Transmitter, receiver, quantum key distribution system, method and program

Abstract

Provided is a transmitter in a quantum key distribution system. The transmitter includes a light source that outputs light, and an optical modulator that modulates the light output from the light source into light for quantum key distribution. The transmitter further includes a bias control unit that controls a bias voltage of the optical modulator on the basis of the light output from the optical modulator. The bias control unit executes a control operation of the bias voltage during a first period, which is a bias voltage control period, and stops the control operation of the bias voltage during a second period, which is a quantum key distribution period.

Description

Transmitter, receiver, quantum key distribution system, method and program

[0001] The present disclosure relates to a transmitter, a receiver, a quantum key distribution system, a method, and a program.

[0002] In recent years, quantum cryptography has been researched as an encryption technology to ensure the security of communications. Quantum cryptography enables secure sharing of secret keys between points using quantum key distribution (QKD).

[0003] Known quantum key distribution methods include discrete variable quantum key distribution (DV-QKD), which uses a photon detector to distribute quantum keys, and continuous-variable quantum key distribution (CV-QKD), which uses coherent detection to distribute quantum keys. For example, Patent Document 1 describes a quantum key distribution method using the BB84 protocol, which is DV-QKD.

[0004] JP 2007-187698 A

[0005] In related technologies such as Patent Document 1, an optical modulator is used in the transmitter of a quantum key distribution system. Since the operating point of an optical modulator fluctuates due to changes in the ambient temperature, etc., it is necessary to control the bias voltage to achieve an optimal operating point. Therefore, a method for effectively controlling the bias of an optical modulator is desired in a quantum key distribution system.

[0006] In view of the above problems, one of the objects of the present disclosure is to provide a transmitter, a receiver, a quantum key distribution system, a method, and a program that are capable of effectively controlling the bias of an optical modulator.

[0007] A transmitter according to one aspect of the present disclosure includes a light source that outputs light, an optical modulator that modulates the light output from the light source into light for quantum key distribution, and bias control means that controls a bias voltage of the optical modulator based on the light output from the optical modulator, wherein the bias control means performs a control operation of the bias voltage during a first period that is a bias voltage control period, and stops the control operation of the bias voltage during a second period that is a quantum key distribution period.

[0008] A receiver according to one aspect of the present disclosure comprises: an opto-electrical conversion means for opto-electrically converting an optical signal received from a transmitter; a demodulation means for demodulating data from the opto-electrically converted signal; and a key generation means for generating a quantum key from the demodulated data by a key distillation process, wherein the key generation means stops generating the quantum key using the data during a first period which is a bias voltage control period of an optical modulator in the transmitter, and performs generation of the quantum key using the data during a second period which is a quantum key distribution period.

[0009] a quantum key distribution system including a transmitter and a receiver, wherein the transmitter includes a light source that outputs light, an optical modulator that modulates the light output from the light source into light for quantum key distribution and transmits the modulated light, and a bias control means that controls a bias voltage of the optical modulator based on the light output from the optical modulator, wherein the bias control means performs a control operation of the bias voltage during a first period that is a bias voltage control period, and stops the control operation of the bias voltage during a second period that is a quantum key distribution period; and the receiver includes photoelectric conversion means that photoelectrically converts an optical signal received from the transmitter, demodulation means that demodulates data from the photoelectrically converted signal, and key generation means that generates a quantum key from the demodulated data by a key distillation process, wherein the key generation means stops generating the quantum key using the data during the first period, and performs generating the quantum key using the data during the second period.

[0010] A method according to one aspect of the present disclosure is a method in a transmitter, which includes controlling a bias voltage of an optical modulator based on light output from the optical modulator, which modulates light from a light source into light for quantum key distribution; executing a control operation of the bias voltage during a first period that is a bias voltage control period; and stopping the control operation of the bias voltage during a second period that is a quantum key distribution period.

[0011] A method according to one aspect of the present disclosure is a method in a receiver, including opto-electrically converting an optical signal received from a transmitter, demodulating data from the opto-electrically converted signal, generating a quantum key from the demodulated data by a key distillation process, stopping generation of the quantum key using the data during a first period that is a bias voltage control period of an optical modulator in the transmitter, and performing generation of the quantum key using the data during a second period that is a quantum key distribution period.

[0012] A program according to one aspect of the present disclosure is a program for causing a computer to execute a method in a transmitter, the method including: controlling a bias voltage of an optical modulator based on light output from the optical modulator, the optical modulator modulating light from a light source into light for quantum key distribution; executing a control operation of the bias voltage during a first period that is a bias voltage control period; and stopping the control operation of the bias voltage during a second period that is a quantum key distribution period.

[0013] A program according to one aspect of the present disclosure is a program for causing a computer to execute a method in a receiver, the method including: photoelectrically converting an optical signal received from a transmitter; demodulating data from the photoelectrically converted signal; generating a quantum key from the demodulated data by a key distillation process; stopping generation of the quantum key using the data during a first period that is a bias voltage control period of an optical modulator in the transmitter; and executing generation of the quantum key using the data during a second period that is a quantum key distribution period.

[0014] According to the present disclosure, the bias control of the optical modulator can be effectively performed.

[0015] FIG. 1 is a block diagram showing an example configuration of a related quantum key distribution system. FIG. 2 is a block diagram showing an example configuration of a transmitter according to some embodiments. FIG. 3 is a block diagram showing an example configuration of a receiver according to some embodiments. FIG. 4 is a block diagram showing an example configuration of a frame according to some embodiments. FIG. 5 is a block diagram showing an example configuration of a quantum key distribution system according to some embodiments. FIG. 6 is a block diagram showing an example configuration of a quantum key distribution system according to some embodiments. FIG. 7 is a block diagram showing an example configuration of a frame in a related quantum key distribution system. FIG. 8 is a block diagram showing an example configuration of a frame in a quantum key distribution system according to some embodiments. FIG. 9 is a flow chart showing an example operation of a quantum key distribution system according to some embodiments. FIG. 10 is a block diagram showing an example operation of a quantum key distribution system according to some embodiments. FIG. 11 is a block diagram showing an example configuration of a quantum key distribution system according to some embodiments. FIG. 12 is a block diagram showing an example configuration of a frame in a related quantum key distribution system. FIG. 13 is a block diagram showing an example configuration of a frame in a quantum key distribution system according to some embodiments. FIG. 14 is a block diagram showing an example operation of a quantum key distribution system according to some embodiments.

[0016] Hereinafter, embodiments will be described with reference to the drawings. In each drawing, the same elements are given the same reference numerals, and duplicate explanations will be omitted as necessary. Note that the arrows shown in each drawing are examples for explanation purposes and do not limit the type or direction of signals.

[0017] (Discussion of Related Technology) Fig. 1 shows an example of the configuration of a related quantum key distribution system 9. In the example of Fig. 1, the quantum key distribution system 9 includes a transmitter 800 and a receiver 900. In the quantum key distribution system 9, quantum key distribution is performed by the transmitter 800 and the receiver 900. The transmitter 800 and the receiver 900 are connected to each other via optical transmission paths 701 and 702 so that they can communicate with each other.

