Measurement device, measurement method, and program

Abstract

The present invention comprises: a generation unit (22) that generates a control signal (s0) that is generated at a time earlier than a synchronization signal (s1) by a prescribed time (tc); a first measurement unit (13) that starts charging a capacitor (C1) from the generation time (t0) of the control signal and measures a first voltage (V1) charged up to a detection time (t1) of the synchronization signal; a second measurement unit (14) that starts charging the capacitor (C1) from the generation time of the control signal and measures a second voltage (T2) charged up to a detection time (t2) of a detection signal (s2); and a calculation unit (23) that calculates a quantum arrival time (T) from the difference between the second voltage and the first voltage.

Description

Measurement device, measurement method, and program

[0001] The present disclosure relates to a measurement device, a measurement method, and a program for measuring quantum arrival times.

[0002] In order to achieve highly confidential communications, for example, Non-Patent Documents 1 and 2 propose a communication system using quantum key distribution (QKD) in which both the transmitter and receiver share a secret key that is difficult for others to eavesdrop on.

[0003] In QKD, the time elapsed from when a frame synchronization signal transmitted from a transmitter is received by a receiver until a quantum is detected (hereinafter referred to as the "quantum arrival time") is measured. As a method for measuring the quantum arrival time, Non-Patent Documents 3 and 4 disclose time-correlated single photon counting (TC-SPC). As TC-SPC, a TAC-ADC (Time-to-Amplitude Converter and Analog-to-Digital Converter) system and a count-type TDC (Time-to-digital converter) system have been proposed.

[0004] In the TAC-ADC method, current is passed from a current source to a capacitor when a synchronization signal is generated, increasing the capacitor voltage, and when a quantum detection signal is detected, the current supply from the current source to the capacitor is stopped.The capacitor voltage at that time is measured, and the time from the generation of the synchronization signal to the acquisition of the quantum detection signal (quantum arrival time) is calculated based on this capacitor voltage.

[0005] Furthermore, Non-Patent Document 5 discloses that in the TAC-ADC system, an integrator is used to measure the quantum arrival time.

[0006] Hiroki Takesueet al. "Quantum key distribution over a 40-dBchannel loss using superconducting single-photon detectors." Nature photonics 1.6 (2007): 343-348. Masahide Sasakiet al. "Field test of quantum key distribution in the TokyoQKD Network." Opticsexpress 19.11 (2011):10387-10409.Wolfgang Becker “The bh TCSPC Handbook”, EighthEditionWilliam R. Leo “Techniques for Nuclear and Particle PhysicsExperiments A How-toApproach”SecondRevised Edition, Springer-Verlag Berlin Heidelberg New York in 1994.Zule Xu et al. “Picosecond Resolution Time-to-Digital Converter Using Gm-C Integrator and SAR-ADC”

[0007] However, with the TAC-ADC methods disclosed in Non-Patent Documents 3 and 4, it is difficult to measure the quantum arrival time with high accuracy when the time difference between the synchronization signal and the detection signal is short. Also, with the technology disclosed in Non-Patent Document 5, there is a problem that the calculation speed and the operating frequency are inversely proportional, which limits the usable frequencies.

[0008] The present disclosure has been made in consideration of the above circumstances, and its purpose is to provide a measurement device, a measurement method, and a program that are capable of measuring quantum arrival time with high accuracy without being affected by the operating frequency.

[0009] A measurement device of one aspect of the present disclosure is a measurement device that measures the quantum arrival time from the detection of a synchronization signal to the detection of a photon detection signal, and includes a generation unit that generates a control signal that is generated at a time that is a predetermined time earlier than the synchronization signal, a first measurement unit that starts charging a capacitor from the time the control signal is generated and measures a first voltage that has been charged by the time the synchronization signal is detected, a second measurement unit that starts charging a capacitor from the time the control signal is generated and measures a second voltage that has been charged by the time the detection signal is detected, and a calculation unit that calculates the quantum arrival time from the difference between the second voltage and the first voltage.

[0010] A measurement method according to one aspect of the present disclosure is a measurement method for measuring the quantum arrival time from the detection of a synchronization signal to the detection of a photon detection signal, which includes generating a control signal to be generated at a time a predetermined time earlier than the synchronization signal, starting charging a capacitor from the time the control signal is generated, measuring a first voltage to which the capacitor has been charged by the time the synchronization signal is detected, starting charging a capacitor from the time the control signal is generated, measuring a second voltage to which the capacitor has been charged by the time the detection signal is detected, and calculating the quantum arrival time from the difference between the second voltage and the second voltage.

[0011] One aspect of the present disclosure is a program for causing a computer to function as the above-described measurement device.

[0012] According to the present disclosure, it is possible to measure the quantum arrival time with high accuracy, without being affected by the operating frequency.

