7,522 research outputs found
Optically Driven Qubits in Artificial Molecules
We present novel models of quantum gates based on coupled quantum dots in
which a qubit is regarded as the superposition of ground states in each dot.
Coherent control on the qubit is performed by both a frequency and a
polarization of a monochromatic light pulse illuminated on the quantum dots. We
also show that a simple combination of two single qubit gates functions as a
controlled NOT gate resulting from an electron-electron interaction. To examine
the decoherence of quantum states, we discuss electronic relaxation contributed
mainly by LA phonon processes.Comment: 11 pages, 4 figures, submitted to Physical Review
Quantum Computer Using Coupled Quantum Dot Molecules
We propose a method for implementation of a quantum computer using artificial
molecules. The artificial molecule consists of two coupled quantum dots stacked
along z direction and one single electron. One-qubit and two-qubit gates are
constructed by one molecule and two coupled molecules, respectively.The ground
state and the first excited state of the molecule are used to encode the |0>
and |1> states of a qubit. The qubit is manipulated by a resonant
electromagnetic wave that is applied directly to the qubit through a microstrip
line. The coupling between two qubits in a quantum controlled NOT gate is
switched on (off) by floating (grounding) the metal film electrodes. We study
the operations of the gates by using a box-shaped quantum dot model and
numerically solving a time-dependent Schridinger equation, and demonstrate that
the quantum gates can perform the quantum computation. The operating speed of
the gates is about one operation per 4ps. The reading operation of the output
of the quantum computer can be performed by detecting the polarization of the
qubits.Comment: 18 pages, 7 figures, submitted to Jpn. J. Appl. Phys, please send
your e-mail to Nan-Jian Wu <[email protected]
Optically pumped intersublevel midinfrared lasers based on InAs-GaAs quantum dots
We propose an optically pumped laser based on intersublevel transitions in InAs-GaAs pyramidal self-Assembled quantum dots. A theoretical rate equations model of the laser is given in order to predict the dependence of the gain on pumping flux and temperature. The energy levels and wave functions were calculated using the 8-band k . p method where the symmetry of the pyramid was exploited to reduce the computational complexity. Carrier dynamics in the laser were modeled by taking both electron-longitudinal optical phonon and electron-longitudinal acoustic phonon interactions into account. The proposed laser emits at 14.6 μm with a gain of g ≈ 570 cm(-1) at the pumping flux Φ= 10(24) cm(-2) s(-1) and a temperature of T = 77 K. By varying the size of the investigated dots, laser emission in the spectral range 13-21 μm is predicted. In comparison to optically pumped lasers based on quantum wells, an advantage of the proposed type of laser is a lower pumping flux, due to the longer carrier lifetime in quantum dots, and also that both surface and edge emission are possible. The appropriate waveguide and cavity designs are presented, and by comparing the calculated values of the gain with the estimated losses, lasing is predicted even at room temperature for all the quantum dots investigated
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
Nanomechanical effects in an Andreev quantum dot
We consider a quantum dot with mechanical degrees of freedom which is coupled
to superconducting electrodes. A Josephson current is generated by applying a
phase difference. In the absence of coupling to vibrations, this setup was
previously proposed as a detector of magnetic flux and we wish here to address
the effect of the phonon coupling to this detection scheme. We compute the
charge on the quantum dot and determine its dependence on the phase difference
in the presence of phonon coupling and Coulomb interaction. This allows to
identify regions in parameter space with the highest charge to phase
sensitivity, which are relevant for flux detection. Further insight about the
interplay of such couplings and subsequent entanglement properties between
electron and phonon degrees of freedom are gained by computing the von Neuman
entropy.Comment: 9 pages, 7 figures; minor corretion
Transport through a vibrating quantum dot: Polaronic effects
We present a Green's function based treatment of the effects of
electron-phonon coupling on transport through a molecular quantum dot in the
quantum limit. Thereby we combine an incomplete variational Lang-Firsov
approach with a perturbative calculation of the electron-phonon self energy in
the framework of generalised Matsubara Green functions and a Landauer-type
transport description. Calculating the ground-state energy, the dot
single-particle spectral function and the linear conductance at finite carrier
density, we study the low-temperature transport properties of the vibrating
quantum dot sandwiched between metallic leads in the whole electron-phonon
coupling strength regime. We discuss corrections to the concept of an
anti-adiabatic dot polaron and show how a deformable quantum dot can act as a
molecular switch.Comment: 10 pages, 8 figures, Proceedings of "Progress in Nonequilibrium
Green's Function IV" Conference, Glasgow 200
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