[0018] The transmitter 800 includes a light source 810 and an optical modulator 820 for quantum key distribution, and a bias control unit 830 for bias control of the optical modulator 820. The receiver 900 includes an optical front end 910, a demodulation DSP (Digital Signal Processor) 920, and a key distillation unit 930 for quantum key distribution, and a BER (Bit Error Rate) measurement unit 940 for bias control of the optical modulator 820.

[0019] The optical modulator 820 used in the transmitter 800 is a phase modulator, which is essential for QKD. The operating point (bias point) of a phase modulator drifts depending on the ambient temperature, and this bias drift must be compensated for. Conventional optical communication systems use a control method called ABC (Auto Bias Control) to control the bias of optical modulators. ABC compensates for the bias drift of optical modulators by applying a bias voltage superimposed with a low-frequency dither signal to the optical modulator and performing feedback control based on the results of monitoring the optical output of the optical modulator. However, in quantum key distribution, a quantum optical signal (weak light) with extremely low intensity is transmitted from the transmitter to the receiver. Therefore, if ABC is simply applied to a quantum key distribution system, the dither signal for ABC becomes noise in the quantum optical signal, making quantum key distribution impossible.

[0020] For this reason, as shown in FIG. 1 , in the related quantum key distribution system 9, bias drift compensation is performed in response to BER measurements at the receiver. In the transmitter 800, the bias control unit 830 applies an arbitrary bias voltage to the optical modulator 820. The quantum optical signal modulated and generated by the optical modulator 820 based on data (modulation data) is transmitted to the receiver 900 via the optical transmission path 701. In the receiver 900, the received quantum optical signal is photoelectrically converted by the optical front end 910 and demodulated by the demodulation DSP. The BER measurement unit 940 measures the BER of the demodulated data using data transmitted from the transmitter 800 via the optical transmission path 702. The BER measurement unit 940 notifies the bias control unit 830 of the transmitter 800 of the BER measurement result via the optical transmission path 702. The bias control unit 830 adjusts the bias voltage in response to the notified BER. In this case, when the BER is measured, the secret key cannot be generated by the key distillation unit 930 .

[0021] As described above, the related quantum key distribution system 9 requires a large adjustment loop including the receiver to compensate for bias drift through BER measurement. Therefore, there are serious problems with the stability of modulator control. Specifically, the large adjustment loop spanning from the transmitter to the receiver results in a very slow response speed of bias control. Because a private key cannot be generated during BER measurement, bias control takes time, which reduces the efficiency of private key generation per unit time. Furthermore, the measured BER is the communication quality of the signal transmitted from the transmitter to the receiver, and therefore fluctuates due to factors other than the transmitter's modulator. This makes it extremely difficult to identify the cause of control failure.

[0022] Therefore, in the embodiment, it is possible to stably and effectively control the bias of an optical modulator in a quantum key distribution system.

[0023] (First Embodiment) Next, a first embodiment will be described. In this embodiment, an outline of several embodiments will be described.

[0024] FIG. 2 shows an example configuration of a transmitter 10 according to some embodiments. FIG. 3 shows an example configuration of a receiver 20 according to some embodiments. For example, the transmitter 10 and the receiver 20 are communicatively connected via an optical transmission path to form a quantum key distribution system. A quantum key distribution system is also a system that shares random numbers that are the source of a quantum key (secret key). The transmitter 10 is a QKD transmitter (QKD transmitter), and the receiver 20 is a QKD receiver (QKD receiver). The QKD may be CV-QKD or DV-QKD.

[0025] In the example of FIG. 2, the transmitter 10 includes a light source 11, an optical modulator 12, and a bias control unit 13.

[0026] The light source 11 outputs light. For example, the light from the light source 11 is light for carrying transmission data, which is a transmission random number.

[0027] The optical modulator 12 modulates the light output from the light source 11 into light for quantum key distribution. For example, the optical modulator 12 modulates the light from the light source 11 in accordance with transmission data and a bias voltage applied from the bias control unit 13. The optical modulator 12 also transmits the modulated light as a quantum optical signal (weak light) to the receiver 20 via an optical transmission path.

[0028] The bias control unit 13 controls the bias voltage of the optical modulator 12 based on the light output (modulated) from the optical modulator 12. For example, the bias control unit 13 controls the bias voltage using ABC. That is, the bias control unit 13 monitors the light output from the optical modulator 12 and searches for an optimal bias voltage based on the monitoring results. For example, the transmitter 10 may include a branching unit that branches the light output from the optical modulator 12 and an optical monitoring unit that monitors the light branched by the branching unit, and the bias control unit 13 may control the bias voltage of the optical modulator 12 based on the light monitored by the optical monitoring unit.

[0029] In the example of FIG. 3, the receiver 20 includes an optical-electrical conversion unit 21, a demodulation unit 22, and a key generation unit 23.

[0030] The photoelectric conversion unit 21 performs photoelectric conversion on the optical signal (weak light) received from the transmitter 10. For example, the photoelectric conversion unit 21 converts the optical signal received from the transmitter 10 into a received signal, which is an electrical signal. In the case of DV-QKD, the photoelectric conversion unit 21 may perform photoelectric conversion using a photon detector, and in the case of CV-QKD, the photoelectric conversion may be performed using a coherent detector.

[0031] The demodulation unit 22 demodulates the received data from the received signal photoelectrically converted by the photoelectric conversion unit 21. For example, the received data received by the receiver 20 may be treated as a received random number, and the transmitted data transmitted by the transmitter 10 may be treated as a transmitted random number, and the transmitted random number and the received random number may be shared between the transmitter 10 and the receiver 20.

[0032] The key generation unit 23 generates a quantum key by key distillation processing from the received data demodulated by the demodulation unit 22. For example, the key generation unit 23 may generate the quantum key by error correction processing and privacy amplification processing.

[0033] 4 shows an example of a frame structure according to some embodiments. A frame represents a time sequence of signals transmitted between the transmitter 10 and the receiver 20, and represents a time sequence of signals processed by the transmitter 10 and the receiver 20, or a time sequence of processing performed by the transmitter 10 and the receiver 20.

[0034] 4, the first period and the second period are repeated. The transmitter 10 and the receiver 20 synchronize with each other and switch between processing during the first period and the second period. The transmitter 10 transmits a time-division multiplexed signal during the first period and the second period, and the receiver 20 receives the time-division multiplexed signal.

[0035] In the transmitter 10, the bias control unit 13 switches the bias voltage control operation depending on the period. For example, the bias control unit 13 executes the bias voltage control operation during a first period and stops the bias voltage control operation during a second period. For example, the first period is a bias voltage control period in which the bias voltage is controlled, and the second period is a quantum key distribution period in which quantum key distribution is performed. The second period is also a period in which random numbers are shared. For example, the bias control unit 13 superimposes a dither signal on the bias voltage during the first period and does not superimpose the dither signal on the bias voltage during the second period. It can also be said that the optical signal transmitted during the first period includes a dither signal, and the optical signal transmitted during the second period does not include a dither signal. In other words, the transmitter 10 time-division multiplexes and transmits an optical signal modulated using a dither signal and an optical signal modulated without using a dither signal, and the receiver 20 receives the time-division multiplexed optical signal.