[0013] FIG. 1 is a block diagram showing the configuration of a measurement device according to a first embodiment. FIG. 2 is a timing chart showing changes in each signal in the measurement device according to the first embodiment. FIG. 3 is a circuit diagram showing the configuration of a voltage measurement circuit. FIG. 4 is a flowchart showing a processing procedure for setting time tc. FIG. 5 is a flowchart showing a process for generating a clock whose phase is synchronized with that of a synchronization signal. FIG. 6 is a flowchart showing a processing procedure for measuring a quantum arriving time using the measurement device according to the first embodiment. FIG. 7 is a block diagram showing the configuration of measurement devices according to second and third embodiments. FIG. 8 is a timing chart showing changes in each signal in the measurement device according to the second embodiment. FIG. 9A is a first subdivision of a flowchart showing a processing procedure for measuring a quantum arriving time using the measurement device according to the second embodiment. FIG. 9B is a second subdivision of a flowchart showing a processing procedure for measuring a quantum arriving time using the measurement device according to the second embodiment. FIG. 10A is a first subdivision of a flowchart showing a processing procedure for measuring a quantum arriving time using the measurement device according to the third embodiment. FIG. 10B is a second subdivision of a flowchart showing a processing procedure for measuring a quantum arriving time using the measurement device according to the third embodiment. Fig. 11 is a timing chart showing changes in each signal in the measurement device according to the third embodiment, and Fig. 12 is a block diagram showing the hardware configuration of this embodiment.

[0014] [Description of First Embodiment] An embodiment will now be described. FIG. 1 is a block diagram showing the configuration of a measurement device 100 according to a first embodiment. FIG. 2 is a timing chart showing changes in a control signal s0, a synchronization signal s1, a detection signal s2, and a capacitor's charging voltage. The measurement device 100 is provided in a receiving unit that receives photons transmitted from a transmitting unit. The measurement device 100 measures the elapsed time (quantum arrival time) from when a frame synchronization signal (hereinafter abbreviated as "synchronization signal") is received in the receiving unit until when a photon detection signal is detected.

[0015] 1 , the measurement device 100 includes a front-end unit 1 and a control unit 2. The front-end unit 1 includes a first detection unit 11, a second detection unit 12, a first measurement unit 13, a second measurement unit 14, and an A / D conversion unit 15. The control unit 2 includes a synchronization unit 21, a generation unit 22, and a calculation unit 23.

[0016] The synchronization unit 21 generates a clock (periodic signal) synchronized with the phase of the synchronization signal s1 based on the synchronization signal s1 detected at a constant period by the first detection unit 11 (details of which will be described later). The synchronization unit 21 outputs the generated clock to the generation unit 22. In other words, the synchronization signal s1 is a signal that is detected periodically, and the synchronization unit 21 generates a periodic signal whose phase is synchronized with the synchronization signal s1.

[0017] The generating unit 22 generates a pulse that occurs a predetermined time earlier than the synchronization signal s1, based on the clock (periodic signal) generated by the synchronizing unit 21. Specifically, as shown in Figures 2(a) and 2(b), the generating unit 22 generates a control signal s0 that occurs at time t0, which is a time tc earlier than time t1 at which the synchronization signal s1 occurs. The generating unit 22 outputs the generated control signal s0 to the first measuring unit 13 and the second measuring unit 14. That is, the generating unit 22 generates a control signal s0 that occurs a predetermined time earlier than the synchronization signal s1. A method for setting the time tc will be described later.

[0018] The calculation unit 23 acquires a voltage measured by the second measurement unit 14 (a second voltage V2 described later) and a voltage measured by the first measurement unit 13 (a first voltage V1 described later). The calculation unit 23 calculates a differential voltage V (=V2-V1) between the second voltage V2 and the first voltage V1. Based on the differential voltage V, the calculation unit 23 calculates the time (quantum arrival time T) from when the synchronization signal s1 is detected to when the detection signal s2 is detected.

[0019] The first detector 11 detects the synchronization signal s1 transmitted from the transmitter. For example, the synchronization signal s1 is detected at time t1 shown in FIG. 2A. The first detector 11 outputs the synchronization signal s1 to the first measuring unit 13.

[0020] The second detector 12 detects the quantum detection signal s2 transmitted from the transmitter. For example, the detection signal s2 is detected at time t2 shown in FIG. 2C. The second detector 12 outputs the detection signal s2 to the second measuring unit 14.

[0021] The first measuring unit 13 measures a voltage (first voltage) corresponding to the time t0 to t1 (referred to as the "first time T1") from when the control signal s0 is output to when the synchronization signal s1 is detected, based on the control signal s0 output from the generating unit 22 and the synchronization signal s1 output from the first detecting unit 11. A method for measuring the first voltage V1 will be described below.

[0022] 3 shows an example of a voltage measurement circuit P1 that measures the first voltage V1 in the first measurement unit 13. The voltage measurement circuit P1 is mounted in the first measurement unit 13. The measurement circuit P1 is also mounted in the second measurement unit 14, the third measurement unit 17, and the fourth measurement unit 18, which will be described later.

[0023] 3 shows an example in which the voltage measurement circuit P1 includes a series-connected circuit of a switch 31, a MOSFET 32, and a current source 33, and a capacitor C1 connected in parallel to the MOSFET 32. Note that a bipolar transistor, an IGBT, or the like may be used instead of the MOSFET 32. A control signal s0 output from the generator 22 shown in FIG. 1 is input to the control terminal (gate) of the MOSFET 32. The switch 31 is normally on (conductive) and is turned off (open) upon input of a detection signal s2. It is preferable to use a contactless switch such as a semiconductor switch as the switch 31.

