2,987 research outputs found
Dynamics of Entanglement Transfer Through Multipartite Dissipative Systems
We study the dynamics of entanglement transfer in a system composed of two
initially correlated three-level atoms, each located in a cavity interacting
with its own reservoir. Instead of tracing out reservoir modes to describe the
dynamics using the master equation approach, we consider explicitly the
dynamics of the reservoirs. In this situation, we show that the entanglement is
completely transferred from atoms to reservoirs. Although the cavities mediate
this entanglement transfer, we show that under certain conditions, no
entanglement is found in cavities throughout the dynamics. Considering the
entanglement dynamics of interacting and non-interacting bipartite subsystems,
we found time windows where the entanglement can only flow through interacting
subsystems, depending on the system parameters.Comment: 8 pages, 11 figures, publishe in Physical Review
Qubit-oscillator systems in the ultrastrong-coupling regime and their potential for preparing nonclassical states
We consider a system composed of a two-level system (i.e. a qubit) and a
harmonic oscillator in the ultrastrong-coupling regime, where the coupling
strength is comparable to the qubit and oscillator energy scales. Special
emphasis is placed on the possibility of preparing nonclassical states in this
system. These nonclassical states include squeezed states, Schrodinger-cat
states and entangled states. We start by comparing the predictions of a number
of analytical methods that can be used to describe the system under different
assumptions, thus analyzing the properties of the system in various parameter
regimes. We then examine the ground state of the system and analyze its
nonclassical properties. We finally discuss some questions related to the
possible experimental observation of the nonclassical states and the effect of
decoherence.Comment: 19 pages (two-column), 11 figure
Decoherence in a system of many two--level atoms
I show that the decoherence in a system of degenerate two--level atoms
interacting with a bosonic heat bath is for any number of atoms governed by
a generalized Hamming distance (called ``decoherence metric'') between the
superposed quantum states, with a time--dependent metric tensor that is
specific for the heat bath.The decoherence metric allows for the complete
characterization of the decoherence of all possible superpositions of
many-particle states, and can be applied to minimize the over-all decoherence
in a quantum memory. For qubits which are far apart, the decoherence is given
by a function describing single-qubit decoherence times the standard Hamming
distance. I apply the theory to cold atoms in an optical lattice interacting
with black body radiation.Comment: replaced with published versio
Analytical solution to position dependent mass Schr\"odinger equation
Using a recently developed technique to solve Schr\"odinger equation for
constant mass, we studied the regime in which mass varies with position i.e
position dependent mass Schr\"odinger equation(PDMSE). We obtained an
analytical solution for the PDMSE and applied our approach to study a position
dependent mass particle scattered by a potential . We
also studied the structural analogy between PDMSE and two-level atomic system
interacting with a classical field.Comment: 5 pages, 4 figure
Heat Capacity Mapping Mission
The Tasman Front was delineated by airborne expendable bathythermograph survey; and an Heat Capacity Mapping Mission (HCMM) IR image on the same day shows the same principal features as determined from ground-truth. It is clear that digital enhancement of HCMM images is necessary to map ocean surface temperatures and when done, the Tasman Front and other oceanographic features can be mapped by this method, even through considerable scattered cloud cover
Repassivation of Pits in Aluminum Thin Films
The effect of metal film thickness on repassivation of pits in sputter-deposited Al thin films was investigated in chloride solutions. The repassivation potential and the critical current density, which is the pit current density below which pits stop growing, were determined for pits in Al thin films ranging from 100 Çș to 43 ÎŒm in thickness. The repassivation potential first decreased as thickness increased from 100 to 4350 Çș, and then increased as the film thickness increased further. This behavior was found to be a consequence of the pit current-density/potential relationship. The critical current density, a more informative parameter, decreased for increasing metal film thickness, even when the repassivation potential increased. The critical current density is the minimum current density needed to maintain the critical pit environment and prevent repassivation. The repassivation potential for a given metal film thickness is the potential at which the pit current density drops below the critical value. Mass-transport and ohmic resistance both increase as the metal film thickness increases, but the former enhances pit stability and the latter destabilizes pitting in this system. Pit repassivation, and thus stability, are strongly influenced by mass-transport considerations for pits in very thin pits, even though dissolution at low potentials is not under pure mass-transport control. Ohmic effects become increasingly important as the film thickness increases.J.R.S. was supported by the NASA-Langley Research Center La^2ST Program and the NSF under DMR-9357463
Signatures of the Unruh effect from electrons accelerated by ultra-strong laser fields
We calculate the radiation resulting from the Unruh effect for strongly
accelerated electrons and show that the photons are created in pairs whose
polarizations are maximally entangled. Apart from the photon statistics, this
quantum radiation can further be discriminated from the classical (Larmor)
radiation via the different spectral and angular distributions. The signatures
of the Unruh effect become significant if the external electromagnetic field
accelerating the electrons is not too far below the Schwinger limit and might
be observable with future facilities. Finally, the corrections due to the
birefringent nature of the QED vacuum at such ultra-high fields are discussed.
