Quantum key distribution system, timing determination method, and program

Abstract

A quantum key distribution system includes: a first light output means for outputting multiple pulses of first light which is pulse light transmitted from a transmission side to a reception side; a phase control means for changing the phase difference between the first light and second light, having a greater light intensity than the first light, so that the phase difference is different for each pulse of the first light; a signal acquisition means for homodyne detection, on the reception side, of the first light using the second light; a peak position detection means for detecting the peak position corresponding to the timing at which the amplitude of a signal obtained by the homodyne detection is greatest or smallest; and a timing determination means for determining, on the basis of the peak position, the detection timing of the pulse light transmitted from the transmission side to the reception side.

Description

Quantum key distribution system, receiving device, timing determination method, and recording medium

[0001] The present invention relates to a quantum key distribution system, a receiving device, a timing determination method, and a recording medium.

[0002] One of the encryption key distribution methods is continuous-variable quantum key distribution (CV-QKD) (see, for example, Patent Document 1).

[0003] Special Publication No. 2019-522394

[0004] In continuous quantum key distribution, it is preferable that the detection timing of the pulsed light can be efficiently detected even when the intensity of the pulsed light received by the receiving device is small (weak).

[0005] An example of an object of the present invention is to provide a quantum key distribution system, a receiving device, a timing determination method, and a recording medium that can solve the above-mentioned problems.

[0006] According to a first aspect of the present invention, a quantum key distribution system includes: first light output means for outputting a plurality of pulses of first light, which is pulsed light to be transmitted from a transmitting side to a receiving side; phase control means for changing the phase difference between the first light and a second light having a light intensity higher than that of the first light, so that the phase difference between the first light and the second light differs for each pulse of the first light; signal acquisition means for performing homodyne detection of the first light using the second light at the receiving side; peak position detection means for detecting a peak position, which is a timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and timing determination means for determining a detection timing of the pulsed light transmitted from the transmitting side to the receiving side based on the peak position.

[0007] According to a second aspect of the present invention, a quantum key distribution system includes a transmitting device and a receiving device, wherein the transmitting device transmits a plurality of pulses of a first light, which is pulsed light, by changing the phase of each pulse, and the receiving device includes: signal acquiring means for homodyne detecting the first light using a second light having a higher optical intensity than the first light; peak position detecting means for detecting a peak position, which is a timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and timing determining means for determining a detection timing of the pulsed light to be transmitted from the transmitting device to the receiving device based on the peak position.

[0008] According to a third aspect of the present invention, a quantum key distribution system includes a transmitting device and a receiving device, wherein the transmitting device transmits a plurality of pulses of first light which is pulsed light, and the receiving device includes: phase control means for changing a phase of second light having a light intensity greater than that of the first light such that a phase of the second light differs for each pulse of the first light; signal acquisition means for homodyne detecting the first light using the second light; peak position detection means for detecting a peak position, which is a timing at which an amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and timing determination means for determining a detection timing of the pulsed light to be transmitted from the transmitting device to the receiving device based on the peak position.

[0009] According to a fourth aspect of the present invention, a receiving device includes a receiving means for receiving a plurality of pulses of first light, which is pulsed light transmitted with a different phase for each pulse; a signal acquiring means for performing homodyne detection of the first light using a second light having a light intensity greater than that of the first light; a peak position detecting means for detecting a peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and a timing determining means for determining the detection timing of the transmitted pulsed light based on the peak position.

[0010] According to a fifth aspect of the present invention, a receiving device includes receiving means for receiving a plurality of pulses of a first light which is pulsed light; phase control means for changing the phase of a second light having a light intensity greater than that of the first light so that the phase of the second light differs for each pulse of the first light; signal acquisition means for homodyne detecting the first light using the second light; peak position detection means for detecting a peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and timing determination means for determining the detection timing of the transmitted pulsed light based on the peak position.

[0011] According to a sixth aspect of the present invention, a timing determination method includes: changing a phase difference between a first light, which is pulsed light transmitted in multiple pulses from a transmitting side to a receiving side, and a second light, which has a light intensity greater than that of the first light, so that the phase difference is different for each pulse of the first light; performing homodyne detection on the first light using the second light; detecting a peak position, which is the timing when the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and determining the detection timing of the pulsed light to be transmitted from the transmitting side to the receiving side based on the peak position.

[0012] According to a seventh aspect of the present invention, a timing determination method includes: a transmitting device transmitting a plurality of pulses of a first light, which is pulsed light, by changing the phase for each pulse; a receiving device performing homodyne detection of the first light using a second light having a light intensity greater than that of the first light; the receiving device detecting a peak position, which is the timing at which the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and the receiving device determining the detection timing of the pulsed light transmitted from the transmitting device to the receiving device based on the peak position.

[0013] According to an eighth aspect of the present invention, a timing determination method includes: a transmitting device transmitting a plurality of pulses of a first light which is pulsed light; a receiving device changing a phase of a second light having a light intensity greater than that of the first light so that the phase of the second light differs for each pulse of the first light; the receiving device homodyne detecting the first light using the second light; the receiving device detecting a peak position which is the timing at which the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and the receiving device determining a detection timing of the pulsed light transmitted from the transmitting device to the receiving device based on the peak position.

[0014] According to a ninth aspect of the present invention, a timing determination method includes receiving a first light which is pulsed light transmitted with a different phase for each pulse, performing homodyne detection on the first light using a second light having a light intensity greater than that of the first light, detecting a peak position which is a timing at which the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum, and determining the detection timing of the transmitted pulsed light based on the peak position.

[0015] According to a tenth aspect of the present invention, a timing determination method includes receiving a plurality of pulses of a first light which is pulsed light, changing the phase of a second light having a light intensity greater than that of the first light so that the phase of the second light differs for each pulse of the first light, performing homodyne detection on the first light using the second light, detecting a peak position which is the timing at which the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum, and determining the detection timing of the pulsed light to be transmitted based on the peak position.

[0016] According to an eleventh aspect of the present invention, a recording medium is a recording medium having recorded thereon a program for causing a computer controlling a receiving device that receives a plurality of pulses of first light, which is pulsed light transmitted with a different phase for each pulse, and performs homodyne detection of the first light using second light having a light intensity greater than that of the first light, to detect a peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum, and to determine the detection timing of the transmitted pulsed light based on the peak position.

[0017] According to a twelfth aspect of the present invention, a recording medium is a recording medium having recorded thereon a program for causing a computer controlling a receiving device that receives a plurality of pulses of first light, which is pulsed light, and performs homodyne detection of the first light using second light having a light intensity greater than that of the first light, to execute the following operations: changing the phase of the second light so that the phase of the second light differs for each pulse of the first light; detecting a peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and determining the detection timing of the transmitted pulsed light based on the peak position.

[0018] According to the present invention, it is expected that the detection timing of the pulsed light can be efficiently detected even when the intensity of the pulsed light received by the receiving device is low.

[0019] 1 is a diagram illustrating an example of the configuration of a quantum key distribution system according to a first embodiment; FIG. 2 is a diagram illustrating an example of a peak position of signal light; FIG. 3 is a diagram illustrating an example of the intensity of signal light when a receiving device cannot perform accurate timing detection using a single photodiode; FIG. 4 is a diagram illustrating a first example of a phase difference between signal light and reference light when a phase modulator according to a first embodiment does not change the phase of signal light; FIG. 5 is a diagram illustrating a first example of the amplitude of a difference signal output by a subtraction unit when a phase modulator according to a first embodiment does not change the phase of signal light; FIG. 6 is a diagram illustrating a second example of a phase difference between signal light and reference light when a phase modulator according to a first embodiment does not change the phase of signal light; FIG. 7 is a diagram illustrating a third example of a phase difference between signal light and reference light when a phase modulator according to a first embodiment does not change the phase of signal light; FIG. 8 is a diagram illustrating a third example of a phase difference between signal light and reference light when a phase modulator according to a first embodiment does not change the phase of signal light; FIG. 10 is a diagram showing a fourth example of the amplitude of the difference signal output by the subtraction unit when the phase modulator according to the first embodiment does not change the phase of the signal light. FIG. 11 is a diagram showing a first example of the phase difference between the signal light and the reference light when the phase modulator according to the first embodiment changes the phase of the signal light. FIG. 12 is a diagram showing a first example of the amplitude of the difference signal output by the subtraction unit when the phase modulator according to the first embodiment changes the phase of the signal light. FIG. 13 is a diagram showing a second example of the phase difference between the signal light and the reference light when the phase modulator according to the first embodiment changes the phase of the signal light. FIG. 14 is a diagram showing a first example of the amplitude of the difference signal output by the subtraction unit when the phase modulator according to the second embodiment changes the phase of the reference light. FIG. 15 is a diagram showing an example of the phase of the reference light when the transmitting device transmits two pulses of signal light when the transmitting device and receiving device according to the second embodiment are synchronized.10 is a diagram showing an example of the phase of the reference light when the phase modulator according to the second embodiment changes the phase of the reference light by a phase difference smaller than 90°. FIG. 11 is a diagram showing an example of the phase of the reference light when the phase modulator according to the second embodiment gradually changes the phase of the reference light. FIG. 12 is a diagram showing a first example of a processing procedure in which the receiving device according to the second embodiment determines the sampling timing of the signal light during quantum key distribution. FIG. 13 is a diagram showing a first example of a processing procedure in which the receiving device according to the second embodiment determines the sampling timing of the signal light during quantum key distribution. FIG. 14 is a diagram showing an example of the configuration of a quantum key distribution system according to the third embodiment. FIG. 15 is a diagram showing an example of the configuration of a quantum key distribution system according to the fourth embodiment. FIG. 16 is a diagram showing an example of the configuration of a quantum key distribution system according to the fifth embodiment. FIG. 17 is a diagram showing an example of the configuration of a receiving device according to the sixth embodiment. FIG. 18 is a diagram showing an example of the configuration of a receiving device according to the seventh embodiment. FIG. 19 is a diagram showing an example of the processing procedure in the timing determination method according to the eighth embodiment. FIG. 19 is a diagram showing an example of the processing procedure in the timing determination method according to the ninth embodiment. FIG. 20 is a diagram showing an example of the processing procedure in the timing determination method according to the tenth embodiment. FIG. 21 is a diagram showing an example of the processing procedure in the timing determination method according to the eleventh embodiment. FIG. 22 is a diagram showing an example of the processing procedure in the timing determination method according to the twelfth embodiment. FIG. 23 is a schematic block diagram showing the configuration of a computer according to at least one embodiment.

[0020] The following describes embodiments of the present invention, but the following embodiments do not limit the scope of the invention as claimed. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0021] <First Embodiment> Fig. 1 is a diagram showing an example of the configuration of a quantum key distribution system according to a first embodiment. In the configuration shown in Fig. 1, the quantum key distribution system 1 includes a transmitting device 100 and a receiving device 200. The transmitting device 100 and the receiving device 200 communicate with each other via a transmission path 300. The transmission path 300 may be a part of the quantum key distribution system 1, or may be an external component of the quantum key distribution system 1.