[0024] When the control signal s0 is input, the MOSFET 32 becomes conductive, a current from the current source 33 flows through the MOSFET 32, charging of the capacitor C1 connected in parallel to the MOSFET 32 begins, and the voltage across the capacitor C1 rises. That is, as shown in FIG. 2(d), the voltage across the capacitor C1 rises over time, with the time t0 at which the control signal s0 is input as the reference point. When the first detector 11 detects the synchronization signal s1 and the switch 31 turns off at time t1, the current from the current source 33 is cut off, and the voltage rise across the capacitor C1 ends. That is, the voltage rise across the capacitor C1 ends at time t1 as shown in FIG. 2(d).

[0025] The first measuring unit 13 measures the voltage (first voltage V1) when the switch 31 is turned off. The first measuring unit 13 outputs the first voltage V1 to the A / D converter 15. The voltage of the capacitor C1 is then discharged. That is, the first measuring unit 13 starts charging the capacitor C1 from time t0 when the control signal s0 is generated, and measures the first voltage V1 charged by time t1 when the synchronization signal s1 is detected.

[0026] That is, the first measuring unit 13 has a circuit including a current source 33 and a capacitor C1, and flows current from the current source 33 to the capacitor C1 at the time t0 when the control signal s0 is generated, and stops the current from the current source 33 at the time t1 when the synchronization signal s1 is detected, and the voltage of the capacitor C1 when the current is stopped is set to the first voltage V1.

[0027] 1 measures a voltage (second voltage V2) corresponding to the time t0 to t2 (hereinafter referred to as the "second time T2") from when the control signal s0 is output until when the detection signal s2 is detected, based on the control signal s0 and the quantum detection signal s2 output from the generation unit 22. The second voltage V2 can also be measured using a circuit similar to that shown in FIG. 3.

[0028] That is, when a control signal s0 is input to the control terminal (gate) of MOSFET 32, a current from current source 33 flows through MOSFET 32, and the voltage of capacitor C1 connected in parallel to MOSFET 32 rises. That is, as shown in Fig. 2(e), the voltage of capacitor C1 rises over time. Furthermore, when second detector 12 detects detection signal s2 and turns off switch 31, the current from current source 33 is cut off, and the voltage rise of capacitor C1 ends.

[0029] The second measuring unit 14 outputs the voltage (second voltage V2) when the switch 31 is turned off to the A / D converter 15. The voltage of the capacitor C1 is then discharged. That is, the second measuring unit 14 starts charging the capacitor C1 from time t0 when the control signal s0 is generated, and measures the second voltage V2 to which the capacitor C1 has been charged by time t2 when the detection signal s2 is detected.

[0030] That is, the second measuring unit 14 has a circuit including a current source 33 and a capacitor C1, and flows current from the current source 33 to the capacitor C1 at the generation time t0 of the control signal s0, and stops the current from the current source 33 at the detection time t2 of the detection signal s2, and the voltage of the capacitor C1 when the current stops is set to the second voltage V2.

[0031] The A / D converter 15 converts the first voltage V1 and the second voltage V2 into digital signals and outputs them to the calculator 23.

[0032] The calculation unit 23 calculates the differential voltage V (=V2-V1) between the second voltage V2 and the first voltage V1. The A / D conversion unit 15 may calculate the differential voltage V between the second voltage V2 and the first voltage V1, convert this differential voltage V into a digital signal, and output it to the calculation unit 23.

[0033] The calculation unit 23 calculates the time (quantum arrival time T) from time t1 when the synchronization signal s1 is received to time t2 when the detection signal s2 is received, based on the differential voltage V. The calculation unit 23 calculates the quantum arrival time T, which is the difference between the second time T2 and the first time T1, based on the differential voltage V (V2-V1), using the following equation (1):

[0034] T=T2-T1=C(V2-V1) / I (1) where I is the current of the current source 33, and C is the capacitance of the capacitor C1.

[0035] The generating unit 22 may set the time tc to approximately half the time required to charge the capacitor C1 in the voltage measuring circuit P1 shown in Fig. 3. Specifically, the generating unit 22 sets the time tc to approximately half the time from time t0 to time t3 shown in Fig. 2(d). Because the synchronization signal s1 is generated at a constant cycle, the synchronization signal s1 may be delayed by the time obtained by subtracting the time tc from the cycle of the synchronization signal.

[0036] 3 is provided in each of the first measuring unit 13 and the second measuring unit 14. Therefore, if there is an individual difference in capacitance between the capacitor C1 of the voltage measuring circuit provided in the first measuring unit 13 and the capacitor C1 of the voltage measuring circuit provided in the second measuring unit 14, the quantum arrival time T cannot be calculated with high accuracy. Therefore, in this embodiment, the offset of the voltage value and the slope of the capacitor capacitance are corrected so as to reduce the influence of the individual difference between the capacitors C1 provided in the first measuring unit 13 and the second measuring unit 14. Note that either the offset of the voltage value or the capacitor capacitance may be corrected.

[0037] The voltage measurement circuit P1 in FIG. 3 shows a TAC-TDC, but it is also possible to use a TDC, which generates electrical signals at regular intervals when a control signal s0 is input, and stops the electrical signals of the TDC when a synchronization signal s1 or a detection signal s2 is input, thereby measuring the time between time t1 and time s2 from the number of electrical signals generated during that time.

