Quantum Cryptography Communication System, Its Communication Device, and Control Method

Abstract

Provided are a communication device and a control method for a quantum cryptography communication system capable of improving the SN ratio in homodyne detection and stabilizing the signal output. In the quantum cryptography communication system, a transmitter (Alice) and a receiver (Bob) are optically connected through an optical transmission line C. The transmitter, among first light and second light generated via splitting by a beam splitter (BS1), generates weak signal light Q having a quantum state by subjecting the first light to phase modulation and intensity attenuation, and outputs the second light as reference light LO together with the signal light Q to the optical transmission line (C). The receiver comprises: an optical amplifier that amplifies the reference light LO while preserving the wavelength and phase; a phase modulator (14) that phase-modulates the amplified reference light; homodyne detectors (BS1, PD1, PD2) that produce a signal output on the basis of the signal light Q and the phase-modulated reference light LO; a level detector (16) that detects a signal output level from the signal output; and an optical amplifier control unit (17) that controls the amplification factor of the optical amplifier (13) on the basis of at least the signal output level.

Description

Technical Field

[0001] The present invention relates to a quantum cryptographic communication system, and more particularly to a communication device for sharing a cryptographic key by quantum cryptographic communication and a control method thereof.

Background Art

[0002] In the field of optical communication, quantum key distribution (QKD) systems are actively being researched and put into practical use as systems that achieve high confidentiality of transmission lines. In such QKD systems, in recent years, continuous variable QKD using continuous variables such as quadrature-phase amplitude of light instead of discrete amounts in photon units has been proposed. In particular, homodyne detection for measuring the quadrature-phase amplitude on the receiving side has attracted attention because it is possible to measure the quantum noise limit even when using an ordinary photodiode at room temperature and achieve high quantum efficiency (Patent Document 1).

[0003] According to Patent Document 1, in continuous variable QKD, at the transmitter (Alice) terminal, a laser beam is split by a beam splitter into a reference light (hereinafter referred to as LO (local oscillator) light) and a signal light, and a weak signal light randomly phase-modulated and the LO light are transmitted to the receiver (Bob) terminal. At the receiver terminal, after randomly phase-modulating the arrived LO light, the same arrived weak signal light as the LO light is detected by two photodetectors through a beam splitter. The phase information of the signal light phase-modulated on the transmission side can be extracted by this homodyne detection.

[0004] At this time, the level average value of the signal light after homodyne detection is 2√n1√n0 as described in the above Patent Document 1, where n1 is the number of photons of the signal light and n0 is the number of photons of the LO light. Since the transmission loss of the optical fiber is 0.2 dB / km or more, at a transmission distance of 50 km, it is 10 dB, that is, the optical power is reduced to 1 / 10, and at a transmission distance of 100 km, it is reduced to 1 / 100. Therefore, the signal level after homodyne detection is also 1 / 10 or less and 1 / 100 or less at transmission distances of 50 km and 100 km, respectively.

[0005] Such attenuation of the signal level degrades the signal-to-noise ratio (SNR) in homodyne detection. To prevent such degradation of the SNR, it is necessary to increase the signal level. However, a countermeasure of providing an optical amplifier in the transmission line cannot be adopted because the signal light is also amplified and affects the information of the encryption key. Also, although the laser output of the transmitter terminal may be increased, if the increase in the laser output is used to compensate for the above-mentioned attenuation of the signal level, for example, it is necessary to significantly increase the laser light source from 10 mW (Class 1) to 1 W (Class 4), which leads to an increase in the size of the device, problems with the durability of optical components, and a decrease in safety during transmission, making it impractical (in the case of the 1.5-μm band of the laser).

[0006] Therefore, in Patent Document 2, a configuration is proposed in which only the LO light is amplified at the receiver terminal in order to improve the SNR in homodyne detection.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in the communication terminal disclosed in the above Patent Document 2, it is only described that only the LO light is amplified in order to improve the SNR in homodyne detection, and it is not described how to perform amplification control to achieve the purpose of improving the SNR.

[0009] Not only the improvement of the SN ratio in homodyne detection, but also the stability of the signal output level obtained based on the LO light and the signal light is important. In the above Patent Document 2, although high-precision timing control can be achieved by using the amplified LO light for the timing control of the phase modulation process, a stable level of signal output cannot be obtained.

[0010] Therefore, an object of the present invention is to provide a quantum key distribution communication system, a communication device, and a control method thereof that can achieve an improvement in the SN ratio and stabilization of the signal output in homodyne detection.