[0022] The transmitting device 100 is also referred to as Alice. The receiving device 200 is also referred to as Bob. The names "transmitting device" and "receiving device" here are names based on the transmitting side and receiving side during quantum key distribution in the following description. A signal may also be transmitted from the receiving device 200 to the transmitting device 100.

[0023] The transmitting device 100 transmits a random number bit string that forms the basis of an encryption key to the receiving device 200 by continuous variable quantum key distribution (CV-QKD) via a transmission path 300. The transmitting device 100 and the receiving device 200 perform key distillation processes such as basis matching, error correction, and privacy amplification on the transmitted and received random number bit string, so that the transmitting device 100 and the receiving device 200 obtain the same encryption key. The transmitted and received random number bit string is also referred to as key information. The obtained encryption key is also referred to as a final key.

[0024] In continuous quantum key distribution, a transmitting device 100 transmits, via a transmission path 300 to a receiving device 200, a signal light carrying a random number bit string (key information) that forms the basis of an encryption key, and a relatively high-intensity (strong) reference light used for homodyne detection of the quadrature phase amplitude of the signal light. Specifically, the transmitting device 100 carries the random number bit string that forms the basis of the encryption key onto the signal light by performing phase modulation on the signal light using pulsed light. In quantum key distribution, homodyne detection is performed on the receiving side to detect the random number bit string carried on the signal light.

[0025] Furthermore, the transmitting device 100 and the receiving device 200 synchronize with each other prior to transmitting and receiving an optical signal carrying a random number bit string that forms the basis of the encryption key. Specifically, the receiving device 200 detects the peak position of the optical signal transmitted by the transmitting device 100 as pulsed light at regular time intervals as the sampling timing of the optical signal. The peak position of the optical signal here refers to the timing at which the intensity of the optical signal becomes maximum or minimum. The receiving device 200 determines the detected peak position of the optical signal within a predetermined time interval during which the transmitting device 100 transmits one optical pulse as the sampling timing of the optical signal.

[0026] The transmitting device 100 and the receiving device 200 are synchronized using light (light carrying the random number bit string) used as signal light when transmitting the random number bit string. Hereinafter, the light used as signal light when transmitting the random number bit string will also be referred to as signal light in the process of synchronizing the transmitting device 100 and the receiving device 200.

[0027] Fig. 2 is a diagram showing an example of peak positions of signal light. The horizontal axis of the graph in Fig. 2 represents time, and the vertical axis represents intensity (light intensity). Line L111 shows an example of the intensity over time of signal light received by the receiving device. Points such as point P111 show example sampling points of signal light by the receiving device.

[0028] In order for the receiving device 200 to detect the signal light with high accuracy during quantum key distribution, it is preferable to sample the signal light at a timing when the intensity of the signal light is as high as possible. Therefore, in the synchronization process, the receiving device 200 detects the timing (time) when the intensity of the signal light generated by the pulsed light is maximized, such as at point P111, as the sampling timing of the signal light.

[0029] Let us now consider a case in which a transmitter can transmit signal light that is much stronger than quantum light. In this case, when synchronizing the transmitter and receiver, the transmitter can transmit only signal light without transmitting reference light. In this case, the receiver measures the intensity of the signal light using a single photodiode and detects the timing when that intensity reaches its maximum.

[0030] However, when the amplitude of the pulsed light transmitted as the signal light is small, the receiving device finds it difficult to obtain the maximum pulse value using the intensity measurement described above. Continuous-quantum key distribution requires that the signal light be weak enough to detect quantum mechanical state changes, and the signal light is transmitted at a low level of intensity. While transmitters are equipped with variable optical attenuators to adjust the intensity of the signal light, there are limits to the amount of attenuation that can be achieved by variable optical attenuators. Even if the variable optical attenuator is set to maximize the transmission of the signal light, as described above, there may be cases where the receiving device is unable to obtain the amplitude required to perform accurate timing detection using a single photodiode.

[0031] Fig. 3 is a diagram showing an example of the intensity of signal light when a receiving device cannot perform accurate timing detection using a single photodiode. The horizontal axis of the graph in Fig. 3 represents time, and the vertical axis represents intensity (light intensity). Line L121 represents an example of the intensity over time of signal light received by the receiving device. As in the example of Fig. 3, it is thought that a low intensity of signal light makes it impossible for the receiving device to accurately detect the peak position of the signal light using a single photodiode, and therefore, cannot perform accurate timing detection.

[0032] Therefore, in the quantum key distribution system 1, even in the process of synchronizing the transmitting device 100 and the receiving device 200, the receiving device 200 performs homodyne detection of the quadrature amplitude of the signal light. The receiving device 200 can amplify the signal indicating the quadrature amplitude of the received light by homodyne detection. This is expected to enable the receiving device 200 to more accurately detect the peak position of the signal light, thereby more accurately synchronizing the transmitting device 100 and the receiving device 200. The signal light used in the process of synchronizing the transmitting device 100 and the receiving device 200 corresponds to an example of the first light. The reference light used in the process of synchronizing the transmitting device 100 and the receiving device 200 corresponds to an example of the second light. The quantum key distribution system 1 uses light with a relatively high intensity as the reference light. In particular, the quantum key distribution system 1 uses light with a higher intensity than the signal light as the reference light.

[0033] In the configuration shown in FIG. 1 , the transmitting device 100 includes a laser light source 101, a beam splitter (BS) 102, a half-wave plate (HWP) 103, an amplitude modulator (AM) 104, a variable optical attenuator (VOA) 105, a phase modulator (PM) 106, and a polarized beam splitter (PBS) 107.

[0034] The laser light source 101 generates and outputs a continuous wave laser beam. Continuous wave laser beams are also called CW beams. The beam splitter 102 splits the CW beam output from the laser light source 101 into two paths. The CW beam output from the beam splitter 102 is used as a reference beam and a signal beam.

[0035] The half-wave plate 103 rotates the polarization of the signal light among the CW light output by the beam splitter 102 by 90°. The intensity modulator 104 pulses the signal light. Specifically, the intensity modulator 104 performs intensity modulation on the polarization-rotated CW light output by the half-wave plate 103, thereby converting the CW light into pulsed light. The intensity modulator 104 corresponds to an example of a first optical output means. When the transmitting device 100 and the receiving device 200 are synchronized, the intensity modulator 104 outputs multiple pulses of the signal light.

[0036] The variable optical attenuator 105 adjusts the intensity of the signal light output from the intensity modulator 104. The phase modulator 106 performs phase modulation on the signal light output from the variable optical attenuator 105.

[0037] During quantum key distribution, i.e., when transmitting a random number bit string that forms the basis of the encryption key from the transmitting device 100 to the receiving device 200, the phase modulator 106 phase-modulates the signal light to superimpose the random number bit string that forms the basis of the encryption key onto the signal light.

[0038] On the other hand, when the transmitting device 100 and the receiving device 200 are synchronized, the phase modulator 106 performs phase modulation on the signal light so that the phase differs for each pulse. The phase modulation here refers to changing the phase. The phase modulator 106 is an example of a phase control means. The phase modulator 106 changes the phase of the signal light for each pulse, thereby changing the phase difference between the signal light and the reference light (the phase difference between the phase of the signal light and the phase of the reference light) so that the phase difference differs for each pulse of the signal light.

[0039] Even if the phase of the signal light and the phase of the reference light are misaligned, it is expected that the receiving device 200 will be able to detect the peak position in any of the pulses by having the phase modulator 106 change the phase of the signal light for each pulse. The phase difference between the signal light and the reference light fluctuates due to disturbances such as vibrations and temperature changes in the quantum key distribution system 1. Below, we consider a case where the time change of disturbances in the quantum key distribution system 1 is gradual and the time change of the fluctuation in the phase difference between the signal light and the reference light is small.

[0040] The polarizing beam splitter 107 combines the reference light and the signal light onto the same optical axis. As a result, the polarizing beam splitter 107 polarization-multiplexes the signal light and the reference light, which are orthogonal to each other by 90°. The light obtained by combining the reference light and the signal light onto the same optical axis by the polarizing beam splitter 107 is transmitted to the receiving device 200. The polarizing beam splitter 107 corresponds to an example of a transmitting means.

[0041] The receiving device 200 includes a polarizing beam splitter 201, a half-wave plate 202, a phase modulator 203, a beam splitter 204, two photodiodes 205 and 206, a subtraction unit 207, an analog / digital converter (ADC) 208, a signal storage unit 209, an absolute value acquisition unit 210, a peak position detection unit 211, a timing determination unit 212, and a timing storage unit 213.

[0042] The combination of the polarizing beam splitter 201, the half-wave plate 202, the phase modulator 203, the beam splitter 204, and the two photodiodes 205 and 206 is also referred to as the optical system of the receiving device 200. The combination of the analog-to-digital conversion unit 208, the signal storage unit 209, the absolute value acquisition unit 210, the peak position detection unit 211, the timing determination unit 212, and the timing storage unit 213 is also referred to as the signal processing system of the receiving device 200. The signal processing system or a part of it may be configured using dedicated hardware. The functions of the signal processing system or a part of it may be executed using a computer.

[0043] The polarizing beam splitter 201 splits the light transmitted from the transmitting device 100 via the transmission path 300 into signal light and reference light. The polarizing beam splitter 201 is an example of a receiving means in that it receives light input from the transmission path 300. The half-wave plate 202 rotates the polarization of the signal light output by the polarizing beam splitter 201 by 90°.

[0044] The phase modulator 203 controls the phase of the reference light output by the polarizing beam splitter 201. During quantum key distribution, that is, when the receiving device 200 performs a receiving process of a random number bit string that forms the basis of an encryption key, the phase modulator 203 performs phase modulation for basis selection on the reference light output by the polarizing beam splitter 201. On the other hand, when the transmitting device 100 and the receiving device 200 are synchronized, the phase modulator 203 does not perform phase modulation on the reference light, and outputs the input reference light as is.

[0045] Beam splitter 204 causes interference between the signal light output from half-wave plate 202 and the reference light output from phase modulator 203. Photodiodes 205 and 206 each detect the light after interference by beam splitter 204. Specifically, photodiodes 205 and 206 photoelectrically convert the light incident thereon.

[0046] The subtraction unit 207 outputs a difference signal obtained by subtracting the result of light detection by the photodiode 206 from the result of light detection by the photodiode 205. Here, the difference signal is a signal indicating the difference in the current values ​​output by the photodiodes when they detect light. The difference signal output by the subtraction unit 207 is converted from a current signal to a voltage signal. For example, a balance detector including the photodiodes 205 and 206 and the subtraction unit 207 amplifies the difference signal, converts it to a voltage, and outputs it to the analog-to-digital conversion unit 208.