[0038] The procedure for correcting the offset of the voltage value and the slope of the capacitor capacitance will be described below with reference to the flowchart shown in Fig. 4. First, in step S11 of Fig. 4, the user transmits a synchronization signal s1 to the first detector 11 at a predetermined time, and then transmits a photon to the second detector 12 after a predetermined reference time Tc has elapsed. The reference time Tc is a known value.

[0039] In step S12, the first measurement unit 13 and the second measurement unit 14 measure the first voltage V1 and the second voltage V2 based on the input synchronization signal s1 and detection signal s2. Specifically, an arbitrary time earlier than the synchronization signal is set as the generation time of the control signal, and the first voltage V1 charged to the capacitor C1 from this time until the synchronization signal s1 is detected is measured. The second voltage V2 charged to the capacitor C1 from the generation time of the control signal until the detection signal s2 is detected is also measured. The measured first voltage V1 and second voltage V2 are digitized by the A / D conversion unit 15 and output to the calculation unit 23.

[0040] In step S13, the calculation unit 23 calculates a differential voltage V between the second voltage V2 calculated in the process of step S12 and the first voltage V1, and further calculates a quantum arrival time T based on the differential voltage V. The calculation unit 23 determines whether the difference between the calculated quantum arrival time T and the reference time Tc is less than a predetermined threshold value. If it is less than the threshold value (S13; YES), the process proceeds to step S16; if not (S13; NO), the process proceeds to step S14.

[0041] In step S14, the user electrically corrects the voltages measured by the first measuring unit 13 and the second measuring unit 14. That is, if the difference between the measured quantum arrival time T and the reference time Tc is large, it is presumed that there is an individual difference between the capacitor C1 of the first measuring unit 13 and the capacitor C1 of the second measuring unit 14. Therefore, a process is performed to correct the offset of the voltage value measured by each capacitor C1 and the capacitor capacitance.

[0042] In step S15, the user changes the interval between the synchronization signal s1 and the control signal s0, that is, the time tc, and then the process returns to step S11.

[0043] In step S16, the output time of the control signal that can be measured with smaller jitter, i.e., the time tc shown in Fig. 2, is calculated. In this way, it becomes possible to set the time tc at which the first voltage V1 and the second voltage V2 become appropriate values.

[0044] Next, the processing procedure by which the control unit 2 generates a control signal will be described with reference to the flowchart shown in Fig. 5. First, in step S31 of Fig. 5, the synchronization unit 21 synchronizes the phase of the synchronization signal s1 with that of the internal clock. As a result, a clock whose phase is synchronized with the periodically generated synchronization signal s1 is generated.

[0045] In step S32, the generation unit 22 generates a control signal based on the clock, which is shifted from the synchronization signal s1 by the time tc set in the process shown in Fig. 4. As a result, the control signal s0 is generated at a point in time earlier than the synchronization signal s1 by the time tc, as shown in Fig. 2(a) and (b).

[0046] In step S33, the generation unit 22 notifies the transmission unit (not shown) that preparation for reception has been completed.

[0047] Next, a procedure for calculating the quantum arrival time T will be described with reference to the flowchart shown in Fig. 6. First, in step S51, a synchronization signal and a photon are transmitted from the transmitter.

[0048] In step S52, the generation unit 22 outputs a control signal s0 to the first measurement unit 13 and the second measurement unit 14. As described above, the control signal s0 is output at a time tc earlier than the synchronization signal s1. The first measurement unit 13 and the second measurement unit 14 start charging the capacitor C1 of the voltage measurement circuit P1 shown in FIG. 3. As a result, as shown in FIGS. 2(d) and 2(e), the first measurement unit 13 and the second measurement unit 14 start charging the capacitor C1 from time t0 when the control signal s0 is supplied.

[0049] In step S53, the first detector 11 determines whether or not the synchronization signal s1 transmitted from the transmitter has been detected. If the synchronization signal s1 has been detected (S53; YES), the process proceeds to step S54.

[0050] In step S54, charging by the first measuring unit 13 is stopped, and as a result, the first voltage V1 shown in FIG.

[0051] In step S55, the second detector 12 determines whether or not the detection signal s2 of the photon transmitted from the transmitter has been detected. If it has been detected (S55; YES), the process proceeds to step S56.

[0052] In step S56, charging by the second measuring unit 14 is stopped, and as a result, the second voltage V2 shown in FIG.

[0053] In step S57, the A / D converter 15 converts the first voltage V1 and the second voltage V2 into digital signals and outputs them to the calculator 23.

[0054] In step S58, the calculation unit 23 calculates the quantum arrival time T (=T2-T1) using the above-mentioned formula (1) based on the differential voltage V (=V2-V1) between the digitized voltages V2 and V1. Then, in step S59, the charge voltages of the first measuring unit 13 and the second measuring unit 14 are reset. In this way, the quantum arrival time T can be calculated.

