2,759 research outputs found
The General Correlation Function in the Schwinger Model on a Torus
In the framework of the Euclidean path integral approach we derive the exact
formula for the general N-point chiral densities correlator in the Schwinger
model on a torusComment: 17 pages, misprints corrected, references adde
Predictability sieve, pointer states, and the classicality of quantum trajectories
We study various measures of classicality of the states of open quantum
systems subject to decoherence. Classical states are expected to be stable in
spite of decoherence, and are thought to leave conspicuous imprints on the
environment. Here these expected features of environment-induced superselection
(einselection) are quantified using four different criteria: predictability
sieve (which selects states that produce least entropy), purification time
(which looks for states that are the easiest to find out from the imprint they
leave on the environment), efficiency threshold (which finds states that can be
deduced from measurements on a smallest fraction of the environment), and
purity loss time (that looks for states for which it takes the longest to lose
a set fraction of their initial purity). We show that when pointer states --
the most predictable states of an open quantum system selected by the
predictability sieve -- are well defined, all four criteria agree that they are
indeed the most classical states. We illustrate this with two examples: an
underdamped harmonic oscillator, for which coherent states are unanimously
chosen by all criteria, and a free particle undergoing quantum Brownian motion,
for which most criteria select almost identical Gaussian states (although, in
this case, predictability sieve does not select well defined pointer states.)Comment: 10 pages, 13 figure
Finite-Time Disentanglement via Spontaneous Emission
We show that under the influence of pure vacuum noise two entangled qubits
become completely disentangled in a finite time, and in a specific example we
find the time to be given by times the
usual spontaneous lifetime.Comment: revtex, 4 pages, 2 figure
Non-Markovianity, Loschmidt echo and criticality: a unified picture
A simple relationship between recently proposed measures of non-Markovianity
and the Loschmidt echo is established, holding for a two-level system (qubit)
undergoing pure dephasing due to a coupling with a many-body environment. We
show that the Loschmidt echo is intimately related to the information flowing
out from and occasionally back into the system. This, in turn, determines the
non-Markovianity of the reduced dynamics. In particular, we consider a central
qubit coupled to a quantum Ising ring in the transverse field. In this context,
the information flux between system and environment is strongly affected by the
environmental criticality; the qubit dynamics is shown to be Markovian exactly
and only at the critical point. Therefore non-Markovianity is an indicator of
criticality in the model considered here
Sudden Death of Entanglement of Two Jaynes-Cummings Atoms
We investigate entanglement dynamics of two isolated atoms, each in its own
Jaynes-Cummings cavity. We show analytically that initial entanglement has an
interesting subsequent time evolution, including the so-called sudden death
effect.Comment: 3 pages, 3 figure
Sum Rules for the Dirac Spectrum of the Schwinger Model
The inverse eigenvalues of the Dirac operator in the Schwinger model satisfy
the same Leutwyler-Smilga sum rules as in the case of QCD with one flavor. In
this paper we give a microscopic derivation of these sum rules in the sector of
arbitrary topological charge. We show that the sum rules can be obtained from
the clustering property of the scalar correlation functions. This argument also
holds for other theories with a mass gap and broken chiral symmetry such as QCD
with one flavor. For QCD with several flavors a modified clustering property is
derived from the low energy chiral Lagrangian. We also obtain sum rules for a
fixed external gauge field and show their relation with the bosonized version
of the Schwinger model. In the sector of topological charge the sum rules
are consistent with a shift of the Dirac spectrum away from zero by
average level spacings. This shift is also required to obtain a nonzero chiral
condensate in the massless limit. Finally, we discuss the Dirac spectrum for a
closely related two-dimensional theory for which the gauge field action is
quadratic in the the gauge fields. This theory of so called random Dirac
fermions has been discussed extensively in the context of the quantum Hall
effect and d-wave super-conductors.Comment: 41 pages, Late
Interaction-induced decoherence in non-Hermitian quantum walks of ultracold Bosons
We study the influence of particle interaction on a quantum walk on a
bipartite one-dimensional lattice with decay from every second site. The
corresponding non-interacting (linear) system has been shown to have a
topological transition described by the average displacement before decay. Here
we use this topological quantity to distinguish coherent quantum dynamics from
incoherent classical dynamics caused by a breaking of the translational
symmetry. We furthermore analyze the behavior by means of a rate equation
providing a quantitative description of the incoherent nonlinear dynamics.Comment: Revised and extended version, 5 pages, 5 figure
Decoherence and the Nature of System-Environment Correlations
We investigate system-environment correlations based on the exact dynamics of
a qubit and its environment in the framework of pure decoherence (phase
damping). We focus on the relation of decoherence and the build-up of
system-reservoir entanglement for an arbitrary (possibly mixed) initial qubit
state. In the commonly employed regime where the qubit dynamics can be
described by a Markov master equation of Lindblad type, we find that for almost
all qubit initial states inside the Bloch sphere, decoherence is complete while
the total state is still separable - no entanglement is involved. In general,
both "separable" and "entangling" decoherence occurs, depending on temperature
and initial qubit state. Moreover, we find situations where classical and
quantum correlations periodically alternate as a function of time in the regime
of low temperatures
Emergence of pointer states in a non-perturbative environment
We show that the pointer basis distinguished by collisional decoherence
consists of exponentially localized, solitonic wave packets. Based on the
orthogonal unraveling of the quantum master equation, we characterize their
formation and dynamics, and we demonstrate that the statistical weights arising
from an initial superposition state are given by the required projection. Since
the spatial width of the pointer states can be obtained by accounting for the
gas environment in a microscopically realistic fashion, one may thus calculate
the coherence length of a strongly interacting gas.Comment: 8 pages, 1 figure; corresponds to published versio
- …