[0047] The amplitude of the difference signal output by the subtraction unit 207 is proportional to the product of the amplitude of the signal light output by the beam splitter 204 and the amplitude of the reference light. The amplitude here refers to the absolute value of the amplitude. Here, the signal light is transmitted as weak light to utilize quantum mechanical properties, whereas the reference light is transmitted as light stronger than the signal light. The difference signal output by the subtraction unit 207 can amplify the weak signal light.

[0048] As described above, the transmitting device 100 transmits the reference light to the receiving device 200 as CW light. Therefore, it is not possible to synchronize the transmitting device 100 and the receiving device 200 using the reference light as is. Furthermore, if the signal light from the transmitting device 100 to the receiving device 200 is used as is, it is considered that sufficient intensity for synchronizing the transmitting device 100 and the receiving device 200 cannot be obtained even if the intensity of the signal light is maximized, as described above. In contrast, in the synchronization process, the receiving device 200 can amplify the signal light transmitted as pulsed light by performing homodyne detection of the signal light using the reference light, and it is expected that synchronization between the transmitting device 100 and the receiving device 200 can be achieved with relatively high accuracy.

[0049] The process in which beam splitter 204 causes interference between signal light and reference light, photodiodes 205 and 206 detect the light after interference, and subtractor 207 outputs a difference signal representing the light detection results by photodiodes 205 and 206 corresponds to an example of homodyne detection. The difference signal output by subtractor 207 is also simply referred to as a difference signal. The difference signal corresponds to an example of a signal obtained by homodyne detection. The combination of beam splitter 204, photodiodes 205 and 206, and subtractor 207 corresponds to an example of signal acquisition means.

[0050] The analog / digital conversion unit 208 converts the difference signal output by the subtraction unit 207 from analog to digital. The signal storage unit 209 stores the digital signal output by the analog / digital conversion unit 208. The signal stored in the signal storage unit 209 corresponds to an example of a signal obtained by homodyne detection (a signal converted from an analog signal to a digital signal). The absolute value acquisition unit 210 takes the absolute value of the signal stored in the signal storage unit 209. In other words, the absolute value acquisition unit 210 takes the absolute value of the amplitude of the signal stored in the signal storage unit 209. Here, taking the absolute value means calculating the absolute value. The absolute value acquisition unit 210 corresponds to an example of absolute value acquisition means.

[0051] The peak position detector 211 detects the peak position of the signal output from the absolute value acquirer 210 and stored in the signal storage unit 209, which is the absolute value of the amplitude of the signal. The peak position of the absolute value signal is the timing at which the amplitude of the signal becomes maximum. The peak position detector 211 is an example of a peak position detector.

[0052] The timing determination unit 212 determines the sampling timing of the signal light during quantum key distribution based on the peak position detected by the peak position detection unit 211. The sampling timing of the signal light during quantum key distribution corresponds to an example of the detection timing of the pulsed light transmitted from the transmitting device 100 to the receiving device 200. The timing determination unit 212 corresponds to an example of timing determination means.

[0053] When the timing determination unit 212 determines that the magnitude of the signal amplitude at the peak position detected by the peak position detection unit 211 is greater than a predetermined threshold, the timing determination unit 212 may determine the peak position as the sampling timing of the signal light during quantum key distribution.

[0054] In this case, the timing determination unit 212 determines whether the magnitude of the signal amplitude at each peak position detected by the peak position detection unit 211 (the absolute value of the amplitude calculated by the absolute value acquisition unit 210) is greater than a threshold value. Then, the timing determination unit 212 determines the peak position at which the magnitude of the signal amplitude is determined to be greater than the threshold value as the sampling timing of the signal light during quantum key distribution.

[0055] If it is determined that the magnitude of the signal amplitude at multiple peak positions is greater than a threshold, the timing determination unit 212 may determine a timing based on the multiple peak positions, such as the average timing of the multiple peak positions, as the sampling timing of the signal light during quantum key distribution.

[0056] Alternatively, the timing determination unit 212 may determine the peak position at which the signal amplitude is greatest among the peak positions detected by the peak position detection unit 211 as the sampling timing for the signal light during quantum key distribution.

[0057] The timing storage unit 213 stores the sampling timing of the signal light during quantum key distribution determined by the timing determination unit 212. The timing storage unit 213 is an example of a timing storage means.

[0058] The phase modulation by the phase modulator 106 when the transmitting device 100 and the receiving device 200 are synchronized will now be described. Fig. 4 is a diagram showing a first example of the phase difference between the signal light and the reference light when the phase modulator 106 does not change the phase of the signal light. The horizontal axis of the graph in Fig. 4 represents the real axis, and the vertical axis represents the imaginary axis. The graph in Fig. 4 represents the phase difference between the signal light and the reference light as a rotation angle relative to the positive direction of the real axis, and point P211 represents the case where the phase difference is approximately 0°.

[0059] Fig. 5 is a diagram showing a first example of the amplitude of the difference signal output by the subtraction unit 207 when the phase modulator 106 does not change the phase of the signal light. Fig. 5 shows an example of the amplitude of the difference signal when the phase difference between the signal light and the reference light is close to 0° as in the example of Fig. 4. The horizontal axis of the graph in Fig. 5 represents time. The vertical axis represents amplitude. Amax represents the maximum value that the difference signal can take. Amin represents the minimum value that the difference signal can take. Line L211 represents an example of the amplitude over time of the difference signal output by the subtraction unit 207.

[0060] 4 and 5, when the phase difference between the signal light and the reference light is close to 0°, a pulse with a large amplitude is obtained as the difference signal on the positive side of the vertical axis. Because the amplitude of the difference signal is large, the change in amplitude near the peak where the amplitude is maximum is relatively large, and it is expected that the receiving device 200 can detect the peak position of the difference signal relatively accurately.

[0061] 6 is a diagram showing a second example of the phase difference between the signal light and the reference light when the phase modulator 106 does not change the phase of the signal light. The horizontal axis of the graph in FIG. 6 represents the real axis, and the vertical axis represents the imaginary axis. The graph in FIG. 6 represents the phase difference between the signal light and the reference light as a rotation angle relative to the positive direction of the real axis, and point P221 represents the case where the phase difference is approximately 180°.

[0062] Fig. 7 is a diagram showing a second example of the amplitude of the difference signal output by the subtraction unit 207 when the phase modulator 106 does not change the phase of the signal light. Fig. 7 shows an example of the amplitude of the difference signal when the phase difference between the signal light and the reference light is close to 180° as in the example of Fig. 6. The horizontal axis of the graph in Fig. 7 represents time. The vertical axis represents amplitude. Amax represents the maximum value that the difference signal can take. Amin represents the minimum value that the difference signal can take. Line L221 represents an example of the amplitude over time of the difference signal output by the subtraction unit 207.

[0063] 6 and 7, when the phase difference between the signal light and the reference light is close to 180°, a pulse with a large amplitude is obtained as the difference signal on the negative side of the vertical axis. Because the amplitude of the difference signal is large, the change in amplitude near the peak where the amplitude is minimal is relatively large, and it is expected that the receiving device 200 can detect the peak position of the difference signal relatively accurately.

[0064] 8 is a diagram showing a third example of the phase difference between the signal light and the reference light when the phase modulator 106 does not change the phase of the signal light. The horizontal axis of the graph in FIG. 8 represents the real axis, and the vertical axis represents the imaginary axis. The graph in FIG. 8 represents the phase difference between the signal light and the reference light as a rotation angle relative to the positive direction of the real axis, and point P231 represents the case where the phase difference is approximately 90°.

[0065] Fig. 9 is a diagram showing a third example of the amplitude of the difference signal output by the subtraction unit 207 when the phase modulator 106 does not change the phase of the signal light. Fig. 9 shows an example of the amplitude of the difference signal when the phase difference between the signal light and the reference light is close to 90° as in the example of Fig. 8. In particular, Fig. 9 shows an example when the phase difference between the signal light and the reference light is slightly greater than 90°. The horizontal axis of the graph in Fig. 9 represents time. The vertical axis represents amplitude. Amax represents the maximum value that the difference signal can take. Amin represents the minimum value that the difference signal can take. Line L231 represents an example of the amplitude over time of the difference signal output by the subtraction unit 207.

[0066] 8 and 9 , when the phase difference between the signal light and the reference light is close to 90°, a pulse with a small amplitude is obtained as the difference signal. For example, when the phase difference between the signal light and the reference light is slightly greater than 90°, a pulse with a small amplitude is obtained as the difference signal on the negative side of the vertical axis. Because the amplitude of the difference signal is small, the change in amplitude near the peak where the amplitude is maximum or minimum is relatively small, which may prevent the receiving device 200 from accurately detecting the peak position.

[0067] Fig. 10 is a diagram showing a fourth example of the phase difference between the signal light and the reference light when the phase modulator 106 does not change the phase of the signal light. The horizontal axis of the graph in Fig. 10 represents the real axis, and the vertical axis represents the imaginary axis. The graph in Fig. 10 represents the phase difference between the signal light and the reference light as a rotation angle relative to the positive direction of the real axis, and point P241 represents the case where the phase difference is approximately -90°.

[0068] Fig. 11 is a diagram showing a fourth example of the amplitude of the difference signal output by the subtraction unit 207 when the phase modulator 106 does not change the phase of the signal light. Fig. 11 shows an example of the amplitude of the difference signal when the phase difference between the signal light and the reference light is close to -90° as in the example of Fig. 10. In particular, Fig. 11 shows an example when the phase difference between the signal light and the reference light is slightly greater than -90°. The horizontal axis of the graph in Fig. 11 represents time. The vertical axis represents amplitude. Amax represents the maximum value that the difference signal can take. Amin represents the minimum value that the difference signal can take. Line L241 represents an example of the amplitude over time of the difference signal output by the subtraction unit 207.

[0069] 10 and 11 , when the phase difference between the signal light and the reference light is close to −90°, a pulse with a small amplitude is obtained as the difference signal. For example, when the phase difference between the signal light and the reference light is slightly greater than −90°, a pulse with a small amplitude is obtained as the difference signal on the positive side of the vertical axis. Because the amplitude of the difference signal is small, the change in amplitude near the peak where the amplitude is maximum or minimum is relatively small, which may prevent the receiving device 200 from accurately detecting the peak position.

[0070] It is expected that the closer the phase difference between the signal light and the reference light is to 0° or 180°, the larger the amplitude of the signal output by the absolute value acquisition unit 210, and the more accurately the timing determination unit 212 can detect the peak position. On the other hand, the closer the phase difference between the signal light and the reference light is to 90° or −90°, the smaller the amplitude of the signal output by the absolute value acquisition unit 210, and the lower the accuracy of the peak position detection performed by the timing determination unit 212, or the timing determination unit 212 may not be able to detect the peak position.

