10 research outputs found
On the analogy between the classical wave optics and the quantum wave phenomena
A striking correspondence between the effects of an auxiliary-mode-assisted
transfer of light power between two waveguides and an auxiliary-state-assisted
transfer of an electron between two quantum dots is highlighted by the example
of an exactly solvable model.Comment: To appear in Solid State Communication
Phonon-induced decoherence of the two-level quantum subsystem due to relaxation and dephasing processes
Phonon-related decoherence effects in a quantum double-well two-level
subsystem coupled to a solid are studied theoretically by the example of
deformation phonons. Expressions for the reduced density matrix at T=0 are
derived beyond the Markovian approximation by means of explicit solution of the
non-stationary Schrodinger equation for the interacting electron-phonon system
at the initial stage of its evolution. It is shown that as long as the
difference between the energies of the electron in the left and the right well
greatly exceeds the energy of the electron tunneling between the minima of the
double-well potential, decoherence is primarily due to dephasing processes.
This case corresponds to a strongly asymmetric potential and spatially
separated eigenfunctions localized in the vicinity of one or another potential
minimum. In the opposite case of the symmetric potential, the decoherence stems
from the relaxation processes, which may be either "resonant" (at relatively
long times) or "nonresonant" (at short times), giving rise to qualitatively
different temporal evolution of the electron state. The results obtained are
discussed in the context of quantum information processing based on the quantum
bits encoded in electron charge degrees of freedom.Comment: 20 pages, no figure
Clusters of interstitial carbon atoms near the graphite surface as a possible origin of dome-like features observed by STM
Formation of clusters of interstitial carbon atoms between the surface and
second atomic layers of graphite is demonstrated by means of molecular dynamics
simulations. It is shown that interstitial clusters result in the dome-like
surface features that may be associated with some of the hillocks observed by
STM on the irradiated graphite surface.Comment: 7 pages, 7 eps figures, submitted to Surface Scienc
Isotope Effect in the Presence of Magnetic and Nonmagnetic Impurities
The effect of impurities on the isotope coefficient is studied theoretically
in the framework of Abrikosov-Gor'kov approach generalized to account for both
potential and spin-flip scattering in anisotropic superconductors. An
expression for the isotope coefficient as a function of the critical
temperature is obtained for a superconductor with an arbitrary contribution of
spin-flip processes to the total scattering rate and an arbitrary degree of
anisotropy of the superconducting order parameter, ranging from isotropic
s-wave to d-wave and including anisotropic s-wave and mixed (s+d)-wave as
particular cases. It is found that both magnetic and nonmagnetic impurities
enhance the isotope coefficient, the enhancement due to magnetic impurities
being generally greater than that due to nonmagnetic impurities. From the
analysis of the experimental results on La-Sr-Cu-M-O high temperature
superconductor, it is concluded that the symmetry of the pairing state in this
system differs from a pure d-wave.Comment: 4 pages, 3 figure
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
Isotope effect in impure high T_c superconductors
The influence of various kinds of impurities on the isotope shift exponent
\alpha of high temperature superconductors has been studied. In these materials
the dopant impurities, like Sr in La_{2-x}Sr_xCuO_4, play different role and
usually occupy different sites than impurities like Zn, Fe, Ni {\it etc}
intentionally introduced into the system to study its superconducting
properties.
In the paper the in-plane and out-of-plane impurities present in layered
superconductors have been considered. They differently affect the
superconducting transition temperature T_c. The relative change of isotope
shift coefficient, however, is an universal function of T_c/T_{c0} (T_{c0}
reffers to impurity free system) {\it i.e.} for angle independent scattering
rate and density of states function it does not depend whether the change of
T_c is due to in- or out-of-plane impurities. The role of the anisotropic
impurity scattering in changing oxygen isotope coefficient of superconductors
with various symmetries of the order parameter is elucidated. The comparison of
the calculated and experimental dependence of \alpha/\alpha_0, where \alpha_0
is the clean system isotope shift coefficient, on T_c/T_{c0} is presented for a
number of cases studied.
The changes of \alpha calculated within stripe model of superconductivity in
copper oxides resonably well describe the data on
La_{1.8}Sr_{0.2}Cu_{1-x}(Fe,Ni)_xO_4, without any fitting parameters.Comment: 8 pages, 6 figures, Phys. Rev. B67 (2003) accepte
Quantitative Treatment of Decoherence
We outline different approaches to define and quantify decoherence. We argue
that a measure based on a properly defined norm of deviation of the density
matrix is appropriate for quantifying decoherence in quantum registers. For a
semiconductor double quantum dot qubit, evaluation of this measure is reviewed.
For a general class of decoherence processes, including those occurring in
semiconductor qubits, we argue that this measure is additive: It scales
linearly with the number of qubits.Comment: Revised version, 26 pages, in LaTeX, 3 EPS figure