Means for Solving the Problems

[0011] A quantum key distribution communication system according to an aspect of the present invention is a quantum key distribution communication system including a transmitter and a receiver connected via a communication network, wherein the transmitter and the receiver are optically connected through an optical transmission line, the transmitter includes a beam splitter that splits coherent light into a first light and a second light, generates a weak signal light having a quantum state by applying phase modulation and intensity attenuation to the first light, uses the second light as a reference light having no quantum state, and outputs the signal light and the reference light to the optical transmission line, the receiver includes an optical receiver that receives the signal light and the reference light that have reached through the optical transmission line, an optical amplifier that amplifies the reference light received by the optical receiver, a phase modulator that applies phase modulation to the reference light output from the optical amplifier, a homodyne detector that generates a signal output based on the signal light that has reached through the optical transmission line and the phase-modulated reference light, a level detector that detects a signal output level from the signal output, and an optical amplifier controller that controls the amplification factor of the optical amplifier based at least on the signal output level. , with a controllable amplification factor It is characterized by including. the an optical amplifier controller that controls the amplification factor of the optical amplifier. A communication device according to an aspect of the present invention is a communication device that acquires a signal output by homodyne detection in a quantum key distribution communication system. The communication device includes an optical receiver that receives, through an optical transmission line, a weak signal light having a quantum state obtained from coherent light by a communication device on the transmission side and a reference light having no quantum state, and the reference light received by the optical receiver is , with a controllable amplification factor an optical amplifier that amplifies the reference light, a phase modulator that performs phase modulation on the reference light output from the optical amplifier, a homodyne detector that generates a signal output based on the signal light that has reached through the optical transmission line and the phase-modulated reference light, a level detector that detects a signal output level from the signal output, and an optical amplifier controller that controls an amplification factor of the optical amplifier based at least on the signal output level. The communication device is characterized by including these components. the A control method for a communication device according to an aspect of the present invention is a control method for a communication device that acquires a signal output by homodyne detection in a quantum key distribution communication system. In the control method, an optical receiver receives, through an optical transmission line, a weak signal light having a quantum state obtained from coherent light by a communication device on the transmission side and a reference light having no quantum state, an optical amplifier amplifies the reference light that has reached through the optical transmission line, a phase modulator performs phase modulation on the reference light output from the optical amplifier, a homodyne detector generates a signal output based on the signal light that has reached through the optical transmission line and the phase-modulated reference light, a level detector detects a signal output level from the signal output, and an optical amplifier controller controls an amplification factor of the optical amplifier based at least on the signal output level. The control method is characterized by these steps. , with a controllable amplification factor the

Advantages of the Invention

[0012] According to the present invention, it is possible to achieve an improvement in the signal-to-noise ratio and stabilization of the signal output in homodyne detection in a quantum key distribution communication system.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2A

Figure 2B

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9A

Figure 9B

Figure 10

Embodiments for Carrying Out the Invention

[0014] <Summary of the Embodiment> According to an embodiment of the present invention, in a system that transmits a weak signal light having a quantum state and a reference light of normal intensity without a quantum state from a transmitting communication device to a receiving communication device, and receives signal information by a homodyne method on the receiving side, an optical amplifier that amplifies only the reference light is provided on the receiving side, and the amplification factor is controlled based on at least the signal output level obtained by homodyne detection. Thereby, the intensity of the reference light can be increased to improve the signal-to-noise ratio in homodyne detection, and further, the stabilization of the signal output can be achieved by controlling the optical amplification factor.

[0015] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the components described in the following embodiments and examples are merely illustrative, and are not intended to limit the technical scope of the present invention thereto.

[0016] 1. Embodiment <First Embodiment> As illustrated in FIG. 1, communication devices including a transmitter (Alice) and a communication device including a receiver (Bob) can perform encrypted communication using a quantum encryption key generated between those communication devices. Here, it is assumed that the transmitter (Alice) and the receiver (Bob) are optically connected by an optical transmission line C. However, as will be described later, the optical transmission line C is a concept including not only an optical fiber but also free space.

[0017] The transmitter (Alice) includes a laser light source 10, a beam splitter BS1, and an optical transmission unit, and the optical transmission unit consists of a phase modulator 11, an attenuator 12, and a mirror M1. The laser light source 10 generates coherent light, and the beam splitter BS1 splits the coherent light into lights in two paths R1 and R2. The light in one path R1 is phase-modulated by the phase modulator 11, and further becomes weak signal light Q having a quantum state by the attenuator 12 and is sent to the optical transmission line C. The light in the other path R2 is reflected by the mirror M1 and sent to the optical transmission line C as reference light LO having a normal intensity without a quantum state. As described in the above-mentioned Patent Document 1, the intensity of the reference light LO is significantly larger than that of the signal light Q. For example, while the signal light Q has an intensity of about one photon, the reference light LO has an intensity of about 10 million photons.

[0018] The receiver (Bob) has an optical amplifier 13, a phase modulator 14, and a mirror M2, a beam splitter BS2 that constitutes a homodyne detector, two optical detectors PD1 and PD2, and a differential arithmetic unit 15, and further has a signal output level detector 16 and an optical amplifier control unit 17.

[0019] The optical amplifier 13 optically amplifies the reference light LO that has reached from the transmitter (Alice) while maintaining the wavelength and phase, the phase modulator 14 phase-modulates the optically amplified reference light LO, and the phase-modulated reference light LO is incident on the beam splitter BS2. Also, the signal light Q that has reached from the transmitter (Alice) is reflected by the mirror M2 and is incident on the beam splitter BS2. The beam splitter BS2 has an equal light transmittance and reflectance, and the phase-modulated reference light LO and the signal light Q reflected by the mirror M2 are input overlapping. In other words, the beam splitter BS1 of the transmitter (Alice) and the beam splitter BS2 of the receiver (Bob) constitute an interferometer consisting of two paths R1 and R2 of equal length.

