Shunichi Muto

Photon-spin qubit-conversion based on Overhauser shift of Zeeman energies in quantum dots

We propose a method to realize the conversion of photon qubit and spin qubit using the effective magnetic field created by the nuclear polarization known as Overhauser field. We discuss the preliminary experiment on an InAlAs∕AlGaAs self-assembled quantum dot and also discuss the effects of electron-hole interaction on the conversion.

Spin Depolarization via Tunneling Effects in Asymmetric Double Quantum Dot Structure

Time-resolved photoluminescence experiments have been performed to investigate exciton spin relaxation processes at low temperatures in InAlAs-InGaAs asymmetric double quantum dots embedded in AlGaAs layers. By decreasing the thickness of the AlGaAs barrier between the dots, the spin relaxation times decreased from 3 ns to 1 ns. The observed spin relaxation as a function of barrier thickness was reasonably described by two components: one was a constant and the other was exponentially depended on barrier thickness. The origin of the latter component is discussed in connection with spin-flip tunneling.

Photon Antibunching Observed from an InAlAs Single Quantum Dot

Single-photon emitters and detectors are key devices to realize secure communications with single-photon-based quantum cryptography and single-photon-based quantum computing. InAlAs quantum dots (QDs) cover the wavelength range with high quantum efficiencies of Si-based single-photon detectors. Clear photon antibunching was observed from an InAlAs single QD under weak excitations. To realize single-photon emitters on demand, complete population of the QD energy states before the photon emission events is necessary, but the measured antibunching properties were dependent substantially on the photo-excitation powers. The physical origin of this problem is discussed. The criterion to distinguish the real deviation from the photon antibunching condition and the artifact of the measurements is clarified. The capability of single-photon emissions on demand will be demonstrated with photon antibunching under pulsed operations.

Long spin relaxation in self-assembled InAlAs quantum dots observed by heterodyne four-wave mixing

Exciton spin relaxation in self-assembled InAlAs quantum dots was investigated by three-pulse four-wave mixing under resonant conditions. The concept of the spin grating holds well for quantum dots and the measurements combined with optical heterodyne detection at 10K demonstrates that the exciton spin relaxation lasts up to a few nanoseconds and the time constant is ∼5 times larger than the exciton recombination time on average.

Peculiar Field-Cycle Dependence of Magnetization Observed for Poly(phenyl)acetylene Prepared with a Rh Complex Catalyst

Magnetic field dependence of magnetization of poly(phenyl)acetylene which is known to show spin-glass-like behavior was studied at temperatures between 2 K and 300 K. Peculiar dependence of magnetization on the history of applied magnetic field, particularly the number of measurement cycles, is reported. The origin of these phenomena is discussed with respect to a possible mechanism of metastability.

Size, density, and shape of InAs quantum dots in closely stacked multilayers grown by the Stranski-Krastanow mode

Closely stacked multilayer structures of InAs islands with intermediate-layer thicknesses d of 3, 6, 10, and 20 nm were grown by the Stranski–Krastanow mode of molecular beam epitaxy and were observed using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The multilayers consisted of five InAs layers each of a thickness of 1.8 monolayers and four GaAs layers each of a thickness d. Columns of coherent islands were observed by cross-sectional TEM. Changes in the size and density of the islands with d, determined by AFM, could be explained in terms of (i) change in the vertical pairing probability of islands, (ii) detachment of In from the top of the island, and (iii) surface segregation of In. The observed AFM images of the islands were elliptical. Their major axis was in the [110] direction, and the length of the minor axis was 80% of that of the major axis.

Critical cluster size of InAs quantum dots formed by Stranski-Krastanow mode

The number of In atoms in a critical cluster, i*, in Stranski–Krastanow (S–K) mode of InAs islands was determined to be 1–10. The i* was determined using an activation energy EA of 2.0 eV determined from an Arrhenius plot of the saturated density of InAs islands formed on a GaAs (001) surface by S–K mode of molecular beam epitaxy [K. Shiramine et al., J. Cryst. Growth 242, 332 (2002)], and an activation energy of 1.6 eV for migration (surface diffusion) of In adatoms, inferred from other references. The common value ∼2.0 eV of EA in S–K mode was ascribed to the small i*.

Quantum gates using tunneling electron spins of a quantum-dot chain

Abstract Quantum gates using tunneling qubits: tunneling of electrons whose spins work as qubits is described by the Hamiltonian using both electron spins and pseudo-spins representing the photon-assisted tunneling. The phase factor during the controlled-NOT operation is also discussed. Preliminary data on a triple quantum dot system realized by closely stacking technique of molecular beam epitaxy are also shown.

Observation of population transfer to dark exciton states by using spin-diffracted four-wave mixing

We observed the exciton spin dynamical processes in GaAs quantum wells at low temperatures by three-pulse spin-diffracted four-wave mixing measurements. After investigating the merits and demerits of this method as compared with the pump–probe method, we discuss the excitation power dependence of exciton spin relaxation and the population transfer to the dark states.We observed the exciton spin dynamical processes in GaAs quantum wells at low temperatures by three-pulse spin-diffracted four-wave mixing measurements. After investigating the merits and demerits of this method as compared with the pump–probe method, we discuss the excitation power dependence of exciton spin relaxation and the population transfer to the dark states.

Quantum Gates Based on Electron Spins of Triple Quantum Dot

Quantum computation using electron spins in three-coupled dots of different sizes is proposed. By using photon-assisted tunnelling, Coulomb blockade and spin rotation of electrons, logic gates are realized by static and rotational magnetic fields and resonant optical pulses. The two-bit CNOT is numerically demonstrated by the time evolution of the density matrix. In addition, the Hamiltonian is described using pseudo-spin representation and the phase change during the gate operation is discussed.