805 research outputs found
Dynamical Casimir-Polder energy between an excited and a ground-state atom
We consider the Casimir-Polder interaction between two atoms, one in the
ground state and the other in its excited state. The interaction is
time-dependent for this system, because of the dynamical self-dressing and the
spontaneous decay of the excited atom. We calculate the dynamical
Casimir-Polder potential between the two atoms using an effective Hamiltonian
approach. The results obtained and their physical meaning are discussed and
compared with previous results based on a time-independent approach which uses
a non-normalizable dressed state for the excited atom.Comment: 11 page
Casimir-Polder potentials as entanglement probe
We have considered the interaction of a pair of spatially separated two-level
atoms with the electromagnetic field in its vacuum state and we have analyzed
the amount of entanglement induced between the two atoms by the non local field
fluctuations. This has allowed us to characterize the quantum nature of the non
local correlations of the electromagnetic field vacuum state as well as to link
the induced quantum entanglement with Casimir-Polder potentials.Comment: Published on Europhysics Letters 78 (2007) 3000
The limits of the rotating wave approximation in the electromagnetic field propagation in a cavity
We consider three two-level atoms inside a one-dimensional cavity,
interacting with the electromagnetic field in the rotating wave approximation
(RWA), commonly used in the atom-radiation interaction. One of the three atoms
is initially excited, and the other two are in their ground state. We
numerically calculate the propagation of the field spontaneously emitted by the
excited atom and scattered by the second atom, as well as the excitation
probability of the second and third atom. The results obtained are analyzed
from the point of view of relativistic causality in the atom-field interaction.
We show that, when the RWA is used, relativistic causality is obtained only if
the integrations over the field frequencies are extended to ; on the
contrary, noncausal tails remain even if the number of field modes is
increased. This clearly shows the limit of the RWA in dealing with subtle
problems such as relativistic causality in the atom-field interaction.Comment: 13 pages, 6 figure
Dynamics of correlations due to a phase noisy laser
We analyze the dynamics of various kinds of correlations present between two
initially entangled independent qubits, each one subject to a local phase noisy
laser. We give explicit expressions of the relevant quantifiers of correlations
for the general case of single-qubit unital evolution, which includes the case
of a phase noisy laser. Although the light field is treated as classical, we
find that this model can describe revivals of quantum correlations. Two
different dynamical regimes of decay of correlations occur, a Markovian one
(exponential decay) and a non-Markovian one (oscillatory decay with revivals)
depending on the values of system parameters. In particular, in the
non-Markovian regime, quantum correlations quantified by quantum discord show
an oscillatory decay faster than that of classical correlations. Moreover,
there are time regions where nonzero discord is present while entanglement is
zero.Comment: 7 pages, 3 figures, accepted for publication in Phys. Scripta,
special issue for CEWQO 2011 proceeding
Entanglement degradation in the solid state: interplay of adiabatic and quantum noise
We study entanglement degradation of two non-interacting qubits subject to
independent baths with broadband spectra typical of solid state nanodevices. We
obtain the analytic form of the concurrence in the presence of adiabatic noise
for classes of entangled initial states presently achievable in experiments. We
find that adiabatic (low frequency) noise affects entanglement reduction
analogously to pure dephasing noise. Due to quantum (high frequency) noise,
entanglement is totally lost in a state-dependent finite time. The possibility
to implement on-chip both local and entangling operations is briefly discussed.Comment: Replaced with published version. Minor change
Revival of quantum correlations without system-environment back-action
Revivals of quantum correlations have often been explained in terms of
back-action on quantum systems by their quantum environment(s). Here we
consider a system of two independently evolving qubits, each locally
interacting with a classical random external field. The environments of the
qubits are also independent, and there is no back-action on the qubits.
Nevertheless, entanglement, quantum discord and classical correlations between
the two qubits may revive in this model. We explain the revivals in terms of
correlations in a classical-quantum state of the environments and the qubits.
Although classical states cannot store entanglement on their own, they can play
a role in storing and reviving entanglement. It is important to know how the
absence of back-action, or modelling an environment as classical, affects the
kind of system time evolutions one is able to describe. We find a class of
global time evolutions where back-action is absent and for which there is no
loss of generality in modelling the environment as classical. Finally, we show
that the revivals can be connected with the increase of a parameter used to
quantify non-Markovianity of the single-qubit dynamics.Comment: 8 pages, 4 figures; this version to appear in Phys. Rev.
Fluctuations of the Casimir-Polder force between an atom and a conducting wall
We consider the quantum fluctuations of the Casimir-Polder force between a
neutral atom and a perfectly conducting wall in the ground state of the system.
In order to obtain the atom-wall force fluctuation we first define an operator
directly associated to the force experienced by the atom considered as a
polarizable body in an electromagnetic field, and we use a time-averaged force
operator in order to avoid ultraviolet divergences appearing in the fluctuation
of the force. This time-averaged force operator takes into account that any
measurement involves a finite time. We also calculate the Casimir-Polder force
fluctuation for an atom between two conducting walls. Experimental
observability of these Casimir-Polder force fluctuations is also discussed, as
well as the dependence of the relative force fluctuation on the duration of the
measurement.Comment: 6 page
Universality of Schmidt decomposition and particle identity
Schmidt decomposition is a widely employed tool of quantum theory which plays a key role for distinguishable particles in scenarios such as entanglement characterization, theory of measurement and state purification. Yet, its formulation for identical particles remains controversial, jeopardizing its application to analyze general many-body quantum systems. Here we prove, using a newly developed approach, a universal Schmidt decomposition which allows faithful quantification of the physical entanglement due to the identity of particles. We find that it is affected by single-particle measurement localization and state overlap. We study paradigmatic two-particle systems where identical qubits and qutrits are located in the same place or in separated places. For the case of two qutrits in the same place, we show that their entanglement behavior, whose physical interpretation is given, differs from that obtained before by different methods. Our results are generalizable to multiparticle systems and open the way for further developments in quantum information processing exploiting particle identity as a resource
Time-dependent Casimir-Polder forces and partially dressed states
A time-dependent Casimir-Polder force is shown to arise during the time
evolution of a partially dressed two-level atom. The partially dressed atom is
obtained by a rapid change of an atomic parameter such as its transition
frequency, due to the action of some external agent. The electromagnetic field
fluctuations around the atom, averaged over the solid angle for simplicity, are
calculated as a function of time, and it is shown that the interaction energy
with a second atom yields a dynamical Casimir-Polder potential between the two
atoms
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