[0020] The two output lights of the beam splitter BS2 are respectively incident on the photodetectors PD1 and PD2 and converted into electrical signals. The detection signals respectively output from the photodetectors PD1 and PD2 are subjected to differential operation in the differential operation unit 15, and the difference signal as a result thereof is the signal output I obtained by homodyne detection. out Note that normal photodiodes can be used for the photodetectors PD1 and PD2 at room temperature.

[0021] The signal output level detector 16 detects the level or average value of the signal output I. out As the signal output level detector 16, for example, a low-pass filter can be used. The optical amplifier control unit 17 inputs the level signal L out obtained by the signal output level detector 16, and controls the amplification factor of the optical amplifier 13 so that the level signal L out is maintained within a predetermined range equal to or higher than the threshold value L TH . For example, when the transmission loss in the optical transmission path C increases and the level signal L out of the signal output I out drops below the threshold value L TH , the optical amplifier control unit 17 can increase the amplification factor of the optical amplifier 13 to compensate for the transmission loss.

[0022] The optical amplifier 13 can amplify the reference light LO received from the transmitter (Alice) while maintaining the wavelength and phase of the light without electrical conversion, and can control the amplification factor (gain). As such an optical amplifier 13, for example, an erbium-doped fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) or a semiconductor optical amplifier (SOA: Semiconductor Optical Amplifier) can be used. When an EDFA is adopted for the optical amplifier 13, the reference light LO can be amplified with high efficiency such that the amplification efficiency with respect to the pump light is 80% or more, and the optical amplification factor of the EDFA can be controlled by controlling the current supplied to the laser that is the light source of the pump light. Also, in the case of the SOA for the optical amplifier 13, the amplification factor can be controlled by the current supplied to the SOA. If the optical amplifier 13 can amplify, for example, with a gain of 20 dB, it can compensate for the attenuation corresponding to 100 km when the optical transmission line C is an optical fiber.

[0023] As described above, according to the first embodiment of the present invention, an optical amplifier 13 that amplifies only the reference light is provided in the receiver (Bob), and the amplification factor of the optical amplifier 13 is controlled based on the signal output level L out obtained by homodyne detection. Thereby, as shown in FIG. 2, the intensity of the reference light can be increased to improve the SN ratio in homodyne detection.

[0024] As shown in FIG. 2A, when the transmission loss of the optical transmission line C is large and the signal output level L out is low, the ratio (SN ratio) of the signal output level L out to the noise levels of the photodetectors PD1 and PD2 becomes low. In contrast, according to the present embodiment, as shown in FIG. 2B, the amplification factor of the optical amplifier 13 is controlled so as to maintain the signal output level L out above a threshold value L TH corresponding to a predetermined SN ratio. Thereby, even when the transmission loss of the optical transmission line C is large, the SN ratio in homodyne detection on the receiving side can be improved without increasing the power of the laser light source 10 or interposing an optical amplifier in the optical transmission line C.

[0025] Furthermore, the optical amplifier control unit 17 adjusts the amplification factor of the optical amplifier 13 according to the comparison result between the signal output level L out and the threshold value L TH to maintain the signal output level L out within a predetermined range and achieve the stabilization of the signal output I out . Furthermore, for example, when the optical transmission path is switched by using an optical switch or the like when eavesdropping on quantum key distribution is detected, it is possible to cope with the difference in transmission loss before and after the switching.

[0026] <Second Embodiment> As illustrated in FIG. 3, the system according to the second embodiment of the present invention has a different configuration of the receiver (Bob) from that of the first embodiment shown in FIG. 1. Hereinafter, the configurations and functions different from those of the first embodiment will be described, and the components having the same functions will be denoted by the same reference numerals and the description thereof will be omitted.

[0027] In FIG. 3, the receiver (Bob) has basically the same configuration as that of the first embodiment, but a transmission loss prediction unit 18 is newly provided, and accordingly, the control function of the optical amplifier control unit 17a is slightly different from that of the optical amplifier control unit 17. The transmission loss prediction unit 18 predicts the change in the loss of the reference light propagating through the optical transmission path C from the environmental data. The optical amplifier control unit 17a controls the amplification factor of the optical amplifier 13 so as to cancel the change in the transmission loss predicted by the transmission loss prediction unit 18 while monitoring the level signal L out of the signal output I out .

[0028] Environmental data are factors that affect the transmission loss of the optical transmission path C, such as data on temperature, humidity, vibration, etc., and further include time data such as date and time of day. As is well known, if the optical transmission path C is an optical fiber, the optical path length and transmission loss may change due to temperature and vibration, and if the optical transmission path C is free space, the transmission loss may vary due to temperature and humidity. Also, since temperature and humidity change with the seasons, rough fluctuations in transmission loss can be predicted based on the date. Moreover, within a day, temperature and humidity change depending on the time of day, and furthermore, the frequency or magnitude of vibration due to transportation, etc. also changes. When the vibration increases, a positional deviation may occur at the input and output portions of the optical transmission path C, and the loss may change accordingly.

