52 research outputs found
Electronic States and Transport Phenomena in Quantum Dot Systems
Electronic states and transport phenomena in semiconductor quantum dots are
studied theoretically. Taking account of the electron-electron Coulomb
interaction by the exact diagonalization method, the ground state and low-lying
excited states are calculated as functions of magnetic field. Using the
obtained many-body states, we discuss the temperature dependence of the
conductance peaks in the Coulomb oscillation. In the Coulomb blockade region,
elastic and inelastic cotunneling currents are evaluated under finite bias
voltages. The cotunneling conductance is markedly enhanced by the Kondo effect.
In coupled quantum dots, molecular orbitals and electronic correlation
influence the transport properties.Comment: Review paper of our work, to appear in Proc. Int. Symp. on Formation,
Physics and Device Application of Quantum Dot Structures (QDS 2000, Sapporo,
Japan), Jpn. J. Appl. Phys. [11 pages, 6 figures
Lasing and antibunching of optical phonons in semiconductor double quantum dots
We theoretically propose optical phonon lasing in a double quantum dot (DQD)
fabricated on a semiconductor substrate. No additional cavity or resonator is
required. An electron in the DQD is found to be coupled to only two
longitudinal optical phonon modes that act as a natural cavity. When the energy
level spacing in the DQD is tuned to the phonon energy, the electron transfer
is accompanied by the emission of the phonon modes. The resulting
non-equilibrium motion of electrons and phonons is analyzed by the rate
equation approach based on the Born-Markov-Secular approximation. We show that
the lasing occurs for pumping the DQD via electron tunneling at rate much
larger than the phonon decay rate, whereas a phonon antibunching is observed in
the opposite regime of slow tunneling. Both effects disappear by an effective
thermalization induced by the Franck-Condon effect in a DQD fabricated in a
suspended carbon nanotube with strong electron-phonon coupling.Comment: 27 pages, 8 figure
Critical current oscillation by magnetic field in semiconductor nanowire Josephson junction
We study theoretically the critical current in semiconductor nanowire
Josephson junction with strong spin-orbit interaction. The critical current
oscillates by an external magnetic field. We reveal that the oscillation of
critical current depends on the orientation of magnetic field in the presence
of spin-orbit interaction. We perform a numerical simulation for the nanowire
by using a tight-binding model. The Andreev levels are calculated as a function
of phase difference between two superconductors. The DC Josephson
current is evaluated from the Andreev levels in the case of short junctions.
The spin-orbit interaction induces the effective magnetic field. When the
external field is parallel with the effective one, the critical current
oscillates accompanying the - like transition. The period of
oscillation is longer as the angle between the external and effective fields is
larger
Quantum State Engineering using Single Nuclear Spin Qubit of Optically Manipulated Ytterbium Atom
A single Yb atom is loaded into a high-finesse optical cavity with a moving
lattice, and its nuclear spin state is manipulated using a nuclear magnetic
resonance technique. A highly reliable quantum state control with fidelity and
purity greater than 0.98 and 0.96, respectively, is confirmed by the full
quantum state tomography; a projective measurement with high speed (500us) and
high efficiency (0.98) is accomplished using the cavity QED technique. Because
a hyperfine coupling is induced only when the projective measurement is
operational, the long coherence times (T_1 = 0.49 s and T_2 = 0.10 s) are
maintained. Our technique can be applied for implementing a scalable one-way
quantum computation with a cluster state in an optical lattice.Comment: 4 figure
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