can be controlled by lowering the temperature of the APD element. For example, in a practical quantum cryptography communication system, the frequency of outputting a photon reception signal by APD is 10%, but when the APD is cooled, the occurrence rate of dark noise in the received signal is 1% or less. It is possible to control to a sufficiently low probability even if not completely.

  The second characteristic is that the phase difference error does not increase rapidly. This is a fluctuation that is sufficiently slow compared to the frequency of the photon signal because the fluctuation of the phase difference can be attributed to frequency drift due to the temperature characteristics of the control circuit and the transmission line as a whole. Therefore, if the change in the arrival timing of the photon signal can be observed, the change in the phase difference can be tracked.

  Therefore, as described below, according to the present invention, it is possible to identify dark noise that could not be identified in the past, and the photon detector drive timing and photon detection that were difficult to detect in the past. It is possible to detect a change in phase error with respect to the photon arrival timing to the detector, or to make the drive timing of the photon detector follow the photon arrival timing.

  The photon receiver according to the present invention includes a driving unit that intermittently supplies a driving signal having a predetermined time length for driving the photon detecting unit to the photon detecting unit at a predetermined timing, and a signal including a change in output of the photon detecting unit. Storage means for storing a plurality of data series in units of a predetermined time length of the drive signal, and a photon arrival timing at the predetermined time length based on a time distribution of signal change occurrence timings of the stored data series A search means for searching for a time range indicating, and a determination means for determining, as a photon detection signal, an output of the photon detection means in which a signal change occurs at a timing within the searched time range. To do.

  According to the first embodiment of the present invention, the search means includes a distribution generation means for generating a time distribution of signal change occurrence timings of the accumulated data series, and a predetermined range including a peak of the time distribution. Peak detection means for identifying the time range.

  According to a second embodiment of the present invention, the search means obtains a temporal change pattern that matches a photon arrival timing based on a time distribution of signal change occurrence timings of the accumulated data series, and the temporal change The temporal range is specified from the pattern.

  More specifically, pattern generation means for generating a temporary temporal change pattern with respect to the signal change occurrence timing of the plurality of accumulated data series, while sequentially changing the temporary temporal change pattern within a predetermined range, Offset processing means for determining whether or not the temporary temporal change pattern matches the photon arrival timing, and the temporary temporal change pattern that best matches the photon arrival timing is changed over time. Select as a pattern.

  As an example of the second embodiment of the present invention, the offset processing means shifts the signal change occurrence timing of each of the accumulated data series in a direction to cancel the temporal change of the temporary temporal change pattern. Timing shift means for generating, a distribution generation means for generating a time distribution of corrected signal change occurrence timing of the plurality of accumulated data series, and a peak detection means for detecting a peak size of the time distribution. The temporary temporal change pattern having the largest peak of the time distribution is selected as the temporal change pattern while sequentially changing the temporary temporal change pattern within a predetermined range.

  According to the third embodiment of the present invention, the photon detection is performed so that the signal change generation timing of each of the accumulated data series is relatively shifted in a direction to cancel the change with time of the change pattern with time. The apparatus further includes offset correction means for causing the drive timing of the photon detecting means to follow the photon arrival timing by shifting the supply timing of the drive signal for driving the means.

  The photon receiver and the photon receiving method according to the present invention are such that a photon arrives from a time distribution obtained by aggregating the phase difference between the change generation timing of the output waveform of the photon detector and the application timing of the drive pulse applied to the photon detector. Identify the timing region. Therefore, changes in the output signal outside the photon arrival timing region can be removed as dark noise, and the SN ratio of the received photon signal can be improved. Moreover, according to the present invention, the photon arrival timing can be identified based on the output of the photon detector without using the reference clock from the photon transmitter.

  Even if the difference in relative time between the application timing of the drive pulse applied to the photon detector and the photon signal arrival timing (specifically, the phase difference between the reference clock and the photon signal) is changing. This can be dealt with by correcting the output change occurrence timing in the past data series stored in the storage means by time offset correction.

  For example, if the drive pulse application timing and photon signal arrival timing are unchanged, the recorded time difference is constant, but if the time difference changes due to changes in the physical properties of the transmission line or changes in the device, the photon signal This means that the arrival timing of GGM is moving from the optimum point of GGM pulse application timing, and if left as it is, photons will reach the APD outside the time range of the GGM pulse, and photon reception will fail. Yes. This phase drift indicates that both the arrival timing of photons and the GGM application timing may fluctuate. However, in order to receive photons, both timings need to match, and one of them is absolute. Need not be fixed. Therefore, it is desirable to follow the phase drift by adjusting the delay amount on the reference clock side, which is easy to control, that is, the GGM timing.

  Therefore, with reference to the photon arrival timing, a variable delay means is inserted into a reference clock obtained from an oscillator included in the receiver or a separately sent clock, and the difference between the GGM pulse application timing and the photon arrival timing is The amount of delay is varied so that the timing of photon arrival and reverse bias pulse application coincide with each other at the optimum phase.

  As described above, according to the present invention, it is possible to improve the SN ratio of the received signal. For example, by searching for the peak of the distribution of signal arrival times in the received waveform and performing time offset processing that maximizes the peak contrast, signals detected outside the time domain where photon arrival can be expected are identified as dark noise. Because it becomes possible.

  In addition, according to the present invention, it is possible to track the application timing of the gate pulse to the arrival timing of the photon. For example, it is possible to correct the change in the phase difference by adjusting the delay of the reference clock based on the ratio of the time offset obtained by the processing described in the above effect.

1. 1. First Embodiment 1.1) Outline of Circuit Configuration FIG. 1 is a block diagram showing a schematic configuration of a photon receiver according to a first embodiment of the present invention. In addition to the GGM power supply circuit 1, the photodetector (APD) 2, and the current-voltage conversion circuit 3, a sampling circuit 4, a signal processing circuit 6, a reference clock generation circuit 70, and a timing generation circuit 71 are provided.

  The GGM power supply circuit 1 generates a GGM bias voltage s4 and applies it to the APD 2. This GGM bias voltage is composed of a DC component equal to or lower than the breakdown voltage VBd of the APD and a reverse bias pulse component equal to or higher than VBd. The reverse bias pulse component is generated in synchronization with the GGM timing signal s20 given from the timing generation circuit 71.

  When a photon is incident on the light receiving surface of the APD 2 at the timing when the reverse bias pulse component is applied to the APD 2, it is multiplied by the avalanche (electron avalanche) effect caused by the generated photoelectrons, and is output as a current s5. However, as described above, the current s5 is output as dark noise even when no photon is incident.

  The output current s5 is converted into a voltage signal (hereinafter referred to as an output voltage signal s51) by the current-voltage conversion circuit 3 and output to the sampling circuit 4. Here, a transimpedance amplifier (hereinafter referred to as TIA) is used as the current-voltage conversion circuit 3.