[0029] By measuring in advance the environmental data that affect the transmission loss of such an optical transmission path C, the relationship between the environment and the transmission loss can be prepared as a conversion table. Therefore, the transmission loss prediction unit 18 can predict the transmission loss of the optical transmission path C by inputting the current environmental data and referring to the conversion table. The optical amplifier control unit 17a can input the transmission loss predicted by the transmission loss prediction unit 18 and control the optical amplification factor so as to compensate for the transmission loss.

[0030] As described above, according to the second embodiment of the present invention, the optical amplifier control unit 17a controls the amplification factor of the optical amplifier 13 based on the current signal output I out level signal L out and the transmission loss predicted by the transmission loss prediction unit 18. Thereby, similar to the first embodiment, the SN ratio in homodyne detection can be improved, and even when the transmission loss of the optical transmission path C increases, the change can be predicted and the signal output I out can be stabilized quickly and with high precision.

[0031] 2. Examples Hereinafter, as an example of the present invention, a system for transmitting the signal light Q and the reference light LO through one transmission path will be described. In the first example, a system using an optical fiber for the optical transmission path will be described, and in the second example, a system using free space for the optical transmission path will be described respectively.

[0032] 2.1) First Embodiment <Configuration> As illustrated in FIG. 4, the quantum key distribution communication system according to the first embodiment of the present invention comprises a communication device 100 including a transmitter (Alice) and a communication device 200 including a receiver (Bob). The transmitter (Alice) includes a laser light source 101, none a polarization beam splitter ( BS ) 102 and polarizing beam splitter (PBS) 103, a mirror 104, a half-wave plate 105, an attenuator 106, a phase modulator 107, a mirror 108, and a control unit 109. Here, the input port of the non-polarizing beam splitter 102 is connected to the output port of the laser light source 101, and the output port of the polarization beam splitter 103 is connected to the optical fiber 300.

[0033] The laser light source 101 outputs a linearly polarized optical pulse P to none the input port of the polarization beam splitter 102. The optical pulse P is split by the non-polarizing beam splitter 102, and one optical pulse is sent to the reference light side path R LO and the other optical pulse is sent to the signal light side path R Q respectively.

[0034] The optical pulse on the reference light side path R LO passes through the polarization beam splitter 103 as it is, and enters the optical fiber 300 as a reference optical pulse P LO having no quantum state and normal intensity. The optical pulse on the signal light side path R Q is reflected by the polarization beam splitter 103 through the mirror 104, the half-wave plate 105, the attenuator 106, the phase modulator 107, and the mirror 108, and enters the optical fiber 300 as a weak signal optical pulse P Q having a quantum state. The half-wave plate 105 rotates the polarization of the optical pulse on the path R Q by 90 degrees, the attenuator 106 attenuates the optical pulse to a weak light having a quantum state, and the phase modulator 107 phase-modulates the weak optical pulse to generate the signal optical pulse P Q . Note that the attenuator 106 and the phase modulator 107 may be arranged in the reverse order with respect to the traveling direction of the optical pulse.

[0035] Here, the path R on the signal light side Q has an optical path longer than that of the path R on the reference light side LO . The difference in optical path length between the path R Q and the path R LO , the half-wave plate 105, and polarization the beam splitter 103 generate a reference light pulse P LO and a signal light pulse P Q that are orthogonally polarized to each other and temporally separated from one optical pulse P. Note that when the non-polarizing beam splitter 102 is used as a polarizing beam splitter, the half-wave plate 105 may not be used

[0036] The control unit 109 controls the communication device 100. Here, it controls the laser light source 101, the attenuator 106, and the phase modulator 107 of the transmitter (Alice), and drives the phase modulator 107 at four phases (0°, 90°, 180°, 270°) according to the original random number of the encryption key. Thereby, the phase modulator 107 generates the signal light pulse P Q by performing phase modulation according to the key information on the weak light pulse output from the attenuator 106. In this way, a pulse train of the reference light pulse P LO with normal intensity and the phase-modulated signal light pulse P Q is transmitted to the receiver (Bob) through the optical fiber 300

[0037] As illustrated in FIG. 5, in the receiver (Bob) of the quantum key distribution system according to the first embodiment of the present invention, the optical fiber 300 is connected to the input port of the polarizing beam splitter 201, and the reference light pulse P LO and the signal light pulse P Q with orthogonal polarizations to each other that reach through the optical fiber 300 from the transmitter (Alice) are received. The signal light pulse P Q directly passes through the polarizing beam splitter 201 and enters the first input port of the non-polarizing beam splitter 203 through the half-wave plate 202 that rotates the polarization by 90 degrees from the first output port. The reference light pulse P LOIt is reflected by the polarization beam splitter 201 and enters the second input port of the non-polarizing beam splitter 203 from the second output port, passing through the mirror 204, the optical amplifier 205, the phase modulator 206, and the mirror 207.

