Transmitter, receiver, quantum key distribution system and method

Abstract

The present invention provides a transmitter, receiver, quantum key distribution system, and method that can suppress the decrease in frequency utilization efficiency. [Solution] The transmitter comprises a first light source that outputs a first light, a first branching unit that splits the first light into a first polarized light with a first polarization and a second polarized light with a second polarization, a first modulation unit that modulates the first polarized light based on a first random number, a second modulation unit that modulates the second polarized light based on data for bit position synchronization of the first random number, and a first polarization multiplexing unit that polarization multiplexes the modulated first polarized light and the modulated second polarized light and transmits the polarization multiplexed first polarization multiplexed light to a receiver.

Description

[Technical Field]

[0001] This disclosure relates to transmitters, receivers, quantum key distribution systems and methods. [Background technology]

[0002] In recent years, research into quantum cryptography has been progressing as an encryption technology to ensure the security of communications. Quantum cryptography enables the secure sharing of private keys between locations through quantum key distribution (QKD).

[0003] Two types of quantum key distribution are known: discrete variable quantum key distribution (DV-QKD), which uses photon detectors for quantum key distribution, and continuous-variable quantum key distribution (CV-QKD), which uses coherent detection for quantum key distribution. For example, in CV-QKD, the wavelength division multiplexing (WDM) technique commonly used in optical communications can be applied.

[0004] As a related technology, for example, Patent Document 1 is known. In Patent Document 1, the optical signal for QKD and the optical signal for other quantum cryptography (Quantum Noise Stream Cipher; QNSC) are wavelength-multiplexed. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2017-050678 [Overview of the project] [Problems that the invention aims to solve]

[0006] In CV-QKD, quantum key distribution (random number sharing) is performed by transmitting weak light (quantum light). Therefore, the receiver cannot determine from the weak light alone which bit in the bit sequence obtained from the weak light is the starting point for the shared random number. For this reason, bit position synchronization is necessary between the transmitter and receiver to synchronize the bit position where the shared random number begins, using a transmission means other than the weak light. For example, when an optical signal for bit position synchronization is transmitted by wavelength division multiplexing, as in related technologies such as Patent Document 1, there is a problem in that the frequency utilization efficiency decreases.

[0007] In view of these challenges, one of the objectives of this disclosure is to provide a transmitter, receiver, quantum key distribution system, and method that can suppress the decrease in frequency utilization efficiency. [Means for solving the problem]

[0008] A transmitter according to one aspect of the present disclosure includes: a first light source that outputs a first light; a first branching unit that branches the first light into a first polarized light with a first polarization and a second polarized light with a second polarization; a first modulation unit that modulates the first polarized light based on a first random number; a second modulation unit that modulates the second polarized light based on data for bit position synchronization of the first random number; and a first polarization multiplexing unit that polarization multiplexes the modulated first polarized light and the modulated second polarized light and transmits the polarization multiplexed first polarization multiplexed light to a receiver.

[0009] A receiver according to one aspect of the present disclosure includes: a first photoelectric conversion unit that receives a first polarization-multiplexed light from a transmitter and converts the received first polarization-multiplexed light into a first electrical signal; a first polarization-demodulation unit that polarization-demodulates the first electrical signal into a first polarization signal with a first polarization and a second polarization signal with a second polarization; a first bit reading unit that reads a first bit sequence including a first random number from the first polarization signal; a second bit reading unit that reads a second bit sequence including data for bit position synchronization of the first random number from the second polarization signal; and a first bit position synchronization unit that synchronizes the bit position of the first random number included in the first bit sequence based on the bit position synchronization data included in the second bit sequence.

[0010] A quantum key distribution system according to one aspect of the present disclosure is a quantum key distribution system comprising a transmitter and a receiver, wherein the transmitter comprises a first light source that outputs first light, a first branching unit that branches the first light into first polarized light with a first polarization and second polarized light with a second polarization, a first modulation unit that modulates the first polarized light based on a first random number, a second modulation unit that modulates the second polarized light based on bit position synchronization data of the first random number, and a first polarization multiplexing unit that polarization multiplexes the modulated first polarized light and the modulated second polarized light and transmits the polarization multiplexed first polarization multiplexed light to the receiver, and the receiver receives the first polarization multiplexed light from the transmitter. The device comprises: a first photoelectric conversion unit that receives light and converts the received first polarization-multiplexed light into a first electrical signal; a first polarization demodulation unit that polarization demodulates the first electrical signal into a first polarization signal with a first polarization and a second polarization signal with a second polarization; a first bit reading unit that reads a first bit sequence containing the first random number from the first polarization signal; a second bit reading unit that reads a second bit sequence containing data for bit position synchronization of the first random number from the second polarization signal; and a first bit position synchronization unit that synchronizes the bit position of the first random number contained in the first bit sequence based on the bit position synchronization data contained in the second bit sequence.

[0011] A method according to one aspect of the present disclosure is a method in a transmitter, comprising: splitting first light from a first light source into first polarized light with a first polarization and second polarized light with a second polarization; modulating the first polarized light based on a first random number; modulating the second polarized light based on bit position synchronization data of the first random number; polarization multiplexing the modulated first polarized light and the modulated second polarized light, and transmitting the polarization multiplexed first polarization multiplexed light to a receiver.

[0012] A method according to one aspect of the present disclosure is a method in a receiver, which includes receiving a first polarization-multiplexed light from a transmitter, converting the received first polarization-multiplexed light into a first electrical signal, polarization-demodulating the first electrical signal into a first polarization signal of a first polarization and a second polarization signal of a second polarization, reading a first bit sequence containing a first random number from the first polarization signal, reading a second bit sequence containing data for bit position synchronization of the first random number from the second polarization signal, and performing bit position synchronization of the first random number contained in the first bit sequence based on the bit position synchronization data contained in the second bit sequence. [Effects of the Invention]

[0013] According to this disclosure, it is possible to suppress the decrease in frequency utilization efficiency. [Brief explanation of the drawing]

[0014] [Figure 1] This is a configuration diagram showing an example of the configuration of a quantum key distribution system according to a basic example of several embodiments. [Figure 2] This is a frequency distribution diagram showing an example of a polarization-multiplexed signal in a quantum key distribution system according to some basic embodiments. [Figure 3] This is a configuration diagram showing an example of the configuration of an optical front end according to a basic example of several embodiments. [Figure 4] This is a configuration diagram illustrating an example of the wavelength configuration in a quantum key distribution system according to a basic example of several embodiments. [Figure 5] This is a wavelength distribution diagram showing an example of the wavelength configuration in a quantum key distribution system according to a basic example of several embodiments. [Figure 6] This is a configuration diagram showing examples of transmitter configurations according to several embodiments. [Figure 7] This is a configuration diagram showing some examples of receiver configurations according to several embodiments. [Figure 8] This is a configuration diagram showing an example of the configuration of a quantum key distribution system according to several embodiments. [Figure 9] This is a frequency distribution diagram showing an example of a polarization-multiplexed signal in a quantum key distribution system according to several embodiments. [Figure 10] This flowchart shows examples of operation of a quantum key distribution system according to several embodiments. [Figure 11] This figure illustrates specific examples of clock synchronization according to several embodiments. [Figure 12] This figure illustrates specific examples of bit position synchronization according to several embodiments. [Figure 13] This diagram illustrates an example of wavelength configuration in a quantum key distribution system according to several embodiments. [Figure 14] This is a wavelength distribution diagram showing examples of wavelength configurations in quantum key distribution systems according to several embodiments. [Figure 15] This is a configuration diagram showing examples of transmitter configurations according to several embodiments. [Figure 16] This is a configuration diagram showing some examples of receiver configurations according to several embodiments. [Figure 17] This diagram illustrates an example of wavelength configuration in a quantum key distribution system according to several embodiments. [Figure 18] This is a wavelength distribution diagram showing examples of wavelength configurations in quantum key distribution systems according to several embodiments. [Figure 19] This is a configuration diagram showing examples of computer hardware configurations according to several embodiments. [Modes for carrying out the invention]

[0015] The embodiments will be described below with reference to the drawings. In each drawing, the same elements are denoted by the same reference numerals, and redundant explanations are omitted where necessary. The arrows shown in each drawing are illustrative for illustrative purposes only and do not limit the type or direction of the signal.

[0016] (Basic example of an embodiment) First, to help understand the problems of the embodiments, we will describe some basic examples that form the basis of several embodiments.

[0017] Figure 1 shows an example configuration of a quantum key distribution system 9 according to a basic example of several embodiments. In the example in Figure 1, the quantum key distribution system 9 comprises a transmitter 800 and a receiver 900. The quantum key distribution system 9 is a system that performs quantum key distribution using the transmitter 800 and the receiver 900. In quantum key distribution, a random number sequence that forms the basis of the encryption key is transmitted using quantum light. This enables secure key sharing between the transmitter 800 and the receiver 900. The quantum key distribution system 9 performs quantum key distribution using light whose intensity has been reduced to a level where quantum behavior can be observed. This makes it possible to quantum mechanically guarantee that the encryption key will not be leaked, thereby achieving high confidentiality.

[0018] In the example shown in Figure 1, the quantum key distribution system 9 is a quantum key distribution system that performs quantum key distribution using CV-QKD. The transmitter 800 is a transmitter for CV-QKD. The receiver 900 is a receiver for CV-QKD.

[0019] In the example shown in Figure 1, the transmitter 800 and the receiver 900 are connected via an optical fiber 2 for communication. The transmitter 800 and the receiver 900 perform quantum key distribution using a quantum channel and a classical channel, which are formed by a transmission path including the optical fiber 2.

[0020] A quantum channel is a communication channel that transmits and receives weak light (quantum light) from a transmitter 800 to a receiver 900. This weak light refers to, for example, light with an optical power of approximately 1 photon / bit or less that behaves quantum mechanically. The quantum channel is constructed using, for example, an optical fiber 2. In the example shown in Figure 1, wavelength division multiplexing is performed via the optical fiber 2, with a wavelength λ QKD The wavelength channel is used as the quantum channel.