  The sampling circuit 4 samples the output voltage signal s51 with a sampling granularity of two or more levels in the amplitude direction according to the reference clock s1, and the discrete output voltage signal s6 (hereinafter referred to as sampling data s6) is a signal processing circuit. 6 is output. The sampling circuit 4 may be a sampling that is identified by binary values, or may be a multi-value sampling that uses analog-digital (AD) conversion. Here, an AD converter (ADC) is used.

  As will be described later, the signal processing circuit 6 performs frequency distribution creation processing, photon reception signal and dark noise signal identification processing, and clock drift processing for the sampling data s6 in accordance with the reference clock s1 and the GGM timing signal s20. Tracking processing is performed, and a photon reception signal s9 and a photon reception timing signal s91 are output.

  The reference clock s1 that the sampling circuit 4 and the signal processing circuit 6 follow is generated by a reference clock generation circuit 70. The reference clock s1 is a high-speed clock signal having sufficient time accuracy compared to the GGM timing signal s20 and the output voltage signal s51, and is generated based on a clock signal sent separately from a photon transmitter (not shown), or It is generated on the basis of an oscillation signal of a highly accurate oscillator arranged locally. As will be described later, it is assumed that the photon signal arrives at the receiver according to a fairly precise period, and that the clock source is also equivalent to the expected time width of the photon in which the jitter amount reaches and sufficiently smaller than the pulse width of the GGM pulse. It is.

  The timing generation circuit 71 generates the GGM timing signal s20 by dividing the reference clock s1 by one or more. The timing of the GGM bias voltage applied to the APD 2 is determined by the GGM timing signal s20. The GGM timing needs to be the same as the period of arrival of photons. For example, when a 1 GHz reference clock s1 is generated by multiplying a clock sent separately from the transmitter and is divided by 20 to generate a 50 MHz GGM timing signal s20, a similar process is performed on the transmitter side. Then, a photon signal is generated and transmitted based on a 50 MHz clock.

1.2) Signal Processing Circuit FIG. 2A is a block diagram illustrating the functional configuration of the signal processing circuit 6 in the photon receiver according to the first embodiment in more detail, and FIG. 2B is a block diagram of a filter. The signal processing circuit 6 has a functional configuration including a clock control unit 72, an arithmetic circuit group 60, and a memory 65. The clock control unit 72 receives the reference clock s 1 and the GGM timing signal s 20 and generates an internal clock that controls the overall processing operation of the signal processing circuit 6.

  The arithmetic circuit group 60 inputs the sampling data s 6 from the sampling circuit 4 through the filter 61. The filter 61 has a signal selection function, and whether or not the input sampling data s6 is no signal (whether there is a rising edge) in a periodic period (GGM bias application period) defined by the GGM timing signal s20. And only the sampling data s6 having a rising edge (not a no-signal) is allowed to pass.

  An example of the configuration of the filter 61 is shown in FIG. The filter 61 includes a rising detector 61.1 and a gate 61.2. The rising detector 61.1 determines whether or not the sampling data s6 is a no signal, and opens the gate 61.2 when the sampling data is not a signal. The sampling data s6 is passed as data s7. In photon communication such as a quantum cryptography system, since the number of photons reaching the APD 2 of the photon receiver is very small, the signal detection frequency is correspondingly reduced. For this reason, the frequency of signal detection is very low compared to the repetition of the GGM timing signal s4, and most of the sampling data s6 is expected to have no signal waveform. Therefore, in order to perform efficient signal processing, only the sampling data s6 in which the rise of the waveform exists is selected by the filter 61 and introduced into the arithmetic circuit group 60 as the signal sampling data s7. Note that the rising edge detection signal of the rising edge detector 61.1 can also be used to specify signal generation timing.

The arithmetic circuit group 60 includes a frequency distribution generation unit 62, a peak detection unit 63, and a comparison determination unit 64. The signal sampled data s7 that has passed through the filter 61 is output to the comparison / determination unit 64 and stored in the memory 65. A predetermined number of signal sampling data s7 at the past GGM timing is stored in the memory 65, and the frequency distribution generation unit 62 generates a rising time frequency distribution using the stored data. The peak detector 63 detects a time region (peak range) T _PK including the distribution peak from the frequency distribution.

As will be described later, the comparison determination unit 64 compares the rising timing of the input signaled sampling data s7 with the peak range T _PK detected by the peak detection unit 63, and determines whether the input signal s6 is a photon signal or dark noise. Determine. If it is a photon signal, a photon reception signal s9 and a photon reception clock s91 synchronized therewith are output to the outside of the photon receiver.

  The arithmetic circuit group 60 can be configured by an electronic circuit for individual functions, but an equivalent function can also be realized by an arithmetic circuit such as a microprocessor and software operating on the arithmetic circuit. is there.

1.3) Reception Operation FIG. 3 is a time chart schematically showing main signal waveforms in the circuit shown in FIG. Based on the reference clock s1 generated by the reference clock generator 70, the timing generation circuit 71 generates a GGM timing signal s20, and the power supply circuit 1 applies the GGM bias signal s4 to the APD 2 accordingly. The GGM bias signal s4 is slightly delayed (corresponding to one clock here) from the GGM timing signal s20 due to a circuit delay inside the power supply circuit 1.

  The waveform of the APD output voltage signal s51 shown in FIG. 3 indicates a case where the photon signal reaches the APD2 at the timing of the arrow while the GGM bias voltage s4 is applied to the APD2. As described above, dark noise may actually occur at the timing of the arrow and cannot be determined at this stage, but here it is due to the arrival of photons from the photon transmitter.

  As the photons arrive, the AGM 2 biased with GGM breaks down, whereby the output voltage signal s51 rises at the timing of the arrow. The output voltage signal s51 continues with the rising amplitude and falls with the stop of the GGM bias voltage s4. A signal obtained by sampling the output voltage signal s51 by the sampling circuit 4 is the sampling data s6. In FIG. 3, the output voltage signal s51 is converted into a discrete data series represented by black circles by sampling, and a delay due to sampling appears.

  The signal processing circuit 6 performs photon reception determination to be described later on the sampling data s6, and after delaying by a corresponding processing time (a delay of 7 clocks in FIG. 3), the photon reception signal s9 and a photon synchronized therewith A reception clock s91 is output. As described above, since it is assumed here that the photon has reached the APD 2 at the timing of the arrow, the signal processing circuit 6 determines that the photon has been received, and raises the photon reception signal s9 and the photon reception clock s91. The rising phase relationship between the photon reception signal s9 and the photon reception clock s91 as shown in FIG. 3 is defined by system requirements other than the photon receiver.