[0038] Here, the signal light pulse P Q has the same path length as the path R LO of the transmitter (Alice), and the reference light pulse P LO has the same path length as the path R Q of the transmitter (Alice). Therefore, the signal light pulse P Q and the reference light pulse P LO incident on the first and second input ports of the non-polarizing beam splitter 203 reach the non-polarizing beam splitter 203 through different optical paths of the same length from the polarization beam splitter 201 of the transmitter (Alice). As a result, the optical configurations of the transmitter (Alice) and the receiver (Bob) form the interferometer described in FIG. 1.

[0039] The optical amplifier 205 is, for example, an EDFA or an SOA, and amplifies the reference light pulse P LO while maintaining the wavelength and phase. The amplification factor of the optical amplifier 205 is controlled by the control unit 210 as described later. The phase modulator 206 phase-modulates the optically amplified reference light pulse P LO . The phase modulation of the phase modulator 206 is controlled by the control unit 210. As described above, the phase modulator 206 of the transmitter (Alice) performs four types of phase modulation (0°, 90°, 180°, 270°) on the signal light pulse P Q to be transmitted, while the phase modulator 206 of the receiver (Bob) performs two types of phase modulation (0°, 90°) on the received reference light pulse P LO .

[0040] Thus, the signal light pulse P Q that has passed through the half-wave plate 202 and the optically amplified and phase-modulated reference light pulse P LO enter the non-polarizing beam splitter 203. The non-polarizing beam splitter 203 has equal light transmittance and reflectance, and the signal light pulse P Qand the reference optical pulse P LO The light emitted from the two output ports after being overlapped is received by the photodetectors PD1 and PD2, respectively. As the photodetectors PD1 and PD2, ordinary photodiodes can be used at room temperature.

[0041] The detection signals respectively output from the photodetectors PD1 and PD2 are subjected to differential operation by the differential operation unit 208, and the difference signal as a result is the signal output I obtained by homodyne detection. out which is output as.

[0042] The signal output I out is averaged by the low-pass filter 209, and the level signal L out is output to the control unit 210. The control unit 210 controls the communication device 200. Here, the phase control of the phase modulator 206 and the amplification rate control of the optical amplifier 205 of the receiver (Bob) are performed. The amplification rate control of the optical amplifier 205 has the same function as the optical amplifier control unit 17 in the first embodiment described above. That is, the level signal L out obtained by the low-pass filter 209 is input, and the amplification rate of the optical amplifier 205 is controlled so that the level signal L out is maintained within a predetermined range equal to or higher than the threshold value L TH . As described with reference to FIG. 2, when the transmission loss in the optical fiber 300 increases and the level signal L out falls below the threshold value L TH , the control unit 210 can increase the amplification rate of the optical amplifier 205 to compensate for the transmission loss.

[0043] <Optical amplification rate control> In the quantum key distribution communication system according to the present embodiment, it is assumed that encrypted communication using a quantum encryption key is performed between the communication device 100 and the communication device 200 based on a predetermined time slot. According to the present embodiment, as illustrated in FIG. 6, the control unit 210 monitors the level signal L out of the signal output I out for each communication time slot, and adjusts the amplification rate of the optical amplifier 205.

[0044] In FIG. 6, the control unit 210 determines whether it is the correction timing for each predetermined time slot (operation 401). If it is the correction timing (YES in operation 401), the control unit 210 inputs the level signal L out from the low-pass filter 209 (operation 402). The control unit 210 determines whether the level signal L out is greater than the threshold value L TH (operation 403). If the level signal L out is less than or equal to the threshold value L TH (NO in operation 403), the control unit 210 increases the amplification factor of the optical amplifier 205 (operation 404). When the level signal L out becomes greater than the threshold value L TH (YES in operation 403), the control unit 210 terminates the optical amplification factor control. If it is not the correction timing (NO in operation 401), the optical amplification factor control is not executed.

[0045] As described above, by monitoring the level signal L out of the signal output I out at a predetermined timing and adjusting the amplification factor of the optical amplifier 205, the level of the signal output I out can be maintained greater than the threshold value L TH . Further, by periodically adjusting the amplification factor, the level of the signal output I out can be stabilized.

[0046] 2.2) Second Embodiment <Configuration> As illustrated in FIG. 7, the quantum key distribution communication system according to the second embodiment of the present invention includes a communication device 100a including a transmitter (Alice) and a communication device 200a including a receiver (Bob), and uses free space 300a as an optical transmission path. In this embodiment, beam expanders 111 and 211 are installed as optical transmission / reception means in the communication device 100a and the communication device 200a, respectively, such that their optical axes coincide with each other, and a reference optical pulse P LO with a normal intensity and a weak signal optical pulse P Q having a quantum state are transmitted through the free space 300a.

[0047] Note that, except for the beam expander 111, the configuration of the transmitter (Alice) is the same as that of the first embodiment in FIG. 4, so the detailed configuration of the transmitter (Alice) is omitted in FIG. 7. Also, except for the beam expander 211, the configuration of the receiver (Bob) is basically the same as that of the first embodiment shown in FIG. 5, so components having the same functions are denoted by the same reference numerals and the description thereof is omitted. Hereinafter, the configurations and functions different mainly from those of the first embodiment will be described.

