Abstract: A user request can be reflected in the degree of security of an updated key in quantum key distribution. A sender and a receiver are connected through optical fiber. A quantum transmitter in the sender and a quantum receiver in the receiver carry out basis reconciliation and error correction through a quantum channel, based on a source of a key sent from the quantum transmitter and on a raw key received by the quantum receiver. Under the control of security control sections in the sender and receiver, the amount of information having the possibility of being intercepted, which is determined in accordance with a degree of security requested by a user, is removed from the key information after error correction, whereby a final cryptographic key is generated. Secret communication is performed between encryption/decryption sections in the sender and receiver by using the cryptographic key thus updated.

Abstract: A random number quality control circuit capable of fast control of the level of random number quality is present. When a “0” output section and a “1” output section generate random numbers by individually receiving a random number signal, a random number quality monitor monitors an unbalance between the numbers of “0”s and “1”s. If a deviation from a desired ratio is found, a drive controller controls the reception characteristics of the “0” output section and “1” output section individually so that the deviation will be compensated for. The amount of information intercepted between a sender and a receiver can be reduced by maintaining the mark ratio of shared random numbers at 50%.

Abstract: In a quantum cryptographic transmitter (11), a phase modulator (1103, 1104) and an LN intensity modulator (1105) apply optical phase modulation and light intensity modulation to an optical signal based on desired data to generate a desired optical signal to be transmitted to a quantum cryptographic receiver (13). Based on the number of photons detected from the desired optical signal, a bias control circuit (1108) controls an operating point in light intensity modulation of the LN intensity modulator (1105).

Abstract: A method for managing shared random numbers in a secret communication network including at least one center node and a plurality of remote nodes connected to the center node, includes: sharing random number sequences between the center node and respective ones of the plurality of remote nodes; when performing random numbers sharing between a first remote node storing a first random number sequence shared with the center node and a second remote node storing a second random number sequence shared with the center node, distributing a part of the second random number sequence from the center node to the first remote node; and sharing the part of the second random number sequence between the first remote node and the second remote node.

Abstract: A sender transmits to a receiver an optical signal that is phase-modulated in accordance with source data and a basis stored in a memory. The receiver phase-modulates the received optical signal in accordance with a basis, obtains detection data through interference, and stores the detection data in a memory. An inter-device address difference (GD) and an intra-device address difference (DI) are provisionally set. The detection data are checked against the source data while sequentially changing the values of GD and DI within a predetermined adjustment range. Based on the result of this checking, GD and DI are determined.

Abstract: A cryptographic key management method and device are provided by which cryptographic keys of multiple nodes can be managed easily and stably. A system includes at least one first node and a plurality of second nodes connected to the first node, and the first node individually generates and consumes a cryptographic key with each of the second nodes connected to the first node itself. A cryptographic key management device in such a system has a monitor that monitors the stored key amounts of cryptographic keys of the individual second nodes, stored at the first node, and a key management control section that performs key generation control on the first node, based on the stored key amounts.

Abstract: A communication system and a timing control method are proposed that optimize timing in a sender and thereby enable information to be stably transmitted at the right timing. Under instructions from a timing controller in a receiver, the timing of driving a phase modulator in a sender is shifted by one step after another, and the then amount of clock shift and result of interference are monitored at the receiver and stored in a memory. The optimum timing is determined based on the stored data. Thus, a clock for driving the phase modulator in the sender can be set at the right timing. This is equivalent to compensating for group velocity dispersion due to wavelength dispersion that occurs when an optical signal channel and a clock signal channel are transmitted by wavelength division multiplexing transmission.

Abstract: For an error rate QBER, threshold values are preset, including a threshold value Qbit for frame synchronization processing, a threshold value Qphase for phase correction processing, and a threshold value QEve for eavesdropping detection. Upon the distribution of a quantum key from a sender to a receiver, when the measurement value of QBER is deteriorated more than Qbit, frame synchronization processing is performed. When the measurement value of QBER is deteriorated more than Qphase, phase correction processing and frame synchronization processing are performed. When QBER does not become better than QEve even after these recovery-processing steps are repeated N times, it is determined that there is a possibility of eavesdropping, and the processing is stopped.

Abstract: An optical transmitter which modulates the phases and intensities of double pulses and then transmits them, includes a branching section which branches each of the input double pulses to first and second paths, a first optical modulator placed in the first path, second and third optical modulators placed in series in the second path, and a combining section which combines the double pulses having traveled through the first path with the double pulses having traveled through the second path to output double pulses. A control section controls such that each of the first and second optical modulators performs any one of relative intensity modulation and relative phase modulation on the double pulses passing through, and the third optical modulator performs relative phase modulation on the double pulses passing through.

Abstract: A secret communications system realizes point-to-multipoint or multipoint-to-multipoint connections of both quantum channels and classical channels. Multiple remote nodes are individually connected to a center node through optical fiber, and random-number strings K1 to KN are individually generated and shared between the respective remote nodes and the center node. Encrypted communication is performed between each remote node and the center node by using the corresponding one of the shared random-number strings K1 to KN as a cryptographic key. The center node is provided with a switch section for quantum channels and a switch section for classical channels. Switching control on each of these switch sections is performed independently of the other by a controller.