[0055] As described above, the measurement device 100 according to this embodiment measures the quantum arrival time T from the detection of the synchronization signal s1 to the detection of the photon detection signal s2, and includes a generation unit 22 that generates a control signal s0 that is generated at a time that is a predetermined time tc earlier than the synchronization signal s1, a first measurement unit 13 that starts charging the capacitor C1 from the generation time t0 of the control signal s0 and measures a first voltage V1 to which the capacitor C1 has been charged by the detection time t1 of the synchronization signal s1, a second measurement unit 14 that starts charging the capacitor C1 from the generation time t0 of the control signal s0 and measures a second voltage V2 to which the capacitor C1 has been charged by the detection time t2 of the detection signal s2, and a calculation unit 23 that calculates the quantum arrival time T from the difference between the second voltage V2 and the first voltage V1.

[0056] Therefore, even if the time from the synchronization signal s1 to the detection signal s2 is short, it is possible to measure the quantum arrival time T with high accuracy. That is, if the control signal s0 is not used, charging of the capacitor C1 starts at time t1, charging stops at time t2, and the quantum arrival time T is calculated based on the charging voltage at this time. If this method is adopted, it is not possible to calculate the quantum arrival time T with high accuracy if the time period from time t1 to t2 is short. In this embodiment, the control signal s0 is generated, and the first voltage V1 and the second voltage V2 are measured based on this control signal s0, so it is possible to measure the quantum arrival time T with high accuracy.

[0057] Furthermore, since no integrating circuit is used, it is possible to measure the quantum arrival time T with high precision even in the high frequency range without being affected by the frequency.

[0058] In the first embodiment described above, an example in which the quantum detection signal s2 is detected by the second detection unit 12 has been described, but in quantum communication, when two outputs from an interferometer are decoded as quantum detection signals or when there are multiple measuring devices such as in a multi-party system, multiple quantum detection signals s2 are detected for the synchronization signal s1. By installing multiple second detection units 12 and second measurement units 14 shown in Fig. 1 (not shown) and detecting multiple detection signals s2, it becomes possible to measure the quantum arrival time T for multiple detection signals s2 detected for one synchronization signal s1.

[0059] Specifically, a plurality of second detection units 12 and second measurement units 14 shown in Fig. 1 are installed in parallel. The detection signal s2 detected by each second detection unit 12 is output to each second measurement unit 14. Then, the A / D conversion unit 15 performs conversion. As a result, it becomes possible to measure the quantum arrival time T for each of the plurality of detection signals s2.

[0060] [Description of Second Embodiment] Next, a second embodiment will be described. Fig. 7 is a block diagram showing the configuration of a measuring device 101 according to the second embodiment. Fig. 8 is a timing chart showing temporal changes in the control signal s0, delay signal s10, synchronization signal s1, detection signal s2, and charging voltage of capacitor C1.

[0061] The measuring device 101 shown in Fig. 7 differs from the measuring device 100 shown in Fig. 1 in that a front end unit 1A is added and a delay unit 24 is added to the control unit 2. The components that differ from the measuring device 100 shown in Fig. 1 will be described in detail below.

[0062] The delay unit 24 generates a delayed signal s10 (see FIG. 8(b)) by delaying the control signal s0 generated by the generation unit 22 by a predetermined delay time td. That is, the delay unit 24 performs processing to generate the delayed signal s10 by delaying the control signal s0 by a predetermined time. The delay signal s10 may be set to, for example, about half the time required for the capacitor C1 to reach full charge.

[0063] The front end unit 1A includes a third measurement unit 17, a fourth measurement unit 18, and an A / D conversion unit 19.

[0064] 3, the third measuring unit 17 starts charging the capacitor C1 by turning on the MOSFET 32 when the delay signal s10 is input, and stops charging when the synchronization signal s1 is input. The charging voltage of the capacitor C1 at this time is defined as the third voltage V3. That is, the third measuring unit 17 starts charging the capacitor C1 from the generation time t10 of the delay signal s10, and measures the third voltage V3 charged by the detection time t1 of the synchronization signal s1.

[0065] 3, the fourth measuring unit 18 starts charging the capacitor C1 by turning on the MOSFET 32 when the delay signal s10 is input, and stops charging when the detection signal s2 is input. The charging voltage of the capacitor C1 at this time is defined as a fourth voltage V4. That is, the fourth measuring unit 18 starts charging the capacitor C1 from time t10 when the delay signal s10 is generated, and measures the fourth voltage V4 to which the capacitor C1 has been charged by time t2 when the detection signal s2 is detected.

[0066] The A / D converter 19 converts the third voltage V3 and the fourth voltage V4 into digital signals and outputs them to the calculator 23.

[0067] The calculation unit 23 calculates the first time T1 based on the first voltage V1 or the third voltage V3, and calculates the second time T2 based on the second voltage V2 or the fourth voltage V4. The calculation unit 23 calculates the quantum arrival time T from the difference between the second time T2 and the first time T1.

[0068] 9A and 9B, a procedure for calculating the quantum arrival time T by the measurement device 101 according to the second embodiment will be described. First, in step S71 shown in FIG. 9A, a synchronization signal and a photon are transmitted from the transmitter.

[0069] In step S72, the generating unit 22 outputs a control signal s0 to the first measuring unit 13 and the second measuring unit 14. The first measuring unit 13 and the second measuring unit 14 start charging the capacitor C1 of the voltage measuring circuit P1 shown in Fig. 3. As a result, as shown in Fig. 8(d), the first measuring unit 13 and the second measuring unit 14 start charging the capacitor C1 from time t0 when the control signal s0 is supplied.