[0048] In the transmitter (Alice), a beam expander 111 is optically connected to the output port of the polarization beam splitter 103. The reference light pulse P LO and the signal light pulse P Q emitted from the output port of the polarization beam splitter 103 are each sent as collimated light with a larger diameter through the free space 300a by the beam expander 111 to the beam expander 211 of the receiver (Bob).

[0049] When the reference light pulse P LO and the signal light pulse P Q are received by the beam expander 211 of the receiver (Bob), a signal output I out is obtained by homodyne detection as described above. The control unit 210a inputs the level signal L out of the signal output I out from the low-pass filter 209 and inputs environmental data from various external sensor units 212. The environmental data is data on factors affecting the transmission loss in the free space 300a as described above, such as temperature, humidity, vibration, etc., and further includes time data such as date and time of day.

[0050] The control unit 210a has the control function of the optical amplifier control unit 17a and the transmission loss prediction function of the transmission loss prediction unit 18 in FIG. 3. That is, the control unit 210a controls the amplification factor of the optical amplifier 205 so as to cancel the predicted change in the transmission loss in the free space 300a while monitoring the level signal L out of the signal output I out .

[0051] The control unit 210a measures in advance environmental data that affects the transmission loss in the free space 300a, and holds the relationship between the environment and the transmission loss as a conversion table. Therefore, the control unit 210a can input the current environmental data from various sensor units 212, refer to the conversion table to predict the transmission loss in the free space 300a, and adjust the optical amplification factor of the optical amplifier 205 so as to compensate for this predicted transmission loss.

[0052] As illustrated in FIG. 8, assume that the output port of the polarization beam splitter 103 and the input port of the beam expander 111 in the transmitter (Alice) are connected by a single-mode (SM) optical fiber. Also assume that the output port of the beam expander 211 and the input port of the polarization beam splitter 201 in the receiver (Bob) are connected by an SM optical fiber. When the laser light is sent from the beam expander 111 through the free space 300a to the beam expander 211, due to disturbances such as the fluctuation of air in the free space 300a, the output light of the beam expander 211 in the receiver (Bob) may not be correctly focused on the core of the SM optical fiber. Also, due to disturbances such as water vapor and fine particles in the free space 300a, the intensity of the output light of the beam expander 211 may be greatly reduced.

[0053] For example, as shown in FIG. 9A, when the output light of the beam expander 211 in the receiver (Bob) is correctly focused on the core of the SM optical fiber, a sufficient light reception intensity can be obtained with the SM optical fiber, and sufficient received light is incident on the input port of the polarization beam splitter 201. However, as shown in FIG. 9B, if the output light of the beam expander 211 is not correctly focused on the core of the SM optical fiber due to the disturbance in the free space 300a, the light reception intensity distribution in the SM optical fiber is greatly disrupted, and sufficient received light does not enter the input port of the polarization beam splitter 201 from the SM optical fiber.

[0054] When using the free space 300a as an optical transmission path in this way, it is necessary to consider fluctuations in the received light intensity due to disturbances. According to the second embodiment of the present invention, the control unit 210a determines the current signal output I out level signal L out and the predicted transmission loss to control the amplification factor of the optical amplifier 205. Therefore, the change in the transmission loss of the free space 300a can be predicted and the amplification factor can be controlled, enabling the signal output I out to be stabilized quickly and with high precision. Furthermore, for example, when the optical transmission path is switched by using an optical switch when eavesdropping on quantum key communication is detected, it is possible to cope with the difference in transmission loss before and after the switching.

[0055] <Optical amplification factor control> In the quantum key communication system according to this embodiment, it is assumed that encrypted communication using a quantum key is performed between the communication device 100a and the communication device 200a based on a predetermined time slot. According to this embodiment, as illustrated in FIG. 10, the control unit 210a monitors the level signal L out of the signal output I out and the environmental data for each communication time slot, and adjusts the amplification factor of the optical amplifier 205.

[0056] In FIG. 10, the control unit 210a determines whether it is the correction timing for each predetermined time slot (operation 501). If it is the correction timing (YES in operation 501), the control unit 210a inputs the level signal L out from the low-pass filter 209 (operation 502). Further, the control unit 210a inputs environmental data from the various sensor units 212, and calculates the transmission loss using the above-described conversion table or the like (operation 503). The control unit 210a increases the amplification factor of the optical amplifier 205 so as to compensate for the calculated transmission loss (operation 504), and determines whether the level signal L out is greater than the threshold value L TH (operation 505). If the level signal L out is less than or equal to the threshold value L TH (NO in operation 503), the optical amplification factor is increased until the level signal L out becomes greater than the threshold value L TH (NO in operation 505), and the level signal Lout is the threshold value L TH If it is greater than (YES in operation 505), the optical amplification rate control is terminated. Note that if it is not the correction timing (NO in operation 501), the optical amplification rate control is not executed.