[0021] Classical channels are more reliable than quantum channels. High reliability of a communication channel means, for example, a low bit error rate (BER). For the sake of explanation, in the following, we will assume an error-free communication channel as a classical channel. Here, "error-free" 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 in a classical channel is not limited to a specific method. For example, a classical channel may be constructed using the same optical fiber 2 as the quantum channel, or it may be constructed using a different transmission path than the quantum channel.

[0022] Note that the terms "transmit" for transmitter 800 and "receive" for receiver 900 are for explanatory purposes only, and data may also be transmitted from receiver 900 to transmitter 800. In particular, receiver 900 uses a classical channel to transmit information to transmitter 800 for processing in quantum key distribution.

[0023] In the example shown in Figure 1, the transmitter 800 includes a QKD signal transmitting unit 801, a synchronization signal transmitting unit 802, and a wavelength division multiplexing (MUX) unit 102. The transmitter 800 may also include a plurality of QKD signal transmitting units 801 that transmit QKD signals of different wavelengths, and a plurality of synchronization signal transmitting units 802 that transmit synchronization signals of different wavelengths.

[0024] The QKD signal transmitter 801 transmits a QKD signal that includes weak light (quantum light) for quantum key distribution by CV-QKD and a reference light for demodulation assistance. In this example, the QKD signal has a wavelength λ QKD This is an optical signal. For example, the QKD signal transmitter 801 modulates the light to be transmitted using a modulation scheme similar to that used in optical transmitters for coherent communication. The QKD signal transmitter 801 modulates weak light using random number data and basis data that serve as key elements, and transmits the modulated weak light to the receiver 900 via a quantum channel (optical fiber 2).

[0025] Furthermore, if the receiver used by the eavesdropper receives quantum light from transmitter 800, it cannot receive the basis data necessary to decode the code before it receives the code from the quantum light. Therefore, according to the quantum non-replica theorem, it is impossible to preserve the quantum light in its quantum light state without leaving a trace, and the eavesdropper's receiver will randomly select one of the two basis sets to decode the code from the stolen quantum light. In this case, the receiver used by the eavesdropper will have a 1 / 2 probability of using a basis set different from the one used by transmitter 800 to decode, and will not be able to decode accurately. In addition, the quantum state changes due to the measurement of a different basis set, and eavesdropping can be detected, making eavesdropping impossible.

[0026] The QKD signal transmission unit 801 includes a light source (Laser Diode; LD) 110, an optical coupler (Coupler; CPL) 120, a modulator 130-x, and a polarizing beam splitter (PBS) 140.

[0027] The light source 110 outputs light used for transmitting QKD signals, including quantum light and reference light. For example, the light source 110 is a laser diode that outputs laser light. The light source 110 has a wavelength λ QKD It outputs light.

[0028] The optical coupler 120 is a polarization separation unit that splits the light output from the light source 110 into X-polarized light and Y-polarized light. The optical coupler 120 outputs the split X-polarized light to the modulator 130-x and the Y-polarized light to the polarization beam splitter 140. In this example, the X-polarized light is modulated to generate weak light (quantum light), and the Y-polarized light is used as the reference light. Alternatively, the Y-polarized light may be used as quantum light, or the X-polarized light may be used as the reference light.

[0029] Modulator 130-x is a modulator that generates the quantum light (weak light) to be transmitted. Modulator 130-x is a phase modulator (PM). For example, modulator 130-x is a DP-QPSK modulator that modulates light using the DP-QPSK modulation scheme (Dual Polarization Quadrature Phase Shift Keying). Note that modulator 130-x is not limited to the DP-QPSK modulation scheme; it may also be phase-modulated using other modulation schemes.

[0030] Modulator 130-x modulates the X-polarized light from the light branched by the optical coupler 120 to generate X-polarized signal light (quantum light). Modulator 130-x receives random number data (shared random numbers) and basis data (basis selection information) as input and modulates the X-polarized light branched by the optical coupler 120 based on the random number data and basis data. For example, a combination of a 0° phase and a 180° phase is used as one basis, and a combination of a 90° phase and a 270° phase is used as the other basis, and a QPSK signal is generated using the bits (Q value) of the basis data and the bits (I value) of the random number data.

[0031] The modulator 130-x further attenuates the modulated X-polarized signal light. For example, modulator 130-x includes a variable optical attenuator (VOA). Modulator 130-x attenuates the modulated X-polarized signal light to a predetermined intensity and outputs the attenuated signal light, which is weak light (quantum light), to the polarization beam splitter 140. Modulator 130-x attenuates the X-polarized signal light to a weak state where the optical power behaves quantum mechanically at approximately 1 photon / bit or less. This makes it possible to determine whether or not eavesdropping is occurring based on the principles of quantum mechanics.

[0032] The polarization beam splitter 140 is a polarization multiplexing unit that polarization multiplexes (polarization mixes) X-polarized signal light and Y-polarized signal light. Figure 2 shows an example of a polarization multiplexed signal that the polarization beam splitter 140 polarization multiplexes in a basic example. The polarization beam splitter 140 polarization multiplexes the X-polarized signal light (modulated quantum light) modulated and attenuated by the modulator 130-x and the Y-polarized signal light (unmodulated reference light) branched from the optical coupler 120. The polarization beam splitter 140 polarization multiplexes the polarization multiplexed signal light with wavelength λ QKD The QKD signal is output to the wavelength division multiplexer 102.

[0033] The synchronization signal transmission unit 802 transmits an optical signal (synchronization signal) for clock synchronization and bit position synchronization between the transmitter 800 and the receiver 900. In this example, the synchronization signal has a wavelength λ sync It is an optical signal.

[0034] The synchronization signal transmission unit 802 includes a light source (LD) 821 and a synchronization modulator 822. The light source 821 outputs light used for transmitting the synchronization signal. For example, the light source 821 is a laser diode that outputs laser light. The light source 821 has a wavelength λ sync It outputs light.

[0035] The synchronous modulator 822 is a modulator that generates a synchronous signal to be transmitted. The synchronous modulator 822 may perform modulation by the QPSK modulation method or other modulation methods. The synchronous modulator 822 modulates the light output from the light source 821 based on data for clock synchronization and bit position synchronization. The synchronous modulator 822 modulates the wavelength λ sync of the synchronous signal and outputs it to the wavelength multiplexing section 102.

[0036] The wavelength multiplexing section 102 wavelength-multiplexes the QKD signal of wavelength λ QKD and the synchronous signal of wavelength λ sync . The wavelength multiplexing section 102 is an optical transmission section that transmits the wavelength-multiplexed wavelength-multiplexed signal light to the receiver 900 via the optical fiber 2.

[0037] In the example of FIG. 1, the receiver 900 includes a QKD signal receiving section 901, a synchronous signal receiving section 902, and a wavelength demultiplexing section (DEMUX) 202. The receiver 900 may include a plurality of QKD signal receiving sections 901 that receive QKD signals of different wavelengths and a plurality of synchronous signal receiving sections 902 that receive synchronous signals of different wavelengths.

[0038] The wavelength demultiplexing section 202 is an optical receiving section that receives the wavelength-multiplexed signal light from the transmitter 800 via the optical fiber 2. The wavelength demultiplexing section 202 separates the received wavelength-multiplexed signal light into an optical signal (QKD signal) of wavelength λ QKD and an optical signal (synchronous signal) of wavelength λ sync .

[0039] The synchronous signal receiving section 902 outputs signals for clock synchronization and bit position synchronization based on the received synchronous signal. The synchronous signal receiving section 902 includes an optical front end (FE) 921, an analog-to-digital converter (ADC) 922, and a skew adjustment section 937.

[0040] The optical front end 921 receives from the transmitter 800 and has a wavelength λ separated by the wavelength demultiplexing section 202 syncThe synchronization signal is photoelectrically converted. The analog-to-digital converter 922 converts the analog electrical signal, which has been photoelectrically converted by the optical front-end 921, into a digital electrical signal.

[0041] The Skew adjustment unit 937 adjusts the timing for clock synchronization and bit position synchronization for the QKD signal receiving unit 901 based on the synchronization signal converted from analog to digital by the analog-to-digital converter 922, and outputs the adjusted synchronization signal to the QKD signal receiving unit 901.

[0042] The QKD signal receiver 901 receives a QKD signal that includes weak light (quantum light) and reference light for quantum key distribution using CV-QKD. In this example, the QKD signal has a wavelength λ QKD This is an optical signal. For example, the QKD signal receiver 901 detects the received light by coherent detection, similar to the optical receiver used in coherent communication. The QKD signal receiver 901 receives weak light from the transmitter 800 via the quantum channel (optical fiber 2), coherently detects the received weak light, and generates a quantum key from the bit sequence obtained by coherent detection.

[0043] The QKD signal receiver 901 generates an encryption key by measuring the state of the optical electric field from the weak light received through the quantum channel using coherent detection. In coherent detection, the signal light is filtered spatially, temporally, and wavelengthly by interfering it with local light to read out the signal state. The local light used here is the laser light from the laser light source provided by the receiver 900.

[0044] In contrast to DV-QKD, another quantum key distribution method, the receiver generates an encryption key from the presence or absence of photons using a photon detector. CV-QKD, on the other hand, can be implemented using common optical components and is less expensive than DV-QKD, which uses a photon detector. Furthermore, CV-QKD enables a quantum key distribution system where general communication light and the transmission path coexist through filtering using localized light. In coherent detection, the signal light can be amplified by interfering it with a localized light source with high optical power. Therefore, even when the signal light power is weak (less than 1 photon / bit), detection is possible using a common photodetector.

[0045] The QKD signal receiving unit 901 includes an optical front end (FE) 210, an analog-to-digital converter (ADC) 220, a clock (CLK) synchronization unit 934, a polarization demodulation unit 231, a reference optical phase demodulation unit 232, a quantum optical phase correction unit 233, a bit reading unit 935, and a bit position synchronization unit 936.