1.4) Determination of Photon Reception FIG. 4 is a time chart showing a photon reception method according to the first embodiment of the present invention. 4A shows a periodic voltage application period AD of the GGM bias voltage s4 applied to the APD 2, and FIG. 4B shows a sampling clock according to the reference clock s1. The pulse width of the GGM bias voltage s4 (A-D on the sampling clock) for a period of 4 clocks (here, between B and C on the sampling clock) where the incidence of the photon signal on the APD2 will be detected. (Between) is wide enough to obtain a meaningful frequency distribution.

  FIG. 4C schematically shows the waveforms of the m groups of the signal sampling data s7 accumulated in the memory 65 in time series. If the signal sampling data s7 of the j-th group is expressed as a data set DS (j) (0 ≦ j ≦ m−1), the data set DS (j) indicates that the signal sampling data s7 is a period of the GGM bias voltage s4. It is the sampling data sequence grouped by typical voltage application period AD. Specifically, the data set DS (0) rises in the period BC, and the data set DS (2) rises in the period AB. Such m past data sets DS (0) to DS (m−1) are stored in the memory 65 in time series.

(Frequency distribution)
FIG. 4D is a graph showing the frequency distribution generated by the frequency distribution generator 62. The frequency distribution generation unit 62 reads the m data sets DS (0) to DS (m−1) stored in the memory 65, and the timing at which the rising edges of the signal waveforms appear, in other words, the GGM bias voltage s4. A time distribution (phase difference) from the rising timing A is totaled to generate a frequency distribution which is a statistical pattern represented by the graph of FIG. The horizontal axis of the graph in FIG. 4D is relative time, and the vertical axis is the number of times.

  As described above, since the photon transmitter transmits photons according to a predetermined timing (GGM timing s20), the arrival timing of the photons in the photon receiver is concentrated in a limited range of the AD period, and the frequency distribution. To make up the peak. On the other hand, since dark noise is distributed at random, the frequency distribution spreads over the entire A-D period and does not constitute a peak. Therefore, when a clear peak appears in the frequency distribution as shown in FIG. 4D, it can be determined that the time region including the peak (here, the BC period) is the arrival timing of the photon. Therefore, it is possible to determine that a signal rises at a timing within the peak range and that it is a photon reception signal, and a signal that rises at other timings is dark noise.

(Peak detection)
The peak detector 63 detects the peak range T _PK indicating the time range of the photon arrival timing from the frequency distribution data as shown in FIG. Statistical processing is used to detect the peak range T _PK . For example, the maximum value of the frequency distribution may be detected, and the full width of the peak distribution or the standard deviation of the peak may be σ, and the range of 2σ (covering about 95%) may be the peak range T _PK .

(Comparison judgment)
As described above, in the state where the data sets DS (0) to DS (m−1) of the past m times are accumulated in the memory 65, the latest signaled sampling data s7 is output from the filter 61, and FIG. ) As a data set DS (m) as shown in FIG.

FIG. 5 is a flowchart showing the photon reception determination operation of the signal processing circuit 6 according to the present embodiment. First, when the data set DS (m) is acquired (step S101), the frequency distribution generation unit 62 uses the preset citation number m to store the past m times of the data sets DS (0) to DS (m−1). Called from the memory 65 (step S102), a frequency distribution is created as described above (step S103). Subsequently, the peak detection unit 63 detects a peak range T _PK indicating a time range of photon arrival timing from the frequency distribution (step S104).

Given the peak range T _PK , the comparison / determination unit 64 compares the signal rise time of the data set DS (m) with the peak range T _PK (step S105). If the data set DS (m) is the waveform shown in FIG. 4E, the rising timing is within the peak range T _PK , so that the latest signaled sampling data s7 is a photon signal (photon detection). Determination is made (step S106). If it is determined that the photon is detected, the comparison / determination unit 64 starts up the photon reception signal s9 and the photon reception clock s91 (step S107).

If the data set DS (m) rises outside the peak range T _PK as in DS (2), for example, the comparison / determination unit 64 determines that it is dark noise (step S108), and receives the photon reception signal s9 and the photon reception. The clock s91 is not raised (step S109).

  When step S107 or S109 ends, DS (j [1: m−1]) is shifted by one cycle, and the latest DS (m) is stored in DS (m−1) (step S110). In other words, the new data set DS (m) is stored in the memory 65. At this time, the oldest DS (0) is deleted, the next old DS (1) becomes DS (0), and so on. Thus, the latest data set DS (m) is stored in the memory 65 as DS (m−1).

  The above processing is sequentially performed according to the repetition period of the GGM timing signal s20. Alternatively, when the capacity of the memory 65 is sufficiently large, it is possible to perform the same processing while storing a large amount of sampling data.

1.5) Modified Example In the process of the photon receiver, DS (0) to DS (m−1) are read out for each period of the GGM timing s20, and the frequency distribution is created, but this embodiment is limited to this. It is not a thing. For example, the created frequency distribution is stored in another memory, and the latest DS (m) is reflected in this frequency distribution, thereby further speeding up the signal processing.

  Furthermore, by using this method, even when the capacity of the memory 65 is limited, it is possible to create a frequency distribution in a range (number of times) exceeding the storage capacity, and an improvement in the accuracy of the frequency distribution can be expected. .

  Alternatively, for the purpose of alleviating the influence of the drift of the relative phase of the GGM timing s20 and the arrival of the photon, the latest DS (m) can be reflected after weighting processing is performed on the previously created frequency distribution. By the weighting process, it is possible to determine the arrival of photons in which the latest data is more important than the past data, and it is possible to compress the frequency distribution error even when the phase drifts.

1.6) Effect As described above, according to the present embodiment, the phase difference between the rising timing of the output waveform of the APD and the GGM bias application timing of the APD is recorded, and the recording of the phase difference is obtained. The photon arrival timing region (peak range T _PK ) is detected from the frequency distribution. Therefore, without using a clock signal sent separately from the transmission side, it is possible to determine that the APD output is a photon signal if it is in the peak range T _PK , and that it is dark noise if the timing is other than that, thus eliminating dark noise. Thus, the S / N ratio of the received photon signal can be improved.

  Therefore, even if dark noise increases by widening the GGM pulse width, deterioration of the SN ratio can be avoided, so that the phase tolerance of photon reception by GGM can be reduced. When the phase tolerance is relaxed, it is not necessary to reach the photon and precise phase adjustment of the GGM pulse, so that the operation is stable and the design is facilitated.