[0057] As described above, the signal output I out level signal L out and the environmental data are monitored at a predetermined timing, the transmission loss is predicted based on the environmental data, and the amplification rate of the optical amplifier 206 is adjusted. As a result, the level of the signal output I out can be maintained greater than the threshold value L TH Furthermore, since the change in the transmission loss of the free space 300a is predicted and the amplification rate is controlled, the signal output I out can be stabilized quickly and with high precision.

[0058] 3. Supplementary Note Some or all of the above-described embodiments and examples may be described as follows in the supplementary note, but are not limited thereto. (Supplementary Note 1) A quantum key distribution communication system comprising a transmitter and a receiver connected via a communication network, wherein the transmitter and the receiver are optically connected through an optical transmission line, the transmitter includes a beam splitter that splits coherent light into first light and second light, an optical transmission unit that generates weak signal light having a quantum state by performing phase modulation and intensity attenuation on the first light, uses the second light as reference light having no quantum state, and outputs the signal light and the reference light to the optical transmission line, and is provided with the receiver includes an optical reception unit that receives the signal light and the reference light that have reached through the optical transmission line, an optical amplifier that amplifies the reference light received by the optical reception unit while maintaining the wavelength and phase, a phase modulator that performs phase modulation on the reference light output from the optical amplifier, A homodyne detector that generates a signal output based on the signal light that has reached through the optical transmission path and the phase-modulated reference light; A level detector that detects a signal output level from the signal output; An optical amplifier control unit that controls the amplification factor of the optical amplifier based at least on the signal output level; A quantum key distribution communication system, characterized by comprising the above. (Appendix 2) The quantum key distribution communication system according to Appendix 1, characterized in that the optical transmission path is an optical fiber. (Appendix 3) The quantum key distribution communication system according to Appendix 1, characterized in that the optical transmission unit of the transmitter and the optical reception unit of the receiver each have an optical transceiver with their optical axes coinciding with each other, and the optical transmission path is a free space between the optical transceivers. (Appendix 4) The quantum key distribution communication system according to any one of Appendices 1 to 3, characterized in that the optical amplifier control unit controls the amplification factor of the optical amplifier so that the signal output level is maintained within a predetermined range. (Appendix 5) The quantum key distribution communication system according to any one of Appendices 1 to 3, characterized in that the optical amplifier control unit sets the amplification factor of the optical amplifier to a value that compensates for the loss in the optical transmission path. (Appendix 6) The quantum key distribution communication system according to any one of Appendices 1 to 5, further comprising an optical amplification factor calculation unit that calculates the amplification factor of the optical amplifier based on the signal output level and environmental data of the optical transmission path; The quantum key distribution communication system according to any one of Appendices 1 to 5, characterized in that the optical amplifier control unit controls the amplification factor of the optical amplifier based on the signal output level and the calculated amplification factor. (Appendix 7) A communication device that acquires a signal output by homodyne detection in a quantum key distribution communication system, An optical reception unit that receives, through an optical transmission path, a weak signal light having a quantum state obtained from coherent light and a reference light having no quantum state in a communication device on the transmission side; An optical amplifier that amplifies the reference light received by the optical receiving unit while maintaining the wavelength and phase; A phase modulator that performs phase modulation on the reference light output from the optical amplifier; A homodyne detector that generates a signal output based on the signal light that has reached through the optical transmission line and the phase-modulated reference light; A level detector that detects a signal output level from the signal output; An optical amplifier control unit that controls the amplification factor of the optical amplifier based at least on the signal output level; A communication device, characterized by comprising the above. (Appendix 8) The communication device according to Appendix 7, wherein the optical amplifier control unit controls the amplification factor of the optical amplifier so that the signal output level is maintained within a predetermined range. (Appendix 9) The communication device according to Appendix 7 or 8, wherein the optical amplifier control unit sets the amplification factor of the optical amplifier to a value that compensates for the loss in the optical transmission line. (Appendix 10) The communication device further includes an optical amplification factor calculation unit that calculates the amplification factor of the optical amplifier based on the signal output level and the environmental data of the optical transmission line, The communication device according to any one of Appendices 7 to 9, wherein the optical amplifier control unit controls the amplification factor of the optical amplifier based on the signal output level and the calculated amplification factor. (Appendix 11) The communication device according to any one of Appendices 7 to 10, wherein the optical receiving unit has an optical receiver whose optical axis coincides with that of the optical transmitter of the transmitting-side communication device, and the optical transmission line is free space between the optical transmitter and receiver. (Appendix 12) A control method for a communication device that obtains a signal output by homodyne detection in a quantum key distribution communication system, The optical receiving unit receives, through an optical transmission line, a weak signal light having a quantum state obtained from coherent light by a transmitting-side communication device and a reference light having no quantum state, The optical amplifier amplifies the reference light that has reached through the optical transmission line while maintaining the wavelength and phase, the phase modulator performs phase modulation on the reference light output from the optical amplifier, the homodyne detector generates a signal output based on the signal light that has reached through the optical transmission line and the phase-modulated reference light, the level detector detects the signal output level from the signal output, the optical amplifier control unit controls the amplification factor of the optical amplifier based at least on the signal output level, A control method for a communication device, characterized in that. (Appendix 13) The control method for a communication device according to Appendix 12, characterized in that the optical amplifier control unit controls the amplification factor of the optical amplifier so that the signal output level is maintained within a predetermined range. (Appendix 14) The optical amplification factor calculation unit calculates the amplification factor of the optical amplifier based on the signal output level and the environmental data of the optical transmission line, The control method for a communication device according to Appendix 12 or 13, characterized in that the optical amplifier control unit controls the amplification factor of the optical amplifier based on the signal output level and the calculated amplification factor.

