25 research outputs found
Fractional plateaus in the Coulomb blockade of coupled quantum dots
Ground-state properties of a double-large-dot sample connected to a reservoir
via a single-mode point contact are investigated. When the interdot
transmission is perfect and the dots controlled by the same dimensionless gate
voltage, we find that for any finite backscattering from the barrier between
the lead and the left dot, the average dot charge exhibits a Coulomb-staircase
behavior with steps of size e/2 and the capacitance peak period is halved. The
interdot electrostatic coupling here is weak. For strong tunneling between the
left dot and the lead, we report a conspicuous intermediate phase in which the
fractional plateaus get substantially altered by an increasing slope.Comment: 6 pages, 4 figures, final versio
Reduction of the Three Dimensional Schrodinger Equation for Multilayered Films
In this paper, we present a method for reducing the three dimensional
Schrodinger equation to study confined metallic states, such as quantum well
states, in a multilayer film geometry. While discussing some approximations
that are employed when dealing with the three dimensionality of the problem, we
derive a one dimensional equation suitable for studying such states using an
envelope function approach. Some applications to the Cu/Co multilayer system
with regard to spin tunneling/rotations and angle resolved photoemission are
discussed.Comment: 14 pages, 1 figur
Measurements with a noninvasive detector and dephasing mechanism
We study dynamics of the measurement process in quantum dot systems, where a
particular state out of coherent superposition is observed. The ballistic
point-contact placed near one of the dots is taken as a noninvasive detector.
We demonstrate that the measurement process is fully described by the
Bloch-type equations applied to the whole system. These equations clearly
reproduce the collapse of the density-matrix into the statistical mixture in
the course of the measurement process. The corresponding dephasing width is
uniquely defined. We show that the continuous observation of one of the states
in a coherent superposition may accelerate decay from this state -- in
contradiction with rapidly repeated observations, which slow down the
transitions between quantum states (the quantum Zeno effect).Comment: The difference between continuous and rapidly repeated observations
is elaborated. To appear in Phys. Rev.
Adiabatic steering and determination of dephasing rates in double dot qubits
We propose a scheme to prepare arbitrary superpositions of quantum states in
double quantum--dots irradiated by coherent microwave pulses. Solving the
equations of motion for the dot density matrix, we find that dephasing rates
for such superpositions can be quantitatively infered from additional electron
current pulses that appear due to a controllable breakdown of coherent
population trapping in the dots.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Resonant multiple Andreev reflections in mesoscopic superconducting junctions
We investigate the properties of subharmonic gap structure (SGS) in
superconducting quantum contacts with normal-electron resonances. We find two
distinct new features of the SGS in resonant junctions which distinguish them
from non-resonant point contacts: (i) The odd-order structures on the
current-voltage characteristics of resonant junctions are strongly enhanced and
have pronounced peaks, while the even-order structures are suppressed, in the
case of a normal electron resonance being close to the Fermi level. (ii)
Tremendous current peaks develop at where indicates a
distance of the resonance to the Fermi level. These properties are determined
by the effect of narrowing of the resonance during multiple Andreev reflections
and by overlap of electron and hole resonances.Comment: 13 pages, 10 figure
Coulomb Blockade of Tunneling Through a Double Quantum Dot
We study the Coulomb blockade of tunneling through a double quantum dot. The
temperature dependence of the linear conductance is strongly affected by the
inter-dot tunneling. As the tunneling grows, a crossover from
temperature-independent peak conductance to a power-law suppression of
conductance at low temperatures is predicted. This suppression is a
manifestation of the Anderson orthogonality catastrophe associated with the
charge re-distribution between the dots, which accompanies the tunneling of an
electron into a dot. We find analytically the shapes of the Coulomb blockade
peaks in conductance as a function of gate voltage.Comment: 11 pages, revtex3.0 and multicols.sty, 4 figures uuencode
Finite-temperature Fermi-edge singularity in tunneling studied using random telegraph signals
We show that random telegraph signals in metal-oxide-silicon transistors at
millikelvin temperatures provide a powerful means of investigating tunneling
between a two-dimensional electron gas and a single defect state. The tunneling
rate shows a peak when the defect level lines up with the Fermi energy, in
excellent agreement with theory of the Fermi-edge singularity at finite
temperature. This theory also indicates that defect levels are the origin of
the dissipative two-state systems observed previously in similar devices.Comment: 5 pages, REVTEX, 3 postscript figures included with epsfi
Quantum gates by coupled asymmetric quantum dots and controlled-NOT-gate operation
A quantum computer based on an asymmetric coupled dot system has been
proposed and shown to operate as the controlled-NOT-gate. The basic idea is (1)
the electron is localized in one of the asymmetric coupled dots. (2)The
electron transfer takes place from one dot to the other when the energy-levels
of the coupled dots are set close. (3)The Coulomb interaction between the
coupled dots mutually affects the energy levels of the other coupled dots. The
decoherence time of the quantum computation and the measurement time are
estimated. The proposed system can be realized by developing the technology of
the single-electron memory using Si nanocrystals and the direct combination of
the quantum circuit and the conventional circuit is possible.Comment: LaTeX, 7 pages, 5 figures, revised content, to appear in Phys. Rev.
Adiabatic Transfer of Electrons in Coupled Quantum Dots
We investigate the influence of dissipation on one- and two-qubit rotations
in coupled semiconductor quantum dots, using a (pseudo) spin-boson model with
adiabatically varying parameters. For weak dissipation, we solve a master
equation, compare with direct perturbation theory, and derive an expression for
the `fidelity loss' during a simple operation that adiabatically moves an
electron between two coupled dots. We discuss the possibility of visualizing
coherent quantum oscillations in electron `pump' currents, combining quantum
adiabaticity and Coulomb blockade. In two-qubit spin-swap operations where the
role of intermediate charge states has been discussed recently, we apply our
formalism to calculate the fidelity loss due to charge tunneling between two
dots.Comment: 13 pages, 8 figures, to appear in Phys. Rev.
Charge Transport Through Open, Driven Two-Level Systems with Dissipation
We derive a Floquet-like formalism to calculate the stationary average
current through an AC driven double quantum dot in presence of dissipation. The
method allows us to take into account arbitrary coupling strengths both of a
time-dependent field and a bosonic environment. We numerical evaluate a
truncation scheme and compare with analytical, perturbative results such as the
Tien-Gordon formula.Comment: 14 pages, 6 figures. To appear in Phys. Rev.