[0036] The receiver 20 switches the quantum key generation operation of the key generation unit 23 depending on the period. The key generation unit 23 stops generating quantum keys using demodulated data during a first period, which is a bias voltage control period, and generates quantum keys using demodulated data during a second period, which is a quantum key distribution period. It can also be said that the key generation unit 23 does not use data received during the first period to generate quantum keys, and uses data received during the second period to generate quantum keys. For example, the key generation unit 23 may discard data received during the first period. For example, the receiver 20 may discard received data during a first period when the optimal bias voltage search function of the bias control unit 13 is enabled, use received data during a second period when the optimal bias voltage search function of the bias control unit 13 is disabled as a received random number, and use transmission data transmitted by the transmitter 10 as a transmitted random number, thereby sharing the transmitted random number and the received random number.

[0037] The receiver 20 may include an optical shutter (light blocking unit) that blocks the optical signal received from the transmitter 100. For example, the first period may include a period during which the optical shutter blocks the received optical signal. The receiver 20 may further include an excess noise measurement unit that performs excess noise measurement. For example, the first period may include a period during which the excess noise measurement unit performs excess noise measurement while the optical shutter blocks the received optical signal. The second period may include a period during which the excess noise measurement unit performs excess noise measurement while the optical shutter transmits the received optical signal, and a period during which the key generation unit 23 generates a quantum key. For example, the receiver 20 may use the received data from a first period when the optimal bias voltage search function of the bias control unit 13 is enabled and the optical shutter is open as data for measuring excess noise, use the received data from a second period when the optimal bias voltage search function of the bias control unit 13 is disabled and the optical shutter is closed as a received random number, and use the transmitted data transmitted by the transmitter 10 during the second period as a transmitted random number, and share the transmitted random number and the received random number.

[0038] As described above, in this embodiment, the transmitter monitors the output of the optical modulator to control the bias voltage, and switches between executing and stopping the bias voltage control between the first period and the second period. The receiver switches between whether to generate a quantum key using received data between the first period and the second period in accordance with the switching of the bias voltage control operation of the transmitter. This makes it possible to introduce automatic bias control (ABC) into the quantum key distribution system, enabling stable and effective control of the bias voltage.

[0039] In the following embodiment, a specific example of the first embodiment will be described.

[0040] (Embodiment 2) Next, a description will be given of embodiment 2. In this embodiment, an example in which bias voltage control is performed by ABC in QKD will be described.

[0041] FIG. 5 shows an example configuration of a quantum key distribution system 1 according to some embodiments. In the example of FIG. 5, the quantum key distribution system 1 includes a transmitter 100 and a receiver 200. The quantum key distribution system 1 is a system that performs quantum key distribution using the transmitter 100 and the receiver 200. In quantum key distribution, random numbers that are the source of an encryption key are transmitted using quantum light. This enables secure key sharing between the transmitter 100 and the receiver 200. The quantum key distribution system 1 performs quantum key distribution using light whose intensity has been reduced to a level at which quantum behavior can be confirmed. This makes it possible to quantum-mechanically guarantee that the encryption key will not be leaked, thereby achieving high confidentiality.

[0042] 5, the quantum key distribution system 1 may perform quantum key distribution using CV-QKD or DV-QKD. That is, the transmitter 100 is a transmitting device for CV-QKD or DV-QKD, and the receiver 200 is a receiving device for CV-QKD or DV-QKD.

[0043] 5, the transmitter 100 and the receiver 200 are communicatively connected via an optical transmission path 2, which is an optical fiber. The transmitter 100 and the receiver 200 perform quantum key distribution using, for example, a quantum channel and a classical channel formed by the optical transmission path 2.

[0044] The quantum channel is a communication channel for transmitting and receiving weak light (quantum light) from the transmitter 100 to the receiver 200. The weak light here refers to light that behaves quantum-wise with an optical power of approximately 1 photon / bit or less. The quantum channel is configured using, for example, an optical transmission line 2.

[0045] A classical channel is a channel with higher reliability than a quantum channel. A highly reliable communication channel means, for example, a low BER. In the following, for convenience of explanation, an error-free communication channel is assumed as the classical channel. The absence of errors here may mean that all communication errors can be corrected by error correction, or that all errors can be detected and retransmitted by error detection. The communication method for the classical channel is not limited to a specific method. For example, the classical channel may be configured using the same optical transmission path 2 as the quantum channel, or may be configured using a transmission path separate from the quantum channel.

[0046] Note that the notation "transmission" of the transmitter 100 and the notation "reception" of the receiver 200 are both for convenience of explanation, and data may be transmitted from the receiver 200 to the transmitter 100. In particular, the receiver 200 transmits information for performing processing in quantum key distribution to the transmitter 100 using a classical channel.

[0047] The transmitter 100 is a quantum key distribution device that transmits weak light (quantum light) for quantum key distribution. For example, the transmitter 100 may modulate the light to be transmitted using the same modulation method as an optical transmitter used in coherent communication. For example, the transmitter 100 modulates the light using a random number sequence (modulation data) and transmits the modulated weak light to the receiver 200 via a quantum channel (optical transmission path 2). The random number sequence may include random number data that is the source of the key and basis data (basis selection information). Note that only the random number data may be transmitted without using the basis data.

[0048] For example, when a receiver used by an eavesdropper receives quantum light from the transmitter 100, it cannot receive the basis data for decoding the code before receiving the quantum light code. Therefore, according to the quantum cloning theorem, it is impossible to maintain the quantum light in its quantum light state without leaving a trace, and the eavesdropper's receiver will randomly select one of two bases and decode the code generated by the quantum light that was just snatched. In this case, there is a 50 / 50 chance that the receiver used by the eavesdropper will decode using a basis different from the basis used by the transmitter 100, and accurate decoding will not be possible. Furthermore, eavesdropping is impossible because the quantum state changes when a different basis is measured, allowing eavesdropping to be detected.

[0049] In the example of Fig. 5, the transmitter 100 includes a light source 110, an IQ modulator 120, and an automatic bias control unit 130. Note that Fig. 5 is just an example, and other configurations are also possible. For example, as shown in Fig. 6, the transmitter 100 may include an optical branching unit 131 that branches the output light of the IQ modulator 120, and an optical monitor 132 that monitors the branched light. The optical branching unit 131 branches the light modulated by the IQ modulator 120 before attenuation or the weak light after attenuation. The optical branching unit 131 is an optical coupler or the like. The optical monitor 132 is a photodetector or the like that detects the power of the light.

[0050] The light source 110 outputs light used for transmitting quantum light. For example, the light source 110 is a laser diode that outputs laser light.

[0051] The IQ modulator 120 is a phase modulator that phase-modulates the light output from the light source 110. The IQ modulator 120 generates quantum light (weak light) to be transmitted from the light output from the light source 110. The IQ modulator 120 modulates the light output from the light source 110 based on modulation data, which is a random number sequence. For example, when the modulation data includes random number data and basis data that are the source of a key, IQ modulation is performed using the bits of the basis data (Q value) and the bits of the random number data (I value).