[0071] Therefore, when the transmitting device 100 and the receiving device 200 are synchronized, the phase modulator 106 changes the phase of the signal light for each pulse. As a result, for one or more pulses of the signal light transmitted by the transmitting device 100, the phase difference between the signal light and the reference light approaches 0° or 180°, and it is expected that the timing determination unit 212 can detect the peak position with relatively high accuracy.

[0072] 12 is a diagram showing a first example of the phase difference between the signal light and the reference light when the phase modulator 106 changes the signal light. The horizontal axis of the graph in FIG. 12 represents the real axis, and the vertical axis represents the imaginary axis. The graph in FIG. 12 represents the phase difference between the signal light and the reference light as a rotation angle with respect to the positive direction of the real axis.

[0073] Point P311 represents the phase difference between the signal light and the reference light at time t11, where the phase difference is approximately 45°. Point P312 represents the phase difference between the signal light and the reference light at time t12, where the phase difference is approximately 135°. Point P313 represents the phase difference between the signal light and the reference light at time t13, where the phase difference is approximately -135°. Point P314 represents the phase difference between the signal light and the reference light at time t14, where the phase difference is approximately -45°.

[0074] For example, consider a case where the phase modulator 106 has a function of modulating the signal light using the QPSK (Quadrature Phase Shift Keying) method of the constellation shown in Fig. 12. In this case, when the transmitting device 100 and the receiving device 200 are synchronized, the phase modulator 106 can use this function to change the phase for each pulse of the signal light as shown in Fig. 12. When the phase of the reference light is approximately 0°, the phase difference between the signal light and the reference light is as shown in the example of Fig. 12.

[0075] Fig. 13 is a diagram showing a first example of the amplitude of the difference signal output by the subtractor 207 when the phase modulator 106 changes the signal light. Fig. 13 shows an example of the amplitude of the difference signal in the example of Fig. 12. The horizontal axis of the graph in Fig. 13 represents time. The vertical axis represents amplitude. Amax represents the maximum value that the difference signal can take. Amin represents the minimum value that the difference signal can take.

[0076] Line L311 represents an example of the amplitude over time of the difference signal output by the subtraction unit 207. At time t11, when the phase difference is approximately 45°, and at time t14, when the phase difference is approximately −45°, the amplitude of the difference signal is positive. At time t12, when the phase difference is approximately 135°, and at time t13, when the phase difference is approximately −135°, the amplitude of the difference signal is negative. From time t11 to t14, the amplitude of the difference signal is relatively large, and it is expected that the peak position detection unit 211 will be able to detect the peak position with relatively high accuracy.

[0077] 14 is a diagram showing a second example of the phase difference between the signal light and the reference light when the phase modulator 106 changes the signal light. The horizontal axis of the graph in FIG. 14 represents the real axis, and the vertical axis represents the imaginary axis. The graph in FIG. 14 represents the phase difference between the signal light and the reference light as a rotation angle relative to the positive direction of the real axis.

[0078] Point P321 represents the phase difference between the signal light and the reference light at time t21, where the phase difference is approximately 90°. Point P322 represents the phase difference between the signal light and the reference light at time t22, where the phase difference is approximately 180° (approximately −180°). Point P323 represents the phase difference between the signal light and the reference light at time t23, where the phase difference is approximately −90°. Point P324 represents the phase difference between the signal light and the reference light at time t24, where the phase difference is approximately 0°.

[0079] For example, if the phase of the signal light changes by 45° from the example in Figure 12, or if the phase of the reference light changes by -45° from the example in Figure 12, the phase difference between the signal light and the reference light may become as shown in the example in Figure 14.

[0080] Fig. 15 is a diagram showing a second example of the amplitude of the difference signal output by the subtractor 207 when the phase modulator 106 changes the signal light. Fig. 15 shows an example of the amplitude of the difference signal for the example in Fig. 14. The horizontal axis of the graph in Fig. 15 represents time. The vertical axis represents amplitude. Amax represents the maximum value that the difference signal can take. Amin represents the minimum value that the difference signal can take.

[0081] Line L321 represents an example of the amplitude over time of the difference signal output by the subtraction unit 207. At time t21, when the phase difference is approximately 90°, and at time t23, when the phase difference is approximately −90°, the amplitude of the difference signal is approximately 0. On the other hand, at time t22, when the phase difference is approximately 180°, the amplitude of the difference signal is close to the minimum value Amin. Furthermore, at time t24, when the phase difference is approximately 0°, the amplitude of the difference signal is close to the maximum value Amax. At times t22 and t24, the amplitude of the difference signal is large, and it is expected that the peak position detection unit 211 will be able to detect the peak position with relatively high accuracy.

[0082] If the phase modulator 106 changes the phase of the two pulses of the signal light by a phase difference of 90°, the amplitude of the difference signal for at least one of the pulses will be equal to or greater than the amplitude when the phase difference between the signal light and the reference light is 45°, and it is expected that the peak position detection unit 211 will be able to detect the peak position with relatively high accuracy.

[0083] However, the phase difference between pulses when the phase modulator 106 changes the phase for each pulse of the signal light is not limited to 90°. For example, if the peak position detector 211 cannot detect the peak position with high accuracy based on the amplitude of the difference signal when the phase difference between the signal light and the reference light is 45°, the phase modulator 106 may change the phase by an amount smaller than 90°, such as by changing the phase by 45° for each pulse of the signal light.

[0084] Alternatively, if the peak position detector 211 can detect the peak position with high accuracy even when the amplitude of the difference signal is relatively small, the phase modulator 106 may change the phase for each pulse of the signal light with a phase difference greater than 90°. For example, if the peak position detector 211 can detect the peak position with high accuracy at the amplitude of the difference signal when the phase difference between the signal light and the reference light is 30°, the phase modulator 106 may change the phase by 120° for each pulse of the signal light.

[0085] Furthermore, the number of pulses of signal light transmitted by the transmitting device 100 when the transmitting device 100 and the receiving device 200 are synchronized may be more than one, and is not limited to a specific number. For example, consider a case where the receiving device 200 does not include the absolute value acquisition unit 210, and the peak position detection unit 211 detects peak positions only when the amplitude of the difference signal is positive. In this case, as shown in the examples of Figures 12 to 15, the transmitting device 100 transmits four pulses of signal light, with the phase shifted by 90° each time. This ensures that, for at least one pulse, the amplitude of the difference signal is positive and the magnitude of the amplitude of the difference signal is equal to or greater than the magnitude when the phase difference between the signal light and the reference light is 45°. This is expected to enable the peak position detection unit 211 to detect peak positions with relatively high accuracy.

[0086] In particular, the amplitude value of the signal obtained by homodyne detection is positive for one of two pulses of the signal light transmitted by the transmitting device 100, in which the phase difference between the signal light and the reference light is 180°. This is expected to enable the quantum key distribution system 1 to detect the peak position without needing to take the absolute value of the signal amplitude.

[0087] On the other hand, when the receiving device 200 includes the absolute value acquiring unit 210, the transmitting device 100 transmits two pulses of signal light with a phase difference of 90°, so that the amplitude of the difference signal in at least one of the pulses is equal to or greater than the amplitude when the phase difference between the signal light and the reference light is 45°. This is expected to enable the peak position detecting unit 211 to detect the peak position with relatively high accuracy.

[0088] Second Embodiment A quantum key distribution system may change the phase difference between signal light and reference light by changing the phase of reference light. In the second embodiment, an example will be described in which the phase of the reference light is changed without changing the phase of the signal light when a transmitting device and a receiving device are synchronized.

[0089] The configuration of the quantum key distribution system 1 in the second embodiment is the same as that in the first embodiment. The second embodiment will also be described using the configuration shown in FIG. 1. In the quantum key distribution system 1 according to the second embodiment, the phase modulator 106 of the transmitting device 100 does not change the phase of the signal light, and the phase modulator 203 of the receiving device 200 changes the phase of the reference light. Except for this point, the second embodiment is the same as the first embodiment. In the second embodiment, the phase modulator 203 corresponds to an example of phase control means.

[0090] Fig. 16 is a diagram showing a first example of the amplitude of the difference signal output by the subtraction unit 207 when the phase modulator 203 changes the phase of the reference light. Part (A) of Fig. 16 shows an example of the relationship between time and the intensity of the signal light. The horizontal axis of the graph in part (A) of Fig. 16 represents time. The vertical axis represents intensity (light intensity). Line L411 represents the intensity of the signal light over time.

[0091] Part (B) of Fig. 16 shows an example of the relationship between time and the phase of the reference light. The horizontal axis of the graph in part (B) of Fig. 16 represents time. The vertical axis represents phase. Line L412 represents the phase of the reference light over time. Part (C) of Fig. 16 shows an example of the relationship between time and the amplitude of the difference signal. The horizontal axis of the graph in part (C) of Fig. 16 represents time. The vertical axis represents amplitude. Line L413 represents the amplitude of the difference signal over time.

[0092] 16 also shows time steps in the receiving device 200. One of these time steps corresponds to an example of a time interval that is predetermined as a time interval during which the transmitting device 100 transmits one pulsed beam. Each time step is also referred to as each time step. The phase modulator 203 changes the phase of the reference beam for each step. At time step ts11, the phase of the reference beam is approximately 45°. At time step ts12, the phase of the reference beam is approximately 135°. At time step ts12, the phase of the reference beam is approximately −135°. At time step ts14, the phase of the reference beam is approximately −45°.

[0093] 16 shows an example where the phase of the reference beam is approximately 0°. The phase difference obtained by subtracting the phase of the reference beam from the phase of the signal beam is approximately −45° at time step ts11, approximately −135° at time step ts12, approximately 135° at time step ts13, and approximately 45° at time step ts14.

[0094] 16(C), the amplitude of the difference signal is positive at time step ts11 when the phase difference is approximately −45° and at time step ts14 when the phase difference is approximately 45°. The amplitude of the difference signal is negative at time step ts12 when the phase difference is approximately −135° and at time step ts13 when the phase difference is approximately 135°. The amplitude of the difference signal is relatively large at all of time steps ts11 to ts14, and it is expected that the peak position detector 211 will be able to detect the peak positions with relatively high accuracy.

[0095] 17 is a diagram showing a second example of the amplitude of the difference signal output by the subtractor 207 when the phase modulator 203 changes the phase of the reference light. Part (A) of FIG. 17 shows an example of the relationship between time and the intensity of the signal light. The horizontal axis of the graph in part (A) of FIG. 17 represents time. The vertical axis represents intensity (light intensity). Line L421 represents the intensity of the signal light over time.