[0046] Furthermore, some functions of the QKD signal receiving unit 901 and the synchronization signal receiving unit 902 are performed by a digital signal processor (DSP) 930. For example, the digital signal processor 930 includes a clock synchronization unit 934, a polarization demodulation unit 231, a reference optical phase demodulation unit 232, a quantum optical phase correction unit 233, a bit reading unit 935, a bit position synchronization unit 936, and a skew adjustment unit 937. In other words, these functions are realized by digital signal processing by the digital signal processor 930. The digital signal processor 930 (QKD signal receiving unit 901) may also include functions necessary for quantum key generation, such as an error correction unit and a security enhancement unit.

[0047] The optical front-end 210 receives the wavelength λ from the transmitter 800 and is separated by the wavelength separation unit 202. QKD The QKD signal is photoelectrically converted. The optical front-end 210 coherently detects the QKD signal, which is a polarization multiplexed signal containing weak light and reference light.

[0048] Figure 3 shows an example configuration of the optical front-end 210 in a basic example. In the example in Figure 3, the optical front-end 210 includes a light source (LD) 211, a 90° hybrid 212, and balanced receivers (Balanced Detectors; BRs) 213-1 to 213-4. The light source 211 outputs local light (local oscillator light) for coherent detection.

[0049] The 90° hybrid 212 and balanced receivers 213-1 to 213-4 use local light output from the light source 211 to coherently detect the received QKD signal, which includes the weak light and reference light. The 90° hybrid 212 interferes the local light output from the light source 211 with the received QKD signal to read out the quadrature phase components. The 90° hybrid 212 projects the received polarization-multiplexed QKD signal, which is the polarized multiplexed signal light, with arbitrary polarizations X' and Y' of the local light output from the light source 211 in phase to generate signal light for the I component of the X' polarization, signal light for the Q component of the X' polarization, signal light for the I component of the Y' polarization, and signal light for the Q component of the Y' polarization.

[0050] Balanced receivers 213-1 to 213-4 convert the quadrature phase components read out by the 90° hybrid 212 into electrical signals. Balanced receivers 213-1 to 213-4 detect the signal light of the I component of the X' polarization, the signal light of the Q component of the X' polarization, the signal light of the I component of the Y' polarization, and the signal light of the Q component of the Y' polarization output from the 90° hybrid 212, convert them into analog electrical signals, and output the converted analog electrical signals of the I component of the X' polarization, the analog electrical signals of the Q component of the X' polarization, the analog electrical signals of the I component of the Y' polarization, and the analog electrical signals of the Q component of the Y' polarization to the analog-to-digital converter 220. Hereafter, the detection of signal light by the balanced receiver will also be referred to as detection.

[0051] The analog-to-digital converter 220 converts the result of coherent detection of the received QKD signal into a digital electrical signal. The analog-to-digital converter 220 quantizes (converts from analog to digital) the analog electrical signal of the I component of the X' polarization, the analog electrical signal of the Q component of the X' polarization, the analog electrical signal of the I component of the Y' polarization, and the analog electrical signal of the Q component of the Y' polarization, which have been detected by the optical front-end 210. The analog-to-digital converter 220 outputs the quantized digital electrical signals of the I component of the X' polarization, the digital electrical signal of the Q component of the X' polarization, the digital electrical signal of the I component of the Y' polarization, and the digital electrical signal of the Q component of the Y' polarization to the clock synchronization unit 934 (digital signal processor 930).

[0052] The clock synchronization unit 934 extracts the clock timing based on the synchronization signal after skew adjustment by the skew adjustment unit 937, and outputs the extracted clock timing to the analog-to-digital converter 220. The analog-to-digital converter 220 samples the extracted clock timing and converts the coherently detected signal from analog to digital.

[0053] The polarization demodulation unit 231 demodulates the received QKD signal to the polarization state before polarization multiplexing at the transmitter 800. That is, the polarization demodulation unit 231 polarizes the digital electrical signal converted from analog to digital by the analog-to-digital converter 220 into X-polarized and Y-polarized signals. For example, known methods can be used for the polarization separation process by the polarization demodulation unit 231. The polarization demodulation unit 231 converts the digital electrical signal of the I component of the X' polarization, the digital electrical signal of the Q component of the X' polarization, the digital electrical signal of the I component of the Y' polarization, and the digital electrical signal of the Q component of the Y' polarization into X-polarized and Y-polarized signals, and generates the digital electrical signal of the I component of the X polarization and the digital electrical signal of the Q component of the X polarization (referred to as the received quantum optical signal), and the digital electrical signal of the I component of the Y polarization and the digital electrical signal of the Q component of the Y polarization (referred to as the received reference optical signal).

[0054] The reference optical phase demodulation unit 232 demodulates (corrects) the phase of the Y-polarized digital electrical signal (received reference optical signal) that has been polarized and demodulated by the polarization demodulation unit 231. The received reference optical signal has phase fluctuations due to the frequency and phase difference between the light from the light source 110 of the transmitter 800 and the light from the light source 211 of the receiver 900. Therefore, the reference optical phase demodulation unit 232 tracks the phase fluctuations of the received reference optical signal due to the frequency and phase difference between the light from the light source 110 of the transmitter 800 and the light from the light source 211 of the receiver 900, extracts a phase reference, and demodulates the original phase modulation value. The reference optical phase demodulation unit 232 outputs the phase correction value used for phase correction of the received reference optical signal to the quantum optical phase correction unit 233.

[0055] The quantum optical phase correction unit 233 corrects the phase of the X-polarized digital electrical signal (received quantum optical signal) that has been polarized and demodulated by the polarization demodulation unit 231. The quantum optical phase correction unit 233 reflects the phase correction value of the received reference optical signal output from the reference optical phase demodulation unit 232 to the received quantum optical signal. After reflecting the phase correction value of the received reference optical signal, the received quantum optical signal has a phase shift due to the phase difference between the quantum light (X-polarized) and the reference light (Y-polarized). Therefore, the quantum optical phase correction unit 233 corrects the phase shift caused by the phase difference between the quantum light and the reference light to the received quantum optical signal after reflecting the phase correction value of the received reference optical signal. The quantum optical phase correction unit 233 outputs the phase-corrected received quantum optical signal to the bit reading unit 935.

[0056] The bit reading unit 935 makes a hard judgment on the received quantum optical signal after phase correction by the quantum optical phase correction unit 233 and reads a bit (quantization key) that is either 0 or 1. For example, the bit reading unit 935 may also make a hard judgment on the signal value after the QPSK signal has been converted to a BPSK signal by a basis matching process and read the bit. In the basis matching process, the receiver receives basis data from the transmitter using a classical channel and performs a basis matching on the received quantum optical signal obtained by coherent detection.

[0057] For example, the bit reading unit 935 receives basis data from the transmitter 800 through the classical channel and rotates the phase of the received quantum optical signal based on the basis data. The bit reading unit 935 performs a hard determination on the phase-rotated received quantum optical signal and outputs a bit sequence of 0 or 1.

[0058] The bit position synchronization unit 936 extracts the starting position of random numbers in the bit sequence based on the synchronization signal after skew adjustment by the skew adjustment unit 937. The bit position synchronization unit 936 outputs a random number sequence starting from the extracted starting position in the bit sequence read from the received quantum optical signal by the bit reading unit 935. The random number sequence after bit position synchronization (after base matching) is called the shift key (selection key).

[0059] A quantum key is obtained by performing error correction and security enhancement on the generated shift key. The error correction and security enhancement processes may be performed in the QKD signal receiver 901 or outside the QKD signal receiver 901. In error correction, a portion of the bits for which basis matching has been completed is exposed on the classical channel between the transmitter 100 and the receiver to measure the error rate, and a portion of the bits is further exposed and used for correction according to the measured error rate, thereby sharing the same bit sequence between the sender and receiver.

[0060] In the enhanced confidentiality process, noise and loss in the quantum channel are measured to estimate the maximum amount of information an eavesdropper could obtain if one were to be present. A portion of the bit sequence is then randomly discarded so that the amount of information the eavesdropper could obtain becomes zero. In other words, only random number sequences that cannot have been intercepted are extracted from the bit sequence obtained through error correction. These extracted random number sequences are used as the final key (quantum key). This allows the transmitter 800 and the receiver 900 to share a random number sequence that is quantum mechanically guaranteed not to have been intercepted.

[0061] Using Figures 4 and 5, an example of the wavelength configuration in the quantum key distribution system 9 relating to the basic example will be explained. A single synchronization signal can be used to synchronize the clock and bit positions of multiple QKD signals. In the example in Figures 4 and 5, one wavelength λsync Using the synchronization signal, four wavelengths λ QKD This performs clock synchronization and bit position synchronization of the QKD signals.

[0062] In the example in Figure 4, the transmitter 800 has a wavelength λ sync_1 A synchronization signal transmission unit 802-1 transmits a synchronization signal, wavelength λ sync_1 Wavelength λ synchronized with the synchronization signal QKD1-1 ~λ QKD1-4 QKD signal transmitting units 801-1-1 to 801-1-4 transmit the respective QKD signals, with wavelength λ sync_2 A synchronization signal transmission unit 802-2 transmits a synchronization signal, wavelength λ sync_2 Wavelength λ synchronized with the synchronization signal QKD2-1 ~λ QKD2-4 QKD signal transmitting units 801-2-1 to 801-2-4 transmit the respective QKD signals, with wavelength λ sync_3 A synchronization signal transmission unit 802-3 transmits a synchronization signal, wavelength λ sync_3 Wavelength λ synchronized with the synchronization signal QKD3-1 ~λ QKD3-4 It includes QKD signal transmitting units 801-3-1 to 801-3-4 that transmit the respective QKD signals.

[0063] The wavelength division multiplexing section 102 is wavelength λ sync_1 , λ sync_2 , λ sync_3 Optical signal (synchronization signal), wavelength λ QKD1-1 ~λ QKD1-4 , λ QKD2-1 ~λ QKD2-4 , λ QKD3-1 ~λ QKD3-4 The optical signal (QKD signal) is wavelength-multiplexed.

