1,492 research outputs found
Decoherence induced by a phase-damping reservoir
A phase damping reservoir composed by -bosons coupled to a system of
interest through a cross-Kerr interaction is proposed and its effects on
quantum superpo sitions are investigated. By means of analytical calculations
we show that: i-) the reservoir induces a Gaussian decay of quantum coherences,
and ii-) the inher ent incommensurate character of the spectral distribution
yields irreversibility . A state-independent decoherence time and a master
equation are both derived an alytically. These results, which have been
extended for the thermodynamic limit, show that nondissipative decoherence can
be suitably contemplated within the EI D approach. Finally, it is shown that
the same mechanism yielding decoherence ar e also responsible for inducing
dynamical disentanglement.Comment: 8 pages, 3 figure
Recoherence in the entanglement dynamics and classical orbits in the N-atom Jaynes-Cummings model
The rise in linear entropy of a subsystem in the N-atom Jaynes-Cummings model
is shown to be strongly influenced by the shape of the classical orbits of the
underlying classical phase space: we find a one-to-one correspondence between
maxima (minima) of the linear entropy and maxima (minima) of the expectation
value of atomic excitation J_z. Since the expectation value of this operator
can be viewed as related to the orbit radius in the classical phase space
projection associated to the atomic degree of freedom, the proximity of the
quantum wave packet to this atomic phase space borderline produces a maximum
rate of entanglement. The consequence of this fact for initial conditions
centered at periodic orbits in regular regions is a clear periodic recoherence.
For chaotic situations the same phenomenon (proximity of the atomic phase space
borderline) is in general responsible for oscillations in the entanglement
properties.Comment: 15 pages (text), 6 figures; to be published in Physical Review
Vibration-enhanced quantum transport
In this paper, we study the role of collective vibrational motion in the
phenomenon of electronic energy transfer (EET) along a chain of coupled
electronic dipoles with varying excitation frequencies. Previous experimental
work on EET in conjugated polymer samples has suggested that the common
structural framework of the macromolecule introduces correlations in the energy
gap fluctuations which cause coherent EET. Inspired by these results, we
present a simple model in which a driven nanomechanical resonator mode
modulates the excitation energy of coupled quantum dots and find that this can
indeed lead to an enhancement in the transport of excitations across the
quantum network. Disorder of the on-site energies is a key requirement for this
to occur. We also show that in this solid state system phase information is
partially retained in the transfer process, as experimentally demonstrated in
conjugated polymer samples. Consequently, this mechanism of vibration enhanced
quantum transport might find applications in quantum information transfer of
qubit states or entanglement.Comment: 7 pages, 6 figures, new material, included references, final
published versio
Methanol formation through reaction of low energy ions with an amorphous solid water surface at low temperature
We have performed experimental investigations of methanol formation via the
reactions of low energy ions with an amorphous solid water (ASW)
surface around 10 K. A newly developed experimental apparatus enabled
irradiation of the ASW surface by several eV ions and detection of trace
amounts of reaction products on the surface. It was found that methanol
molecules were produced by low-energy irradiation of the ASW
surface and that hydroxy groups in produced methanol originated from water
molecules in ASW, as predicted in a previous theoretical study. Little
temperature dependence of observed methanol intensity is apparent in the
temperature range 12 - 60 K. Ab-initio molecular dynamics simulations under
constant temperature conditions of 10 K suggested that this reaction
spontaneously produced a methanol molecule and an ion, regardless
of the contact point of on the ASW surface. We have performed
simulation with an astrochemical model under molecular-cloud conditions, where
the reaction between and ice, leading to methanol
formation, was included. We found that the impact of the reaction on methanol
abundance was limited only at the edge of the molecular cloud (< 1 mag) because
of the low abundance of in the gas phase, whereas the reaction
between the abundant molecular ion and ice, which has not
yet been confirmed experimentally, can considerably affect the abundance of a
complex organic molecule. This work sheds light on a new type of reaction
between molecular ions and ice surfaces that should be included in
astrochemical models.Comment: 5 figures and Appendix, accepted to Ap
Entanglement production in a chaotic quantum dot
It has recently been shown theoretically that elastic scattering in the Fermi
sea produces quantum mechanically entangled states. The mechanism is similar to
entanglement by a beam splitter in optics, but a key distinction is that the
electronic mechanism works even if the source is in local thermal equilibrium.