[0052] For example, the IQ modulator 120 may include an MZ modulator for Ich (in-phase component) and an MZ modulator for Qch (quadrature component). Ich modulation data (e.g., random number data) is applied as a drive signal to the Ich MZ modulator, and the light from the light source 110 is modulated in accordance with the Ich modulation data. A bias voltage for adjusting an operating point is applied from the automatic bias control unit 130 to the Ich MZ modulator. Qch modulation data (e.g., base data) is applied as a drive signal to the Qch MZ modulator, and the light from the light source 110 is modulated in accordance with the Qch modulation data. A bias voltage for adjusting an operating point is applied from the automatic bias control unit 130 to the Qch MZ modulator.

[0053] Furthermore, the optical power of the signal light modulated by the IQ modulator 120 is attenuated to a weak state where the signal light behaves quantum-wise at approximately 1 photon / bit or less. This makes it possible to determine whether or not eavesdropping has occurred based on the principles of quantum mechanics. The attenuation of the signal light may be performed by the IQ modulator 120, or may be performed by a variable attenuator separate from the IQ modulator 120. The transmitter 100 transmits the attenuated signal light, that is, weak light (quantum light), to the receiver 200 via an optical transmission path 2 (quantum channel). The transmitter 100 includes a transmitting unit that transmits the weak light.

[0054] The automatic bias control unit 130 performs automatic bias control (ABC) on the IQ modulator 120. The automatic bias control unit 130 monitors the output of the IQ modulator 120 and controls the bias voltage of the IQ modulator 120 based on the monitoring results. For example, the automatic bias control unit 130 may acquire the monitoring results from the optical monitor 132. The automatic bias control unit 130 applies a bias voltage onto which a dither signal is superimposed to the IQ modulator 120 and adjusts the bias voltage to compensate for drift of the operating point (bias point) based on the monitoring results. In other words, the bias drift is compensated for by an adjustment loop closed in the transmitter 100. The automatic bias control unit 130 may monitor the light IQ-modulated by the IQ modulator 120 as is, or may monitor weak light after attenuation. The automatic bias control unit 130 also switches the automatic bias control on and off depending on the period. That is, the automatic bias control unit 130 switches the bias voltage applied to the IQ modulator 120 between a bias voltage with a dither signal superimposed thereon and a bias voltage without a dither signal, depending on the period.

[0055] The receiver 200 is a quantum key distribution device that receives weak light (quantum light) for quantum key distribution. For example, the receiver 200 receives the weak light from the transmitter 100 via a quantum channel (optical transmission path 2), detects the received weak light, and generates a quantum key (private key) from a bit string obtained by the detection.

[0056] 5, receiver 200 includes an optical front end 210, a demodulation DSP 220, and a key distillation unit 230. In the case of DV-QKD, demodulation DSP 220 does not necessarily have to be included.

[0057] The optical front end 210 is a conversion unit that performs photoelectric conversion on the received weak light (quantum light). The receiver 200 includes a receiving unit that receives the weak light via the quantum channel (optical transmission path 2), and the optical front end 210 may perform photoelectric conversion on the weak light received by the receiving unit.

[0058] In the case of DV-QKD, the optical front end 210 may be a photon detector, such as an avalanche photodiode (APD), which converts weak light into an electrical signal depending on the presence or absence of photons in the weak light.

[0059] In the case of CV-QKD, the optical front-end 210 may be a coherent detector. In the case of a coherent detector, for example, a 90° hybrid circuit may be used to cause interference between weak light and localized light, reading out the quadrature-phase component, and a photodetector may convert the readout quadrature-phase component into an electrical signal. CV-QKD can be implemented using standard optical components and is less costly than DV-QKD, which uses a photon detector. Furthermore, CV-QKD can realize a quantum key distribution system in which general communication light and transmission paths coexist by filtering using localized light. In coherent detection, the signal light can be optically amplified by interfering localized light with high optical power with the signal light. Therefore, even when the signal light power is weak (less than 1 photon / bit), it can be detected using a standard photodetector.

[0060] The demodulation DSP 220 is a demodulation unit that performs digital signal processing on the electrical signal converted from the weak light by the optical front end 210 and demodulates it into a sifted key. For example, the demodulation DSP 220 may generate a quantized raw key, which is a bit of a random number sequence, from the detection result of the weak light, and generate a sifted key (screened key) from the quantized raw key through a basis matching process. In the basis matching process, the sifted key is generated by screening the quantized raw key using a classical channel. In the basis matching process, for example, basis data is received from the transmitter 100 through the classical channel, and the detection result for the phase and amplitude of the read quantum light is projected onto the I axis or Q axis to convert it into a bit value of 0 or 1.

[0061] The key distillation unit 230 is a quantum key generation unit that performs key distillation processing based on the sifted key generated by the demodulation DSP to generate a quantum key (private key). In the case of DV-QKD, the key distillation processing may be performed on the detection results of the photon detector in the optical front end 210. Furthermore, the key distillation unit 230 switches whether or not to discard the demodulated sifted key depending on the period. In other words, the key distillation unit 230 switches whether or not to generate a quantum key (private key) using the demodulated sifted key depending on the period.

[0062] For example, the key distillation process includes an error correction process and a privacy amplification process. That is, the key distillation unit 230 may include an error correction processing unit that performs an error correction process and a privacy amplification processing unit that performs a privacy amplification process. In the error correction process, a classical channel is used to perform error correction on the sifted key obtained by basis matching. For example, a portion of the bits for which basis matching has been completed is made public via the classical channel between the transmitter 100 and the sender, and an error rate is measured. Depending on the measured error rate, a further portion of the bits is made public and used for correction, thereby allowing the sender and the receiver to share the same bit string.

[0063] In the privacy amplification process, only a random number sequence that has no possibility of being eavesdropped is extracted from the error-corrected sifted key, and a secret key (quantum key) is output. In the privacy amplification process, noise and loss in the quantum channel are measured to estimate the maximum amount of information that an eavesdropper can obtain if an eavesdropper is present, and a portion of the bit sequence is randomly discarded so that the amount of information that the eavesdropper can obtain is zero. The privacy amplification process includes an eavesdropping level estimation process that estimates the amount of eavesdropping. This allows the transmitter 100 and receiver 200 to share a random number sequence that is quantum-mechanically guaranteed not to have been eavesdropped.

[0064] Fig. 7 shows an example of a frame structure in a related quantum key distribution system 9. Fig. 8 shows an example of a frame structure in a quantum key distribution system 1 according to some embodiments.

[0065] As shown in Figure 7, in the related quantum key distribution system 9, periods T10 and T11 are repeated. That is, bias control (optimal bias search) based on BER measurement is performed in period T10, and then private key generation is performed in period T11, and this process is repeated. As described above, in the related quantum key distribution system 9, bias control based on BER measurement poses the problem that bias drift compensation takes time and is unstable. Bias control based on BER measurement requires a large loop including the transmitter and receiver, and may take, for example, approximately 10 minutes.

[0066] In contrast, as shown in FIG. 8 , the quantum key distribution system 1 repeats periods T12 and T11. That is, bias control (optimal bias search) using ABC is performed during period T12, followed by private key generation during period T11, and this process is repeated. Period T12 is a period during which the optimal bias search in the transmitter 100 is active, and the received data is discarded in the receiver 200. Period T11 is a period during which the optimal bias search in the transmitter 100 is inactive, and the receiver 200 shares random numbers based on the received data (generates a private key). In the quantum key distribution system 1, bias control using ABC allows bias drift to be compensated stably and in a short time. In ABC bias control, control is completed in a closed loop on the transmitter side, so processing is completed in, for example, several seconds. For example, bias control using ABC during period T12 may be performed every hour or every few hours. Since the operating point shifts depending on the temperature, in an environment where the temperature changes drastically, bias control may be performed more frequently.