[0096] Part (B) of Fig. 17 shows an example of the relationship between time and the phase of the reference light. The horizontal axis of the graph in part (B) of Fig. 17 represents time. The vertical axis represents phase. Line L422 represents the phase of the reference light over time. Part (C) of Fig. 17 shows an example of the relationship between time and the amplitude of the difference signal. The horizontal axis of the graph in part (C) of Fig. 17 represents time. The vertical axis represents amplitude. Line L423 represents the amplitude of the difference signal over time.

[0097] 16, Fig. 17 also shows time steps in the receiving device 200. The relationship between time and the intensity of the signal light shown in part (A) of Fig. 17 and the relationship between time and the phase of the reference light shown in part (B) of Fig. 17 are the same as in the example of Fig. 16.

[0098] On the other hand, in the example of Fig. 16, the phase of the signal light is approximately 0°, whereas in the example of Fig. 17, the phase of the signal light is approximately −45°. The phase difference obtained by subtracting the phase of the reference light from the phase of the signal light is approximately −90° at time step ts21, approximately 180° (approximately −180°) at time step ts22, approximately 90° at time step ts23, and approximately 0° at time step ts14.

[0099] As shown in part (C) of Figure 17, the amplitude of the difference signal is approximately 0 at time step ts21, where the phase difference is approximately 90°, and at time step ts23, where the phase difference is approximately -90°. On the other hand, at time step ts22, where the phase difference is approximately 180°, the amplitude of the difference signal is close to the minimum value Amin. Furthermore, at time step ts24, where the phase difference is approximately 0°, the amplitude of the difference signal is close to the maximum value Amax. At time steps ts22 and ts24, the amplitude of the difference signal is large, and it is expected that the peak position detector 211 will be able to detect the peak position with relatively high accuracy.

[0100] As in the case where the phase modulator 106 changes the phase of the signal light, when the phase modulator 203 changes the phase of the reference light, if the phase of the reference light for two pulses of the signal light is changed by a phase difference of 90°, the magnitude of the amplitude of the difference signal for at least one of the pulses will be equal to or greater than the magnitude when the phase difference between the signal light and the reference light is 45°, and it is expected that the peak position detection unit 211 will be able to detect the peak position with relatively high accuracy.

[0101] However, when the phase modulator 203 changes the phase of each pulse of the reference light, the phase difference of the reference light between pulses of the signal light is not limited to 90°. Also, when the phase modulator 203 changes the phase of the reference light, the number of pulses of the signal light transmitted by the transmitting device 100 when the transmitting device 100 and the receiving device 200 are synchronized may be plural, and is not limited to a specific number.

[0102] FIG. 18 is a diagram showing an example of the phase of the reference light when the transmitting device 100 transmits two pulses of signal light when synchronizing the transmitting device 100 and the receiving device 200. The horizontal axis of the graph in FIG. 18 represents time. The vertical axis represents phase. In the example of FIG. 18, the transmitting device 100 transmits one pulse of signal light at each of time steps ts31 and ts32. The phase modulator 203 sets the phase of the reference light to 45° at time step ts31 and to 135° at time step ts32. In the example of FIG. 18, for at least one of the pulses, the magnitude of the amplitude of the difference signal is equal to or greater than the magnitude when the phase difference between the signal light and the reference light is 45°, and it is expected that the peak position detector 211 can detect the peak position with relatively high accuracy.

[0103] 19 is a diagram showing an example of the phase of the reference light when the phase modulator 203 changes the phase of the reference light by a phase difference smaller than 90°. The horizontal axis of the graph in FIG. 19 represents time. The vertical axis represents phase. In the example of FIG. 19, the transmitting device 100 transmits one pulse of signal light for each time step from ts41 to ts44. The phase modulator 203 sets the phase of the reference light to 45° at time step ts41, to 90° at time step ts42, to 135° at time step ts43, and to 180° at time step ts42.

[0104] In the example of Fig. 19, for at least one of the pulses, the magnitude of the amplitude of the difference signal is equal to or greater than the magnitude when the phase difference between the signal light and the reference light is 22.5°. Even if the peak position detector 211 cannot detect the peak position with high accuracy for the magnitude of the amplitude of the difference signal when the phase difference between the signal light and the reference light is 45°, in the example of Fig. 19, it is expected that the peak position detector 211 can detect the peak position with relatively high accuracy.

[0105] FIG. 20 is a diagram showing an example of the phase of the reference light when the phase modulator 203 gradually changes the phase of the reference light. The horizontal axis of the graph in FIG. 20 represents time. The vertical axis represents phase. In the example of FIG. 20, the transmitting device 100 transmits one pulse of signal light for each time step from ts51 to ts54. The phase modulator 203 gradually changes the phase of the reference light from 0° to 180° at a constant rate during the time interval from time step ts51 to ts52. Furthermore, the phase modulator 203 gradually changes the phase of the reference light from 0° to −180° at a constant rate during the time interval from time step ts53 to ts54.

[0106] 20 , for at least one pulse of the signal light, the magnitude of the amplitude of the difference signal is equal to or greater than the magnitude when the phase difference between the signal light and the reference light is 45°, and it is expected that the peak position detector 211 will be able to detect the peak position with relatively high accuracy. Moreover, because the phase modulator 203 shifts the phase of the reference light by 180° to both the positive and negative sides, it is expected that the peak position detector 211 will be able to detect the peak position with relatively high accuracy even if the receiving device 200 does not include the absolute value acquisition unit 210 and the peak position detector 211 detects the peak position only when the amplitude of the difference signal is positive.

[0107] When the receiving device 200 includes the absolute value acquisition unit 210, the transmitting device 100 only needs to transmit two pulses of signal light. The phase modulator 203 only needs to gradually change the phase of the reference light at a constant rate so that the magnitude of the change in the phase of the reference light is 180° in a time interval corresponding to two time steps, such as the time interval from time step ts51 to time step ts52 in FIG. 20 .

[0108] 21 is a diagram showing a first example of a processing procedure for determining the sampling timing of the signal light during quantum key distribution by the receiving device 200. Fig. 21 shows an example in which the timing determiner 212 determines, from among the peak positions detected by the peak position detector 211, the peak position at which the signal amplitude is maximum, as the sampling timing of the signal light during quantum key distribution.

[0109] 21, the receiving device 200 starts a loop L11 in which the receiving device 200 performs processing for each time step (step S11). The receiving device 200 receives one pulse of signal light from the transmitting device 100 for each time step and performs homodyne detection on the received pulse.

[0110] In the processing of loop L11, the analog-to-digital converter 208 converts the signal obtained by homodyne detection (the difference signal output by the subtractor 207) from an analog signal to a digital signal (step S12).Then, the analog-to-digital converter 208 stores the obtained digital signal in the signal storage unit 209 (step S13).

[0111] Next, the absolute value acquiring unit 210 acquires the absolute value of the amplitude of the signal stored in the signal storage unit 209 (step S14). Next, the peak position detecting unit 211 detects the peak position of the signal output by the absolute value acquiring unit 210, which is the absolute value of the signal stored in the signal storage unit 209 (step S15).

[0112] Next, the receiving device 200 performs termination processing of the loop L11 (step S16). Specifically, the receiving device 200 determines whether or not the processing of the loop L11 has been performed for each of a predetermined number of time steps. If it is determined that there are steps for which the processing of the loop L11 has not yet been performed, the receiving device 200 continues to perform the processing of the loop L11 for the steps for which the processing of the loop L11 has not yet been performed. On the other hand, if it is determined that the processing of the loop L11 has been performed for each of a predetermined number of steps, the receiving device 200 terminates the loop L11.

[0113] When the receiving device 200 finishes the loop L11, the timing determination unit 212 detects the peak position having the maximum absolute value of the signal amplitude from among the peak positions detected by the peak position detection unit 211 (step S17). The peak position having the maximum absolute value of the signal amplitude is also referred to as the maximum position. The maximum position detected by the timing determination unit 212 is used as the sampling timing of the signal light during quantum key distribution.

[0114] Then, the timing determination unit 212 stores the detected maximum position in the timing storage unit 213 (step S18). After step S18, the receiving device 200 ends the process of FIG.

[0115] In step S17, the method by which the timing determiner 212 determines the sampling timing of the signal light during quantum key distribution based on the peak position is not limited to the method of detecting the maximum position. For example, the timing determiner 212 may calculate the average or median value of the timing within one time step indicated by the peak position detected by the peak position detector 211.

[0116] 22 is a diagram showing a first example of a processing procedure for determining the sampling timing of the signal light during quantum key distribution by the receiving device 200. Fig. 22 shows an example in which the timing determiner 212 determines, from among the peak positions detected by the peak position detector 211, a peak position at which the magnitude of the signal amplitude is determined to be greater than a threshold value, as the sampling timing of the signal light during quantum key distribution.

[0117] 22 , the receiving device 200 receives one pulse of signal light from the transmitting device 100 for each time step and performs homodyne detection on the received pulse (step S21). Next, the analog-to-digital converter 208 converts the difference signal obtained by homodyne detection from an analog signal to a digital signal (step S22). The analog-to-digital converter 208 then stores the obtained digital signal in the signal storage unit 209 (step S23).

[0118] Next, the absolute value acquiring unit 210 acquires the absolute value of the amplitude of the signal stored in the signal storage unit 209 (step S24). Next, the peak position detecting unit 211 detects the peak position of the signal output by the absolute value acquiring unit 210, which is the absolute value of the signal stored in the signal storage unit 209 (step S25).

[0119] Next, the timing determination unit 212 determines whether the absolute value of the signal amplitude at the peak position detected by the peak position detection unit 211 is greater than a predetermined threshold (step S26). If the timing determination unit 212 determines that the absolute value of the amplitude is equal to or less than the threshold (step S26: NO), the process returns to step S21.

[0120] On the other hand, if it is determined that the absolute value of the amplitude is greater than the threshold (step S26: YES), the timing determiner 212 stores the peak position (the peak position at which it is determined that the absolute value of the signal amplitude is greater than the threshold) in the timing memory 213. After step S213, the receiver 200 ends the process of FIG.

[0121] It is also possible to change both the phase of the signal light and the phase of the reference light. For example, in the configuration of Fig. 1, the phase of the signal light may be adjusted by both the phase modulator 106 of the transmitting device 100 and the phase modulator 203 of the receiving device 200, thereby adjusting the phase difference between the signal light and the reference light.

[0122] As described above, the intensity modulator 104 outputs multiple pulses of signal light. The phase modulator 106 or the phase modulator 203 changes the phase difference between the signal light and the reference light so that the phase difference between the signal light and the reference light varies for each pulse of the signal light. The combination of the beam splitter 204, the photodiodes 205 and 206, and the subtractor 207 performs homodyne detection of the signal light using the reference light. The peak position detector 211 detects the peak position. The peak position is the timing at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. The timing determiner 212 determines the detection timing of the pulsed light (signal light during quantum key distribution) transmitted from the transmitter to the receiver based on the peak position.