PACS: 04.62.+v, 12.20.Fv, 41.60.-m, 42.25.Lc.Comment: 4 pages, 1 figur
Production of a sterile species via active-sterile mixing: an exactly solvable model
The production of a sterile species via active-sterile mixing in a thermal
medium is studied in an exactly solvable model. The \emph{exact} time evolution
of the sterile distribution function is determined by the dispersion relations
and damping rates for the quasiparticle modes. These depend on
\wtg = \Gamma_{aa}/2\Delta E, with the interaction rate of the
active species in absence of mixing and the oscillation frequency in
the medium without damping. \wtg \ll1,\wtg \gg 1 describe the weak and strong
damping limits respectively. For \wtg\ll1, \Gamma_1 = \Gamma_{aa}\cos^2\tm ;
\Gamma_{2}=\Gamma_{aa}\sin^2\tm where \tm is the mixing angle in the medium
and the sterile distribution function \emph{does not} obey a simple rate
equation. For \wtg \gg 1, and \Gamma_2 = \Gamma_{aa}
\sin^22\tm/4\wtg^2, is the sterile production rate. In this regime sterile
production is suppressed and the oscillation frequency \emph{vanishes} at an
MSW resonance, with a breakdown of adiabaticity. These are consequences of
quantum Zeno suppression. For active neutrinos with standard model interactions
the strong damping limit is \emph{only} available near an MSW resonance
\emph{if} with the vacuum mixing angle.
The full set of quantum kinetic equations for sterile production for arbitrary
\wtg are obtained from the quantum master equation. Cosmological resonant
sterile neutrino production is quantum Zeno suppressed relieving potential
uncertainties associated with the QCD phase transition.Comment: To appear in Phys. Rev.
Dynamics of the excitations of a quantum dot in a microcavity
We study the dynamics of a quantum dot embedded in a three-dimensional
microcavity in the strong coupling regime in which the quantum dot exciton has
an energy close to the frequency of a confined cavity mode. Under the
continuous pumping of the system, confined electron and hole can recombine
either by spontaneous emission through a leaky mode or by stimulated emission
of a cavity mode that can escape from the cavity. The numerical integration of
a master equation including all these effects gives the dynamics of the density
matrix. By using the quantum regression theorem, we compute the first and
second order coherence functions required to calculate the photon statistics
and the spectrum of the emitted light. Our main result is the determination of
a range of parameters in which a state of cavity modes with poissonian or
sub-poissonian (non-classical) statistics can be built up within the
microcavity. Depending on the relative values of pumping and rate of stimulated
emission, either one or two peaks close to the excitation energy of the dot
and/or to the natural frequency of the cavity are observed in the emission
spectrum. The physics behind these results is discussed
Breakdown of the few-level approximation in collective systems
The validity of the few-level approximation in dipole-dipole interacting
collective systems is discussed. As example system, we study the archetype case
of two dipole-dipole interacting atoms, each modelled by two complete sets of
angular momentum multiplets. We establish the breakdown of the few-level
approximation by first proving the intuitive result that the dipole-dipole
induced energy shifts between collective two-atom states depend on the length
of the vector connecting the atoms, but not on its orientation, if complete and
degenerate multiplets are considered. A careful analysis of our findings
reveals that the simplification of the atomic level scheme by artificially
omitting Zeeman sublevels in a few-level approximation generally leads to
incorrect predictions. We find that this breakdown can be traced back to the
dipole-dipole coupling of transitions with orthogonal dipole moments. Our
interpretation enables us to identify special geometries in which partial
few-level approximations to two- or three-level systems are valid
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