[0067] In this manner, in this embodiment, the transmitter 100 and the receiver 200 switch between automatic bias control (optimal bias search) and secret key generation at the same time. The transmitter 100 and the receiver 200 may each include a control unit that controls the timing of each process. For example, the control unit of the transmitter 100 and the control unit of the receiver 200 may synchronize to switch the process of each device.

[0068] 9 shows an example of operation during automatic bias control (period T12) in the quantum key distribution system 1 according to some embodiments. In the example of FIG. 9, the transmitter 100 executes S101 to S104, and the receiver 200 executes S105 to S107.

[0069] During automatic bias control, the transmitter 100 turns on the dither signal to execute bias voltage control by ABC (S101). The automatic bias control unit 130 determines whether the current time (timing) is in the automatic bias control period T12 or the secret key generation period T11. If it is in the period T12, the automatic bias control is started and a bias voltage onto which the dither signal is superimposed is applied to the IQ modulator 120.

[0070] Next, the transmitter 100 performs modulation based on the modulation data (S102). The IQ modulator 120 modulates the light output from the light source 110 based on the input modulation data and a bias voltage on which a dither signal is superimposed. The IQ modulator 120 performs IQ modulation at an operating point set by the applied bias voltage.

[0071] Next, the transmitter 100 performs automatic bias control (S103). The automatic bias control unit 130 monitors the output light of the IQ modulator 120 and adjusts the bias voltage applied to the IQ modulator 120 based on the monitoring results. The automatic bias control unit 130 repeatedly adjusts the bias voltage until the power of the output light of the IQ modulator 120 falls within a predetermined range, searching for the bias voltage at the optimal operating point. For example, the automatic bias control may be performed until the end of the period T12.

[0072] The transmitter 100 also transmits the weak light (S104). The transmitter 100 transmits the weak light modulated and attenuated by the IQ modulator 120 to the receiver 200 via the optical transmission path 2 (quantum channel). The weak light transmitted here is light modulated using a bias voltage on which a dither signal is superimposed, as described above.

[0073] Next, the receiver 200 detects the received weak light (S105). The optical front end 210 receives the weak light via the optical transmission path 2 (quantum channel) and converts the received weak light into an electrical signal. For example, in the case of DV-QKD, the weak light is converted into an electrical signal by a photon detector, and in the case of CV-QKD, the weak light is converted into an electrical signal by a coherent detector.

[0074] Next, the receiver 200 demodulates the detected signal to generate data (S106). The demodulation DSP 220 performs digital signal processing on the electrical signal obtained by the detection to demodulate the data. For example, in the case of CV-QKD, the demodulation DSP 220 performs a basis matching process on the quantized raw key obtained by coherent detection to generate a sifted key.

[0075] Next, the receiver 200 discards the demodulated data (S107). The key distillation unit 230 determines whether the current time (timing) is in the automatic bias control period T12 or the private key generation period T11, and if it is in the period T12, discards the demodulated data (shift key). That is, the key distillation unit 230 does not generate a private key if it is in the automatic bias control period T12. Note that the receiver 200 may discard data by any process. For example, the data may be discarded before error correction or before privacy amplification.

[0076] 10 and 11 show an example of operation during private key generation (period T11) in the quantum key distribution system 1 according to some embodiments. In the example of Fig. 10 and Fig. 11, the transmitter 100 executes S111 and S102 to S104, and the receiver 200 executes S105 to S108.

[0077] When generating the private key, the transmitter 100 does not execute bias voltage control by ABC and therefore turns off the dither signal (S111). The automatic bias control unit 130 determines whether the current time (timing) is in the automatic bias control period T12 or the private key generation period T11, and if it is in the period T11, stops the automatic bias control and applies a predetermined bias voltage without a dither signal to the IQ modulator 120. For example, the automatic bias control unit 130 applies the optimal bias voltage found by the automatic bias control.

[0078] Next, the transmitter 100 performs modulation based on the modulation data (S102). The IQ modulator 120 modulates the light output from the light source 110 based on the input modulation data and a bias voltage without a dither signal.

[0079] 9, the transmitter 100 transmits weak light (S104). The transmitted weak light is modulated using a bias voltage without a dither signal, as described above. The receiver 200 receives and detects the weak light (S105) and demodulates the detected signal (S106).

[0080] Next, the receiver 200 performs key distillation (S108). The key distillation unit 230 determines whether the current time (timing) is in the automatic bias control period T12 or the private key generation period T11, and if it is in the period T11, performs key distillation processing using the demodulated data (shift key). That is, if it is in the private key generation period, the key distillation unit 230 generates a private key.

[0081] 11 , as the key distillation process, the key distillation unit 230 performs error correction (S109) and privacy amplification (S110). The key distillation unit 230 performs error correction on the sifted key obtained by demodulation using a classical channel. By privacy amplification, the key distillation unit 230 extracts only a random number sequence that is unlikely to have been intercepted from the error-corrected sifted key, and generates a secret key.

[0082] As described above, in this embodiment, ABC is introduced into QKD by time division multiplexing to stabilize bias drift compensation control. In this embodiment, in order to eliminate the influence of the dither signal superimposed on the bias voltage of the modulator, time division multiplexing is performed in which a secret key is not generated when bias control by ABC is performed, and a secret key is generated when bias control by ABC is not performed. This allows ABC to be introduced into QKD, improving the stability of bias control of the modulator. In other words, the adjustment loop can be simplified, allowing bias adjustment to be closed at the transmitter, shortening the pull-in time of bias control and significantly improving the stability of bias control.

[0083] (Embodiment 3) Next, a description will be given of embodiment 3. In this embodiment, an example will be described in which bias voltage control by ABC and excess noise measurement are performed simultaneously in CV-QKD.

[0084] Fig. 12 shows an example of the configuration of a quantum key distribution system 1 according to some embodiments. In the example of Fig. 12, the quantum key distribution system 1 performs quantum key distribution by CV-QKD. That is, the transmitter 100 is a transmitting device for CV-QKD, and the receiver 200 is a receiving device for CV-QKD. The configuration of the transmitter 100 is the same as that of Fig. 5.

[0085] 5, receiver 200 includes an optical shutter 240 and an excess noise measurement unit 250. The rest is the same as in FIG.

[0086] The optical shutter 240 is an optical blocking unit that blocks the received weak light (quantum light). The receiver 200 includes a receiving unit that receives the weak light (quantum light) via a quantum channel (optical transmission path 2), and the optical shutter 240 may block the weak light received by the receiving unit. The optical shutter 240 is disposed between the optical transmission path 2 and the optical front end 210 and the excess noise measurement unit 250. The optical shutter 240 can be opened and closed. When opened, the received weak light is input to the optical front end 210 and the excess noise measurement unit 250, and when closed, the received weak light is blocked and is not input to the optical front end 210 and the excess noise measurement unit 250 (zero input light). A control unit that controls the opening and closing of the optical shutter 240 may be provided.

