Toshio Ohshima

Tunneling Hot Electron Transistor Using GaAs/AlGaAs Heterojunctions

The first tunneling hot electron transistor (HET) using semiconductor heterojunctions has been achieved. This device uses GaAs/AlGaAs heterojunctions grown by molecular beam epitaxy and sophisticated process technology. Transfer efficiency for hot electrons through 100 nm thick n-GaAs was measured as 0.28.

All-optical electron spin quantum computer with ancilla bits for operations in each coupled-dot cell

A cellular quantum computer with a spin qubit and ancilla bits in each cell is proposed. The whole circuit works only with the help of external optical pulse sequences. In the operation, some of the ancilla bits are activated, and autonomous single-and two-qubit operations are made. In the sleep mode of a cell, the decoherence of the qubit is negligibly small. Since only two cells at most are active at once, the coherence can be maintained for a sufficiently long time for practical purposes. A device structure using a coupled-quantum-dot array with possible operation and measurement schemes is also proposed.

Quantum gates using spin states of triple quantum dot

Abstract We present a new model of quantum gates using electron spins of triplely coupled quantum dots. By using the energy selectivity of both photon-assisted tunneling and spin rotation of electrons, logic gates are realized by static and rotational magnetic field and resonant optical pulses. We used what we call a “tunneling qubit”, which realized the switching of interaction between quantum bits. Possibility of the extension to many-qubit-system is also discussed.

Quasi-One-Dimensional Channel GaAs/AlGaAs Modulation Doped FET Using a Corrugated Gate Structure

This paper reports on the fabrication and characterization of a modulation-doped GaAs/AlGaAs FET with a corrugated gate structure in which the electron gas confinement has been changed from two-dimensional to quasi-one-dimensional with a negatively biased gate voltage. We observed enhanced field effect mobility and transconductance oscillations in a strictly-confined one-dimensional channel regime.

Electron transport properties in GaAs/AlGaAs quasi one-dimensional fets

Abstract A GaAs/AlGaAs modulation-doped FET with an n+-GaAs grating cap layer has been fabricated and characterized. By applying a negative gate voltage, the two-dimensional channel changes into an array of quasi one-dimensional channels. The quasi one-dimensionality of the channels is confirmed with I-V, C-V, and magnetoconductance measurements. Mobility in the channels is found to be enhanced and oscillatory which we think is the result of one-dimensional size-quantization.

Quantum repeaters using orbitals in quantum dot molecules

We propose quantum repeaters using quantum dot molecules, in which matter-photon entanglement is generated by Raman scatterings in lambda systems composed of various coherent exciton levels formed in the ensembles of asymmetric coupled quantum dots. In our scheme, the wavelength of Stokes and anti-Stokes photons can be chosen to fulfill the requirements of optical fiber communication. Further, the relative superposition phase in the entangled states can be stabilized by the active feedback to the gate voltage in quantum dot system. These characteristics are favorable for implementing our scheme in practice.

Molecular states observed in a single pair of strongly coupled self-assembled InAs quantum dots

Molecular states in a single pair of strongly coupled self-assembled InAs quantum dots are investigated using a sub-micron sized single-electron transistor containing just a few pairs of coupled InAs dots embedded in a GaAs matrix. We observe a series of well-formed Coulomb diamonds with charging energy of less than 5 meV, which are much smaller than those reported previously. This is because electrons are occupied in molecular states, which are spread over both dots and occupy a large volume. In the measurement of ground and excited state single-electron transport spectra with a magnetic field, we find that the electrons are sequentially trapped in symmetric and anti-symmetric states. This result is well explained by numerical calculation using an exact diagonalization method.

Effect of geometrical irregularity on transport in wires

Abstract Geometrical irregularity in ultranarrow wires longer than the phase coherence length produces positive magnetoresistances and smears the magnetic depopulation oscillation occurring as subband edges pass through the Fermi level. Even a slight boundary roughness in the ballistic regime adversely affects the accuracy of conductance quantization, and conductance varies with Fermi energy. The statistical character of the roughness cannot be neglected in either case.