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 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 with the possibility of being intercepted that 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 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 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: 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: 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: 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: The objectives of this invention are to provide a novel Nile red luminescent compound capable of emitting red light at a high luminance and at high color purity, and a luminescence element capable of emitting light at a high luminance. To achieve these objectives, the present invention provides a Nile red luminescent compound emitting red light represented by formula (1) and a luminescence element that includes the compound in the light-emitting layer thereof.

Abstract: The objectives of the present invention are to provide a red light-emitting compound capable of emitting red light at high color purity and strong luminance, and excellent in fastness, and to provide a luminescent element capable of emitting red light at strong luminance. The objectives are achieved by the Nile Red luminescent compound emitting red light represented by formula (1): wherein X is cyano group or a fluorohydrocarbyl group, and a luminescent element, the light-emitting layer of which includes the compound.

Abstract: In a photo detecting circuit using an avalanche photodiode (APD), a pulse noise mask signal indicating the timing of occurrence of a pulse noise is generated by XORing a reverse-bias-pulse application timing signal and its delayed signal. Pulse noises are eliminated by ANDing an output signal of the APD and the pulse noise mask signal. Alternatively, a pulse noise is estimated by taking the average for the output current signal a given number of times. Pulse noises are eliminated by subtracting the estimated pulse noise from the APD output signal.

Abstract: A sender sends original random-number data to a receiver through a quantum channel. The receiver generates a raw key from information received through the quantum channel and notifies the received information to the sender. The sender performs received-bit comparison and basis reconciliation based on the received information and provisionally shares a sifted key with the receiver. The receiver sends part of its version of the sifted key to the sender, by which an error rate is calculated. The calculated error rate is compared with a predetermined threshold value for bit position synchronization determination. When the calculated error rate is larger, the sender notifies the receiver that bit position synchronization is not established. The receiver reassigns bit numbers to the sifted key, and received-bit comparison and basis reconciliation are performed again. This procedure is repeated until the calculated error rate becomes smaller than the threshold value.

Abstract: In a system where a quantum channel and a classical channel are multiplexed on a single optical transmission line and information is transmitted from a transmitter to a receiver through the quantum channel, the classical channel is inhibited from affecting the quantum channel. To this end, the transmission characteristics of a transmitter-side wavelength multiplexer/demultiplexer for the classical channel, the transmission characteristics of a receiver-side wavelength multiplexer/demultiplexer for the quantum channel, and the optical power of a light source for the classical channel are designed so that crosstalk light due to spontaneous emission light and crosstalk light due to nonlinear optical effects can be suppressed, and the classical channel does not affect the quantum channel.

Abstract: An optical packet exchange apparatus and an optical switch in which search for a connection pattern between an input unit devoid of a packet to be transmitted and an output unit devoid of a packet to be received is reduced to enable fast switch control even in cases wherein the number of channels of the exchange apparatus is increased or network speed is higher. A plurality of input units, a plurality of output units and an optical switch are provided. Each input unit includes an input buffer unit, a parallel/serial conversion unit, an electrical/optical conversion unit, and a dummy packet insertion unit for sending a dummy packet if there is no packet to be transmitted. Each output unit includes an exchange counterpart contention resolution unit for controlling the exchange counterpart, an optical/electrical conversion unit, a serial/parallel conversion unit, and a packet eliminating unit. The exchange counterpart contention resolution unit controls the packet eliminating unit to eliminate a dummy packet.

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: A sender's encrypted communication apparatus and a recipient's encrypted communication apparatus autonomously generate keys for encryption with respective key generators, store the generated encryption keys in respective encryption key memories, and store part of the generated encryption keys in respective authentication memories. The keys stored in the authentication memories are used for mutual authentication when the sender's encrypted communication apparatus and a recipient's encrypted communication apparatus are connected to each other.

Abstract: A transmitter-receiver having, a means for automatically determining the status of transmission medium such as optical fiber, and a means for automatically setting and resetting the transmission rate and/or output power according to the status of the transmission medium, a transmission loss and gain measurement method, and a transmitting-receiving system. A transmitter-receiver comprises at least: an output power controller for controlling the output power of a transmitter; an input power measuring section for measuring the strength of input signals; and an information processor for deriving the loss or gain of a path to change the output power of the transmitter and/or the rate of data transmission according to the derived loss or gain of the path.

Abstract: The objectives of this invention are to provide a novel Nile red luminescent compound capable of emitting red light at a high luminance and at high color purity, and a luminescence element capable of emitting light at a high luminance. To achieve these objectives, the present invention provides a Nile red luminescent compound emitting red light represented by formula (1) and a luminescence element that includes the compound in the light-emitting layer thereof.

Abstract: A quantum cryptography key distributing system includes an optical fiber; a transmission unit and a reception unit. The transmission unit is connected with the optical fiber, generates a transmission optical pulse signal from an optical pulse signal based on a first data in synchronism with an optical clock signal and transmits the transmission optical pulse signal to the reception unit via the optical fiber. Polarization of the transmission optical pulse signal is different from that of the optical pulse signal. The reception unit is connected with the optical fiber, transmits the optical pulse signal to the transmission unit via the optical fiber, phase-modulates a part of the transmission optical pulse signal based on a second data in synchronism with the optical clock signal, and detects a reception data corresponding to the first data based on the transmission optical pulse signal in synchronism with the optical clock signal.

Abstract: In view of the fact that luminescent compounds and polymers capable of emitting blue light for a long time and excellent in durability have not been developed, the present invention provides blue light-emitting compounds having special structures, blue light-emitting polymers having special repeating units, processes of producing the compounds, and luminescent elements including the polymers.

Abstract: The present invention provides a single fibre bidirectional optical transmission system capable of realizing the extension of a single fibre bidirectional long distance at a moderate price.