2. Second Embodiment In the first embodiment described above, the case where the photon arrival timing range (peak range T _PK ) does not change as shown in FIG. 4 has been described. However, in actuality, the photon is affected by the temperature change of the optical fiber. The arrival timing of can vary. When the photon arrival timing changes with time, it is sufficient to read out DS (0) to DS (m−1) for each period of the GGM timing s20 with the time width of the GGM timing s20 and create a frequency distribution. A clear peak may not appear. This is because, as time changes, the generation of relatively frequent photon signals is gradually shifted and the peaks are dispersed.

  The main cause of such a phase change is a delay variation due to a temperature change of the fiber or electronic component, and does not become a sudden change. From the time scale of signal processing, it can be approximated as a linear change. In addition, even in a system with a highly accurate clock source without separately sending clocks between transmission and reception, if the frequency difference between the clock sources is very small, it should be similarly approximated as a linear change. Can do. For example, if the amount of change in the phase difference for each cycle of the clock signals from the two clock sources can be considered to be very small relative to the AGM GGM pulse width, it can be approximated to a linear change.

  Therefore, when the peaks of the frequency distribution are dispersed, when the top of the peak is flat, when the peak distribution width is wider than the predetermined value, meaningful timing detection cannot be performed, or the frequency distribution If the peak-to-background ratio (contrast ratio) is smaller than a predetermined threshold, the phase difference is assumed to change linearly at a certain rate with respect to time, and the change Find the percentage. If the rising rate of the output signal of the APD is corrected using the rate of change as the correction coefficient α, it is possible to identify whether the signal is a photon signal as in the first embodiment. According to the second embodiment of the present invention, the offset process using the correction coefficient α corresponds to the temporal change of the photon arrival timing.

2.1) Outline of Circuit Configuration FIG. 6 is a block diagram illustrating in more detail the functional configuration of the signal processing circuit 6 in the photon receiver according to the second embodiment of the present invention. The arithmetic circuit group 60 of the signal processing circuit 6 in this embodiment has a configuration in which an offset processing unit 66 and a counter 67 are added to the configuration shown in FIG. The other blocks have the same functions as the blocks shown in FIG. 1 and FIG.

  Similarly to the first embodiment, m DS (0) to DS (m−1) are stored in the memory 65. At this time, the GGM timing signal s20 corresponding to each data set DS (j) is stored. Are counted by the counter 67 and stored in the memory 65. As described above, since the no-signal sampling data s6 is not accumulated in the memory 65 by the filter 61, each data set DS (j) and the actual time can be stored by associating the count value corresponding to that time. Corresponding to the course. The counter 67 is assumed to have a count capacity that can sufficiently cover the actual time passage of m DS (0) to DS (m−1).

As described in the first embodiment, the frequency distribution generation unit 62 of the arithmetic circuit group 60 generates a frequency distribution from the data sets DS (0) to DS (m−1) stored in the memory 65, and performs peak detection. The unit 63 tries to detect a time region (peak range) T _PK including a distribution peak from the frequency distribution. At this time, if the peak detection unit 63 cannot detect a peak from the frequency distribution, the fact is notified to the offset processing unit 66.

  Upon receiving the notification that the peak cannot be detected, the offset processing unit 66 reads m DS (0) to DS (m−1) and the count value indicating the passage of time from the memory 65, and the rising edge of the data set as will be described later. A correction coefficient α that best matches the change in timing with time is obtained by linear approximation. Then, the offset processing unit 66 determines whether the rising edge is a photon reception or dark noise depending on whether the rising timing of the new data set DS (m) matches the change defined by the correction coefficient α. Hereinafter, a specific example of the offset process will be described in detail.

2.2) Offset processing (Example 1)
FIG. 7 is a block diagram showing an internal functional configuration of the offset processing unit, and FIG. 8 is a time chart showing an example of the rising timing of the data set DS that is not subjected to the offset processing.

  7, the offset processing unit 66 includes a program control processor 66.1 that controls the offset processing, and a memory 66.2 that stores a data set group illustrated in FIG. 8, and further includes a pattern detection unit 66.3 and An inclination α determining unit 66.4 is functionally included. The pattern detection unit 66.3 and the inclination α determination unit 66.4 can also be realized by executing a program on the processor 66.1.

  As shown in FIG. 8, the processor 66.1 records the time T (i) of signal generation timing in the memory 66.2 as a clock value for the DS (i) acquired periodically. When plotting in a space composed of the sequence of equally spaced periods (data set number) i and the signal generation time T (i) in each period, for example, a distribution as shown in FIG. 8 is obtained. The generation of the signal exists in the range of A to D of the clock corresponding to the width of the GGM pulse. In the example of FIG. 8, the period in which no signal is generated is i = 2, 5-7.

  As described above, the drifted signal generation timing is distributed obliquely with the inclination α, and the noise generation is randomly distributed around. Therefore, if the plotted data (i, T (i)) can be approximated by a linear function by any method, the signal generation can be distinguished from the noise generation using the slope α.

  As a method of performing the linear function approximation, a least square method can be used, but the method is not limited thereto. As shown in Fig. 8, the plotted data (i, T (i)) can be regarded as two-dimensionally distributed discrete signals. It is also possible to use an image processing technique. For example, a two-dimensional Fourier transform can be used to extract a sequence of signals inclined to α from noise distributed over the entire width A to D. Further, if a discrete signal distributed two-dimensionally is used as an image, a Hough Transform that extracts a straight line from the image can be used.

  FIG. 9 is a flowchart showing an example of the photon reception determination operation of the signal processing circuit 6 according to the present embodiment. FIG. 9 is the same as the step in FIG. 5 except that a pattern detection step S201 and an α determination step S202 by linear function approximation are arranged instead of step S103 in FIG.

  First, the processor 66.1 stores the signal generation timing time T (i) in the memory 66.2 as a clock value for the periodically acquired DS (i) (step S102). Subsequently, the signal pattern detection unit 66.3 approximates the distribution data stored in the memory 66.2 with a linear function using a method such as a least square method or a two-dimensional Fourier transform (step S201). Then, the inclination α determining unit 66.4 determines the inclination α from the obtained linear function (step S202).

When the inclination α is determined in this way, the peak detector 63 detects the peak range T _PK from the frequency distribution after the offset process (step S104). When the peak range T _PK is given, the comparison / determination unit 64 compares the signal rise time of the offset-processed data set DS (α, m) with the peak range T _PK (step S105). Hereinafter, as described with reference to FIG. 5, if the rising timing of the data set DS (α, m) is within the peak range T _PK , the latest signaled sampling data s7 is determined to be a photon signal (photon detection). (Step S106) If it has risen outside the peak range T _PK , it is determined that it is dark noise (Step S108).

2.3) Offset processing (example 2)
The determination of the slope α of the linear function corresponding to the above-described drift can be performed while monitoring the frequency distribution.