[0087] Excess noise measurement unit 250 measures the excess noise using light input to receiver 200. Excess noise measurement unit 250 measures the dispersion in a state where there is no input light with optical shutter 240 closed, measures the dispersion of the input weak light with optical shutter 240 open, and measures the excess noise by comparing the dispersion in a state where there is no input light with the dispersion of the weak light.

[0088] Figure 13 shows an example of a frame structure in a related quantum key distribution system 9. Figure 14 shows an example of a frame structure in a quantum key distribution system 1 according to some embodiments. In the case of CV-QKD, in addition to bias control and secret key generation, excess noise measurement is required.

[0089] 13, in the related quantum key distribution system 9, periods T20 to T23 are repeated. That is, bias control (optimal bias search) is performed based on BER measurement in period T20, excess noise measurement is performed with the optical shutter 240 closed in period T21, excess noise measurement is performed with the optical shutter 240 open in period T22, and private key generation is performed in period T23, and this process is repeated. In the related quantum key distribution system 9, bias control is performed based on BER measurement, so bias drift compensation is time-consuming and unstable, and the need for excess noise measurement also results in poor key generation efficiency.

[0090] In contrast, as shown in FIG. 14 , the quantum key distribution system 1 repeats periods T21 to T23. That is, during period T21 when the optical shutter 240 is closed, excess noise measurement is performed and bias control by ABC (optimal bias search) is performed, and bias control completed by the transmitter and excess noise measurement completed by the receiver are simultaneously performed. Furthermore, during period T22, excess noise measurement is performed with the optical shutter 240 open, and during period T23, private key generation is performed, and this process is repeated. Period T21 is a period during which the optimal bias search in the transmitter 100 is valid, and the receiver 200 performs excess noise measurement with the optical shutter 240 closed. Period T22 is a period during which the optimal bias search in the transmitter 100 is invalid, and the receiver 200 performs excess noise measurement with the optical shutter 240 open. Period T23 is a period during which the optimal bias search in the transmitter 100 is invalid, and the receiver 200 opens the optical shutter 240 and performs excess noise measurement. For example, to share the key securely, the excess noise measurement may be performed periodically, such as every hour, over periods T21 and T22. In this manner, in this embodiment, the transmitter 100 and the receiver 200 switch between processing so that automatic bias control (optimum bias search) and excess noise measurement with the optical shutter closed are performed at the same time.

[0091] 15 shows an example of operation of the quantum key distribution system 1 according to some embodiments. In Fig. 15, excess noise measurement processing in S201 to S205 is added to the example of operation in Fig. 11.

[0092] 9 and 10 . That is, the transmitter 100 (automatic bias control unit 130) determines whether the current time (timing) is during period T21, when the optical shutter 240 of the receiver 200 is closed, or during periods T22 and T23, when the optical shutter 240 of the receiver 200 is open. If the current time is during period T21, the transmitter 100 turns on the dither signal and performs automatic bias control. If the current time is during periods T22 and T23, the transmitter 100 turns off the dither signal and does not perform automatic bias control. That is, during period T21, the transmitter 100 transmits weak light modulated using a bias voltage superimposed with a dither signal, and during periods T22 and T23, the transmitter 100 transmits weak light modulated using a bias voltage without a dither signal (S104).

[0093] 15 , first, during an excessive noise measurement period, receiver 200 closes optical shutter 240 (S201) and performs excessive noise measurement with optical shutter 240 closed (S202). Receiver 200 (controller of optical shutter 240) determines whether the current time (timing) is in period T21, when automatic bias control is performed, or in periods T22 and T23, when automatic bias control is not performed, and if it is period T21, closes optical shutter 240 to block input light. Excess noise measurement unit 250 determines whether the current time (timing) is in period T21, when excessive noise measurement is performed with optical shutter 240 closed, and if it is period T21, measures dispersion in a state with zero input light.

[0094] Next, the receiver 200 opens the optical shutter 240 (S203), performs demodulation (S204), and performs excess noise measurement with the optical shutter 240 open (S205). The receiver 200 (controller of the optical shutter 240) determines whether the current time (timing) is in period T21, when automatic bias control is performed, or in periods T22 and T23, when automatic bias control is not performed. If it is in periods T22 and T23, the receiver 200 opens the optical shutter 240 and inputs the received weak light to the optical front end 210. The optical front end 210 detects the weak light received via the optical shutter 240, and the demodulation DSP 220 demodulates the detected signal. The excess noise measurement unit 250 determines whether the current time (timing) is in period T22, when excessive noise measurement is performed with the optical shutter 240 open. If it is in period T22, the receiver 200 measures the variance of the received weak light based on the demodulated data. Furthermore, the excess noise measurement unit 250 compares the variance in the zero input light state with the variance of the weak light, and outputs the comparison result to the key distillation unit 230 .

[0095] Next, in period T23, which is the time for generating a private key, similarly to FIGS. 10 and 11 , the receiver 200 receives and detects weak light (S105) and demodulates the detected signal (S106). Furthermore, in the key distillation process (S108), the receiver 200 performs error correction (S109) and privacy amplification (S110). The key distillation unit 230 determines whether the current time (timing) is in periods T21 and T22, during which excessive noise measurement is performed, or in period T23, during which a private key is generated. If it is in periods T21 and T22, the key distillation unit 230 does not use the demodulated data (shift key) to generate a private key. As in the second embodiment, the data may be discarded. Furthermore, if it is in period T23, the key distillation unit 230 uses the demodulated data (shift key) to perform error correction and privacy amplification and generate a private key. In privacy amplification, the key distillation unit 230 uses the results of excess noise measurement by the excess noise measurement unit 250 during periods T21 and T22. For example, if the variance in the zero-input light state matches the variance of weak light, it is determined that there is no excess noise and no eavesdropping has occurred, and the amount of eavesdropping is estimated. As a result, only random number sequences that are unlikely to have been eavesdropped are extracted from the error-corrected sifted key, and a secret key is generated.

[0096] As described above, in this embodiment, the excess noise measurement essential for CV-QKD and the bias control by ABC are performed in the same period. That is, the bias control by ABC, which is completed only by the transmitter, and the excess noise measurement with the optical shutter closed, which is completed only by the receiver, are performed simultaneously. This makes it possible to suppress the reduction in the efficiency of private key generation due to bias drift compensation in CV-QKD to zero.

[0097] The present disclosure is not limited to the above-described embodiments, and can be modified as appropriate within the scope of the present disclosure.

[0098] Each component in the above-described embodiments may be configured with hardware or software, or both, and may be configured with a single piece of hardware or software, or may be configured with multiple pieces of hardware or software. Each function of each device (error correction, privacy amplification, etc.) may be realized by a computer 30 having a processor 31 such as a CPU and a memory 32 serving as a storage device, as shown in FIG. 16. For example, a program for performing the method in the embodiment may be stored in the memory 32, and each function may be realized by the processor 31 executing the program stored in the memory 32.