[0064] In the receiver 900, the wavelength separation unit 202 separates the wavelength multiplexed optical signal by wavelength λ sync_1 , λ sync_2 , λ sync_3 Optical signal (synchronization signal), wavelength λ QKD1-1 ~λ QKD1-4 , wavelength λ QKD2-1 ~λ QKD2-4 , wavelength λ QKD3-1 ~λ QKD3-4 It is separated into optical signals (QKD signals).

[0065] Receiver 900 uses wavelength λ sync_1 A synchronization signal receiving unit 902-1 receives the synchronization signal, wavelength λ sync_1 Wavelength λ synchronized with the synchronization signal QKD1-1 ~λ QKD1-4 QKD signal receiving units 901-1-1 to 901-1-4, which receive the respective QKD signals, and wavelength λ sync_2 A synchronization signal receiving unit 902-2 receives the synchronization signal, wavelength λ sync_2 Wavelength λ synchronized with the synchronization signal QKD2-1 ~λ QKD2-4 QKD signal receiving units 901-2-1 to 901-2-4, which receive the respective QKD signals, and wavelength λ sync_3 A synchronization signal receiving unit 902-3 receives the synchronization signal, wavelength λ sync_3 Wavelength λ synchronized with the synchronization signal QKD3-1 ~λ QKD3-4 It includes QKD signal receiving units 901-3-1 to 901-3-4 that receive the respective QKD signals.

[0066] In this case, as shown in Figure 5, one of the five wavelengths is used for the synchronization signal. For example, wavelength λ QKD1-1 ~λ QKD1-4 For clock synchronization and bit position synchronization of the QKD signal, wavelength λ QKD1-1 ~λ QKD1-4 The central wavelength λ sync_1 The synchronization signal is used. Wavelength λ QKD2-1 ~λ QKD2-4 For clock synchronization and bit position synchronization of the QKD signal, wavelength λ QKD2-1 ~λ QKD2-4 The central wavelength λ sync_2 The synchronization signal is used. Wavelength λ QKD3-1 ~λ QKD3-4 For clock synchronization and bit position synchronization of the QKD signal, wavelength λ QKD3-1 ~λ QKD3-4 The central wavelength λ sync_3 The synchronization signal is used.

[0067] Thus, CV-QKD transmits weak coherent light to enable random number sharing between two distant parties without eavesdropping. In CV-QKD, where the intensity of the transmitted light is weak, a different wavelength light signal is essential for bit position synchronization. For example, in the basic example, as shown in Figure 5, the wavelength λ sync A synchronization signal is required. This reduces the frequency utilization efficiency of the quantum key distribution system. A major feature of CV-QKD is that it is wavelength multiplexed, and therefore, this problem of reducing the frequency utilization of CV-QKD systems is a very important issue.

[0068] (Embodiment 1) Next, Embodiment 1 will be described. This embodiment will outline several embodiments.

[0069] Figure 6 shows an example configuration of a transmitter 10 according to several embodiments. Figure 7 shows an example configuration of a receiver 20 according to several embodiments. For example, the transmitter 10 and the receiver 20 are connected communicably via an optical transmission path to constitute a quantum key distribution system. A quantum key distribution system is also a system that shares random numbers that form the basis 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). In this example, the QKD is CV-QKD.

[0070] In the example shown in Figure 6, the transmitter 10 includes a light source 11, a branching unit 12, a first modulation unit 13, a second modulation unit 14, and a polarization multiplexing unit 15.

[0071] A light source (e.g., a first light source) 11 outputs light (e.g., first light). A branching section (e.g., a first branching section) 12 branches the light output from the light source 11 into a first polarized light with a first polarization and a second polarized light with a second polarization. The first polarization may be either X-polarization or Y-polarization, and the second polarization may be the other of X-polarization or Y-polarization.

[0072] The first modulation unit 13 modulates the first polarization light branched by the branching unit 12 based on a random number (for example, a first random number). The second modulation unit 14 modulates the second polarization light branched by the branching unit 12 based on data for synchronizing the bit positions of the random number. The data for synchronizing the bit positions of the random number used for modulation by the first modulation unit 13 is used to synchronize the bit positions between the transmitter 10 and the receiver 20. For example, the data for synchronizing the bit positions indicates the starting position of the random number in the bit sequence modulated (transmitted) by the first modulation unit 13. The second polarization light modulated by the second modulation unit 14 may also indicate the clock timing for transmitting the first polarization light modulated by the first modulation unit 13. The second polarization light modulated by the second modulation unit 14 may also be a reference light for phase demodulation of the first polarization light modulated by the first modulation unit 13.

[0073] The polarization multiplexing unit 15 polarization multiplexes the modulated first polarization light and the modulated second polarization light, and transmits the polarization multiplexed light (for example, the first polarization multiplexed light) to the receiver 20. The transmitter 10 may also include a wavelength multiplexing unit that wavelength multiplexes the polarization multiplexed light polarized by the polarization multiplexing unit 15 and transmits the wavelength multiplexed light to the receiver 20.

[0074] In the example shown in Figure 7, the receiver 20 includes a photoelectric conversion unit 21, a polarization demodulation unit 22, a first bit reading unit 23, a second bit reading unit 24, and a bit position synchronization unit 25.

[0075] The photoelectric conversion unit (for example, the first photoelectric conversion unit) 21 receives polarization-multiplexed light from the transmitter 10 and converts the received polarization-multiplexed light into an electrical signal (for example, the first electrical signal). For example, the photoelectric conversion unit 21 may perform coherent detection on the received polarization-multiplexed light and convert it into an electrical signal.

[0076] The receiver 20 may also include a wavelength separation unit that receives wavelength-division multiplexed light from the transmitter 10 and separates the polarization-divisioned light from the received wavelength-division multiplexed light. In this case, the photoelectric conversion unit 21 performs photoelectric conversion on the wavelength-separated polarization-divisioned light. The receiver 20 may also include an analog-to-digital conversion unit that performs analog-to-digital conversion on the photoelectrically converted electrical signal, and a clock synchronization unit that extracts clock timing based on the analog-to-digital converted digital signal. In this case, the analog-to-digital conversion unit may perform analog-to-digital conversion based on the extracted clock timing.

[0077] The polarization demodulation unit (for example, the first polarization demodulation unit) 22 polarization demodulates the electrical signal converted by the photoelectric conversion unit 21 into a first polarization signal with a first polarization and a second polarization signal with a second polarization. The polarization demodulation unit 22 demodulates the signal to have the same polarization as the first and second polarizations that were polarization multiplexed in the transmitter 10.

[0078] The receiver 20 may also include a phase demodulation unit that demodulates the phases of the first and second polarization signals based on the first and second polarization signals that have been polarized and demodulated by the polarization demodulation unit 22. For example, the phase demodulation unit may demodulate the phase of the second polarization signal and then demodulate the phase of the first polarization signal based on the demodulated phase.

[0079] The first bit reading unit 23 reads a first bit sequence containing random numbers from the first polarization signal that has been polarization-demodulated by the polarization-demodulation unit 22. The second bit reading unit 24 reads a second bit sequence containing data for synchronizing the bit positions of random numbers from the second polarization signal that has been polarization-demodulated by the polarization-demodulation unit 22.

[0080] The bit position synchronization unit (for example, the first bit position synchronization unit) 25 synchronizes the bit positions of random numbers in the first bit sequence read by the first bit reading unit 23 based on the bit position synchronization data in the second bit sequence read by the second bit reading unit 24. For example, the bit position synchronization unit 25 may perform bit position synchronization based on the starting position of the random numbers indicated by the bit position synchronization data. That is, the bit position synchronization unit 25 outputs data in the first bit sequence that starts from the bit corresponding to the starting position indicated by the bit position synchronization data as a random number shared with the transmitter 10.

[0081] In this embodiment, bit position synchronization is performed between the transmitter and receiver by polarizing and transmitting a first polarization optical signal modulated by random numbers and a second polarization optical signal modulated by data for bit position synchronization. As a result, a wavelength for bit position synchronization is not required, and thus the decrease in frequency utilization efficiency can be suppressed.

[0082] The following embodiments will describe specific examples of Embodiment 1.

[0083] (Embodiment 2) Next, Embodiment 2 will be described. This embodiment allows data for bit position synchronization to be transmitted at the wavelength used for QKD, compared to the configuration of the basic example. Therefore, it can be implemented in combination with the basic example, and each configuration shown in the basic example may be used as appropriate.

[0084] Figure 8 shows some configuration examples of the quantum key distribution system 1 according to several embodiments. The quantum key distribution system 1 is a quantum key distribution system that performs quantum key distribution by CV-QKD, similar to the basic example in Figure 1.

[0085] In the example shown in Figure 8, the quantum key distribution system 1 comprises a transmitter 100 and a receiver 200. Similar to the basic example in Figure 1, the transmitter 100 is a QKD transmitter (transmitter-side quantum key distribution device) that performs quantum key distribution by CV-QKD, and the receiver 200 is a QKD receiver (receiver-side quantum key distribution device) that performs quantum key distribution by CV-QKD. The transmitter 100 and the receiver 200 are connected via an optical fiber 2 for communication, similar to the basic example in Figure 1.

[0086] In the example shown in Figure 8, the transmitter 100 includes a QKD signal transmitting unit 101 and a wavelength division multiplexing unit (MUX) 102. The transmitter 100 may also include multiple QKD signal transmitting units 101 that transmit QKD signals of different wavelengths.

[0087] The QKD signal transmission unit 101, similar to the basic example in Figure 1, polarizes and multiplexes weak light (quantum light) and reference light for quantum key distribution using CV-QKD, and polarizes and multiplexes the wavelength λ QKD The QKD signal is transmitted to the receiver 200 via optical fiber 2 (quantum channel).

[0088] In the example shown in Figure 8, the QKD signal transmitter 101 includes a light source (LD) 110, an optical coupler (CPL) 120, a modulator 130-x, a polarization beam splitter (PBS) 140, and, similar to Figure 1, a modulator 130-y. Here, we will mainly describe the configuration that differs from that in Figure 1.

