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. Use of the polarization element enables one-to-one conversion of one Bell state to another.

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 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 semiconductor laser is provided having excellent characteristics such as long life, less variable threshold current due to the temperature change, and low noise, by effectively preventing leakage of electrons from the active layer. The semiconductor laser is provided with a electron barrier layer and the guide layer of the laser is doped with an n-type dopant, such that a high electron barrier is constructed between the guide layer and the cladding layer.

Abstract: A semiconductor laser includes a substrate and an active layer. The substrate has a lattice constant which falls within the range between 0.565 nm and 0.56 nm. The active layer has a quantum well structure in which a GaInP alloy crystal with a lattice constant larger than the lattice constant of the substrate is formed as a well layer and an AlGaInP alloy crystal with a lattice constant smaller than the lattice constant of the substrate is formed as a barrier layer.

Abstract: The present invention provides a quantum-well semiconductor laser device having a substrate, a clad layer on the substrate, an optical confinement layer on the clad layer, an active layer on the optical confinement layer, an optical confinement layer on the active layer and a clad layer on the optical confinement layer each of which formed of a semiconductor wherein the active layer formed of a multi-layer quantum-well structure of which each layer comprising a quantum-well layer, a first barrier layer adjacent the quantum-well layer and a second barrier layer adjacent the first barrier layer wherein the semiconductor of the first barrier has a higher energy level at a .GAMMA. point of a valence band than that of the second barrier layer.

Abstract: A stripe structure including an MQW active layer has a width equal to or smaller than twice the diffusion length of holes, and a p type semiconductor layer for injecting holes into the MQW active layer is formed on both sides of the stripe structure in contact with the sides of the stripe structure. Even when any MQW structure is used as the MQW active layer in order to reduce the temperature dependency of the threshold current, holes are injected into QW layers from the p type semiconductor layer which is in direct contact with all the QW layers in the MQW active layer, so that no local presence of holes in some QW layers occurs. Since the width of the stripe structure is equal to or smaller than twice the diffusion length of holes, the holes are uniformly injected in the direction parallel to the QW surface.

Abstract: In order to provide a semiconductor laser with high differential gain and low nonlinear gain parameter and is capable of modulation at high speed, a p-type impurity is doped in a multi-quantum well barrier layer 151 which forms an active layer 15, and a spacer layer 152 undoped with impurity and thickness in the range of 2 to 4 nm is inserted between the barrier layer and a well layer 153. By setting the thickness of the spacer layer 152 in the above-mentioned range, the wave function of the electron leaks to the barrier layer 151 beyond the spacer layer 152, whereas the wave function of the hole is localized in the well layer 153 and does not leak to the barrier layer 151. Therefore, electrons alone are scattered and their intraband relaxation time is reduced. Since the intra-band relaxation time of the hole does not change, the nonlinear gain parameter alone is reduced while maintaining the differential gain at a high value, and the maximum modulation frequency can be increased.

Abstract: An avalanche photodiode comprises an electron multiplication layer of superlattice including InAlAs mixed crystal layers which are lattice-matched with InP, and InGaAsP mixed crystal which are lattice-matched with InP and are provided with a bandgap energy of 1.0 eV to 1.2 eV at the room temperature. In the electron multiplication, a conduction band discontinuity is much greater than a valence band discontinuity to increase a ratio between ion densities .alpha. and .beta. for electron and hole, so that a noise level is decreased and a response characteristic is improved.