  FIG. 10A is a time chart showing an example of the rising timing of the data set DS that has not been subjected to the offset processing, and FIG. 10B is a data set DS (α, j) that has been offset processed by photon reception according to the present embodiment. It is a time chart which shows the rise timing. The explanation of waveforms a) to e) in FIGS. 10A and 10B is the same as that in FIG. 4 except for timing drift due to offset processing and its correction, but each waveform c) is shown in FIG. Similarly to FIG. 8, it is assumed that the time T (i) of signal generation timing is stored in the memory as a clock value for DS (i) acquired periodically.

  When the frequency distribution is generated from the data sets DS (0) to DS (m−1) that are not subjected to the offset processing shown in the waveform c) of FIG. 10A, the frequency distribution is clear as shown in the waveform d) of FIG. Therefore, the peak detection unit 63 cannot detect the arrival timing of the photons. In this case, the offset processing unit 66 searches for an oblique straight line in which the rising timings of the data sets DS (0) to DS (m−1) stored in the memory 65 are linear to some extent. Although the details of the search method will be described later, a method of finding an inclination (proportional constant) α that maximizes the peak value of the peak detection unit 63 while changing the inclination of the straight line is used.

  In the example shown in FIG. 10A, if there is a phase drift of a speed approximated on a straight line with a slope α with respect to the time passage from DS (0) to DS (m−1). By offset processing the rising timing of each data set with a constant α proportional to time, the peak detector 63 is the largest in a specific timing range (here, BC period) as shown in FIG. Contrast should be obtained.

As shown in FIG. 10B, by selecting the optimum correction coefficient α and applying the same offset process to all data sets, a clear peak is found in the frequency distribution between B and C of the sampling clock. Appear. The peak range is output to the comparison determination unit 64 as T _PK . The new data set DS (m) is converted into the GGM timing signal s20 by comparing the DS (α, m) obtained by performing the same offset processing on the new data set DS (m) with the peak range T _PK. Although it is a very small pulse that appears just before the end of, it can be identified as a photon signal.

  FIG. 11A is a flowchart showing the photon reception determination operation of the signal processing circuit 6 according to this embodiment, and FIG. 11B is a flowchart showing the optimum frequency distribution generation operation as its subprocedure. FIG. 11A is the same as the step in FIG. 5 except that an optimum frequency distribution generation step S300 is arranged instead of step S103 in FIG. 5, and hence the optimum frequency distribution generation step in FIG. S300 will be described.

  In FIG. 11B, the offset processing unit 66 uses the variable a to determine the range of the offset coefficient as the range a1 to a2 that will include the optimum value α, and sequentially changes the variable a with accuracy d. The following steps are executed (step S301).

  First, all data sets DS (0) to DS (m−1) are subjected to offset processing determined by a variable a to generate DS (a, 0) to DS (a, m−1) ( Step S302). The frequency distribution generation unit 62 creates a frequency distribution from DS (a, 0) to DS (a, m-1) (step S303). From this frequency distribution, the peak detector 63 calculates and stores the peak position and its peak contrast (step S304). Here, the peak contrast refers to the frequency ratio between the peak and the background. When the peak position and the peak contrast are obtained for the value of a certain variable a, the variable a is changed by d, and steps S302 to S304 are executed in the same manner.

  When the peak positions and peak contrasts are obtained for all the variables a1 to a2 in this way (end of step S301), the offset processing unit 66 offsets the value of the variable a that maximizes the peak contrast of the frequency distribution. The coefficient α is determined (step S305). The offset coefficient α thus obtained corresponds to the optimum inclination α in FIG. 8 or FIG.

  Subsequently, the frequency distribution generation unit 62 creates a frequency distribution from DS (α, 0) to DS (α, m−1) offset using the offset coefficient α (step S306), and returns to the main procedure. Thereafter, steps S104 to S110 described above are executed.

  In this way, the contrast of the frequency distribution is calculated while the variable a is a proportional constant of the offset and is changed with sufficient accuracy d within the finite range a1 to a2. When the relative phase between the APD arrival of the photon signal and the GGM pulse is fixed as in the first embodiment, a clear peak appears in the frequency distribution without changing the variable a, and the photon arrival The phase is revealed. On the other hand, when the relative phase drifts, the maximum peak contrast appears in the frequency distribution when the variable a is set to a certain value. In this way, the peak contrast of the frequency distribution is calculated while changing the variable a in a certain finite range, and the value of the variable a giving the maximum peak contrast is determined as the optimum correction coefficient α. The latest data set DS (m) is offset using this α, and the same photon reception determination as in the first embodiment is performed on DS (α, m).

3. Third Embodiment A photon receiver according to a third embodiment of the present invention has a configuration in which the GGM timing signal s20 follows the phase drift by using the correction coefficient α obtained in the second embodiment.

  FIG. 12 is a block diagram illustrating in more detail the functional configuration of the signal processing circuit 6 in the photon receiver according to the third embodiment of the present invention. FIG. 6 shows that the drift correction circuit 71a is connected to the timing generation circuit 71 and the drift-corrected GGM timing signal s20α is supplied to the power supply circuit 1 and the signal processing circuit 6 in the photon receiver in this embodiment. It is different from the circuit. Since other configurations and operations are the same as those of the second embodiment shown in FIG. 6, those having the same functions are denoted by the same reference numerals and description thereof is omitted.

  The offset processing unit 66 calculates a correction coefficient α for correcting the relative phase drift between the arrival of the photon and the GGM timing and uses it for photon reception determination. However, the photon receiver according to the present embodiment is provided with the drift correction circuit 71a. And a feedback function for correcting the GGM timing so that α, which is the proportionality constant, becomes zero.

  The drift correction circuit 71 a receives the GGM timing signal s 20 from the timing generation circuit 71 and the correction coefficient α from the offset processing unit 66. The drift correction circuit 71a corrects the GGM timing signal s20 so that the correction coefficient α becomes zero. For example, when a correction coefficient α as shown in FIG. 8 is searched, the phase of the GGM timing signal s20 is changed so that α becomes zero. As a result, a GGM bias voltage synchronized with the drift-corrected GGM timing signal s20α is applied to the APD 2, and as a result, as shown in FIG. 10B, the signal is output in a certain timing region (B-C period) of the GGM timing signal. Rising edge is detected.

  By using this drift correction method, it is possible to follow the phase of the GGM timing with respect to the photon arrival timing without using a complicated method such as extracting a clock signal from the photon or performing another clock transmission. It becomes.

  Specifically, the drift correction circuit 71a can be realized by using a variable delay circuit. Alternatively, a phase lock loop PLL or a delay lock loop DLL can be used. It is also possible to obtain the same effect by inserting the same drift correction circuit 71a immediately after the reference clock generation circuit 70 and correcting the drift of the reference clock s1.