[0099] These programs include instructions (or software code) that, when loaded into a computer, cause the computer to perform one or more functions described in the embodiments. The programs may be stored on a non-transitory computer-readable medium or a tangible storage medium. By way of example and not limitation, computer-readable media or tangible storage media include random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD) or other memory technology, CD-ROM, digital versatile disc (DVD), Blu-ray disc or other optical disk storage, magnetic cassette, magnetic tape, magnetic disk storage or other magnetic storage device. The programs may also be transmitted on a transitory computer-readable medium or communication medium. By way of example and not limitation, transitory computer-readable media or communication media include electrical, optical, acoustic, or other forms of propagated signals.

[0100] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0101] Each drawing is merely an example for describing one or more embodiments. Each drawing may not relate to only one particular embodiment, but may also relate to one or more other embodiments. As will be understood by those skilled in the art, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings to create, for example, an embodiment not explicitly shown or described. Not all features or steps shown in any one drawing are necessary to describe an exemplary embodiment, and some features or steps may be omitted. The order of steps described in any drawing may be changed as appropriate.

[0102] Some or all of the above embodiments can be described as, but are not limited to, the following supplementary notes. (Supplementary Note 1) A transmitter comprising: a light source that outputs light; an optical modulator that modulates the light output from the light source into light for quantum key distribution; and bias control means that controls a bias voltage of the optical modulator based on the light output from the optical modulator, wherein the bias control means performs a control operation of the bias voltage during a first period that is a bias voltage control period, and stops the control operation of the bias voltage during a second period that is a quantum key distribution period. (Supplementary Note 2) The transmitter according to Supplementary Note 1, wherein the first period includes a period in which a quantum key is not generated in a receiver, and the second period includes a period in which a quantum key is generated in the receiver. (Supplementary Note 3) The transmitter according to Supplementary Note 1 or 2, wherein the first period includes a period in which an optical signal received by the receiver is blocked and excess noise measurement is performed. (Supplementary Note 4) The transmitter according to any one of Supplements 1 to 3, wherein the bias control means superimposes a dither signal on the bias voltage during the first period and does not superimpose the dither signal on the bias voltage during the second period. (Supplementary Note 5) The transmitter according to any one of Supplements 1 to 4, comprising: branching means for branching light output from the optical modulator; and optical monitoring means for monitoring the branched light, wherein the bias control means controls a bias voltage of the optical modulator based on the monitored light. (Supplementary Note 6) A receiver comprising: photoelectric conversion means for photoelectrically converting an optical signal received from a transmitter; demodulation means for demodulating data from the photoelectrically converted signal; and key generation means for generating a quantum key from the demodulated data by a key distillation process, wherein the key generation means stops generating the quantum key using the data during a first period that is a bias voltage control period of the optical modulator in the transmitter, and performs quantum key generation using the data during a second period that is a quantum key distribution period. (Supplementary Note 7) The receiver according to Supplementary Note 6, wherein the key generating means discards the data during the first period. (Supplementary Note 8) The receiver according to Supplementary Note 6 or 7, further comprising: an optical blocking means for blocking the received optical signal, and the first period includes a period during which the optical blocking means blocks the received optical signal.(Supplementary Note 9) The receiver according to Supplementary Note 8, comprising an excess noise measurement means for performing excess noise measurement, wherein the first period includes a period during which the excess noise measurement means performs the excess noise measurement in a state in which the optical blocking means blocks the received optical signal. (Supplementary Note 10) The receiver according to Supplementary Note 9, wherein the second period includes a period during which the excess noise measurement means performs the excess noise measurement in a state in which the optical blocking means transmits the received optical signal, and a period during which the key generation means generates the quantum key. and a receiver configured to generate a quantum key from the demodulated data by a key distillation process, wherein the key generation means stops generating the quantum key using the data during the first period and generates the quantum key using the data during the second period. (Supplementary Note 12) The quantum key distribution system according to Supplementary Note 11, wherein the bias control means superimposes a dither signal on the bias voltage during the first period and does not superimpose the dither signal on the bias voltage during the second period. (Supplementary Note 13) The quantum key distribution system according to Supplementary Note 11 or 12, wherein the transmitter comprises: branching means for branching light output from the optical modulator; and optical monitoring means for monitoring the branched light, and the bias control means controls the bias voltage of the optical modulator based on the monitored light. (Supplementary Note 14) The quantum key distribution system according to any one of Supplements 11 to 13, wherein the key generation means discards the data during the first period.(Supplementary Note 15) The quantum key distribution system according to any one of Supplementary Notes 11 to 14, wherein the receiver comprises an optical blocking means for blocking the received optical signal, and the first period includes a period during which the optical blocking means blocks the received optical signal. (Supplementary Note 16) The quantum key distribution system according to Supplementary Note 15, wherein the receiver comprises an excess noise measurement means for performing excess noise measurement, and the first period includes a period during which the excess noise measurement means performs the excess noise measurement in a state in which the optical blocking means blocks the received optical signal. (Supplementary Note 17) A method in a transmitter, comprising: controlling a bias voltage of an optical modulator based on light output from the optical modulator that modulates light from a light source to light for quantum key distribution; and performing a control operation of the bias voltage during a first period that is a bias voltage control period, and stopping the control operation of the bias voltage during a second period that is a quantum key distribution period. (Supplementary Note 18) A method in a receiver, comprising: performing opto-electrical conversion on an optical signal received from a transmitter; demodulating the opto-electrically converted signal into data; generating a quantum key from the demodulated data by a key distillation process; stopping generation of the quantum key using the data during a first period that is a bias voltage control period of an optical modulator in the transmitter, and performing generation of the quantum key using the data during a second period that is a quantum key distribution period. (Supplementary Note 19) A program for causing a computer to execute the method in a transmitter, comprising: controlling a bias voltage of the optical modulator based on light output from an optical modulator that modulates light from a light source to light for quantum key distribution; and performing a control operation of the bias voltage during a first period that is a bias voltage control period, and stopping the control operation of the bias voltage during a second period that is a quantum key distribution period.(Supplementary Note 20) A program for causing a computer to execute a method in a receiver, the method comprising: opto-electrically converting an optical signal received from a transmitter; demodulating data from the opto-electrically converted signal; generating a quantum key from the demodulated data by a key distillation process; and stopping generation of the quantum key using the data during a first period that is a bias voltage control period of an optical modulator in the transmitter, and executing generation of the quantum key using the data during a second period that is a quantum key distribution period. and a demodulator that demodulates the received signal and outputs received data, wherein the receiver discards the received data during a first period when an optimal bias voltage search function of the optical modulator bias controller is enabled, and regards the received data during a second period when the optimal bias voltage search function of the optical modulator bias controller is disabled as a received random number, and realizes sharing of the transmitted random number and the received random number by using the transmitted data transmitted during the second period as a transmitted random number.(Supplementary Note 22) A random number sharing system using continuous quantum key distribution in which a transmitter and a receiver are connected via an optical transmission line, the transmitter comprising: a light source that outputs light carrying transmission data; an optical modulator that modulates the light output from the light source according to an input bias voltage by superimposing the transmission data and a dither signal on the light output from the light source; and an optical modulator bias controller that outputs a bias voltage for the optical modulator and performs an optimal bias voltage search by outputting the dither signal and monitoring the output of the optical modulator; the receiver comprising: an optical shutter that sets the intensity of light input to the receiver to zero for excess noise measurement; an optical front-end unit that converts a received optical signal into a received signal which is an electrical signal; and a demodulator that demodulates the received signal and outputs received data; the received data in a first period in which the optimal bias voltage search function of the optical modulator bias controller is active and the optical shutter is open is used as excess noise measurement data; A random number sharing system using quantum key distribution, characterized in that the received data during a second period in which the optimal bias voltage search function of the optical modulator bias controller is disabled and the optical shutter is closed is used as a received random number, and the transmitted data transmitted during the second period is used as a transmitted random number, thereby realizing sharing of the transmitted random number and the received random number.

