Ryosaku Kaji

10.5 km Fiber-Optic Quantum Key Distribution at 1550 nm with a Key Rate of 45 kHz

High-speed single-photon detectors are crucial for efficiently performing quantum key distribution. We demonstrate fiber-optic quantum key distribution at 1550 nm using single-photon detectors operating at 10 MHz. Discharge pulse counting is adopted for single-photon detection in order to suppress after-pulse generation. Furthermore, by setting a dead time in single-photon detection, we discard after-pulses to reduce the number of bit errors in the quantum key distribution. Using discharge pulse counting and after-pulse discarding in the quantum key distribution, we can achieve a key rate of 45 kHz with a bit-error rate of 2%. The fiber length is 10.5 km.

Continuous Output Beam Steering in Vertical-Cavity Surface-Emitting Lasers with Two p-Type Electrodes by Controlling Injection Current Profile.

Using vertical-cavity surface-emitting lasers with two p-type electrodes, continuous output beam steering was achieved for the first time by controlling the injection current profile. The far-field peak position can be shifted linearly by varying the ratio between injection currents into the two p-type electrodes. The deflection angle measured from the surface normal direction ranges from -1.3° to +1.0°. Also, we investigated operating characteristics that are important for optical processing such as optical fuzzy inference.

Combination of beam propagation method and mode expansion propagation method for bidirectional optical beam propagation analysis

A combined method of the beam propagation method (BPM) and the mode expansion propagation method (MEPM) is proposed for the analysis of reflections of both transverse electric (TE) and transverse magnetic (TM) polarized waves in waveguiding structures containing longitudinal discontinuities. BPM based on the finite-element method (FEM) is applied to slowly varying regions and MEPM is applied only to regions including abrupt discontinuities. FEM is also utilized for evaluating the eigenmodes necessary for the MEPM analysis. To show the validity and usefulness of the present approach, numerical examples for a semiconductor laser facet with antireflection coating and an optical directional coupler are presented.

Equivalent network approach for multistep discontinuities in electron waveguides

An analysis method is described for multistep discontinuities in electron waveguides composed of semiconductor heterostructures. The method is based on a rigorous mode-matching procedure that takes into account the effects of external finite potential and spatially varying effective mass. Stress is placed on network representations to establish physical pictures of the quantum effects and to yield insight; in addition, a systematic microwave network approach is employed. The equivalent network representation is given for the waveguide modes propagating normally to the step discontinuity, and the mode-matching procedure is described for the boundary-value problem of the step discontinuity. Combining the above two procedures, the super-cascade matrix formulation is carried out, from which the reflection and transmission characteristics of multistep discontinuities in electron waveguides can be estimated. In order to show the validity and usefulness of this approach, examples are computed for constriction structures composed of three-layered electron waveguides. >

A Method of Discarding After-Pulses in Single-Photon Detection for Quantum Key Distribution

We evaluate the after-pulse probability of a gated-mode single-photon detector operating at 1550 nm. The experimental results are applied to the simulation of quantum key distribution to estimate the obtainable key rate, which decreases at higher gate frequencies because of a large number of after-pulse-related errors. In this paper, we present a postselection method of discarding after-pulses to improve the key rate.

A comprehensive analysis of multilayer channel waveguides

Using a simple approach based on the scalar finite element method, propagation characteristics of multilayer channel waveguides are calculated. The effective index, modal field, confinement factor, far-field intensity pattern, and radiation angle of the far-field pattern (full width at half maximum intensity) for multilayer channel waveguides formed with multiple quantum well (MQW) materials and with the MQW materials replaced by a single homogeneous material with the root mean square value of the refractive indexes are compared. Numerical results confirm that the root-mean-square-value approximation, which has been widely used for planar MQW (two-dimensional) waveguides, is useful also for MQW channel (three-dimensional) waveguides with a large number of layers. >

A complete set of normal modes in three-layered electron waveguides

Mathematical expressions are given for guided and radiation modes including nonpropagating parts in three-layered electron waveguides, in addition to gaining a clear understanding of the difference between electron and optical waveguides in terms of normal modes. According to the dispersion diagrams, it is shown that a complete set of normal modes for electron waveguides depends on the electrons total energy and that in the quantum-well-type waveguide the discrete modes and the continuous modes radiating to one side of the film are reduced to nonpropagating modes in a certain energy range, whereas a complete set of normal modes for optical waveguides always consists of three types of modes: guided modes, substrate radiation modes, and substrate-cover radiation modes. In addition, the discrete modes and the continuous modes radiating to one side of the film are not reduced to nonpropagating modes in the entire range of frequencies. Also, it is shown that the guided modes always exist in electron waveguides composed of arbitrarily different Al-concentration ratios in the film, substrate, and cover regions, and a new waveguiding quantum-step-type structure, which utilizes the total reflection at both the potential rise and drop is proposed, whereas in optical waveguides, the guided modes exist only in the structure in which the refractive index of the film is higher than those of the substrate and of the cover. >

Equivalent network approach for guided electron waves in quantum-well structures and its application to electron-wave directional couplers

By utilizing an analogy in electromagnetic wave propagation, an equivalent network procedure is described for guided electron waves in quantum-well structures. It relies heavily on the earlier achievements of the microwave network approach, and it involves the application of concepts and techniques in electromagnetic waves to corresponding categories of electron waves. To demonstrate the validity and usefulness of this approach, it is used to derive the properties of electron waveguides consisting of single or double quantum wells. Based on the results of the analysis of propagation characteristics of the guided electron waves in these structures, the coupling properties of the double-quantum-well electron waveguide are also examined under the coupled-mode procedure. >

Two-photon Interference at 1550 nm Using Two Periodically Poled Lithium Niobate Waveguides

We have observed two-photon interference at 1550 nm using two periodically poled lithium niobate waveguides. Two indium-gallium-arsenide gated-mode single-photon detectors were used for coincidence detection. The measured visibility was 95% (after subtraction of accidental coincidences) with a coincidence detection rate of 0.8 kHz. The ratio of single-photon to coincidence detection rate was 30.

Quantum Interference Phenomena in an Electron-Wave Directional Coupler with Finite Coupling Length.

A new type of quantum field effect directional coupler with finite coupling length is studied with the equivalent network approach. The coupler analyzed is composed of GaAs/Ga1-xAlxAs and has finite external potential barriers and longitudinal discontinuities. Although this structure is analogous to that in an optical counterpart, there is no radiation loss. Numerical results show that the effects of the longitudinal interference appear as a short-periodic behavior of the transmission probability and the effects of the beat of two normal modes appear as a long-periodic behavior of the transmission probability.