Abstract: A method and apparatus for measuring the optical power of very weak light arriving at a receiver, by using a photon detector, are provided. A photon detector detects the presence or absence of the arrival of a photon in accordance with bias application timing. For a train of optical pulses coming in at an arbitrary timing in respective time slots, the bias application timing is sequentially shifted within the range of a time slot. Each time a shift is made, the number of photons detected is counted by a photon counter. Based on this number of photons, the optical power of the train of the optical pulses is measured.

Abstract: For an error rate QBER, threshold values are preset, including a threshold value Qbit for frame synchronization processing, a threshold value Qphase for phase correction processing, and a threshold value QEve for eavesdropping detection. Upon the distribution of a quantum key from a sender to a receiver, when the measurement value of QBER is deteriorated more than Qbit, frame synchronization processing is performed. When the measurement value of QBER is deteriorated more than Qphase, phase correction processing and frame synchronization processing are performed. When QBER does not become better than QEve even after these recovery-processing steps are repeated N times, it is determined that there is a possibility of eavesdropping, and the processing is stopped.

Abstract: A communication system capable of employing polarization-dependent phase modulators with a reversing configuration that preserves security against disturbance of a polarization state at a transmission path but without using Faraday mirrors and a communication method using the same are provided. A quantum cryptography system of the present invention includes a first station 1, a transmission path 2, and a second station 3. The first station 1 has means for emitting time-divided optical pulses into the transmission path 2 and measuring a phase difference between the optical pulses returning from the transmission path 2. The transmission path 2 is a medium of light.

Abstract: For an error rate QBER, threshold values are preset, including a threshold value Qbit for frame synchronization processing, a threshold value Qphase for phase correction processing, and a threshold value QEve for eavesdropping detection. Upon the distribution of a quantum key from a sender to a receiver, when the measurement value of QBER is deteriorated more than Qbit, frame synchronization processing is performed. When the measurement value of QBER is deteriorated more than Qphase, phase correction processing and frame synchronization processing are performed. When QBER does not become better than QEve even after these recovery-processing steps are repeated N times, it is determined that there is a possibility of eavesdropping, and the processing is stopped.

Abstract: An optical communication apparatus that can perform stable intensity and phase modulation on an optical pulse at high speed is provided, as well as a quantum key distribution system using the apparatus. Using multilevel signals for the electric signals (RF1, RF2) to be applied to two arms of a two-electrode Mach-Zehnder modulator, phase modulation is performed on an optical pulse in accordance with the average of the levels of the signals (RF1, RF2), and intensity modulation is performed on the optical pulse in accordance with the voltage difference between the signals (RF1, RF2), whereby stable high-speed multilevel modulation can be realized. The cryptographic key generation rate in a decoy quantum key distribution system is enhanced.

Abstract: A clock signal of a master clock of a sender is transmitted to a receiver through a classical channel and is returned from the receiver. The clock signal is transmitted with strong light from a sender-side quantum unit to a receiver-side quantum unit through a quantum channel. A sender-side synchronization section establishes phase synchronization between the clock signal returned from the receiver and the clock signal detected by the sender-side quantum unit, and generates a calibration clock signal. At the receiver as well, a receiver-side synchronization section establishes phase synchronization between the clock signal detected from the classical channel and the clock signal detected by the receiver-side quantum unit, and generates a calibration clock signal.

Abstract: In a quantum cryptographic transmitter (11), a phase modulator (1103, 1104) and an LN intensity modulator (1105) apply optical phase modulation and light intensity modulation to an optical signal based on desired data to generate a desired optical signal to be transmitted to a quantum cryptographic receiver (13). Based on the number of photons detected from the desired optical signal, a bias control circuit (1108) controls an operating point in light intensity modulation of the LN intensity modulator (1105).

Abstract: A method and system for allowing communication devices to synchronously manage shared information are provided. A sender sends single-photon pulses modulated with original random numbers to a receiver and also sends frame pulses by using ordinary optical pulses. Bit comparison and basis reconciliation are performed by the frame which is defined by the frame pulses, whereby sifted keys, which are aggregated as a file, are generated by the sender and the receiver individually. The sifted keys are subjected to error correction, privacy amplification, and file sharing processing by the file, whereby common cryptographic keys are synchronously stored in the sender and the receiver individually. The generated cryptographic keys are managed as encryption keys and decryption keys separately. A newly generated key is preferentially placed in the encryption keys or decryption keys that have a smaller stored amount.

Abstract: The present invention provides a single fiber bidirectional optical transmission system capable of realizing the extension of a single fiber bidirectional long distance at a moderate price. An optical signal outputted from a second optical transmitter is incident on an optical amplifying portion passing through an optical circulator, a single fiber bidirectional transmission path, an optical circulator and an optical Blue/Red filter. The optical signal outputted from a first optical transmitter is incident on the optical amplifying portion passing through a dispersion compensator and the optical Blue/Red filter.

Abstract: A cryptographic key management method and device are provided by which cryptographic keys of multiple nodes can be managed easily and stably. A system includes at least one first node and a plurality of second nodes connected to the first node, and the first node individually generates and consumes a cryptographic key with each of the second nodes connected to the first node itself. A cryptographic key management device in such a system has a monitor that monitors the stored key amounts of cryptographic keys of the individual second nodes, stored at the first node, and a key management control section that performs key generation control on the first node, based on the stored key amounts.