4). Application example (1)
The photon receiver according to the present invention can be applied to a receiving-side photon detection circuit of a quantum key distribution system. In particular, the Plug & Play quantum encryption key distribution system can compensate for polarization fluctuations in an optical fiber transmission line, and thus is expected as a method for putting a polarization sensitive quantum encryption key distribution system into practical use.

  FIG. 13 is a block diagram showing a schematic configuration of a transmitter of the Plug & Play quantum key distribution system, and FIG. 14 is a receiver of the Plug & Play quantum encryption key distribution system to which the photon receiver according to the third embodiment of the present invention is applied. It is a block diagram which shows schematic structure of these.

  As shown in FIG. 14, the receiver 50 is provided with a clock source 51, and the laser light source 52 generates an optical pulse P according to the reference clock signal. The reference clock signal is supplied to the timing generation circuit 71. However, it is assumed that the timing generation circuit 71 includes a drift correction circuit 71a. The timing generation circuit 71 generates the GGM timing signal s20α according to the correction coefficient α from the signal processing circuit 6, and as described above, the GGM bias voltage is applied to the two APD0 and APD1 according to the GGM timing signal s20α. Therefore, the generation timing of the optical pulse P is synchronized with the reference clock signal, and the GGM timings of APD0 and APD1 are subordinate to the reference clock signal and compensate for delay variation caused by the transmission path 24.

  First, in FIG. 14, an optical pulse P emitted from a laser light source 52 is divided into two by an optical coupler CPL through an optical circulator CIR, and one optical pulse P1 passes through a short path as it is to a polarization beam splitter PBS. The other optical pulse P2 is guided to the polarization beam splitter PBS through a long path having a phase modulator 54, and is transmitted to the transmission line 24 as a double pulse. In addition, the reference clock signal is sent to the transmitter 10 through another channel, and the transmitter 10 and the receiver 50 operate in synchronization by being returned by being returned by the transmitter 10.

In FIG. 13, the transmitter 10 is provided with a Faraday mirror 21 and a phase modulator 22, and the received optical pulses P 1 and P 2 are reflected by the Faraday mirror 21, so that the polarization state is rotated by 90 degrees and passes through the variable optical attenuator 13. Returned to the receiver 50. At that time, the phase modulator 22 phase-modulates the optical pulse P2 at a timing when the optical pulse P2 passes, and the phase-modulated optical pulse P2 ^{* a} is returned to the receiver 50.

Since the polarization state of the optical pulses P1 and P2 ^{* a} received from the transmitter 10 is rotated by 90 degrees, the polarization beam splitter PBS of the receiver 50 guides these received pulses to paths different from those at the time of transmission. That is, the received optical pulse P1 passes through a long path and is phase-modulated at a timing when it passes through the phase modulator 54, and the phase-modulated optical pulse P1 ^{* b} reaches the optical coupler CPL. On the other hand, the optical pulse P2 ^{* a} phase-modulated by the transmitter 10 similarly reaches the optical coupler CPL through a short path different from that at the time of transmission. Therefore, the optical pulse P2 ^{* a} phase-modulated by the transmitter 10 interferes with the optical pulse P1 ^{* b} phase-modulated by the receiver 50, and the result is detected by the photon detector APD0 or APD1.

  When the photon receiver according to the third embodiment of the present invention is applied to the Plug & Play receiver 50 having such a configuration, the arrival timing of the photons changes due to the temperature change of the optical fiber transmission line 24 or the like. Even so, the GGM bias voltage can be applied to the APD0 and APD1 in accordance with the actual photon arrival timing, and the encryption key generation efficiency can be greatly improved.

5. Application example (2)
FIG. 15 is a block diagram showing a schematic configuration of a transmitter of a unidirectional transmission quantum encryption key distribution system, and FIG. 16 is a unidirectional transmission quantum encryption to which a photon receiver according to a third embodiment of the present invention is applied. It is a block diagram which shows the schematic structure of the receiver of a key distribution system.

  As shown in FIG. 15, the transmitter 10 is provided with a clock source 11, an optical pulse train is emitted from the laser light source LD in synchronization with the reference clock signal, and phase-modulated by a phase modulator 22 provided in a short path. The optical pulse passing through the long path is sent to the optical fiber transmission line 24 through the variable optical attenuator 13 in tandem with the optical pulse. The reference clock signal is sent to the receiver 50 through another channel.

  As shown in FIG. 16, the receiver 50 supplies the reference clock signal received from the transmitter 10 to the timing generation circuit 71. However, it is assumed that the timing generation circuit 71 includes a drift correction circuit 71a. The timing generation circuit 71 generates the GGM timing signal s20α according to the correction coefficient α from the signal processing circuit 6, and as described above, the GGM bias voltage is applied to the two APD0 and APD1 according to the GGM timing signal s20α. Therefore, the generation timing of the optical pulse P is synchronized with the reference clock signal in the transmitter 10, and the GGM timing of APD0 and APD1 is dependent on the reference clock signal in the receiver 50 and compensates for delay variation and the like due to the transmission path 24. .

6). Application example (3)
FIG. 17 is a block diagram showing a schematic configuration of a receiver of a unidirectional transmission quantum encryption key distribution system to which a photon receiver according to a third embodiment of the present invention is applied. The transmitter 10 is the same as that shown in FIG.

  In FIG. 17, the receiver 50 is provided with a clock source 51, and the reference clock signal is supplied to the timing generation circuit 71. However, it is assumed that the timing generation circuit 71 includes a drift correction circuit 71a. The timing generation circuit 71 generates the GGM timing signal s20α according to the correction coefficient α from the signal processing circuit 6, and as described above, the GGM bias voltage is applied to the two APD0 and APD1 according to the GGM timing signal s20α. Accordingly, an optical pulse is generated in the transmitter 10 in synchronization with the clock source 11, and in the receiver 50, the GGM timing of the APD0 and APD1 is subordinate to the reference clock signal of the clock source 51 to compensate for delay variation due to the transmission line 24, etc. It will be a thing.

  As already described, even in the system having the high-accuracy clock sources 11 and 51 individually in the transmitter 10 and the receiver 50, when the frequency difference between the clock sources is very small, the second embodiment As described above, the phase drift can be approximated as a linear change. For example, when the change amount of the phase difference for each period of the reference clock signals of the two clock sources 11 and 51 can be regarded as being very small with respect to the GGM pulse width of APD0 and APD1, it can be approximated to a linear change. .

  Note that the photon receivers of the application examples (1) to (3) may be those according to the first embodiment or the second embodiment of the present invention.

  Further, the photodetector 2 is not limited to an avalanche photodiode (APD). For example, a single photon detector using a photomultiplier tube (photomultiplier) or a cryogenic PIN photodiode can be used. However, GGM is required when using an InGaAs-APD in the 1.55 μm band. Visible Si-APD does not require GGM, but the present invention is applicable. The present invention is effective in a fiber transmission band of 1.55 μm.