[0123] According to the quantum key distribution system 1, it is expected that the detection timing of the signal light during quantum key distribution can be determined with relatively high accuracy using any pulse of the signal light, even when fluctuations occur in the phase difference between the signal light and the reference light. In this respect, according to the quantum key distribution system 1, it is expected that the detection timing of the pulse light can be efficiently detected even when the intensity of the pulse light received by the receiving device 200 is low.

[0124] Furthermore, the phase modulator 106 or the phase modulator 203 changes the phase difference by 90 degrees for each time step. The combination of the beam splitter 204, the photodiodes 205 and 206, and the subtractor 207 homodyne detects the first light for each time step.

[0125] In the quantum key distribution system 1, for any pulse of the signal light, the amplitude of the signal obtained by homodyne detection is greater than the amplitude when the phase difference between the signal light and the reference light is 45°. In this respect, the quantum key distribution system 1 is expected to require a relatively short time for processing to determine the detection timing of the signal light, and to be able to determine the detection timing of the signal light during quantum key distribution with relatively high accuracy.

[0126] Furthermore, when the timing determiner 212 determines that the magnitude of the signal amplitude at the peak position detected by the peak position detector 211 is greater than a predetermined threshold, the timing determiner 212 determines the peak position as the detection timing of the signal light during quantum key distribution. According to the quantum key distribution system 1, when a peak position where the magnitude of the signal amplitude is greater than the threshold is detected, detection of subsequent peak positions can be discontinued. In this respect, according to the quantum key distribution system 1, it is expected that the time required for the process of determining the detection timing of the signal light will be relatively short.

[0127] Furthermore, the timing determiner 212 determines the peak position at which the signal amplitude is maximized, among the peak positions detected by the peak position detector 211, as the detection timing of the signal light during quantum key distribution. According to the quantum key distribution system 1, it is expected that the detection timing of the signal light can be determined with relatively high accuracy, since the peak position at which the signal amplitude is maximized is determined as the detection timing of the signal light.

[0128] Furthermore, the absolute value acquisition unit 210 obtains the absolute value of the amplitude of the signal obtained by homodyne detection. The peak position detection unit 211 detects the timing at which the absolute value of the amplitude of the signal obtained by homodyne detection becomes maximum as the peak position. According to the quantum key distribution system 1, it is possible to detect the peak position at both the timing at which the signal amplitude becomes maximum and the timing at which it becomes minimum, and it is expected that the number of signal light pulses required to determine the detection timing of the signal light will be relatively small. In this respect, it is expected that the quantum key distribution system 1 will require a relatively short time for the process of determining the detection timing of the signal light.

[0129] Furthermore, the phase modulator 106 or the phase modulator 203 changes the phase difference between the signal light and the reference light so that the phase difference between the signal light and the reference light includes a phase difference whose magnitude is 180 degrees. The peak position detector 211 detects, as the peak position, either the timing at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum.

[0130] In the quantum key distribution system 1, among the multiple pulses of signal light transmitted by the transmitting device 100, for one of two pulses in which the phase difference between the signal light and the reference light is 180 degrees, the amplitude value of the signal obtained by homodyne detection is positive. As a result, in the quantum key distribution system 1, it is expected that the peak position can be detected without the need to take the absolute value of the signal amplitude. Therefore, it is expected that the detection timing of the signal light can be determined even if the receiving device 200 is configured without including the absolute value acquisition unit 210. In this respect, it is expected that the quantum key distribution system 1 can make the configuration of the receiving device 200 relatively simple.

[0131] Furthermore, the phase modulator 106 changes the phase of the signal light for each pulse. According to the quantum key distribution system 1, the phase modulator 106 used for quantum key distribution can be used as a phase control means for synchronizing the transmitting device 100 and the receiving device 200, and there is no need to provide a separate configuration for the phase control means. In this respect, according to the quantum key distribution system 1, the configuration of the quantum key distribution system 1 can be made relatively simple.

[0132] Furthermore, the phase modulator 203 changes the phase of the reference light so that the phase of the reference light differs for each pulse of the signal light. According to the quantum key distribution system 1, the phase modulator 203 used in quantum key distribution can be used as a phase control means for synchronizing the transmitting device 100 and the receiving device 200, and there is no need to provide a separate configuration for the phase control means. In this respect, according to the quantum key distribution system 1, the configuration of the quantum key distribution system 1 can be relatively simple.

[0133] 23 is a diagram showing an example of the configuration of a quantum key distribution system according to the third embodiment. In the configuration shown in Fig. 23, a quantum key distribution system 610 includes a first optical output unit 611, a phase control unit 612, a signal acquisition unit 613, a peak position detection unit 614, and a timing determination unit 615.

[0134] In this configuration, the first light output unit 611 outputs multiple pulses of first light, which is pulsed light transmitted from the transmitting side to the receiving side. The phase control unit 612 changes the phase difference between the first light and the second light so that the phase difference between the first light and the second light, which has a higher optical intensity than the first light, varies for each pulse of the first light. The signal acquisition unit 613 performs homodyne detection of the first light using the second light on the receiving side. The peak position detection unit 614 detects peak positions, which are the timings at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. The timing determination unit 615 determines the detection timing of the pulsed light transmitted from the transmitting side to the receiving side based on the peak positions.

[0135] The first optical output unit 611 corresponds to an example of first optical output means. The phase control unit 612 corresponds to an example of phase control means. The signal acquisition unit 613 corresponds to an example of signal acquisition means. The peak position detection unit 614 corresponds to an example of peak position detection means. The timing determination unit 615 corresponds to an example of timing determination means.

[0136] According to the quantum key distribution system 610, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the pulsed light transmitted from the transmitting side to the receiving side can be determined with relatively high accuracy by using any pulse of the first light. In this respect, according to the quantum key distribution system 610, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving side is low.

[0137] 24 is a diagram showing an example of the configuration of a quantum key distribution system according to a fourth embodiment. In the configuration shown in Fig. 24, a quantum key distribution system 620 includes a transmitting device 621 and a receiving device 622. The receiving device 622 includes a signal acquiring unit 623, a peak position detecting unit 624, and a timing determining unit 625.

[0138] With this configuration, the transmitting device 621 transmits multiple pulses of first light, which is pulsed light, by changing the phase for each pulse. The signal acquiring unit 623 performs homodyne detection of the first light using second light, which has a higher optical intensity than the first light. The peak position detecting unit 624 detects peak positions, which are the timings at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum. The timing determining unit 625 determines the detection timing of the pulsed light transmitted from the transmitting device to the receiving device based on the peak positions.

[0139] The signal acquiring unit 623 is an example of a signal acquiring means, the peak position detecting unit 624 is an example of a peak position detecting means, and the timing determining unit 625 is an example of a timing determining means.

[0140] According to the quantum key distribution system 620, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the pulsed light transmitted from the transmitting device 621 to the receiving device 622 can be determined with relatively high accuracy by using any pulse of the first light. In this respect, according to the quantum key distribution system 620, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving device 622 is low.

[0141] 25 is a diagram showing an example of the configuration of a quantum key distribution system according to a fifth embodiment. In the configuration shown in Fig. 25, a quantum key distribution system 630 includes a transmitting device 631 and a receiving device 632. The receiving device 632 includes a phase control unit 633, a signal acquisition unit 634, a peak position detection unit 635, and a timing determination unit 636.

[0142] With this configuration, the transmitting device 631 transmits multiple pulses of first light, which is pulsed light. The phase control unit 633 changes the phase of the second light, which has a higher optical intensity than the first light, so that the phase of the second light differs for each pulse of the first light. The signal acquiring unit 634 performs homodyne detection of the first light using the second light. The peak position detecting unit 635 detects peak positions, which are the timings at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. The timing determining unit 636 determines the detection timing of the pulsed light transmitted from the transmitting device 631 to the receiving device 632 based on the peak positions.

[0143] The phase control unit 633 corresponds to an example of a phase control means. The signal acquisition unit 634 corresponds to an example of a signal acquisition means. The peak position detection unit 635 corresponds to an example of a peak position detection means. The timing determination unit 636 corresponds to an example of a timing determination means.

[0144] According to the quantum key distribution system 630, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the pulsed light transmitted from the transmitting device 631 to the receiving device 632 can be determined with relatively high accuracy by using any pulse of the first light. In this respect, according to the quantum key distribution system 630, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving device 632 is low.

[0145] 26 is a diagram showing an example of the configuration of a receiving device according to the sixth embodiment. In the configuration shown in Fig. 26, a receiving device 640 includes a receiving unit 641, a signal acquiring unit 642, a peak position detecting unit 643, and a timing determining unit 644.

[0146] With this configuration, the receiving unit 641 receives multiple pulses of first light, which is pulsed light transmitted with a different phase for each pulse. The signal acquiring unit 642 performs homodyne detection of the first light using second light, which has a higher optical intensity than the first light. The peak position detecting unit 643 detects peak positions, which are the timings at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. The timing determining unit 644 determines the detection timing of the transmitted pulsed light based on the peak positions.

[0147] The receiving unit 641 is an example of a receiving means. The signal acquiring unit 642 is an example of a signal acquiring means. The peak position detecting unit 643 is an example of a peak position detecting means. The timing determining unit 644 is an example of a timing determining means.

[0148] According to the receiving device 640, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the transmitted pulsed light can be determined with relatively high accuracy by using any pulse of the first light. In this respect, according to the receiving device 640, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving device 640 is low.

[0149] Seventh Embodiment Fig. 27 is a diagram showing an example of the configuration of a receiving device according to a seventh embodiment. In the configuration shown in Fig. 27, a receiving device 650 includes a receiving unit 651, a phase control unit 652, a signal acquisition unit 653, a peak position detection unit 654, and a timing determination unit 655.

[0150] With this configuration, the receiving unit 651 receives multiple pulses of first light, which is pulsed light. The phase control unit 652 changes the phase of the second light, which has a higher optical intensity than the first light, so that the phase of the second light differs for each pulse of the first light. The signal acquiring unit 653 performs homodyne detection of the first light using the second light. The peak position detecting unit 654 detects the peak position, which is the timing at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. The timing determining unit 655 determines the detection timing of the transmitted pulsed light based on the peak position.

[0151] The receiving unit 651 corresponds to an example of receiving means. The phase control unit 652 corresponds to an example of phase control means. The signal acquiring unit 653 corresponds to an example of signal acquiring means. The peak position detecting unit 654 corresponds to an example of peak position detecting means. The timing determining unit 655 corresponds to an example of timing determining means.

[0152] According to the receiving device 650, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the transmitted pulsed light can be determined with relatively high accuracy by using any pulse of the first light. In this respect, according to the receiving device 650, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving device 650 is low.

[0153] Eighth Embodiment Fig. 28 is a diagram showing an example of a processing procedure in a timing determination method according to an eighth embodiment. The timing determination method shown in Fig. 28 includes controlling a phase difference (step S611), performing homodyne detection (step S612), detecting a peak position (step S613), and determining timing (step S614).