[0070] In step S73, the first detector 11 determines whether or not the synchronization signal s1 transmitted from the transmitter has been detected. If the synchronization signal s1 has been detected (S73; YES), the process proceeds to step S76. If not (S73; NO), the process proceeds to step S74.

[0071] In step S74, the delay unit 24 determines whether or not the delay time td has elapsed since time t0. If the delay time td has elapsed (S74; YES), the process proceeds to step S75; if not (S74; NO), the process returns to step S73.

[0072] In step S75, the delay unit 24 outputs a delay signal s10 to the third measuring unit 17 and the fourth measuring unit 18. The third measuring unit 17 and the fourth measuring unit 18 start charging the capacitor C1. That is, as shown in FIG. 8(e), charging of the capacitor C1 starts at time t10. Then, the process returns to step S73.

[0073] In step S76, the first measurement unit 13 and the third measurement unit 17 stop charging the capacitor C1 and measure the first voltage V1 or the third voltage V3, which is the charging voltage of the capacitor C1 at this time. That is, if the synchronization signal s1 is detected at time t1, which is earlier than time t10, the first measurement unit 13 measures the first voltage V1. On the other hand, if the synchronization signal s1 is detected at a time later than time t10, the third measurement unit 17 measures the third voltage V3. The first voltage V1 or the third voltage V3 is converted into a digital signal by the A / D conversion units 15 and 19 and output to the calculation unit 23. Thereafter, the voltage charged in the capacitor C1 is discharged.

[0074] 9B, the second measurement unit 14 determines whether or not the detection signal s2 of the photon transmitted from the transmitter has been detected. If the detection signal s2 has been detected (S77; YES), the process proceeds to step S81. If not (S77; NO), the process proceeds to step S78.

[0075] In step S78, the delay unit 24 determines whether or not the delay time td has elapsed since time t0. If the delay time td has elapsed (S78; YES), the process proceeds to step S79; if not (S78; NO), the process returns to step S77.

[0076] In step S79, the delay unit 24 determines whether or not charging of the capacitor C1 has already started in the third measuring unit 17 and the fourth measuring unit 18. If charging has already started (S79; YES), the process returns to step S77, and if charging has not started (S79; NO), the process proceeds to step S80.

[0077] In step S80, the delay unit 24 outputs a delay signal s10 to the third measuring unit 17 and the fourth measuring unit 18. The third measuring unit 17 and the fourth measuring unit 18 start charging their capacitors. That is, as shown in FIG. 8(e), charging of the capacitor C1 starts at time t10. Then, the process returns to step S77.

[0078] In step S81, the second measuring unit 14 and the fourth measuring unit 18 stop charging the capacitor C1 and measure the second voltage V2 or the fourth voltage V4, which is the charging voltage of the capacitor C1 at this time. That is, if the detection signal s2 is detected at time t1, which is earlier than time t10, the second measuring unit 14 measures the second voltage V2. On the other hand, if the detection signal s2 is detected at a time later than time t10, the fourth measuring unit 18 measures the fourth voltage V4. The second voltage V2 or the fourth voltage V4 is output to the A / D conversion units 15 and 19. Thereafter, the voltage charged in the capacitor C1 is discharged.

[0079] In step S82, the A / D converters 15 and 19 convert the first voltage V1 or the third voltage V3, and the second voltage V2 or the fourth voltage V4 into digital signals and output them to the calculation unit 23.

[0080] In step S83, the calculation unit 23 calculates the quantum arrival time T based on the differential voltage between the first voltage V1 or the third voltage V3 and the second voltage V2 or the fourth voltage V4. For example, if the synchronization signal s1 is detected at time t1 as shown in Fig. 8(a) and the detection signal s2 is detected at time t2 as shown in Fig. 8(c), the calculation unit 23 calculates the quantum arrival time T based on the first voltage V1 and the fourth voltage V4.

[0081] Specifically, the calculation unit 23 calculates a first time T1 from time t0 to time t1 based on the first voltage V1, and calculates a time T3 from time t10 to time t2 based on the fourth voltage V4. Furthermore, the calculation unit 23 calculates a second time T2 by adding a delay time td to the time T3. The quantum arrival time T from time t1 to time t2 can be calculated by "T = T2 - T1".

[0082] In step S84, the first to fourth measuring units 13, 14, 17, and 18 discharge the voltage charged in the capacitor C1 to reset it.

[0083] In this way, in the measuring device 101 according to the second embodiment, even if the elapsed time from when the synchronization signal s1 is received until when the detection signal s2 is detected (t1 to t2 shown in FIG. 8) is long, it is possible to measure the first time T1 and the second time T2 with high accuracy.

[0084] 8, if the detection time t2 of the detection signal s2 is later than the time when the capacitor C1 of the second measuring unit 14 is fully charged, the second measuring unit 14 will not be able to detect the detection signal s2 before the capacitor C1 is fully charged. In this embodiment, the third measuring unit 17 and fourth measuring unit 18 are provided, which start charging the capacitor C1 based on the delayed signal s10, and therefore it is possible to detect the detection time t2 of the detection signal s2, as shown in FIG.