  The photon receiver according to the present invention is applicable not only to photon receivers for quantum cryptography communication but also to photon receivers for general optical communication and optical measurement. The optical measurement can be applied to an optical time domain reflectometer (OTDR), a photoluminescence spectroscopic instrument, and the like.

It is a block diagram which shows schematic structure of the photon receiver by 1st Embodiment of this invention. It is the block diagram which described the functional structure of the signal processing circuit 6 in the photon receiver by 1st Embodiment in detail. 3 is a time chart schematically showing main signal waveforms in the circuit shown in FIG. 2. It is a time chart which shows the photon receiving method by 1st Embodiment of this invention. It is a flowchart which shows the photon reception determination operation | movement of the signal processing circuit 6 by this embodiment. It is the block diagram which described in detail the function structure of the signal processing circuit 6 in the photon receiver by 2nd Embodiment of this invention. It is a block diagram which shows the internal function structure of an offset process part. It is a time chart which shows an example of the rising timing of the data set DS which has not performed offset processing. It is a flowchart which shows an example of the photon reception determination operation | movement of the signal processing circuit 6 by this embodiment. (A) is a time chart showing the rise timing of the data set DS (j) before performing the offset process, and (B) is a time chart showing the rise timing of the data set DS (α, j) after the offset process. is there. (A) is a flowchart showing a photon reception determination operation of the signal processing circuit 6 according to the present embodiment, and (B) is a flowchart showing an optimum frequency distribution generation operation as its subprocedure. It is the block diagram which described in detail the function structure of the signal processing circuit 6 in the photon receiver by 3rd Embodiment of this invention. It is a block diagram which shows schematic structure of the transmitter of a Plug & Play system quantum encryption key distribution system. It is a block diagram which shows schematic structure of the receiver of the Plug & Play system quantum key distribution system to which the photon receiver by 3rd Embodiment of this invention is applied. It is a block diagram which shows schematic structure of the transmitter of the quantum encryption key distribution system of unidirectional transmission. It is a block diagram which shows schematic structure of the receiver of the quantum key distribution system of the unidirectional transmission to which the photon receiver by 3rd Embodiment of this invention is applied. It is a block diagram which shows schematic structure of the receiver of the quantum key distribution system of the unidirectional transmission to which the photon receiver by 3rd Embodiment of this invention is applied.

Explanation of symbols

1 Power supply circuit 2 Avalanche photodiode (APD)
3 Current-voltage converter (TIA)
4 sampling circuit 6 signal processing circuit 61 filter 62 frequency distribution generation unit 63 peak detection unit 64 comparison determination unit 65 memory 66 offset processing unit 67 counter 70 reference clock circuit 71 timing generation circuit 71a drift correction circuit 72 clock control unit

Claims (32)