Industrial Applicability

[0059] The present invention can be used in optical communication systems, particularly in the optical receiver of quantum key distribution systems.

Explanation of Signs

[0060] 10 Transmitter (Alice) 11 Phase modulator 12 Attenuator 13 Optical amplifier 14 Phase modulator 15 Differential calculator 16 Signal output level detector 17, 17a Optical amplifier control unit 18 Transmission loss prediction unit BS1, BS2 Beam splitter Mirrors M1 and M2 Optical transmission path C Photodetectors PD1 and PD2 Communication devices (transmitters) 100 and 100a Laser light source 101 Non-polarizing beam splitter 102 Polarizing beam splitter 103 Mirror 104 Half-wave plate 105 Attenuator 106 Phase modulator 107 Mirror 108 Control unit 109 Beam expander 111 Communication devices (receivers) 200 and 200a Polarizing beam splitter 201 Half-wave plate 202 Non-polarizing beam splitter 203 Mirror 204 Optical amplifier 205 Phase modulator 206 Mirror 207 Differential calculator 208 Low-pass filter 209 Control units 210 and 210a Beam expander 211 Various sensor units 212 Optical fiber 300 Free space 300a

Claims

1. A quantum key distribution communication system comprising a transmitter and a receiver connected via a communication network, wherein the transmitter and the receiver are optically connected through an optical transmission line, the transmitter includes an optical transmitter that generates first light and second light from coherent light, generates weak signal light having a quantum state by attenuating the intensity of the first light, uses the second light as reference light having no quantum state, and outputs the signal light and the reference light to the optical transmission line, the receiver includes an optical receiver that receives the signal light and the reference light that have reached through the optical transmission line, an optical amplifier that amplifies the reference light received by the optical receiver with a controllable amplification factor, a homodyne detector that generates a signal output based on the signal light that has reached through the optical transmission line and the amplified reference light, a level detector that detects a signal output level from the signal output, an optical amplifier controller that controls the amplification factor of the optical amplifier based at least on the signal output level, and characterized by comprising the above.

2. The quantum key distribution communication system according to claim 1, wherein the optical transmission line is an optical fiber.

3. The quantum key distribution communication system according to claim 1, wherein the optical transmitter of the transmitter and the optical receiver of the receiver each have an optical transceiver with their optical axes coinciding with each other, and the optical transmission line is free space between the optical transceivers.

4. A communication device that obtains a signal output by homodyne detection in a quantum key distribution communication system, including an optical receiver that receives, through an optical transmission line, weak signal light having a quantum state and reference light having no quantum state obtained from coherent light by a communication device on the transmission side, an optical amplifier that amplifies the reference light received by the optical receiver with a controllable amplification factor, a homodyne detector that generates a signal output based on the signal light that has reached through the optical transmission line and the amplified reference light, a level detector that detects a signal output level from the signal output, an optical amplifier controller that controls the amplification factor of the optical amplifier based at least on the signal output level, and characterized by comprising the above.

5. The communication device according to claim 4, wherein the optical amplifier controller controls the amplification factor of the optical amplifier so that the signal output level is maintained within a predetermined range.

6. The communication device according to claim 4 or 5, wherein the optical amplifier control unit sets the amplification factor of the optical amplifier to a value that compensates for the loss in the optical transmission line.

7. The communication device further includes an optical amplification factor calculation unit that calculates the amplification factor of the optical amplifier based on the signal output level and the environmental data of the optical transmission line, The communication device according to any one of claims 4 to 6, wherein the optical amplifier control unit controls the amplification factor of the optical amplifier based on the signal output level and the calculated amplification factor.

8. A control method for a communication device that acquires a signal output by homodyne detection in a quantum key distribution communication system, An optical receiver receives, through an optical transmission line, a weak signal light having a quantum state obtained from coherent light in a transmitting-side communication device and a reference light having no quantum state, An optical amplifier amplifies the reference light that has reached through the optical transmission line with a controllable amplification factor, A homodyne detector generates a signal output based on the signal light that has reached through the optical transmission line and the amplified reference light, A level detector detects a signal output level from the signal output, An optical amplifier control unit controls the amplification factor of the optical amplifier based at least on the signal output level, characterizing the control method of the communication device.

9. The control method of the communication device according to claim 8, wherein the optical amplifier control unit controls the amplification factor of the optical amplifier so that the signal output level is maintained within a predetermined range.

10. An optical amplification factor calculation unit calculates the amplification factor of the optical amplifier based on the signal output level and the environmental data of the optical transmission line, The control method of the communication device according to claim 8 or 9, wherein the optical amplifier control unit controls the amplification factor of the optical amplifier based on the signal output level and the calculated amplification factor.

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