[0154] In controlling the phase difference (step S611), the phase difference between the first light, which is pulsed light transmitted from the transmitting side to the receiving side in multiple pulses, and the second light, which has a light intensity greater than that of the first light, is changed so that the phase difference between the first light and the second light differs for each pulse of the first light.

[0155] In performing homodyne detection (step S612), the first light is homodyne detected using the second light. In detecting peak positions (step S613), peak positions are detected, which are timings at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. In determining timing (step S614), the detection timing of the pulsed light transmitted from the transmitting side to the receiving side is determined based on the peak positions.

[0156] According to the timing determination method shown in Fig. 28, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the pulsed light transmitted from the transmitting side to the receiving side can be determined with relatively high accuracy by using any pulse of the first light. In this respect, according to the timing determination method shown in Fig. 28, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving side is low.

[0157] 29 is a diagram showing an example of a processing procedure in a timing determination method according to a ninth embodiment. The timing determination method shown in Fig. 29 includes transmitting a first light (step S621), performing homodyne detection (step S622), detecting a peak position (step S623), and determining timing (step S624).

[0158] In transmitting the first light (step S621), the transmitting device transmits a plurality of pulses of the first light, which is pulsed light, by changing the phase for each pulse. In performing homodyne detection (step S622), the receiving device performs homodyne detection of the first light using a second light having a higher optical intensity than the first light.

[0159] In detecting the peak position (step S623), the receiving device detects the peak position, which is the timing at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. In determining the timing (step S624), the receiving device determines the detection timing of the pulsed light transmitted from the transmitting device to the receiving device based on the peak position.

[0160] According to the timing determination method shown in Fig. 29, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the pulsed light transmitted from the transmitting device to the receiving device can be determined with relatively high accuracy by any pulse of the first light. In this respect, according to the timing determination method shown in Fig. 29, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving device is low.

[0161] 30 is a diagram showing an example of a processing procedure in a timing determination method according to a tenth embodiment. The timing determination method shown in Fig. 30 includes transmitting a first light (step S631), controlling the phase of a second light (step S632), performing homodyne detection (step S633), detecting a peak position (step S634), and determining timing (step S635).

[0162] In transmitting the first light (step S631), the transmitting device transmits a plurality of pulses of the first light, which is pulsed light. In controlling the phase of the second light (step S632), the receiving device changes the phase of the second light, which has a higher optical intensity than the first light, so that the phase of the second light differs for each pulse of the first light.

[0163] In performing homodyne detection (step S633), the receiving device homodyne detects the first light using the second light. In detecting peak positions (step S634), the receiving device detects peak positions, which are timings at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. In determining timing (step S635), the receiving device determines the detection timing of the pulsed light transmitted from the transmitting device to the receiving device based on the peak positions.

[0164] According to the timing determination method shown in Fig. 30, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the pulsed light transmitted from the transmitting device to the receiving device can be determined with relatively high accuracy by any pulse of the first light. In this respect, according to the timing determination method shown in Fig. 30, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving device is low.

[0165] 31 is a diagram showing an example of a processing procedure in a timing determination method according to an eleventh embodiment. The timing determination method shown in Fig. 31 includes receiving a first light (step S641), performing homodyne detection (step S642), detecting a peak position (step S643), and determining timing (step S644).

[0166] In receiving first light (step S641), the first light is received, which is pulsed light transmitted with a different phase for each pulse. In performing homodyne detection (step S642), the first light is homodyne detected using second light having a higher optical intensity than the first light. In detecting peak positions (step S643), peak positions are detected, which are the timings at which the amplitude of the signal obtained by homodyne detection becomes maximum or minimum. In determining timing (step S644), the detection timing of the transmitted pulsed light is determined based on the peak positions.

[0167] According to the timing determination method shown in Fig. 31, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the pulsed light transmitted from the transmitting device to the receiving device can be determined with relatively high accuracy by using any pulse of the first light. In this respect, according to the timing determination method shown in Fig. 31, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving device is low.

[0168] 32 is a diagram showing an example of a processing procedure in a timing determination method according to a twelfth embodiment. The timing determination method shown in Fig. 32 includes receiving a first light (step S651), controlling the phase of a second light (step S652), performing homodyne detection (step S653), detecting a peak position (step S654), and determining timing (step S655).

[0169] In receiving the first light (step S651), a plurality of pulses of the first light, which is pulsed light, are received. In controlling the phase of the second light (step S652), the phase of the second light, which has a light intensity greater than that of the first light, is changed so that the phase of the second light differs for each pulse of the first light.

[0170] In performing homodyne detection (step S653), the first light is homodyne detected using the second light. In detecting peak positions (step S654), peak positions are detected, which are timings at which the amplitude of the signal obtained by homodyne detection reaches a maximum or minimum. In determining timing (step S655), the detection timing of the transmitted pulsed light is determined based on the peak positions.

[0171] According to the timing determination method shown in Fig. 32, even if fluctuations occur in the phase difference between the first light and the second light, it is expected that the detection timing of the pulsed light transmitted from the transmitting device to the receiving device can be determined with relatively high accuracy by using any pulse of the first light. In this respect, according to the timing determination method shown in Fig. 32, it is expected that the detection timing of the pulsed light can be efficiently detected even if the intensity of the pulsed light received by the receiving device is low.

[0172] 33 is a schematic block diagram illustrating the configuration of a computer according to at least one embodiment. In the configuration shown in FIG. 33, a computer 700 includes a CPU 710, a main memory device 720, an auxiliary memory device 730, an interface 740, a non-volatile recording medium 750, and a quantum chip 760.

[0173] The processing of one or more of the above-described transmitting device 100, receiving device 200, quantum key distribution system 610, transmitting device 621, receiving device 622, transmitting device 631, receiving device 632, receiving device 640, and receiving device 650, or a part thereof, may be implemented in a computer 700. In this case, the operation of each of the above-described processing units is stored in the auxiliary storage device 730 in the form of a program. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the above-described processing in accordance with the program. The CPU 710 also allocates storage areas in the main storage device 720 corresponding to each of the above-described storage units in accordance with the program. Communication between each device and other devices is executed by the interface 740, which has a communication function, and performs communication under the control of the CPU 710.

[0174] When the processing of the transmitting device 100 is implemented in the computer 700, the operation of each unit of the transmitting device 100 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the above processing in accordance with the program.

[0175] Furthermore, the CPU 710 allocates a storage area in the main storage device 720 for the transmission device 100 to perform processing in accordance with the program. Communication between the transmission device 100 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the transmission device 100 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0176] When the processing of the receiving device 200 is implemented in the computer 700, the operation of each unit of the receiving device 200 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the processing described above in accordance with the program.

[0177] Furthermore, the CPU 710 allocates a storage area in the main storage device 720 for the receiving device 200 to perform processing in accordance with a program. Communication between the receiving device 200 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the receiving device 200 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0178] When the processing of the quantum key distribution system 610 is implemented in the computer 700, the operation of each unit of the quantum key distribution system 610 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the above processing in accordance with the program.

[0179] Furthermore, the CPU 710 allocates a storage area in the main storage device 720 for the quantum key distribution system 610 to perform processing in accordance with the program. Communication between the quantum key distribution system 610 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the quantum key distribution system 610 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0180] When the processing of the transmitting device 621 is implemented in the computer 700, the operation of each unit of the transmitting device 621 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the processing described above in accordance with the program.

[0181] Furthermore, the CPU 710, in accordance with the program, allocates a storage area in the main storage device 720 for the transmission device 621 to perform processing. Communication between the transmission device 621 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the transmission device 621 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0182] When the processing of the receiving device 622 is implemented in the computer 700, the operation of each unit of the receiving device 622 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the processing described above in accordance with the program.

[0183] Furthermore, the CPU 710, in accordance with the program, allocates a storage area in the main storage device 720 for the receiving device 622 to perform processing. Communication between the receiving device 622 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the receiving device 622 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0184] When the processing of the transmitting device 631 is implemented in the computer 700, the operation of each part of the transmitting device 631 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the above processing in accordance with the program.

[0185] Furthermore, the CPU 710, in accordance with the program, allocates a storage area in the main storage device 720 for the transmission device 631 to perform processing. Communication between the transmission device 631 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the transmission device 631 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0186] When the processing of the receiving device 632 is implemented in the computer 700, the operation of each unit of the receiving device 632 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the processing described above in accordance with the program.

[0187] Furthermore, the CPU 710, in accordance with the program, allocates a storage area in the main storage device 720 for the receiving device 632 to perform processing. Communication between the receiving device 632 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the receiving device 632 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0188] When the processing of the receiving device 640 is implemented in the computer 700, the operation of each unit of the receiving device 640 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the processing described above in accordance with the program.

[0189] Furthermore, the CPU 710, in accordance with the program, allocates a storage area in the main storage device 720 for the receiving device 640 to perform processing. Communication between the receiving device 640 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the receiving device 640 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0190] When the processing of the receiving device 650 is implemented in the computer 700, the operation of each unit of the receiving device 650 is stored in the form of a program in the auxiliary storage device 730. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main storage device 720, and executes the processing described above in accordance with the program.

[0191] Furthermore, the CPU 710, in accordance with the program, allocates a storage area in the main storage device 720 for the receiving device 650 to perform processing. Communication between the receiving device 650 and other devices is performed by the interface 740, which has a communication function and operates under the control of the CPU 710. Interaction between the receiving device 650 and a user is performed by the interface 740, which has a display device and an input device, displaying various images under the control of the CPU 710 and accepting user operations.

[0192] One or more of the above-described programs may be recorded on nonvolatile recording medium 750. In this case, interface 740 may read the programs from nonvolatile recording medium 750. Then, CPU 710 may directly execute the programs read by interface 740, or may temporarily store the programs in main storage device 720 or auxiliary storage device 730 and then execute them.

[0193] Note that a program for executing all or part of the processing performed by the transmitting device 100, the receiving device 200, the quantum key distribution system 610, the transmitting device 621, the receiving device 622, the transmitting device 631, the receiving device 632, the receiving device 640, and the receiving device 650 may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be loaded into a computer system and executed to perform the processing of each unit. Note that the term "computer system" herein includes hardware such as an OS and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), a CD-ROM (Compact Disc Read Only Memory), and a storage device such as a hard disk built into a computer system. Furthermore, the program may be for realizing part of the above-described functions, or may be capable of realizing the above-described functions in combination with a program already recorded in the computer system.

[0194] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0195] Some or all of the above-described embodiments can be described as, but are not limited to, the following supplementary notes.