[0085] Furthermore, by providing the third measuring unit 17 and the fourth measuring unit 18, it becomes possible to detect the detection signal s2 during the dead time period when the capacitor C1 of the first measuring unit 13 and the second measuring unit 14 is discharging. That is, after the capacitor C1 is fully charged, the first measuring unit 13 and the second measuring unit 14 need to discharge the accumulated voltage before performing the next measurement. During this discharge time, the detection signal s2 cannot be detected. The second embodiment can solve this problem.

[0086] In the second embodiment, an example in which two front end units 1 and 1A are provided as shown in FIG. 7 has been described, but it is also possible to provide a configuration in which three or more front end units are provided.

[0087] [Description of Third Embodiment] Next, a third embodiment will be described. The measurement device according to the third embodiment has the same configuration as that shown in Fig. 7. In the third embodiment, the quantum arrival time T is measured with higher accuracy by changing the time period for measuring voltage in the voltage measurement circuit P1 shown in Fig. 3.

[0088] 2(d), (e), 8(d), and 8(e) show cases where the voltage supplied to capacitor C1 increases linearly as it is charged. However, the voltage actually charged to capacitor C1 may deviate from the linearity in the high frequency range except for the central region, and the A / D conversion unit 15 also has a large error in the region other than the central region.

[0089] Therefore, if the charging voltage of the capacitor C1 is measured during the central period of the voltage rise of the capacitor C1, it is possible to measure the quantum arrival time with higher accuracy. In the third embodiment, the synchronization signal s1 and the detection signal s2 are set to be detected during the central period of the voltage rise period.

[0090] The procedure for measuring the quantum arrival time T according to the third embodiment will be described below with reference to the flowcharts shown in Figures 10A and 10B and the timing chart shown in Figure 11. Note that, for simplicity of explanation, Figures 11(d) and 11(e) show graphs in which the charging voltage of the capacitor C1 is changed linearly.

[0091] First, in step S101 of FIG. 10A, a synchronization signal and a photon are transmitted from the transmitting unit.

[0092] In step S102, the generating unit 22 outputs a control signal s0 to the first measuring unit 13 and the second measuring unit 14 at time t0 as shown in Fig. 11(b). The first measuring unit 13 and the second measuring unit 14 start charging the capacitor C1 of the voltage measuring circuit P1 shown in Fig. 3. As a result, the charging voltage of the capacitor C1 increases as shown in Fig. 11(d).

[0093] 11(e), at time t10, which is delayed by delay time td from time t0, the delay unit 24 outputs a delay signal s10 to the third measuring unit 17 and the fourth measuring unit 18. The third measuring unit 17 and the fourth measuring unit 18 start charging the capacitor C1 of the voltage measuring circuit P1 shown in FIG.

[0094] In step S104, the first detector 11 determines whether or not the synchronization signal s1 transmitted from the transmitter has been detected. If the synchronization signal s1 has been detected (YES in step S104), the process proceeds to step S105. In the example shown in FIG. 11A, the synchronization signal s1 is detected at time t1.

[0095] In step S105, the first measuring unit 13 and the third measuring unit 17 stop supplying current to the capacitor C1.

[0096] In step S106, the first measuring unit 13 and the third measuring unit 17 output the first voltage V1 and the third voltage V3 to the A / D conversion units 15 and 19. The A / D conversion units 15 and 19 convert the first voltage V1 and the third voltage V3 into digital signals and output them to the calculation unit 23.

[0097] In step S107, the second detector 12 determines whether or not a detection signal s2 of a photon transmitted from the transmitter has been detected. If a detection signal s2 has been detected (S107; YES), the process proceeds to step S108 in Fig. 10B. In the example shown in Fig. 11(c), the detection signal s2 is detected at time t2.

[0098] In step S108, the second measuring unit 14 and the fourth measuring unit 18 stop supplying current to the capacitor C1.

[0099] In step S109, the second measuring unit 14 and the fourth measuring unit 18 output the second voltage V2 and the fourth voltage V4 to the A / D conversion units 15 and 19. The A / D conversion units 15 and 19 convert the second voltage V2 and the fourth voltage V4 into digital signals and output them to the calculation unit 23.

[0100] In step S110, the calculation unit 23 calculates the quantum arrival time T based on either the first voltage V1 and the second voltage V2, or the third voltage V3 and the fourth voltage V4. As described above, when the voltage rise time period shown from t1 to t2 in FIG. 11(d) is compared with the voltage rise time period shown from t1 to t2 in FIG. 11(e), the third voltage V3 and the fourth voltage V4 shown in FIG. 11(e) are closer to the central time period. Therefore, the calculation unit 23 calculates the quantum arrival time T using the third voltage V3 and the fourth voltage V4.

[0101] That is, when the first voltage V1, the second voltage V2, the third voltage V3, and the fourth voltage V4 are all detected, the calculation unit 23 selects either the difference between the second voltage V2 and the first voltage V1 or the difference between the fourth voltage V4 and the third voltage V3, and calculates the quantum arrival time T.

[0102] In step S111, the first to fourth measuring units 13, 14, 17, and 18 reset the voltage charged in the capacitor C1.

