104 research outputs found
Quantum recoil effects in finite-time disentanglement of two distinguishable atoms
Starting from the requirement of distinguishability of two atoms by their
positions, it is shown that photon recoil has a strong influence on finite-time
disentanglement and in some cases prevents its appearance. At near-field inter
atomic distances well localized atoms, with maximally one atom being initially
excited, may suffer disentanglement at a single finite time or even at a series
of equidistant finite times, depending on their mean inter atomic distance and
their initial electronic preparation.Comment: 13 pages, 1 figure, submitted to Physical Review on august 2
Entanglement, recoherence and information flow in an accelerated detector - quantum field system: Implications for black hole information issue
We study an exactly solvable model where an uniformly accelerated detector is
linearly coupled to a massless scalar field initially in the Minkowski vacuum.
Using the exact correlation functions we show that as soon as the coupling is
switched on one can see information flowing from the detector to the field and
propagating with the radiation into null infinity. By expressing the reduced
density matrix of the detector in terms of the two-point functions, we
calculate the purity function in the detector and study the evolution of
quantum entanglement between the detector and the field. Only in the ultraweak
coupling regime could some degree of recoherence in the detector appear at late
times, but never in full restoration. We explicitly show that under the most
general conditions the detector never recovers its quantum coherence and the
entanglement between the detector and the field remains large at late times. To
the extent this model can be used as an analog to the system of a black hole
interacting with a quantum field, our result seems to suggest in the prevalent
non-Markovian regime, assuming unitarity for the combined system, that black
hole information is not lost but transferred to the quantum field degrees of
freedom. Our combined system will evolve into a highly entangled state between
a remnant of large area (in Bekenstein's black hole atom analog) without any
information of its initial state, and the quantum field, now imbued with
complex information content not-so-easily retrievable by a local observer.Comment: 16 pages, 12 figures; minor change
The Rotating-Wave Approximation: Consistency and Applicability from an Open Quantum System Analysis
We provide an in-depth and thorough treatment of the validity of the
rotating-wave approximation (RWA) in an open quantum system. We find that when
it is introduced after tracing out the environment, all timescales of the open
system are correctly reproduced, but the details of the quantum state may not
be. The RWA made before the trace is more problematic: it results in incorrect
values for environmentally-induced shifts to system frequencies, and the
resulting theory has no Markovian limit. We point out that great care must be
taken when coupling two open systems together under the RWA. Though the RWA can
yield a master equation of Lindblad form similar to what one might get in the
Markovian limit with white noise, the master equation for the two coupled
systems is not a simple combination of the master equation for each system, as
is possible in the Markovian limit. Such a naive combination yields inaccurate
dynamics. To obtain the correct master equation for the composite system a
proper consideration of the non-Markovian dynamics is required.Comment: 17 pages, 0 figures
Non-Markovian Dynamics and Entanglement of Two-level Atoms in a Common Field
We derive the stochastic equations and consider the non-Markovian dynamics of
a system of multiple two-level atoms in a common quantum field. We make only
the dipole approximation for the atoms and assume weak atom-field interactions.
From these assumptions we use a combination of non-secular open- and
closed-system perturbation theory, and we abstain from any additional
approximation schemes. These more accurate solutions are necessary to explore
several regimes: in particular, near-resonance dynamics and low-temperature
behavior. In detuned atomic systems, small variations in the system energy
levels engender timescales which, in general, cannot be safely ignored, as
would be the case in the rotating-wave approximation (RWA). More problematic
are the second-order solutions, which, as has been recently pointed out, cannot
be accurately calculated using any second-order perturbative master equation,
whether RWA, Born-Markov, Redfield, etc.. This latter problem, which applies to
all perturbative open-system master equations, has a profound effect upon
calculation of entanglement at low temperatures. We find that even at zero
temperature all initial states will undergo finite-time disentanglement
(sometimes termed "sudden death"), in contrast to previous work. We also use
our solution, without invoking RWA, to characterize the necessary conditions
for Dickie subradiance at finite temperature. We find that the subradiant
states fall into two categories at finite temperature: one that is temperature
independent and one that acquires temperature dependence. With the RWA there is
no temperature dependence in any case.Comment: 17 pages, 13 figures, v2 updated references, v3 clarified results and
corrected renormalization, v4 further clarified results and new Fig. 8-1
Moving Atom-Field Interactions: Quantum Motional Decoherence and Relaxation
The reduced dynamics of an atomic qubit coupled both to its own quantized
center of mass motion through the spatial mode functions of the electromagnetic
field, as well as the vacuum modes, is calculated in the influence functional
formalism. The formalism chosen can describe the entangled non-Markovian
evolution of the system with a full account of the coherent back-action of the
environment on the qubit. We find a slight increase in the decoherence due to
the quantized center of mass motion and give a condition on the mass and qubit
resonant frequency for which the effect is important. In optically resonant
alkali-metal atom systems, we find the effect to be negligibly small. The
framework presented here can nevertheless be used for general considerations of
the coherent evolution of qubits in moving atoms in an electromagnetic field.Comment: 9 pages, 1 figure, minor change
Relativistic Quantum Information in Detectors-Field Interactions
We review Unruh-DeWitt detectors and other models of detector-field
interaction in a relativistic quantum field theory setting as a tool for
extracting detector-detector, field-field and detector-field correlation
functions of interest in quantum information science, from entanglement
dynamics to quantum teleportation. We in particular highlight the contrast
between the results obtained from linear perturbation theory which can be
justified provided switching effects are properly accounted for, and the
nonperturbative effects from available analytic expressions which incorporate
the backreaction effects of the quantum field on the detector behaviour.Comment: 21 pages, 3 figures. Prepared for the special focus issue on RQ
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