[0103] Some or all of the elements (e.g., configurations and functions) described in Supplementary Notes 2 to 5 and Supplementary Notes 7 to 10, which are dependent on Supplementary Note 1 (transmitter) and Supplementary Note 6 (receiver), may also be dependent on Supplementary Note 11 (system), Supplementary Note 17 (method), Supplementary Note 18 (method), Supplementary Note 19 (program), Supplementary Note 20 (program), Supplementary Note 21 (system), and Supplementary Note 22 (system) in the same dependent relationship as Supplementary Note 2 to Supplementary Note 5 and Supplementary Note 7 to Supplementary Note 10. Some or all of the elements described in any Supplementary Note may be applicable to various hardware, software, recording means for recording software, systems, and methods.

[0104] REFERENCE SIGNS LIST 1 quantum key distribution system 2 optical transmission path 10 transmitter 11 light source 12 optical modulator 13 bias control unit 20 receiver 21 photoelectric conversion unit 22 demodulation unit 23 key generation unit 30 computer 31 processor 32 memory 100 transmitter 110 light source 120 IQ modulator 130 automatic bias control unit 131 optical branching unit 132 optical monitor 200 receiver 210 optical front end 220 demodulation DSP 230 key distillation unit 240 optical shutter 250 excess noise measurement unit

Claims

1. A transmitter comprising: a light source that outputs light; an optical modulator that modulates the light output from the light source into light for quantum key distribution; and bias control means that controls a bias voltage of the optical modulator based on the light output from the optical modulator, wherein the bias control means executes a control operation of the bias voltage during a first period that is a bias voltage control period, and stops the control operation of the bias voltage during a second period that is a quantum key distribution period.

2. The transmitter of claim 1, wherein the first period includes a period during which a quantum key is not generated at the receiver, and the second period includes a period during which a quantum key is generated at the receiver.

3. The transmitter according to claim 1 or 2, wherein the first period includes a period during which the optical signal received by the receiver is blocked and excess noise is measured.

4. A transmitter according to any one of claims 1 to 3, wherein the bias control means superimposes a dither signal on the bias voltage during the first period, and does not superimpose the dither signal on the bias voltage during the second period.

5. A transmitter according to any one of claims 1 to 4, comprising: branching means for branching light output from said optical modulator; and optical monitoring means for monitoring said branched light, wherein said bias control means controls the bias voltage of said optical modulator based on said monitored light.

6. A receiver comprising: photoelectric conversion means for photoelectrically converting an optical signal received from a transmitter; demodulation means for demodulating data from the photoelectrically converted signal; and key generation means for generating a quantum key from the demodulated data by key distillation processing, wherein the key generation means stops generating the quantum key using the data during a first period which is a bias voltage control period for an optical modulator in the transmitter, and performs generation of the quantum key using the data during a second period which is a quantum key distribution period.

7. The receiver according to claim 6, wherein the key generating means discards the data during the first period.

8. A receiver according to claim 6 or 7, further comprising an optical blocking means for blocking the received optical signal, and wherein the first period includes a period during which the optical blocking means blocks the received optical signal.

9. The receiver according to claim 8, further comprising excess noise measurement means for performing excess noise measurement, wherein the first period includes a period during which the excess noise measurement means performs the excess noise measurement while the optical blocking means blocks the received optical signal.

10. The receiver according to claim 9, wherein the second period includes a period during which the excess noise measurement means performs the excess noise measurement while the optical blocking means transmits the received optical signal, and a period during which the key generation means generates the quantum key.

11. A quantum key distribution system comprising a transmitter and a receiver, wherein the transmitter comprises: a light source that outputs light; an optical modulator that modulates the light output from the light source into light for quantum key distribution and transmits the modulated light; and bias control means that controls a bias voltage of the optical modulator based on the light output from the optical modulator, wherein the bias control means performs a control operation of the bias voltage during a first period that is a bias voltage control period, and stops the control operation of the bias voltage during a second period that is a quantum key distribution period, and the receiver comprises: photoelectric conversion means that photoelectrically converts an optical signal received from the transmitter, demodulation means that demodulates data from the photoelectrically converted signal, and key generation means that generates a quantum key from the demodulated data by key distillation processing, wherein the key generation means stops generating the quantum key using the data during the first period, and performs generation of the quantum key using the data during the second period.

12. The quantum key distribution system according to claim 11, wherein the bias control means superimposes a dither signal on the bias voltage during the first period and does not superimpose the dither signal on the bias voltage during the second period.

13. The quantum key distribution system according to claim 11 or 12, wherein the transmitter comprises: branching means for branching the light output from the optical modulator; and optical monitoring means for monitoring the branched light; and the bias control means controls the bias voltage of the optical modulator based on the monitored light.

14. The quantum key distribution system according to any one of claims 11 to 13, wherein the key generation means discards the data during the first period.

15. A quantum key distribution system according to any one of claims 11 to 14, wherein the receiver comprises an optical blocking means for blocking the received optical signal, and the first period includes a period during which the optical blocking means blocks the received optical signal.

16. The quantum key distribution system according to claim 15, wherein the receiver comprises excess noise measurement means for performing excess noise measurement, and the first period includes a period during which the excess noise measurement means performs the excess noise measurement while the optical blocking means blocks the received optical signal.

17. A method in a transmitter, comprising: controlling a bias voltage of an optical modulator based on light output from the optical modulator, the optical modulator modulating light from a light source into light for quantum key distribution; and executing a control operation of the bias voltage during a first period that is a bias voltage control period, and stopping the control operation of the bias voltage during a second period that is a quantum key distribution period.

18. A method in a receiver, comprising: opto-electrically converting an optical signal received from a transmitter; demodulating data from the opto-electrically converted signal; generating a quantum key from the demodulated data by key distillation processing; and stopping generation of the quantum key using the data during a first period, which is a bias voltage control period for an optical modulator in the transmitter, and performing generation of the quantum key using the data during a second period, which is a quantum key distribution period.

19. A program for causing a computer to execute a method in a transmitter, the method including: controlling a bias voltage of an optical modulator based on light output from the optical modulator, which modulates light from a light source into light for quantum key distribution; and executing a control operation of the bias voltage during a first period, which is a bias voltage control period, and stopping the control operation of the bias voltage during a second period, which is a quantum key distribution period.

20. A program for causing a computer to execute a method in a receiver, the method comprising: opto-electrically converting an optical signal received from a transmitter; demodulating data from the opto-electrically converted signal; generating a quantum key from the demodulated data by key distillation processing; and stopping generation of the quantum key using the data during a first period, which is a bias voltage control period for an optical modulator in the transmitter, and executing generation of the quantum key using the data during a second period, which is a quantum key distribution period.

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