[0089] Modulator 130-y is a modulator that generates the reference light to be transmitted. For example, modulator 130-y may be a phase modulator. Modulator 130-y may be a DP-QPSK modulator, like modulator 130-x, or a modulator of another modulation scheme. Modulator 130-y modulates the Y-polarized light from the light branched by the optical coupler 120 to generate a Y-polarized signal light (reference light), and outputs the modulated Y-polarized signal light to the polarization beam splitter 140.

[0090] Modulator 130-y receives data for bit position synchronization and modulates the Y-polarized light branched by the optical coupler 120 based on the bit position synchronization data. The bit position synchronization data is data indicating the starting position of a random number transmitted by quantum light and is shared in advance between transmitter 100 and receiver 200. The starting position of the random number modulated and transmitted by modulator 130-x is synchronized with the starting position indicated by the bit position synchronization data modulated and transmitted by modulator 130-y. The bit position synchronization data can be any data as long as it can indicate the starting position of a random number. The bit position synchronization data may be a bit sequence of a predetermined length, for example, 2 to the power of 15. For example, if the length is only a few bits, synchronization will not be possible if there is a timing difference across data of that length, so it is preferable that the bit length be greater than the predetermined length.

[0091] The polarization beam splitter 140 polarization multiplexes the modulated X-polarized signal light and the modulated Y-polarized signal light. Figure 9 shows examples of polarization multiplexed signals that the polarization beam splitter 140 polarization multiplexes in several embodiments. The polarization beam splitter 140 polarization multiplexes the X-polarized signal light (modulated quantum light) modulated and attenuated by modulator 130-x and the Y-polarized signal light (modulated reference light) modulated by modulator 130-y. The polarization beam splitter 140 polarization multiplexes the polarization multiplexed signal light with wavelength λ QKD The QKD signal is output to the wavelength division multiplexer 102.

[0092] The wavelength division multiplexing unit 102, as in the basic example, uses the wavelength λ generated by the QKD signal transmission unit 101. QKD The QKD signals are wavelength-multiplexed. Multiple QKD signal transmitters 101 transmit at different wavelengths λ. QKD When multiple QKD signals are generated, the wavelength division multiplexer 102 generates multiple wavelengths λ QKD The QKD signal is wavelength-multiplexed. The wavelength-multiplexed unit 102 transmits the wavelength-multiplexed signal light to the receiver 200 via the optical fiber 2.

[0093] In the example shown in Figure 8, the receiver 200 includes a QKD signal receiving unit 201 and a wavelength separation unit (DEMUX) 202. The receiver 200 may also include multiple QKD signal receiving units 201 that receive QKD signals of different wavelengths.

[0094] The wavelength separation unit 202, as in the basic example, receives wavelength-division multiplexed signal light from the transmitter 100 via the optical fiber 2, and from the received wavelength-division multiplexed signal light, wavelength λ QKD The optical signals (QKD signals) are separated. Wavelength division multiplexing signals have different wavelengths λ. QKD When multiple QKD signals are included, the wavelength separation unit 202 separates multiple wavelengths λ QKD Separate into QKD signals.

[0095] The QKD signal receiver 201 receives weak light (quantum light) and reference light for quantum key distribution using CV-QKD, similar to the basic example in Figure 1. The QKD signal receiver 201 also includes an optical front-end (FE) 210, an analog-to-digital converter (ADC) 220, a polarization demodulation unit 231, a reference light phase demodulation unit 232, and a quantum light phase correction unit 233, similar to Figure 1. It also includes a clock (CLK) synchronization unit 234, a bit reading unit 235, a bit position synchronization unit 236, and a shift key output unit 237. For example, the polarization demodulation unit 231, the reference light phase demodulation unit 232, the quantum light phase correction unit 233, the clock synchronization unit 234, the bit reading unit 235, the bit position synchronization unit 236, and the shift key output unit 237 are all composed of a digital signal processor 230. Here, we will mainly describe a configuration that differs from Figure 1.

[0096] The clock synchronization unit 234 extracts the clock timing based on the digital electrical signal of the Y-polarized optical component (reference optical component) of the quantized, pre-polarization demodulated optical signal converted by the analog-to-digital converter 220, and outputs the extracted clock timing to the analog-to-digital converter 220. The clock synchronization unit 234 may also extract the clock timing based on the digital electrical signal of the X-polarized optical component (quantum optical component) of the pre-polarization demodulated optical signal. The analog-to-digital converter 220 performs sampling at the extracted clock timing, as in the basic example, and converts the coherently detected signal from analog to digital. The analog-to-digital converter 220 may also perform the same clock extraction process as the clock synchronization unit 234.

[0097] The bit reading unit 235 reads bits from the received quantum optical signal after phase correction by the quantum optical phase correction unit 233, and also reads bits from the received reference optical signal after phase demodulation by the reference optical phase demodulation unit 232. For example, the bit reading unit 235 includes a bit reading unit 235a (e.g., a first bit reading unit) that reads bits from the received quantum optical signal after phase correction, and a bit reading unit 235b (e.g., a second bit reading unit) that reads bits from the received reference optical signal after phase demodulation.

[0098] The bit reading unit 235a is the same as the bit reading unit 935 in the basic example. For example, the bit reading unit 235a performs hard determination on the received quantum optical signal after phase correction and generates a bit sequence of 0s or 1s (called the quantum optical bit sequence). The bit reading unit 235a may also perform base matching processing using a classical channel and read the quantum optical bit sequence. Alternatively, the bit reading unit 235b performs hard determination on the received reference optical signal after phase demodulation and generates a bit sequence of 0s or 1s (called the reference optical bit sequence).

[0099] The bit position synchronization unit 236 extracts bit position synchronization data from the reference optical bit sequence read from the received reference optical signal after phase demodulation by the bit reading unit 235b, and uses the bit position synchronization data to determine the starting position of the random number. The bit position synchronization unit 236 outputs the determined starting position of the random number to the shift key output unit 237.

[0100] The shift key output unit 237 outputs a bit sequence as a shift key that starts from the random number starting position identified by the bit position synchronization unit 236, from the quantum optical bit sequence read out from the received quantum optical signal after phase correction by the bit reading unit 235a.

[0101] Figure 10 shows an example of the operation of a quantum key distribution system 1 according to several embodiments. In the example in Figure 10, the transmitter 100 generates quantum light (S101) and also generates reference light (S102). The transmitter 100 synchronously generates (modulates) quantum light using random numbers and generates (modulates) reference light using data for bit position synchronization.

[0102] Modulator 130-x modulates the X-polarized light output from light source 110 and branched by optical coupler 120 to generate X-polarized quantum light (weak light). Modulator 130-x modulates the X-polarized light branched by optical coupler 120 based on random data and basis data (basis selection information), and attenuates the power of the modulated light to make it weak light.

[0103] Furthermore, modulator 130-y modulates the Y-polarized light output from light source 110 and branched by optical coupler 120 to generate a Y-polarized reference light. In synchronization with the modulation by modulator 130-x, modulator 130-y modulates the Y-polarized light branched by optical coupler 120 based on data for bit position synchronization.

[0104] Next, the transmitter 100 polarization-multiplexes the generated quantum light and the reference light (S103). The polarization beam splitter 140 polarization-multiplexes the X-polarized quantum light modulated and attenuated by modulator 130-x and the Y-polarized reference light modulated by modulator 130-y, and the polarization-multiplexed signal light has a wavelength λ QKD Generates a QKD signal.

[0105] Next, the transmitter 100 transmits the wavelength-multiplexed signal light (S104). The wavelength division multiplexing unit 102 transmits the wavelength λ generated by the QKD signal transmission unit 101. QKDThe QKD signal is wavelength-multiplexed, and the wavelength-multiplexed signal light is transmitted to the receiver 200 via optical fiber 2.

[0106] Next, the receiver 200 performs coherent detection on the transmitted wavelength-division multiplexed signal light (S105). The wavelength separation unit 202 receives the wavelength-division multiplexed signal light from the transmitter 800 via the optical fiber 2 and separates the received wavelength-division multiplexed signal light into wavelength λ QKD The QKD signals are separated. The optical front-end 210 uses the local light output from the light source 211 to separate the wavelengths λ. QKD The QKD signal (polarization multiplexed signal) is coherently detected. The optical front-end 210 interferes the local light output from the light source 211 with the received QKD signal to read out the quadrature phase components, and converts the read-out signal light of the I component of the X' polarization, the Q component of the X' polarization, the I component of the Y' polarization, and the Q component of the Y' polarization into electrical signals.

[0107] Next, the receiver 200 performs analog-to-digital conversion of the coherent detection result (S106) and clock synchronization (S107). The analog-to-digital converter 220 converts the coherent detected analog electrical signals of the I component of X' polarization, the Q component of X' polarization, the I component of Y' polarization, and the Q component of Y' polarization into digital electrical signals, respectively.

[0108] For example, the clock synchronization unit 234 extracts the clock timing based on the digital electrical signal of the Y-polarized optical component (reference optical component) of the optical signal before polarization demodulation, which has been converted from analog to digital.

[0109] Figure 11 shows a specific example of clock synchronization in the clock synchronization unit 234 according to several embodiments. Since the quantum light received by the receiver 200 has a very low intensity, it is difficult to extract the clock timing from the quantum light alone. Therefore, the quantum key distribution system 1 uses the clock timing of the Y-polarized optical component (reference optical component), which has a high intensity and is modulated in synchronization with the quantum light, as a trigger to enable accurate extraction of the clock timing. As shown in Figure 11, for example, unlike an optical signal in which the reference optical component is not polarization-multiplexed with the quantum light, a high signal-to-noise ratio can be achieved in the received digital signal of an optical signal in which the reference optical component, which has a high intensity and is modulated in synchronization with the quantum light, is polarization-multiplexed with the quantum light. For example, as shown in Figure 11, the clock synchronization unit 234 extracts the timing of the center of the rectangular pulse from the received digital signal of the reference optical component (received reference optical digital signal) as the clock timing. The analog-to-digital converter 220 samples using the clock timing extracted from the received reference optical digital signal as the sampling timing and performs analog-to-digital conversion of the coherently detected signal.

