Abstract: An object is to prevent eavesdropping in quantum key distribution. A decoding unit decodes a quantum signal incident thereinto. A plurality of detect photons of the decoded quantum signal output from the decoding unit. A signal processing unit detects bits of the decoded quantum signal based on photon detection results of the plurality of detectors. A control unit perform switching processing by switching destinations to which two decoded quantum signals corresponding to one encoding basis are output between the plurality of detectors, and switching the bits detected by the signal processing unit based on the respective photon detection results of the plurality of detectors.

Abstract: An object is to prevent eavesdropping in quantum key distribution. A photon detector outputs an output current indicating a result of detecting a quantum signal. A current-voltage conversion unit converts the output current into an output voltage signal. An analog-to-digital converter outputs an output voltage signal obtained by analog-digital conversion of the output voltage signal. A signal processing unit performs predetermined signal processing on the output voltage signal, and outputs a photon detection signal indicating a result of detecting the quantum signal. When a time difference between a timing at which the quantum signal is incident into the photon detector and a reference timing determined based on a clock signal is not within a determination range, the photon detection signal is not output from the signal processing unit.

Abstract: A quantum key distribution device is provided with an encoding unit which encodes an optical pulse train; an intensity modulating unit which subjects the encoded optical pulse train to N (where N is an integer at least equal to 3) types of intensity modulation having mutually different intensities, with different timings; and a first key distillation processing unit which generates an encryption key on the basis of a data sequence obtained by removing data obtained from an optical pulse having a specific modulation pattern from a data sequence used by the encoding unit and the intensity modulating unit.

Abstract: A quantum cryptographic key output apparatus includes a semiconductor laser device that repeatedly generates pulsed laser light, an encoder that encodes the pulsed laser light based on a quantum cryptographic key, an optical branching unit that branches the pulsed laser light, and an attenuator that attenuates a light intensity of first pulsed laser light so that the number of photons of the first pulsed laser light has any one of a plurality of candidate values that are values equal to or smaller than 1. Further, the output apparatus includes a light intensity determination unit that determines whether or not a light intensity of a second pulsed laser light is in a predetermined range, and an information output unit that outputs specifying information for specifying the first pulsed laser light corresponding to second pulsed laser light of which the light intensity is not in the predetermined range to an input apparatus.

Abstract: A random number sequence generation apparatus includes: a semiconductor laser device repeatedly generating a pulsed laser beam having a disordered phase; an interferometer including a first transmission line and a second transmission line, a first port connected to an input terminal side and to which the pulsed laser beam is input, a second port connected to an output terminal side and outputs the pulsed laser beam, and a third port connected to the input terminal side; a Faraday mirror connected to the second port and reflecting the pulsed laser beam; a photodiode connected to the third port and outputs an electrical signal in accordance with interference light of the pulsed laser beam that is reflected by the Faraday mirror and passes through one of the transmission lines; and an AD converter configured to generate a random number sequence on the basis of the electrical signal and a threshold.

Abstract: A quantum key distribution device is provided with an encoding unit which encodes an optical pulse train; an intensity modulating unit which subjects the encoded optical pulse train to N (where N is an integer at least equal to 3) types of intensity modulation having mutually different intensities, with different timings; and a first key distillation processing unit which generates an encryption key on the basis of a data sequence obtained by removing data obtained from an optical pulse having a specific modulation pattern from a data sequence used by the encoding unit and the intensity modulating unit.

Abstract: A quantum cryptographic key output apparatus includes a semiconductor laser device that repeatedly generates pulsed laser light, an encoder that encodes the pulsed laser light based on a quantum cryptographic key, an optical branching unit that branches the pulsed laser light, and an attenuator that attenuates a light intensity of first pulsed laser light so that the number of photons of the first pulsed laser light has any one of a plurality of candidate values that are values equal to or smaller than 1. Further, the output apparatus includes a light intensity determination unit that determines whether or not a light intensity of a second pulsed laser light is in a predetermined range, and an information output unit that outputs specifying information for specifying the first pulsed laser light corresponding to second pulsed laser light of which the light intensity is not in the predetermined range to an input apparatus.

Abstract: A random number sequence generation apparatus includes: a semiconductor laser device repeatedly generating a pulsed laser beam having a disordered phase; an interferometer including a first transmission line and a second transmission line, a first port connected to an input terminal side and to which the pulsed laser beam is input, a second port connected to an output terminal side and outputs the pulsed laser beam, and a third port connected to the input terminal side; a Faraday mirror connected to the second port and reflecting the pulsed laser beam; a photodiode connected to the third port and outputs an electrical signal in accordance with interference light of the pulsed laser beam that is reflected by the Faraday mirror and passes through one of the transmission lines; and an AD converter configured to generate a random number sequence on the basis of the electrical signal and a threshold.

Abstract: It is an object of the present invention to provide a network system for quantum key distribution (QKD) for free space and fiber networks. The system of the present invention generates a couple of photons which have different wavelength and inputs each of the photons into the asymmetric Mach-Zehnder interferometer to obtain time-bin entangled state. It provides polarization information with one part of the photons. Then it can obtain hybrid quantum entanglement. The system of the present invention may be used hybrid quantum key distribution system applied for both free space and fibers.