[0103] In this way, in the measurement device according to the third embodiment, the synchronization signal s1 and the detection signal s2 are measured during the first half of the time period in which the capacitor C1 is charged, which enables voltage measurement in a linear region with little jitter and enables more accurate measurement of the quantum arrival time T. Furthermore, it becomes possible to calculate the quantum arrival time T at a high count rate.

[0104] In the third embodiment, an example of measuring voltage using the first measuring unit 13, the second measuring unit 14, the third measuring unit 17, and the fourth measuring unit 18 has been described, but it is also possible to configure the device so that a selector switch (not shown) is installed on the output side of the second detecting unit 12 shown in Figure 7, allowing selection of either the second measuring unit 14 or the fourth measuring unit 18.

[0105] The measuring devices 100 and 101 of the above-described embodiment can be implemented as a general-purpose computer system including, for example, a CPU (Central Processing Unit, processor) 901, a memory 902, a storage 903 (HDD: Hard Disk Drive, SSD: Solid State Drive), a communication device 904, an input device 905, and an output device 906, as shown in Fig. 12. The memory 902 and the storage 903 are storage devices. In this computer system, the CPU 901 executes a predetermined program loaded onto the memory 902, thereby realizing each function of the measuring devices 100 and 101.

[0106] The measurement devices 100 and 101 may be implemented in one computer or in multiple computers. Furthermore, the measurement devices 100 and 101 may be virtual machines implemented in a computer.

[0107] The programs for the measuring devices 100 and 101 can be stored in a computer-readable recording medium such as a HDD, SSD, USB (Universal Serial Bus) memory, CD (Compact Disc), or DVD (Digital Versatile Disc), or can be distributed via a network. The computer-readable recording medium is, for example, a non-transitory recording medium.

[0108] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the present disclosure.

[0109] REFERENCE SIGNS LIST 1, 1A Front end unit 2 Control unit 11 First detection unit 12 Second detection unit 13 First measurement unit 14 Second measurement unit 15, 19 A / D conversion unit 17 Third measurement unit 18 Fourth measurement unit 21 Synchronization unit 22 Generation unit 23 Calculation unit 24 Delay unit 100, 101 Measurement device s0 Control signal s1 Synchronization signal s2 Detection signal s10 Delay signal V Differential voltage V1 First voltage V2 Second voltage V3 Third voltage V4 Fourth voltage

Claims

1. A measurement device for measuring the quantum arrival time from the detection of a synchronization signal to the detection of a photon detection signal, comprising: a generation unit that generates a control signal to be generated at a time a predetermined time earlier than the synchronization signal; a first measurement unit that starts charging a capacitor from the time the control signal is generated and measures a first voltage to which the capacitor has been charged by the time the synchronization signal is detected; a second measurement unit that starts charging a capacitor from the time the control signal is generated and measures a second voltage to which the capacitor has been charged by the time the detection signal is detected; and a calculation unit that calculates the quantum arrival time from the difference between the second voltage and the first voltage.

2. The measuring device according to claim 1, wherein the first measuring section has a circuit including a current source and a capacitor, flows a current from the current source to the capacitor when the control signal is generated, stops the current from the current source when the synchronization signal is detected, and defines the voltage of the capacitor when the current is stopped as the first voltage.

3. The measuring device according to claim 1 or 2, wherein the second measuring section has a circuit including a current source and a capacitor, flows a current from the current source to the capacitor at the time when the control signal is generated, stops the current from the current source at the time when the detection signal is detected, and defines the voltage of the capacitor at the time when the current is stopped as the second voltage.

4. The measurement device according to claim 1, further comprising a synchronization unit that generates a periodic signal whose phase is synchronized with the synchronization signal, wherein the synchronization signal is a signal that is detected periodically, and the generation unit generates the control signal based on the periodic signal.

5. The measuring device according to claim 1, further comprising: a delay unit which generates a delayed signal by delaying the control signal by a predetermined time; a third measurement unit which starts charging a capacitor from the generation time of the delayed signal and measures a third voltage to which the capacitor has been charged by the detection time of the synchronization signal; and a fourth measurement unit which starts charging a capacitor from the generation time of the delayed signal and measures a fourth voltage to which the capacitor has been charged by the detection time of the detection signal, wherein the calculation unit calculates the quantum arrival time based on the first voltage or the third voltage, and the second voltage or the fourth voltage.

6. The measurement device described in claim 5, wherein the calculation unit, when the first voltage, the second voltage, the third voltage, and the fourth voltage are all detected, selects either the difference between the second voltage and the first voltage or the difference between the fourth voltage and the third voltage to calculate the quantum arrival time.

7. A method for measuring a quantum arrival time from detection of a synchronization signal to detection of a photon detection signal, comprising: generating a control signal to be generated at a time a predetermined time earlier than the synchronization signal; starting charging a capacitor from the time the control signal is generated and measuring a first voltage to which the capacitor has been charged by the time the synchronization signal is detected; starting charging a capacitor from the time the control signal is generated and measuring a second voltage to which the capacitor has been charged by the time the detection signal is detected; and calculating the quantum arrival time from the second voltage and a difference between the second voltage and the second voltage.

8. A program for causing a computer to function as the measuring device according to claim 1.

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