[0110] Next, the receiver 200 demodulates the polarization of the analog-to-digital converted signal (S108). The polarization demodulation unit 231 polarizes the analog-to-digital converted digital electrical signal into X-polarized and Y-polarized signals. Specifically, the polarization demodulation unit 231 converts the digital electrical signal of the I component of the X' polarization, the digital electrical signal of the Q component of the X' polarization, the digital electrical signal of the I component of the Y' polarization, and the digital electrical signal of the Q component of the Y' polarization into X-polarized and Y-polarized signals, generating the digital electrical signal of the I component of the X polarization and the digital electrical signal of the Q component of the X polarization (received quantum optical signal), and the digital electrical signal of the I component of the Y polarization and the digital electrical signal of the Q component of the Y polarization (received reference optical signal).

[0111] Next, the receiver 200 demodulates the phase of the Y-polarized reference light (S109). The reference light phase demodulation unit 232 demodulates (corrects) the phase of the Y-polarized digital electrical signal (received reference light signal) that has been demodulated. The reference light phase demodulation unit 232 tracks the phase variation due to the frequency and phase difference between the light from the light source 110 of the transmitter 100 and the light from the light source 211 of the receiver 200 with respect to the received reference light signal, extracts a phase reference, and demodulates the original phase modulation value. For example, the reference light phase demodulation unit 232 corrects the phase of the reference light signal so that the constellation of the IQ plane becomes a QPSK pattern.

[0112] Next, the receiver 200 corrects the phase of the X-polarized quantum light that has been demodulated (S110). The quantum light phase correction unit 233 corrects the phase of the X-polarized digital electrical signal (received quantum light signal) that has been demodulated. The quantum light phase correction unit 233 corrects the phase of the received quantum light signal by reflecting the phase correction value of the received reference optical signal used by the reference optical phase demodulation unit 232 for phase correction. The quantum light phase correction unit 233 corrects the phase shift caused by the phase difference between the quantum light and the reference optical signal in the received quantum light signal after the phase correction value of the received reference optical signal has been reflected. The quantum light phase correction unit 233 corrects the slope of the constellation by exposing some of the data of the quantum light signal that reflects the phase correction value of the reference optical signal via the classical channel. The quantum light phase correction unit 233 corrects the phase of the quantum light signal so that the constellation becomes a QPSK pattern.

[0113] Next, the receiver 200 reads the bits of the phase-corrected quantum light (S111). For example, the bit reading unit 235a receives basis data from the transmitter 100 through the classical channel, and rotates the phase of the phase-corrected quantum light signal based on the received basis data to generate a bit sequence of 0s or 1s (quantum light bit sequence).

[0114] The receiver 200 also reads the bits of the phase-demodulated reference light (S112). The bit reading unit 235b performs a hard determination on the received reference light signal after phase demodulation and generates a bit sequence of 0s or 1s (reference light bit sequence). For example, in the case of a QPSK signal, the bit reading unit 235b reads out one of 00, 01, 10, or 11 based on the symbol position on the constellation.

[0115] Next, the receiver 200 performs bit position synchronization (S113). The bit position synchronization unit 236 extracts bit position synchronization data from the reference optical bit sequence read from the received reference optical signal after phase demodulation, and outputs the starting position of the random number indicated by the bit position synchronization data. The shift key output unit 237 outputs the bit sequence that starts from the starting position indicated by the bit position synchronization data among the quantum optical bit sequence read from the received quantum optical signal after phase correction as a shift key.

[0116] Figure 12 shows a specific example of bit position synchronization in the bit position synchronization unit 236 according to several embodiments. Since quantum light has a weak intensity and the received qubit (quantum light bit sequence) contains many errors, it is difficult to perform bit position synchronization with the received qubit alone. Therefore, in the quantum key distribution system 1, bit position synchronization is performed using data from a modulated reference light that has a high intensity and is modulated in synchronization with the quantum light. For example, as shown in Figure 12, the read received reference light data (reference light bit sequence) includes a repetition of "ransuukaishi," which is data for bit position synchronization, and the first character "ra" of "ransuukaishi" indicates the starting position of the random number. The bit position synchronization unit 236 outputs the timing of this random number's starting position, and the shift key output unit 237 outputs a bit sequence (e.g., "1101010") starting from the starting position as a shift key from the read received qubit. After that, error correction and confidentiality enhancement are performed on the shift key to obtain the quantum key. Error correction and confidentiality enhancement processing may be performed in the QKD signal receiving unit 201 or outside the QKD signal receiving unit 201.

[0117] Using FIGS. 13 and 14, a wavelength configuration example in the quantum key distribution system 1 according to some embodiments will be described. In the example of FIG. 13, the transmitter 100 includes QKD signal transmission units 101-1 to 101-15 that respectively transmit QKD signals having wavelengths λ QKD1 ~λ QKD15 including quantum light and reference light (for bit position synchronization). The wavelength multiplexing unit 102 multiplexes optical signals (QKD signals) having wavelengths λ QKD1 ~λ QKD15 . Also, in the receiver 200, the wavelength separation unit 202 separates the wavelength-multiplexed optical signal into optical signals (QKD signals) having wavelengths λ QKD1 ~λ QKD15 . The receiver 200 includes QKD signal reception units 201-1 to 201-15 that respectively receive QKD signals having wavelengths λ QKD1 ~λ QKD15 . In this case, as shown in FIG. 14, all wavelengths from λ QKD1 ~λ QKD15 can be used for QKD signals.

[0118] As described above, in this embodiment, bit position synchronization is enabled using the reference light that was polarization multiplexed with the quantum light for phase demodulation in the basic example. That is, bit position synchronization is performed by modulating the polarization-multiplexed reference light and carrying data for bit position synchronization for transmission. As a result, as shown in FIG. 14, the need for a separate wavelength light for bit position synchronization is eliminated, and the frequency utilization efficiency of the CV-QKD system can be improved. Also, eliminating the need for a separate wavelength light for bit position synchronization simplifies, for example, the configuration of the receiver optical system.

[0119] (Embodiment 3) Next, Embodiment 3 will be described. In this embodiment, an example of performing bit position synchronization at other wavelengths for QKD based on data for bit position synchronization transmitted at a wavelength for QKD will be described. This embodiment is an example that combines the basic example and Embodiment 2, and each configuration shown in the basic example and each configuration shown in Embodiment 2 may be appropriately used.

[0120] FIG. 15 shows a configuration example of the transmitter 100 according to some embodiments. In the example of FIG. 15, the transmitter 100 includes the QKD signal transmitter 801 of the basic example, the QKD signal transmitter 101 of Embodiment 2, and a wavelength multiplexer (MUX) 102. The configuration of the QKD signal transmitter 801 is the same as that in FIG. 1. The configuration of the QKD signal transmitter 101 is the same as that in FIG. 8. For example, in the QKD signal transmitter 101, the light source 110 may be referred to as the first light source, the optical coupler 120 as the first branching section, the modulator 130-x as the first modulation section, the modulator 130-y as the second modulation section, and the polarization beam splitter 140 as the first polarization multiplexing section. For example, in the QKD signal transmitter 801, the light source 110 may be referred to as the second light source, the optical coupler 120 as the second branching section, the modulator 130-x as the third modulation section, and the polarization beam splitter 140 as the second polarization multiplexing section. The transmitter 100 may include a plurality of QKD signal transmitters 801 and a plurality of QKD signal transmitters 101 that transmit QKD signals of different wavelengths. Here, the wavelength of the QKD signal generated by the QKD signal transmitter 101 is λ QKDs is referred to as.

[0121] For example, the transmission of random numbers by the QKD signal transmitter 801 and the transmission of bit position synchronization data by the QKD signal transmitter 101 are synchronized. Specifically, the modulation using random numbers by the modulator 130-x of the QKD signal transmitter 801 and the modulation using data for bit position synchronization by the modulator 130-y of the QKD signal transmitter 101 are synchronized.

[0122] The wavelength multiplexer 102 wavelength-multiplexes the QKD signal of wavelength λ QKD generated by the QKD signal transmitter 801 and the QKD signal of wavelength λ QKDs generated by the QKD signal transmitter 101.

[0123] Figure 16 shows an example configuration of a receiver 200 according to several embodiments. In the example in Figure 16, the receiver 200 comprises a QKD signal receiving unit 901 of the basic example, a QKD signal receiving unit 201 of embodiment 2, and a wavelength separation unit (DEMUX) 202. The configuration of the QKD signal receiving unit 901 is the same as in Figure 1. The configuration of the QKD signal receiving unit 201 is the same as in Figure 8. For example, in the QKD signal receiving unit 201, the optical front end 210 may be called the first photoelectric conversion unit, the analog-to-digital converter 220 the first analog-to-digital conversion unit, the clock synchronization unit 234 the first clock synchronization unit, the polarization demodulation unit 231 the first polarization demodulation unit, the reference optical phase demodulation unit 232 and the quantum optical phase correction unit 233 the first phase demodulation unit, the bit reading unit 235a the first bit reading unit, the bit reading unit 235b the second bit reading unit, and the bit position synchronization unit 236 the first bit position synchronization unit. For example, in the QKD signal receiving unit 901, the optical front end 210 may be called the second photoelectric conversion unit, the analog-to-digital converter 220 the second analog-to-digital conversion unit, the clock synchronization unit 934 the second clock synchronization unit, the polarization demodulation unit 231 the second polarization demodulation unit, the reference optical phase demodulation unit 232 and the quantum optical phase correction unit 233 the second phase demodulation unit, the bit reading unit 935 the third bit reading unit, and the bit position synchronization unit 936 the second bit position synchronization unit.

[0124] The wavelength separation unit 202 separates the received wavelength division multiplexed signal light from wavelength λ QKD QKD signal, wavelength λ QKDs Separate the QKD signals.