In a photon receiver in a photon communication system,
Photon detection means capable of receiving light with a small number of photons to the extent that timing cannot be extracted;
Storage means for storing a plurality of signal data series including a change in output of the photon detection means in a predetermined time length of a predetermined period in units of the predetermined time length;
Search means for searching a temporal range indicating photon arrival timing in the predetermined time length based on a time distribution of signal change occurrence timings of the plurality of accumulated data series,
A determination unit that determines an output of the photon detection unit in which a signal change occurs at a timing within the searched time range as a photon detection signal;
A photon receiver comprising: The search means includes
A distribution generating means for generating the time distribution of the signal change occurrence timing of the plurality of accumulated data series;
Peak detection means for identifying a predetermined range including the peak of the time distribution as the temporal range;
The photon receiver of claim 1, comprising:   2. The photon according to claim 1, wherein the search unit includes a pattern detection unit that detects a time-varying pattern that matches a photon arrival timing based on a space defined by the plurality of accumulated data series. Receiver.   The search means obtains a temporal change pattern that matches a photon arrival timing based on a time distribution of signal change occurrence timings of the accumulated data series, and specifies the temporal range from the temporal change pattern. The photon receiver of claim 1, wherein The search means includes
Pattern generating means for generating a temporary temporal change pattern for the signal change occurrence timing of the plurality of accumulated data series;
Offset processing means for determining whether or not the temporary temporal change pattern matches the photon arrival timing while sequentially changing the temporary temporal change pattern within a predetermined range;
5. The photon receiver according to claim 4, wherein the provisional temporal change pattern that best matches the photon arrival timing is selected as the temporal change pattern. The offset processing means includes
Timing shift means for shifting the signal change occurrence timing of each of the accumulated data series in a direction to cancel the temporal change of the temporary temporal change pattern;
A distribution generating means for generating a time distribution of corrected signal change occurrence timings of the plurality of accumulated data series;
Peak detection means for detecting the peak size of the time distribution;
Have
6. The temporary temporal change pattern having the largest peak of the time distribution is selected as the temporal change pattern while sequentially changing the temporary temporal change pattern within a predetermined range. Photon receiver.   Supply of the drive signal for driving the photon detection means so that the signal change occurrence timing of each of the accumulated data series is relatively shifted in a direction to cancel the change with time of the change pattern with time The photon receiver according to any one of claims 3 to 6, further comprising offset correction means that shifts the timing so that the drive timing of the photon detection means follows the photon arrival timing. In a photon receiver in a photon communication system,
Photon detection means capable of receiving light with a small number of photons to the extent that timing cannot be extracted;
Drive means for intermittently supplying a drive signal of a predetermined time length for driving the photon detection means to the photon detection means at a predetermined timing;
Storage means for storing a plurality of data series of signals including output changes of the photon detection means in units of a predetermined time length of the drive signal;
A distribution generating means for generating a time distribution of signal change occurrence timing of the plurality of accumulated data series;
Peak detection means for determining whether or not there is a significant peak in the time distribution;
When the peak is detected in the time distribution, a time range indicating the photon arrival timing in the predetermined time length is searched based on the time distribution of the signal change occurrence timing of the accumulated data series, When the peak is not detected in the time distribution, a time-varying pattern matching the photon arrival timing is obtained based on the accumulated signal change occurrence distribution pattern of the plurality of data series, and the time change pattern is calculated from the time-varying pattern. A search means for identifying the target range;
A determination unit that determines an output of the photon detection unit in which a signal change occurs at a timing within the searched time range as a photon detection signal;
A photon receiver comprising:   Supply of the drive signal for driving the photon detection means so that the signal change occurrence timing of each of the accumulated data series is relatively shifted in a direction to cancel the change with time of the change pattern with time 9. The photon receiver according to claim 8, further comprising an offset correction unit that shifts the timing so that the drive timing of the photon detection unit follows the photon arrival timing.   10. The photon receiver according to claim 3, wherein the time-varying pattern is linearly approximated in a space defined by the accumulated data series. In a photon receiving method in a photon communication system,
a) Accumulating a plurality of data series of signals including output changes of the photon detector in units of a predetermined time length of a predetermined period;
b) Based on the time distribution of the signal change occurrence timing of the plurality of accumulated data series, a time range indicating the photon arrival timing in the predetermined time length is searched,
c) determining an output of the photon detector in which a signal change occurs at a timing within the searched time range as a photon detection signal;
A photon receiving method comprising: Said step b)
Generating a time distribution of signal change occurrence timing of the plurality of accumulated data series;
Identifying a predetermined range including the peak of the time distribution as the temporal range;
The photon receiving method according to claim 11. Said step b)
b. 1) Detecting a time-varying pattern that matches the photon arrival timing based on a space defined by the accumulated data series,
b. 2) Identify the temporal range from the time course pattern,
The photon receiving method according to claim 11. Said step b)
b. 1) Obtain a temporal change pattern that matches the photon arrival timing based on the time distribution of the signal change occurrence timing of the plurality of accumulated data series,
b. 2) Identify the temporal range from the time course pattern,
The photon receiving method according to claim 11. Said step b. 1)
A temporary temporal change pattern is generated for the signal change occurrence timing of the plurality of accumulated data series,
While sequentially changing the temporary aging pattern within a predetermined range, determine whether the temporary aging pattern is aligned with the photon arrival timing,
Selecting the temporary time-varying pattern that best matches the photon arrival timing as the time-varying pattern;
The photon receiving method according to claim 14. Said step b)
b. 1) Generate a temporal change pattern with respect to the signal change occurrence timing of the plurality of accumulated data series,
b. 2) Shift the signal change occurrence timing of each of the accumulated data series in a direction to cancel the temporal change of the temporary temporal change pattern,
b. 3) generating a time distribution of corrected signal change occurrence timings of the plurality of accumulated data series;
b. 4) detecting the peak size of the time distribution;
b. 5) While changing the provisional temporal change pattern sequentially within a predetermined range, b. 2) to b. 4) is repeated, and the temporary change pattern with the largest peak of the time distribution is selected as the change pattern with time,
b. 6) Identify the temporal range from the time course pattern,
The photon receiving method according to claim 11. Supply of the drive signal for driving the photon detector so that the signal change generation timing of each of the accumulated data series is relatively shifted in a direction to cancel the change with time of the change pattern with time Shift the timing,
Making the drive timing of the photon detector follow the photon arrival timing,
The photon receiving method according to any one of claims 13 to 16, In a photon receiving method in a photon communication system,
The photon detector capable of receiving light with a small number of photons to the extent that timing cannot be extracted is intermittently driven at a predetermined timing for a predetermined time length,
A plurality of data series of signals including output changes of the photon detector are stored in units of the predetermined time length,
Generating a time distribution of signal change occurrence timing of the plurality of accumulated data series;
Determine whether there is a significant peak in the time distribution;
When the peak is detected in the time distribution, a time range indicating a photon arrival timing in the predetermined time length is searched based on a signal change occurrence distribution pattern of the plurality of accumulated data series,
When the peak is not detected in the time distribution, a time-varying pattern that matches the photon arrival timing is obtained based on the time distribution of the signal change occurrence timing of the plurality of accumulated data series, and the time-varying pattern is calculated from the time-varying pattern. Identify the time range,
The output of the photon detector in which a signal change occurs at a timing within the searched time range is determined as a photon detection signal.
A photon receiving method comprising:   Supply of the drive signal for driving the photon detector so that the signal change generation timing of each of the accumulated data series is relatively shifted in a direction to cancel the change with time of the change pattern with time 19. The photon receiving method according to claim 18, wherein the drive timing of the photon detector is made to follow the photon arrival timing by shifting the timing.   20. The photon receiving method according to claim 13, wherein the temporal change pattern is linearly approximated in a space defined by the plurality of accumulated data series.   11. The photon receiver according to claim 1, wherein the photon detection means is an avalanche photodiode (APD).   20. The photon receiving method according to claim 11, wherein the photon detector is an avalanche photodiode (APD). In a program for causing a computer to execute photon reception by driving a photon detector capable of receiving light with a small number of photons to the extent that timing cannot be extracted,
a) accumulating a plurality of data series of signals including output changes of the photon detector in a memory in units of a predetermined time length of a predetermined period;
b) searching a time range indicating a photon arrival timing in the predetermined time length based on a time distribution of signal change occurrence timings of the plurality of accumulated data series;
c) determining the output of the photon detector in which a signal change occurs at a timing within the searched time range as a photon detection signal;
A computer program characterized by comprising: Said step b)
Generating a time distribution of signal change occurrence timing of the plurality of accumulated data series;
Identifying a predetermined range including the peak of the time distribution as the temporal range;
24. The computer program according to claim 23. Said step b)
b. 1) Detecting a time-varying pattern that matches the photon arrival timing based on a space defined by the accumulated data series,
b. 2) Identify the temporal range from the time course pattern,
24. The computer program according to claim 23. Said step b)
b. 1) Obtain a temporal change pattern that matches the photon arrival timing based on the time distribution of the signal change occurrence timing of the plurality of accumulated data series,
b. 2) Identify the temporal range from the time course pattern,
24. The computer program according to claim 23. Said step b. 1)
A temporary temporal change pattern is generated for the signal change occurrence timing of the plurality of accumulated data series,
While sequentially changing the temporary aging pattern within a predetermined range, determine whether the temporary aging pattern is aligned with the photon arrival timing,
Selecting the temporary time-varying pattern that best matches the photon arrival timing as the time-varying pattern;
The computer program according to claim 26. Said step b)
b. 1) Generate a temporal change pattern with respect to the signal change occurrence timing of the plurality of accumulated data series,
b. 2) Shift the signal change occurrence timing of each of the accumulated data series in a direction to cancel the temporal change of the temporary temporal change pattern,
b. 3) generating a time distribution of corrected signal change occurrence timings of the plurality of accumulated data series;
b. 4) detecting the peak size of the time distribution;
b. 5) While changing the provisional temporal change pattern sequentially within a predetermined range, b. 2) to b. 4) is repeated, and the temporal change pattern that maximizes the peak of the time distribution is selected as the time change pattern,
b. 6) Identify the temporal range from the time course pattern,
24. The computer program according to claim 23.   A quantum key distribution system comprising the photon receiver according to claim 1.   21. A quantum key distribution system for executing the photon receiving method according to claim 11.   An optical measuring instrument comprising the photon receiver according to claim 1. An optical measuring instrument for executing the photon receiving method according to claim 11.

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