2,446 research outputs found
All (qubit) decoherences: Complete characterization and physical implementation
We investigate decoherence channels that are modelled as a sequence of
collisions of a quantum system (e.g., a qubit) with particles (e.g., qubits) of
the environment. We show that collisions induce decoherence when a bi-partite
interaction between the system qubit and an environment (reservoir) qubit is
described by the controlled-U unitary transformation (gate). We characterize
decoherence channels and in the case of a qubit we specify the most general
decoherence channel and derive a corresponding master equation. Finally, we
analyze entanglement that is generated during the process of decoherence
between the system and its environment.Comment: 10 pages, 3 figure
Generalized Kinetic Theory of Electrons and Phonons
A Generalized Kinetic Theory was proposed in order to have the possibility to
treat particles which obey a very general statistics. By adopting the same
approach, we generalize here the Kinetic Theory of electrons and phonons.
Equilibrium solutions and their stability are investigated.Comment: Proceedings of the International School and Workshop on Nonextensive
Thermodynamics and Physical Applications, NEXT 2001, 23-30 May 2001, Cagliari
(Italy) (To appear in Physica A
Space fragment in studies of the Earth
The fragment apparatus, mounted on board the artificial earth satellite Meteor, was created for the operational study of the natural resources of the Earth in the optical range of electromagnetic waves. The orbit of the satellite at an altitude of about 650 km makes it possible to observe the same sectors of the Earth's surface at the same time of day with a periodicity of 15 days
Concurrence vs. purity: Influence of local channels on Bell states of two qubits
We analyze how a maximally entangled state of two-qubits (e.g., the singlet
) is affected by action of local channels described by completely
positive maps \cE . We analyze the concurrence and the purity of states
\varrho_\cE=\cE\otimes\cI[\psi_s].Using the concurrence-{\it vs}-purity phase
diagram we characterize local channels \cE by their action on the singlet
state . We specify a region of the concurrence-{\it vs.}-purity diagram
that is achievable from the singlet state via the action of unital channels. We
show that even most general (including non-unital) local channels acting just
on a single qubit of the original singlet state cannot generate the maximally
entangled mixed states (MEMS). We study in detail various time evolutions of
the original singlet state induced by local Markovian semigroups. We show that
the decoherence process is represented in the concurrence-{\it vs.}-purity
diagram by a line that forms the lower bound of the achievable region for
unital maps. On the other hand, the depolarization process is represented by a
line that forms the upper bound of the region of maps induced by unital maps.Comment: 9 pages, 6 figure
Implementation of quantum maps by programmable quantum processors
A quantum processor is a device with a data register and a program register.
The input to the program register determines the operation, which is a
completely positive linear map, that will be performed on the state in the data
register. We develop a mathematical description for these devices, and apply it
to several different examples of processors. The problem of finding a processor
that will be able to implement a given set of mappings is also examined, and it
is shown that while it is possible to design a finite processor to realize the
phase-damping channel, it is not possible to do so for the amplitude-damping
channel.Comment: 10 revtex pages, no figure
Lattice thermal conductivity of disordered binary alloys : a formulation
We present here a formulation for the calculation of the configuration
averaged lattice thermal conductivity in random alloys. Our formulation is
based on the augmented-space theorem, introduced by one of us, combined with a
generalized diagrammatic technique. The diagrammatic approach simplifies the
problem of including effects of disorder corrections to a great extent. The
approach allows us to obtain an expression for the effective heat current in
case of disordered alloys, which in turn is used in a Kubo-Greenwood type
formula for the thermal conductivity. We show that disorder scattering
renormalizes the phonon propagators as well as the heat currents. The
corrections to the current terms have been shown to be related to the
self-energy of the propagators. We also study the effect of vertex corrections
in a simplified ladder diagram approximation. A mode dependent diffusivity
and then a total thermal diffusivity averaged over different modes
are defined. Schemes for implementing the said formalism are discussed. A few
initial numerical results on the frequency and temperature dependence of
lattice thermal conductivity are presented for NiPd alloy and are also compared
with experiment. We also display numerical results on the frequency dependence
of thermal diffusivity averaged over modes.Comment: 16 pages, 17 figure
The dominant spin relaxation mechanism in compound organic semiconductors
Despite the recent interest in "organic spintronics", the dominant spin
relaxation mechanism of electrons or holes in an organic compound semiconductor
has not been conclusively identified. There have been sporadic suggestions that
it might be hyperfine interaction caused by background nuclear spins, but no
confirmatory evidence to support this has ever been presented. Here, we report
the electric-field dependence of the spin diffusion length in an organic
spin-valve structure consisting of an Alq3 spacer layer, and argue that this
data, as well as available data on the temperature dependence of this length,
contradict the notion that hyperfine interactions relax spin. Instead, they
suggest that the Elliott-Yafet mechanism, arising from spin-orbit interaction,
is more likely the dominant spin relaxing mechanism.Comment: Accepted for publication in Physical Review
Deviations from Matthiessen's Rule for and
We have measured the change in the resistivity of thin films of and upon introducing point defects by electron
irradiation at low temperatures, and we find significant deviations from
Matthiessen's rule. For a fixed irradiation dose, the induced change in
resistivity {\it decreases} with increasing temperature. Moreover, for a fixed
temperature, the increase in resistivity with irradiation is found to be {\it
sublinear}. We suggest that the observed behavior is due to the marked
anisotropic scattering of the electrons together with their relatively short
mean free path (both characteristic of many metallic oxides including cuprates)
which amplify effects related to the Pippard ineffectiveness condition
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