An experimental realization was proposed using tunneling between two edge
channels in a strong magnetic field. Here we investigate a low-magnetic field
alternative, using multiple scattering in a quantum dot. Two pairs of
single-channel point contacts define a pair of qubits. If the scattering is
chaotic, a universal statistical description of the entanglement production
(quantified by the concurrence) is possible. The mean concurrence turns out to
be almost independent on whether time-reversal symmetry is broken or not. We
show how the concurrence can be extracted from a Bell inequality using
low-frequency noise measurements, without requiring the tunneling assumption of
earlier work.Comment: 12 pages, 2 figures, Kluwer style file include
Optical properties of correlated materials -- Generalized Peierls approach and its application to VO2
The aim of the present paper is to present a versatile scheme for the
computation of optical properties of solids, with particular emphasis on
realistic many-body calculations for correlated materials. Geared at the use
with localized basis sets, we extend the commonly known lattice "Peierls
substitution" approach to the case of multi-atomic unit cells. We show in how
far this generalization can be deployed as an approximation to the full Fermi
velocity matrix elements that enter the continuum description of the response
of a solid to incident light. We further devise an upfolding scheme to
incorporate optical transitions, that involve high energy orbitals that had
been downfolded in the underlying many-body calculation of the electronic
structure. As an application of the scheme, we present results on a material of
longstanding interest, vanadium dioxide, VO2. Using dynamical mean-field data
of both, the metallic and the insulating phase, we calculate the corresponding
optical conductivities, elucidate optical transitions and find good agreement
with experimental results.Comment: 15 pages, 6 figure
The magnetic field of IRAS 16293-2422 as traced by shock-induced H2O masers
Shock-induced H2O masers are important magnetic field tracers at very high
density gas. Water masers are found in both high- and low-mass star-forming
regions, acting as a powerful tool to compare magnetic field morphologies in
both mass regimes. In this paper, we show one of the first magnetic field
determinations in the low-mass protostellar core IRAS 16293-2422 at volume
densities as high as 10^(8-10) cm^-3. Our goal is to discern if the collapsing
regime of this source is controlled by magnetic fields or other factors like
turbulence. We used the Very Large Array (VLA) to carry out
spectro-polarimetric observations in the 22 GHz Zeeman emission of H2O masers.
From the Stokes V line profile, we can estimate the magnetic field strength in
the dense regions around the protostar. A blend of at least three maser
features can be inferred from our relatively high spatial resolution data set
(~ 0.1"), which is reproduced in a clear non-Gaussian line profile. The
emission is very stable in polarization fraction and position angle across the
channels. The maser spots are aligned with some components of the complex
outflow configuration of IRAS 16293-2422, and they are excited in zones of
compressed gas produced by shocks. The post-shock particle density is in the
range of 1-3 x 10^9 cm^-3, consistent with typical water masers pumping
densities. Zeeman emission is produced by a very strong line-of-sight magnetic
field (B ~ 113 mG). The magnetic field pressure derived from our data is
comparable to the ram pressure of the outflow dynamics. This indicates that the
magnetic field is energetically important in the dynamical evolution of IRAS
16293-2422.Comment: 7 pages, 6 figures, accepted for publication in A&
Investigating the basis of substrate recognition in the pC221 relaxosome
The nicking of the origin of transfer (oriT) is an essential initial step in the conjugative mobilization of plasmid DNA. In the case of staphylococcal plasmid pC221, nicking by the plasmid-specific MobA relaxase is facilitated by the DNA-binding accessory protein MobC; however, the role of MobC in this process is currently unknown. In this study, the site of MobC binding was determined by DNase I footprinting. MobC interacts with oriT DNA at two directly repeated 9 bp sequences, mcb1 and mcb2, upstream of the oriT nic site, and additionally at a third, degenerate repeat within the mobC gene, mcb3. The binding activity of the conserved sequences was confirmed indirectly by competitive electrophoretic mobility shift assays and directly by Surface Plasmon Resonance studies. Mutation at mcb2 abolished detectable nicking activity, suggesting that binding of this site by MobC is a prerequisite for nicking by MobA. Sequential site-directed mutagenesis of each binding site in pC221 has demonstrated that all three are required for mobilization. The MobA relaxase, while unable to bind to oriT DNA alone, was found to associate with a MobC–oriT complex and alter the MobC binding profile in a region between mcb2 and the nic site. Mutagenesis of oriT in this region defines a 7 bp sequence, sra, which was essential for nicking by MobA. Exchange of four divergent bases between the sra of pC221 and the related plasmid pC223 was sufficient to swap their substrate identity in a MobA-specific nicking assay. Based on these observations we propose a model of layered specificity in the assembly of pC221-family relaxosomes, whereby a common MobC:mcb complex presents the oriT substrate, which is then nicked only by the cognate MobA
Entangling power of quantized chaotic systems
We study the quantum entanglement caused by unitary operators that have
classical limits that can range from the near integrable to the completely
chaotic. Entanglement in the eigenstates and time-evolving arbitrary states is
studied through the von Neumann entropy of the reduced density matrices. We
demonstrate that classical chaos can lead to substantially enhanced
entanglement. Conversely, entanglement provides a novel and useful
characterization of quantum states in higher dimensional chaotic or complex
systems. Information about eigenfunction localization is stored in a graded
manner in the Schmidt vectors, and the principal Schmidt vectors can be scarred
by the projections of classical periodic orbits onto subspaces. The eigenvalues
of the reduced density matrices are sensitive to the degree of wavefunction
localization, and are roughly exponentially arranged. We also point out the
analogy with decoherence, as reduced density matrices corresponding to
subsystems of fully chaotic systems are diagonally dominant.Comment: 21 pages including 9 figs. (revtex
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