[0125] The QKD signal receiver 901 performs clock synchronization and bit position synchronization based on the clock synchronization and bit position synchronization timing extracted by the QKD signal receiver 201. Skew adjustment may be performed on the clock synchronization and bit position synchronization timing of the QKD signal receiver 201.

[0126] The clock synchronization unit 234 of the QKD signal receiver 201 extracts the clock timing based on the digital electrical signal of the Y-polarized optical component (reference optical component) of the optical signal before polarization demodulation, which has been converted from analog to digital, and outputs the extracted clock timing to the clock synchronization unit 934 of the QKD signal receiver 901. The clock synchronization unit 934 outputs the clock timing extracted by the clock synchronization unit 234 to the analog-to-digital converter 220.

[0127] The bit position synchronization unit 236 of the QKD signal receiver 201 outputs the starting position of a random number, indicated by the bit position synchronization data in the reference optical bit sequence read from the received reference optical signal after phase demodulation, to the bit position synchronization unit 936 of the QKD signal receiver 901. The bit position synchronization unit 936 outputs a bit sequence starting from the starting position extracted by the bit position synchronization unit 236 as a random number sequence.

[0128] Using Figures 17 and 18, examples of wavelength configurations in a quantum key distribution system 1 according to several embodiments will be explained. In the examples in Figures 17 and 18, similar to Figures 4 and 5, one wavelength λ QKDs Using the QKD signal, four wavelengths λ QKD This synchronizes the QKD signals.

[0129] In the example shown in Figure 17, the transmitter 100 has a wavelength λ QKDs_1 QKD signal transmitting unit 101-1 transmits a QKD signal, wavelength λ QKDs_1 Wavelength λ synchronized with the QKD signal QKD1 -1~λ QKD1-4 QKD signal transmitting units 801-1-1 to 801-1-4 transmit the respective QKD signals, with wavelength λ QKDs_2 QKD signal transmitting unit 101-2 transmits a QKD signal, wavelength λ QKDs_2 Wavelength λ synchronized with the QKD signal QKD2-1 ~λ QKD2-4 QKD signal transmitting units 801-2-1 to 801-2-4 transmit the respective QKD signals, with wavelength λ QKDs_3 QKD signal transmitting unit 101-3 transmits a QKD signal, wavelength λ QKDs_3 Wavelength λ synchronized with the QKD signal QKD3-1 ~λQKD3-4 It includes QKD signal transmitting units 801-3-1 to 801-3-4 that transmit the respective QKD signals.

[0130] The wavelength division multiplexing section 102 is wavelength λ QKDs_1 , λ QKDs_2 , λ QKDs_3 , λ QKD1-1 ~λ QKD1-4 , λ QKD2-1 ~λ QKD2-4 , λ QKD3-1 ~λ QKD3-4 The optical signal (QKD signal) is wavelength-multiplexed.

[0131] In the receiver 200, the wavelength separation unit 202 separates the wavelength multiplexed optical signal by wavelength λ QKDs_1 , λ QKDs_2 , λ QKDs_3 , λ QKD1-1 ~λ QKD1-4 , λ QKD2-1 ~λ QKD2-4 , λ QKD3-1 ~λ QKD3-4 It is separated into optical signals (QKD signals).

[0132] Receiver 200 has a wavelength λ QKDs_1 QKD signal receiving unit 201-1, receiving the QKD signal, wavelength λ QKDs_1 Wavelength λ synchronized with the QKD signal QKD1-1 ~λ QKD1-4 QKD signal receiving units 901-1-1 to 901-1-4, which receive the respective QKD signals, and wavelength λ QKDs_2 QKD signal receiving unit 201-2 that receives the QKD signal, wavelength λ QKDs_2 Wavelength λ synchronized with the QKD signal QKD2-1 ~λ QKD2-4 QKD signal receiving units 901-2-1 to 901-2-4, which receive the respective QKD signals, and wavelength λ QKDs_3 A synchronization signal receiving unit 902-3 receives the QKD signal, wavelength λ QKDs_3 Wavelength λ synchronized with the QKD signal QKD3-1 ~λ QKD3-4 It includes QKD signal receiving units 901-3-1 to 901-3-4 that receive the respective QKD signals.

[0133] In this case, as shown in Figure 18, one of the five wavelengths is used for the QKD signal that transmits data for bit position synchronization. For example, wavelength λ QKD1-1 ~λ QKD1-4 The wavelength λ in the center is the wavelength used to transmit data for bit position synchronization. QKDs_1 Let's assume the wavelength is λ. QKD1-1 ~λ QKD1-4 and wavelength λ QKDs_ They are adjacent. Wavelength λ QKD2-1 ~λ QKD2-4 The wavelength λ in the center is the wavelength used to transmit data for bit position synchronization. QKDs_2 Let's assume the wavelength is λ. QKD2-1 ~λ QKD2-4 and wavelength λ QKDs_2 They are adjacent. Wavelength λ QKD3-1 ~λ QKD3-4 The wavelength λ in the center is the wavelength used to transmit data for bit position synchronization. QKDs_3 Let's assume the wavelength is λ. QKD3-1 ~λ QKD3-4 and wavelength λ QKDs_3 They are adjacent.

[0134] Thus, bit position synchronization may be performed at other QKD wavelengths based on bit position synchronization data transmitted at the QKD wavelength. In this case as well, frequency utilization efficiency can be improved, similar to Embodiment 2. Furthermore, the device configuration can be made even simpler.

[0135] This disclosure is not limited to the embodiments described above, and may be modified as appropriate without departing from its intent.

[0136] Each configuration in the above-described embodiment is composed of hardware, software, or both, and may consist of one piece of hardware or software, or multiple pieces of hardware or software. Each function of each device (transmitter, receiver, etc.) (such as processing by a digital signal processor) may be realized by a computer 30 having a processor 31 such as a CPU (Central Processing Unit) and a memory 32 as a storage device, as shown in Figure 19. 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 executing the program stored in the memory 32 with the processor 31.

[0137] These programs, when loaded into a computer, include a set of instructions (or software code) for causing the computer to perform one or more of the functions described in the embodiments. The programs may be stored on non-temporary computer-readable media or tangible storage media. Examples, but not limited to, include random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drives (SSDs), or other memory technologies, CD-ROMs, digital versatile discs (DVDs), Blu-ray® discs, or other optical disc storage, magnetic cassettes, magnetic tapes, magnetic disk storage, or other magnetic storage devices. The programs may be transmitted over temporary computer-readable media or communication media. Examples, but not limited to, include electrical, optical, acoustic, or other forms of propagating signals.

[0138] Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as can be understood by those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0139] Each drawing is merely illustrative to illustrate one or more embodiments. Each drawing may be associated with one or more other embodiments rather than with only one specific embodiment. As those skilled in the art will understand, 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, for example, to create embodiments not explicitly shown or described. Not all features or steps shown in any one drawing to illustrate an exemplary embodiment are necessarily required, and some features or steps may be omitted. The order of steps shown in any of the drawings may be changed as appropriate.

[0140] Some or all of the above embodiments may also be described as follows, but are not limited to the following:

[0141] (Note 1) A first light source that outputs a first light, A first branching section that splits the first light into a first polarized light with a first polarization and a second polarized light with a second polarization, A first modulation unit modulates the first polarized light based on a first random number, A second modulation unit modulates the second polarization light based on the bit position synchronization data of the first random number, A first polarization multiplexing unit that polarization multiplexes the modulated first polarization light and the modulated second polarization light, and transmits the polarization multiplexed first polarization multiplexed light to a receiver, A transmitter equipped with the following features. (Note 2) The data for bit position synchronization indicates the starting position of the first random number. The transmitter described in Appendix 1. (Note 3) The modulated second polarization light indicates the clock timing for transmitting the modulated first polarization light. The transmitter described in Appendix 1 or 2. (Note 4) The modulated second polarization light is a reference light for phase demodulation of the modulated first polarization light. The transmitter described in Appendix 1 or 2. (Note 5) The system includes a wavelength multiplexing unit that wavelength multiplexes the first polarization multiplexed light and transmits the wavelength multiplexed light to the receiver. The transmitter described in Appendix 1 or 2. (Note 6) A second light source that outputs a second light, A second branching section that splits the second light into a third polarized light of the first polarization and a fourth polarized light of the second polarization, A third modulation unit modulates the third polarized light based on a second random number, A second polarization multiplexing unit that polarization multiplexes the modulated third polarization light and the fourth polarization light to generate a second polarization multiplexed light, Equipped with, The wavelength division multiplexing unit wavelength-multiplexes the first polarization-multiplexed light and the second polarization-multiplexed light, and transmits the wavelength-multiplexed wavelength-multiplexed light to the receiver. The transmitter described in Appendix 5. (Note 7) A first photoelectric conversion unit that receives a first polarization-multiplexed light from a transmitter and converts the received first polarization-multiplexed light into a first electrical signal, A first polarization demodulator that polarizes and demodulates the first electrical signal into a first polarization signal with a first polarization and a second polarization signal with a second polarization, A first bit reading unit reads a first bit sequence containing a first random number from the first polarization signal, A second bit reading unit reads a second bit sequence containing data for bit position synchronization of the first random number from the second polarization signal, A first bit position synchronization unit performs bit position synchronization of the first random number contained in the first bit sequence based on the bit position synchronization data contained in the second bit sequence, A receiver equipped with the following features. (Note 8) The bit position synchronization unit performs bit position synchronization of the first random number based on the starting position of the first random number indicated by the bit position synchronization data. The receiver described in Appendix 7. (Note 9) A first analog-to-digital conversion unit that converts the first electrical signal from analog to digital and generates a first digital signal, A first clock synchronization unit that extracts clock timing based on the first digital signal, Equipped with, The first analog-to-digital conversion unit performs the analog-to-digital conversion based on the extracted clock timing. The receiver described in Appendix 7 or 8. (Note 10) The system includes a first phase demodulation unit that demodulates the phase of the second polarization signal and demodulates the phase of the first polarization signal based on the demodulated phase. The receiver described in Appendix 7 or 8. (Note 11) The system includes a wavelength separation unit that receives wavelength-division multiplexed light from the transmitter and wavelength-separates the first polarization-division multiplexed light from the received wavelength-division multiplexed light. The receiver described in Appendix 7 or 8. (Note 12) The wavelength separation unit separates the first polarization-multiplexed light and the second polarization-multiplexed light from the received wavelength-multiplexed light by wavelength. A second photoelectric conversion unit that converts the second polarization-multiplexed light into a second electrical signal, A second polarization demodulation unit that polarization demodulates the second electrical signal into a third polarization signal of the first polarization and a fourth polarization signal of the second polarization, A third bit reading unit reads a third bit sequence containing a second random number from the third polarization signal, A second bit position synchronization unit performs bit position synchronization of the second random number contained in the third bit sequence based on the bit position synchronization data contained in the second bit sequence, A receiver as described in Appendix 11, comprising the following features. (Note 13) A quantum key distribution system comprising a transmitter and a receiver, The aforementioned transmitter is A first light source that outputs a first light, A first branching section that splits the first light into a first polarized light with a first polarization and a second polarized light with a second polarization, A first modulation unit modulates the first polarized light based on a first random number, A second modulation unit modulates the second polarization light based on the bit position synchronization data of the first random number, A first polarization multiplexing unit that polarization multiplexes the modulated first polarization light and the modulated second polarization light, and transmits the polarization multiplexed first polarization multiplexed light to a receiver, Equipped with The aforementioned receiver is A first photoelectric conversion unit that receives the first polarization-multiplexed light from the transmitter and converts the received first polarization-multiplexed light into a first electrical signal, A first polarization demodulator that polarizes and demodulates the first electrical signal into a first polarization signal with a first polarization and a second polarization signal with a second polarization, A first bit reading unit reads a first bit sequence containing the first random number from the first polarization signal, A second bit reading unit reads a second bit sequence from the second polarization signal, which includes the data for bit position synchronization of the first random number, A first bit position synchronization unit performs bit position synchronization of the first random number contained in the first bit sequence based on the bit position synchronization data contained in the second bit sequence, A quantum key distribution system equipped with [the following features]. (Note 14) The first light from the first light source is split into a first polarized light with a first polarization and a second polarized light with a second polarization, Modulation of the first polarized light based on a first random number, The second polarization light is modulated based on the data for bit position synchronization of the first random number, The modulated first polarized light and the modulated second polarized light are polarized multiplexed, and the polarized multiplexed first polarized multiplexed light is transmitted to the receiver. A method in a transmitter, including the method described above. (Note 15) The system receives a first polarization-multiplexed light from a transmitter and converts the received first polarization-multiplexed light into a first electrical signal. The first electrical signal is polarized and demodulated into a first polarization signal with a first polarization and a second polarization signal with a second polarization. Reading a first bit sequence containing a first random number from the first polarization signal, Reading a second bit sequence from the second polarization signal, which includes data for bit position synchronization of the first random number, Based on the bit position synchronization data included in the second bit sequence, the bit position synchronization of the first random number included in the first bit sequence is performed. A method in a receiver, including the method described above. (Note 16) The first light from the first light source is split into a first polarized light with a first polarization and a second polarized light with a second polarization, Modulation of the first polarized light based on a first random number, The second polarization light is modulated based on the data for bit position synchronization of the first random number, The modulated first polarized light and the modulated second polarized light are polarized multiplexed, and the polarized multiplexed first polarized multiplexed light is transmitted to the receiver. A program that causes a computer to execute a method in a transmitter, including the method described. (Note 17) The system receives a first polarization-multiplexed light from a transmitter and converts the received first polarization-multiplexed light into a first electrical signal. The first electrical signal is polarized and demodulated into a first polarization signal with a first polarization and a second polarization signal with a second polarization. Reading a first bit sequence containing a first random number from the first polarization signal, Reading a second bit sequence from the second polarization signal, which includes data for bit position synchronization of the first random number, Based on the bit position synchronization data included in the second bit sequence, the bit position synchronization of the first random number included in the first bit sequence is performed. A program that causes a computer to perform a method in a receiver, including the method described above.

[0142] Some or all of the elements (e.g., configuration and function) described in Appendices 2-6 and 8-12, which are dependent on Appendice 1 (Transmitter) and Appendice 7 (Receiver), may also be dependent on Appendice 13 (Quantum Key Distribution System), Appendices 14 and 15 (Method), and Appendices 16 and 17 (Program) in the same manner as those described in Appendices 2-6 and 8-12. Some or all of the elements described in any appendice may be applied to various hardware, software, recording means, systems, and methods for recording software. [Explanation of Symbols]

[0143] 1.9 Quantum Key Distribution System 2 Optical Fibers 2 Optical Fibers 10 Transmitters 11 Light source 12 Branching point 13. First Modulation Section 14. Second Modulation Section 15 Polarization Multiplexing 20 receivers 21 Photoelectric conversion unit 22 Polarization demodulation section 23 First bit reading section 24 Second bit reading section 25-bit position synchronization section 30 Computers 31 processors 32 memory 100, 800 Transmitters 101, 801 QKD signal transmitter 102 Wavelength multiplexing section 110 Light source 120 Optical Coupler 130-x, 130-y modulator 140 Polarization Beam Splitter 200, 900 receiver 201, 901 QKD signal receiver 202 Wavelength separation section 210 Optical Frontend 211 Light source 212 90° Hybrid 213 Balanced Receiver 220 Analog-to-Digital Converter 230,930 Digital Signal Processors 231 Polarization demodulation section 232 Reference Optical Phase Demodulation Unit 233 Quantum optical phase correction unit 234, 934 Clock synchronization section 235, 235a, 235b, 935 bit read section 236,936 bit position synchronization section 237 Shift key output section 802 Synchronization signal transmission unit 821 Light source 822 Synchronous modulator 902 Synchronization signal receiving unit 921 Optical Frontend 922 Analog-to-Digital Converter 937 Skew adjustment section

Claims

1. A first light source that outputs a first light, A first branching section that splits the first light into a first polarized light with a first polarization and a second polarized light with a second polarization, A first modulation unit modulates the first polarized light based on a first random number, A second modulation unit modulates the second polarization light based on the bit position synchronization data of the first random number, A first polarization multiplexing unit that polarization multiplexes the modulated first polarization light and the modulated second polarization light, and transmits the polarization multiplexed first polarization multiplexed light to a receiver, A transmitter equipped with the following features.

2. The data for bit position synchronization indicates the starting position of the first random number. The transmitter according to claim 1.

3. The modulated second polarization light indicates the clock timing for transmitting the modulated first polarization light. The transmitter according to claim 1 or 2.

4. The modulated second polarization light is a reference light for phase demodulation of the modulated first polarization light. The transmitter according to claim 1 or 2.

5. A first photoelectric conversion unit that receives a first polarization-multiplexed light from a transmitter and converts the received first polarization-multiplexed light into a first electrical signal, A first polarization demodulator that polarizes and demodulates the first electrical signal into a first polarization signal with a first polarization and a second polarization signal with a second polarization, A first bit reading unit reads a first bit sequence containing a first random number from the first polarization signal, A second bit reading unit reads a second bit sequence containing data for bit position synchronization of the first random number from the second polarization signal, A first bit position synchronization unit performs bit position synchronization of the first random number included in the first bit sequence based on the bit position synchronization data included in the second bit sequence, A receiver equipped with the following features.

6. The bit position synchronization unit performs bit position synchronization of the first random number based on the starting position of the first random number indicated by the bit position synchronization data. The receiver according to claim 5.

7. A first analog-to-digital conversion unit that converts the first electrical signal from analog to digital and generates a first digital signal, A first clock synchronization unit that extracts clock timing based on the first digital signal, Equipped with, The first analog-to-digital conversion unit performs the analog-to-digital conversion based on the extracted clock timing. The receiver according to claim 5 or 6.

8. A quantum key distribution system comprising a transmitter and a receiver, The aforementioned transmitter is A first light source that outputs a first light, A first branching section that splits the first light into a first polarized light with a first polarization and a second polarized light with a second polarization, A first modulation unit modulates the first polarized light based on a first random number, A second modulation unit modulates the second polarization light based on the bit position synchronization data of the first random number, A first polarization multiplexing unit that polarization multiplexes the modulated first polarization light and the modulated second polarization light, and transmits the polarization multiplexed first polarization multiplexed light to a receiver, Equipped with The aforementioned receiver is A first photoelectric conversion unit that receives the first polarization-multiplexed light from the transmitter and converts the received first polarization-multiplexed light into a first electrical signal, A first polarization demodulator that polarizes and demodulates the first electrical signal into a first polarization signal with a first polarization and a second polarization signal with a second polarization, A first bit reading unit reads a first bit sequence containing the first random number from the first polarization signal, A second bit reading unit reads a second bit sequence from the second polarization signal, which includes the data for bit position synchronization of the first random number, A first bit position synchronization unit performs bit position synchronization of the first random number included in the first bit sequence based on the bit position synchronization data included in the second bit sequence, A quantum key distribution system equipped with [the following features].

9. The first light from the first light source is split into a first polarized light with a first polarization and a second polarized light with a second polarization, Modulation of the first polarized light based on a first random number, The second polarization light is modulated based on the data for bit position synchronization of the first random number, The modulated first polarized light and the modulated second polarized light are polarized multiplexed, and the polarized multiplexed first polarized multiplexed light is transmitted to the receiver. A method in a transmitter, including the method described above.

10. The system receives a first polarization-multiplexed light from a transmitter and converts the received first polarization-multiplexed light into a first electrical signal. The first electrical signal is polarized and demodulated into a first polarization signal with a first polarization and a second polarization signal with a second polarization. Reading a first bit sequence containing a first random number from the first polarization signal, Reading a second bit sequence from the second polarization signal, which includes data for bit position synchronization of the first random number, Based on the bit position synchronization data included in the second bit sequence, the bit position synchronization of the first random number included in the first bit sequence is performed. A method in a receiver, including the method described above.

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