[0196] (Supplementary Note 1) A quantum key distribution system comprising: first light output means for outputting a plurality of pulses of first light, which is pulsed light to be transmitted from a transmitting side to a receiving side; phase control means for changing the phase difference between the first light and a second light having a light intensity greater than that of the first light, so that the phase difference between the first light and the second light differs for each pulse of the first light; signal acquisition means for performing homodyne detection of the first light using the second light at the receiving side; peak position detection means for detecting a peak position, which is a timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and timing determination means for determining the detection timing of the pulsed light transmitted from the transmitting side to the receiving side based on the peak position.

[0197] (Supplementary Note 2) The quantum key distribution system according to Supplementary Note 1, wherein the phase control means changes the phase difference by 90 degrees for each time step, and the signal acquisition means homodyne detects the first light for each time step.

[0198] (Supplementary Note 3) The quantum key distribution system according to Supplementary Note 1 or Supplementary Note 2, wherein the timing determination means determines the peak position as the detection timing when it determines that the magnitude of the amplitude at the peak position detected by the peak position detection means is greater than a predetermined threshold.

[0199] (Supplementary Note 4) The quantum key distribution system according to Supplementary Note 1 or Supplementary Note 2, wherein the timing determination means determines, as the detection timing, a peak position at which the amplitude is maximum among the peak positions detected by the peak position detection means.

[0200] (Supplementary Note 5) The quantum key distribution system according to any one of Supplementary Notes 1 to 4, further comprising: absolute value acquisition means for acquiring an absolute value of the amplitude of the signal obtained by the homodyne detection; and the peak position detection means detects, as the peak position, a timing at which the absolute value of the amplitude of the signal obtained by the homodyne detection becomes maximum.

[0201] (Supplementary Note 6) The quantum key distribution system according to any one of Supplementary Notes 1 to 4, wherein the phase control means changes the phase difference between the first light and the second light so that the phase difference between the first light and the second light includes a phase difference whose magnitude is 180 degrees, and the peak position detection means detects, as the peak position, either a timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum.

[0202] (Supplementary Note 7) The quantum key distribution system according to any one of Supplementary Notes 1 to 6, wherein the phase control means changes the phase of the first light for each pulse.

[0203] (Supplementary Note 8) The quantum key distribution system according to any one of Supplementary Notes 1 to 6, wherein the phase control means changes the phase of the second light so that the phase of the second light differs for each pulse of the first light.

[0204] (Supplementary Note 9) A quantum key distribution system comprising: a transmitting device and a receiving device, wherein the transmitting device transmits a plurality of pulses of a first light, which is pulsed light, by changing the phase for each pulse; and the receiving device comprises: signal acquiring means for performing homodyne detection of the first light using a second light having a light intensity greater than that of the first light; peak position detecting means for detecting a peak position, which is a timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and timing determining means for determining a detection timing of the pulsed light to be transmitted from the transmitting device to the receiving device based on the peak position.

[0205] (Supplementary Note 10) A quantum key distribution system comprising: a transmitting device and a receiving device, wherein the transmitting device transmits a plurality of pulses of a first light which is pulsed light; and the receiving device comprises: a phase control means for changing a phase of a second light having a light intensity greater than that of the first light such that a phase of the second light differs for each pulse of the first light; a signal acquiring means for performing homodyne detection of the first light using the second light; a peak position detecting means for detecting a peak position which is a timing at which an amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and a timing determining means for determining a detection timing of the pulsed light transmitted from the transmitting device to the receiving device based on the peak position.

[0206] (Supplementary Note 11) A receiving device comprising: receiving means for receiving a plurality of pulses of first light, which is pulsed light transmitted with a different phase for each pulse; signal acquiring means for performing homodyne detection of the first light using second light having a light intensity greater than that of the first light; peak position detecting means for detecting a peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and timing determining means for determining the detection timing of the transmitted pulsed light based on the peak position.

[0207] (Supplementary Note 12) A receiving device comprising: a receiving means for receiving a plurality of pulses of a first light which is pulsed light; a phase control means for changing the phase of a second light having a light intensity greater than that of the first light so that the phase of the second light differs for each pulse of the first light; a signal acquisition means for performing homodyne detection of the first light using the second light; a peak position detection means for detecting a peak position which is a timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and a timing determination means for determining the detection timing of the transmitted pulsed light based on the peak position.

[0208] (Supplementary Note 13) A timing determination method including: changing a phase difference between a first light, which is pulsed light transmitted in multiple pulses from a transmitting side to a receiving side, and a second light, which has a light intensity greater than that of the first light, so that the phase difference differs for each pulse of the first light; performing homodyne detection on the first light using the second light; detecting a peak position, which is a timing when the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and determining the detection timing of the pulsed light transmitted from the transmitting side to the receiving side based on the peak position.

[0209] (Supplementary Note 14) A timing determination method including: a transmitting device transmitting a plurality of pulses of a first light, which is pulsed light, by changing the phase for each pulse; a receiving device performing homodyne detection of the first light using a second light having a light intensity greater than that of the first light; the receiving device detecting a peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and the receiving device determining the detection timing of the pulsed light transmitted from the transmitting device to the receiving device based on the peak position.

[0210] (Supplementary Note 15) A timing determination method including: a transmitting device transmitting a plurality of pulses of a first light which is pulsed light; a receiving device changing a phase of a second light having a light intensity greater than that of the first light so that the phase of the second light differs for each pulse of the first light; the receiving device homodyne detecting the first light using the second light; the receiving device detecting a peak position which is a timing at which the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and the receiving device determining a detection timing of the pulsed light transmitted from the transmitting device to the receiving device based on the peak position.

[0211] (Supplementary Note 16) A timing determination method including: receiving first light, which is pulsed light transmitted with a different phase for each pulse; homodyne detecting the first light using second light having a light intensity greater than that of the first light; detecting a peak position, which is a timing at which the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and determining the detection timing of the transmitted pulsed light based on the peak position.

[0212] (Supplementary Note 17) A timing determination method including: receiving a plurality of pulses of a first light which is pulsed light; changing the phase of a second light having a light intensity greater than that of the first light so that the phase of the second light differs for each pulse of the first light; performing homodyne detection on the first light using the second light; detecting a peak position, which is a timing when the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and determining the detection timing of the transmitted pulsed light based on the peak position.

[0213] (Supplementary Note 18) A recording medium having recorded thereon a program for causing a computer controlling a receiving device that receives a plurality of pulses of first light, which is pulsed light transmitted with a different phase for each pulse, and performs homodyne detection of the first light using second light having a higher optical intensity than the first light, to: detect a peak position, which is the timing at which the amplitude of a signal obtained by the homodyne detection becomes maximum or minimum; and determine the detection timing of the transmitted pulsed light based on the peak position.

[0214] (Supplementary Note 19) A recording medium having recorded thereon a program for causing a computer to control a receiving device that receives a plurality of pulses of first light, which is pulsed light, and performs homodyne detection of the first light using second light having a light intensity greater than that of the first light, to execute the following: changing the phase of the second light so that the phase of the second light differs for each pulse of the first light; detecting a peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection becomes maximum or minimum; and determining the detection timing of the transmitted pulsed light based on the peak position.

[0215] The present invention may be applied to a quantum key distribution system, a receiving device, a timing determination method, and a recording medium.

[0216] 1, 610, 620, 630 Quantum key distribution system 100, 621, 631 Transmitting device 200, 622, 632, 640, 650 Receiving device 101 Laser light source 102 Beam splitter 103 Half-wave plate 104 Intensity modulator 105 Variable optical attenuator 106 Phase modulator 107 Polarizing beam splitter 201 Polarizing beam splitter 202 Half-wave plate 203 Phase modulator 204 Beam splitter 205, 206 Photodiode 207 Subtraction unit 208 Analog / digital conversion unit 209 Signal storage unit 210 Absolute value acquisition unit 211, 614, 624, 635, 643, 654 Peak position detection unit 212, 615, 625, 636, 644, 655 Timing determination unit 213 Timing storage unit 611 First optical output unit 612, 633, 652 Phase control unit 613, 623, 634, 642, 653 Signal acquisition unit 641, 651 Receiving unit

Claims

1. A first optical output means that outputs multiple pulses of the first light, which is pulsed light transmitted from the transmitting side to the receiving side, A phase control means for changing the phase difference such that the phase difference between the first light and the second light, which has a greater light intensity than the first light, differs for each pulse of the first light; The receiving side includes signal acquisition means for homodyne detection of the first light using the second light, A peak position detection means for detecting the peak position which is the timing at which the amplitude of the signal obtained by the homodyne detection is at its maximum or minimum, A timing determination means for determining the detection timing of pulse light transmitted from the transmitting side to the receiving side based on the peak position, A quantum key distribution system equipped with [the following features].

2. The phase control means changes the phase difference by 90 degrees at each time step. The signal acquisition means performs homodyne detection of the first light at each time step. The quantum key distribution system according to claim 1.

3. The timing determination means determines the peak position at the detection timing when it is determined that the amplitude at the peak position detected by the peak position detection means is greater than a predetermined threshold. A quantum key distribution system according to claim 1 or claim 2.

4. The timing determination means determines the peak position where the amplitude is maximum among the peak positions detected by the peak position detection means as the detection timing. A quantum key distribution system according to claim 1 or claim 2.

5. The system further includes an absolute value acquisition means for obtaining the absolute value of the amplitude of the signal obtained by the homodyne detection, The peak position detection means detects the timing at which the absolute value of the amplitude of the signal obtained by homodyne detection becomes maximum as the peak position. The quantum key distribution system according to claim 1.

6. The phase control means changes the phase difference between the first light and the second light such that the phase difference between the first light and the second light includes a phase difference whose magnitude is 180 degrees. The peak position detection means detects the peak position as either the timing at which the amplitude of the signal obtained by homodyne detection is maximum or minimum. The quantum key distribution system according to claim 1.

7. The phase control means changes the phase of the first light pulse by pulse. The quantum key distribution system according to claim 1.

8. The phase control means changes the phase of the second light such that the phase of the second light is different for each pulse of the first light. The quantum key distribution system according to claim 1.

9. The phase difference between a first light, which is pulsed light transmitted from the transmitting side to the receiving side for multiple pulses, and a second light, which has a greater light intensity than the first light, is changed so that the phase difference differs for each pulse of the first light. The first light is homodyne detected using the second light, The peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection is at its maximum or minimum, is detected. The detection timing of the pulse light transmitted from the transmitting side to the receiving side is determined based on the peak position. A method for determining timing, including the following.

10. The computer provided in the receiving device: Receiving a first pulse of light output in multiple pulses transmitted from a transmitting device, and a second pulse of light controlled to change the phase difference between the first light and a second light having a greater light intensity than the first light for each pulse of the first light, Using the second light to perform homodyne detection on the first light, The process involves detecting the peak position, which is the timing at which the amplitude of the signal obtained by the homodyne detection is at its maximum or minimum. The detection timing of the pulsed light transmitted from the transmitting device is determined based on the peak position. A program that executes the command.