Abstract: It is an object of the present invention to provide a network system for quantum key distribution (QKD) for free space and fiber networks. The system of the present invention generates a couple of photons which have different wavelength and inputs each of the photons into the asymmetric Mach-Zehnder interferometer to obtain time-bin entangled state. It provides polarization information with one part of the photons. Then it can obtain hybrid quantum entanglement. The system of the present invention may be used hybrid quantum key distribution system applied for both free space and fibers.

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: A quantum circuit and a quantum computer are capable of performing multi-bit quantum computation. In the quantum circuit, a quantum bit is represented by polarization directions of light, a sequence of polarized light pulses representing a quantum bit string is sequentially supplied to the quantum circuit, and an amount of polarization rotation and phase difference applied to a certain light pulse are determined on the basis of a result of a polarization measurement of a preceding input light pulse sequence, thus realizing a controlled-unitary transform. In addition, regarding the light pulses representing the quantum bits, the number of photons included in one pulse is larger than 1, resulting in a reduction of the influence of error.

Abstract: Signature requestor terminal device (10) codes a plurality of messages to a quantum state and transmits it to signer terminal device (20). Signer terminal device (20) issues a blind signature for the plurality of the messages coded to the quantum state, codes the messages and the signature to a quantum state and sends them back to signature requestor terminal device (10).

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: The present invention realizes a quantum circuit and a quantum computer capable of performing multi-bit quantum computation. A quantum bit is represented by the polarization directions of light, a sequence of polarized light pulses representing a quantum bit string is sequentially supplied, and the amount of polarization rotation applied to a certain light pulse and the amount of phase difference are determined on the basis of a result of the measurement of polarization of the preceding input light pulse sequence, thus realizing a controlled-unitary transform. In addition, regarding the light pulses representing the quantum bits, the number of photons included in one pulse is larger than 1, resulting in a reduction of the influence of error.

Abstract: A cryptographic key distribution method, in which coherent light being suitable for optical fiber communication network is used and high security is secured, is provided. A sending end encodes random numbers so that symmetry probability distributions can be obtained at a receiving end, and also sets light intensity and a modulation index of signal light radiating from the sending end so that the SNR of an eavesdropper is less than 2 dB even when said eavesdropper uses a most suitable receiving equipment at the sending end, and also so that the SNR of the receiving end is more than ?10 dB, and transmits signals. The receiving end calculates probability distributions of obtained signals and sets a discrimination threshold value after a set of random numbers was transmitted from the sending end. When the probability distributions have some abnormal states, it is judged that the eavesdropper exists, and distributing the cryptographic key is stopped and a fresh cryptographic key is distributed again.

Abstract: A method for testing the reliability of a quantum key distribution apparatus is provided. The method includes the steps of: producing a set of quanta by the sender, the set of quanta comprising first, second, and third quantum, the first, second, and third quantum having a quantum correlation; measuring the first and second quantum at a sender using one of two prearranged bases; transmitting the third quantum to the receiver over the quantum channel; measuring the third quantum at a receiver using one of the two prearranged bases; and exchanging information regarding the measured bases between the sender and receiver over a public channel to check for a known behavior of the quantum apparatus based upon the quantum correlation, wherein if the quantum apparatus behaves as is known or within a tolerable limit the reliability of the quantum apparatus is confirmed. A similar method for quantum key distribution is also disclosed.

Abstract: A single photon generating apparatus includes an optical waveguide, an active medium section and a resonator section. In the active medium section, a single electron is excited in response to application of exciting energy, and a single photon is emitted from the electron. The resonator section optically resonates with the active medium section, holds the photon emitted from the electron in the resonator, and transfers the held photon to the optical waveguide in response to a first control signal.

Abstract: A quantum cryptography multi-node communication system includes a quantum communication channel and a plurality of nodes including a transmission node and a reception node and connected with the quantum communication channel. The transmission node transmits a light signal as a time series of photons to the reception node through the quantum communication channel, a quantum state of the photons is modulated, and transmits a quantum state sequence to the reception node. The reception node predetermines a quantum state sequence, receives the light signal transmitted from the transmission node, measures quantum states of the received light signal, and determines presence or absence of interception based on the predetermined quantum state sequence, the transmitted quantum state sequence and the measured quantum states.

Abstract: With the object of providing a practical quantum circuit capable of discriminating Bell states in order to realize transmission of quantum states with high fidelity, a quantum circuit comprises: a two-photon absorbing crystal that selectively absorbs, in accordance with known selection rules, a photon pair of a Bell state that is determined depending on crystal symmetry of said two-photon absorbing crystal; a two-photon absorption detector that detects absorption of photon pairs by said two-photon absorbing crystal; and a polarization element that converts the Bell state of a polarized photon pair. The two-photon absorbing crystal makes two-photon absorption of a photon pair of a specific Bell state only. Electrons that have been excited by the two-photon absorption are detected by the detector.