460 research outputs found
Bounds to unitary evolution
Upper and lower bounds are established for the survival probability
of a quantum state, in terms of the energy moments
. Introducing a cut-off in the energy generally
enables considerable improvement in these bounds and allows the method to be
used where the exact energy moments do not exist.Comment: 5 pages, 8 figure
Gauge transformation through an accelerated frame of reference
The Schr\"{o}dinger equation of a charged particle in a uniform electric
field can be specified in either a time-independent or a time-dependent gauge.
The wave-function solutions in these two gauges are related by a phase-factor
reflecting the gauge symmetry of the problem. In this article we show that the
effect of such a gauge transformation connecting the two wave-functions can be
mimicked by the effect of two successive extended Galilean transformations
connecting the two wave-function. An extended Galilean transformation connects
two reference frames out of which one is accelerating with respect to the
other.Comment: 7 Pages, Latex fil
A fast and robust approach to long-distance quantum communication with atomic ensembles
Quantum repeaters create long-distance entanglement between quantum systems
while overcoming difficulties such as the attenuation of single photons in a
fiber. Recently, an implementation of a repeater protocol based on single
qubits in atomic ensembles and linear optics has been proposed [Nature 414, 413
(2001)]. Motivated by rapid experimental progress towards implementing that
protocol, here we develop a more efficient scheme compatible with active
purification of arbitrary errors. Using similar resources as the earlier
protocol, our approach intrinsically purifies leakage out of the logical
subspace and all errors within the logical subspace, leading to greatly
improved performance in the presence of experimental inefficiencies. Our
analysis indicates that our scheme could generate approximately one pair per 3
minutes over 1280 km distance with fidelity (F>78%) sufficient to violate
Bell's inequality.Comment: 10 pages, 4 figures, 5 tables (Two appendixes are added to justify
two claims used in the maintext.
Role of the relative phase in the merging of two independent Bose-Einstein condensates
We study the merging of two independent Bose-Einstein condensates with
arbitrary initial phase difference, in the framework of a one-dimensional
time-dependent Gross-Pitaevskii model. The role of the initial phase difference
in the process is discussed, and various types of phase-sensitive excitations
are identified.Comment: 19 Pages, 7 figure
Delay Time in Quaternionic Quantum Mechanics
In looking for quaternionic violations of quantum mechanics, we discuss the
delay time for pure quaternionic potentials. The study shows in which energy
region it is possible to amplify the difference between quaternionic and
complex quantum mechanics.Comment: 9 pages, 5 figure
Classical and Quantum Fluctuation Theorems for Heat Exchange
The statistics of heat exchange between two classical or quantum finite
systems initially prepared at different temperatures are shown to obey a
fluctuation theorem.Comment: 4 pages, 1 included figure, to appear in Phys Rev Let
Disentanglement and Decoherence without dissipation at non-zero temperatures
Decoherence is well understood, in contrast to disentanglement. According to
common lore, irreversible coupling to a dissipative environment is the
mechanism for loss of entanglement. Here, we show that, on the contrary,
disentanglement can in fact occur at large enough temperatures even for
vanishingly small dissipation (as we have shown previously for decoherence).
However, whereas the effect of on decoherence increases exponentially with
time, the effect of on disentanglement is constant for all times,
reflecting a fundamental difference between the two phenomena. Also, the
possibility of disentanglement at a particular increases with decreasing
initial entanglement.Comment: 3 page
Smooth double barriers in quantum mechanics
Quantum mechanical tunneling across smooth double barrier potentials modeled
using Gaussian functions, is analyzed numerically and by using the WKB
approximation. The transmission probability, resonances as a function of
incident particle energy, and their dependence on the barrier parameters are
obtained for various cases. We also discuss the tunneling time, for which we
obtain generalizations of the known results for rectangular barriers.Comment: 23 pages, 8 figures, a slightly reduced version to appear in American
Journal of Physics, references correcte
Mapping of strongly correlated steady-state nonequilibrium to an effective equilibrium
By mapping steady-state nonequilibrium to an effective equilibrium, we
formulate nonequilibrium problems within an equilibrium picture where we can
apply existing equilibrium many-body techniques to steady-state electron
transport problems. We study the analytic properties of many-body scattering
states, reduce the boundary condition operator in a simple form and prove that
this mapping is equivalent to the correct linear-response theory. In an example
of infinite-U Anderson impurity model, we approximately solve for the
scattering state creation operators, based on which we derive the bias operator
Y to construct the nonequilibrium ensemble in the form of the Boltzmann factor
exp(-beta(H-Y)). The resulting Hamiltonian is solved by the non-crossing
approximation. We obtain the Kondo anomaly conductance at zero bias, inelastic
transport via the charge excitation on the quantum dot and significant
inelastic current background over a wide range of bias. Finally, we propose a
self-consistent algorithm of mapping general steady-state nonequilibrium.Comment: 15 pages, 9 figure
A scaling theory of quantum breakdown in solids
We propose a new scaling theory for general quantum breakdown phenomena. We
show, taking Landau-Zener type breakdown as a particular example, that the
breakdown phenomena can be viewed as a quantum phase transition for which the
scaling theory is developed. The application of this new scaling theory to
Zener type breakdown in Anderson insulators, and quantum quenching has been
discussed.Comment: 3 page
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