3,297 research outputs found
Dynamics of quantum correlations in colored environments
We address the dynamics of entanglement and quantum discord for two non
interacting qubits initially prepared in a maximally entangled state and then
subjected to a classical colored noise, i.e. coupled with an external
environment characterized by a noise spectrum of the form . More
specifically, we address systems where the Gaussian approximation fails, i.e.
the sole knowledge of the spectrum is not enough to determine the dynamics of
quantum correlations. We thus investigate the dynamics for two different
configurations of the environment: in the first case the noise spectrum is due
to the interaction of each qubit with a single bistable fluctuator with an
undetermined switching rate, whereas in the second case we consider a
collection of classical fluctuators with fixed switching rates. In both cases
we found analytical expressions for the time dependence of entanglement and
quantum discord, which may be also extended to a collection of flcutuators with
random switching rates. The environmental noise is introduced by means of
stochastic time-dependent terms in the Hamiltonian and this allows us to
describe the effects of both separate and common environments. We show that the
non-Gaussian character of the noise may lead to significant effects, e.g.
environments with the same power spectrum, but different configurations, give
raise to opposite behavior for the quantum correlations. In particular,
depending on the characteristics of the environmental noise considered, both
entanglement and discord display either a monotonic decay or the phenomena of
sudden death and revivals. Our results show that the microscopic structure of
environment, besides its noise spectrum, is relevant for the dynamics of
quantum correlations, and may be a valid starting point for the engineering of
non-Gaussian colored environments.Comment: 8 pages, 3 figure
Loopholes in Bell Inequality Tests of Local Realism
Bell inequalities are intended to show that local realist theories cannot
describe the world. A local realist theory is one where physical properties are
defined prior to and independent of measurement, and no physical influence can
propagate faster than the speed of light. Quantum-mechanical predictions for
certain experiments violate the Bell inequality while a local realist theory
cannot, and this shows that a local realist theory cannot give those
quantum-mechanical predictions. However, because of unexpected circumstances or
"loopholes" in available experiment tests, local realist theories can reproduce
the data from these experiments. This paper reviews such loopholes, what effect
they have on Bell inequality tests, and how to avoid them in experiment.
Avoiding all these simultaneously in one experiment, usually called a
"loophole-free" or "definitive" Bell test, remains an open task, but is very
important for technological tasks such as device-independent security of
quantum cryptography, and ultimately for our understanding of the world.Comment: 42 pages, 2 figure
Multimode entanglement in coupled cavity arrays
We study a driven-dissipative array of coupled nonlinear optical resonators
by numerically solving the Von Neumann equation for the density matrix. We
demonstrate that quantum correlated states of many photons can be generated
also in the limit where the nonlinearity is much smaller than the losses,
contrarily to common expectations. Quantum correlations in this case arise from
interference between different pathways that the system can follow in the
Hilbert space to reach its steady state under the effect of coherent driving
fields. We characterize in particular two systems: a linear chain of three
coupled cavities and an array of eight coupled cavities. We demonstrate the
existence of a parameter range where the system emits photons with
continuous-variable bipartite and quadripartite entanglement, in the case of
the first and the second system respectively. This entanglement is shown to
survive realistic rates of pure dephasing and opens a new perspective for the
realization of quantum simulators or entangled photon sources without the
challenging requirement of strong optical nonlinearities.Comment: 20 pages, 7 figure
Measuring and engineering entropy and spin squeezing in weakly linked Bose-Einstein condensates
We propose a method to infer the single-particle entropy of bosonic atoms in
an optical lattice and to study the local evolution of entropy, spin squeezing,
and entropic inequalities for entanglement detection in such systems. This
method is based on experimentally feasible measurements of
non-nearest-neighbour coherences. We study a specific example of dynamically
controlling atom tunneling between selected sites and show that this could
potentially also improve the metrologically relevant spin squeezing
Entanglement dynamics in a non-Markovian environment: an exactly solvable model
We study the non-Markovian effects on the dynamics of entanglement in an
exactly-solvable model that involves two independent oscillators each coupled
to its own stochastic noise source. First, we develop Lie algebraic and
functional integral methods to find an exact solution to the single-oscillator
problem which includes an analytic expression for the density matrix and the
complete statistics, i.e., the probability distribution functions for
observables. For long bath time-correlations, we see non-monotonic evolution of
the uncertainties in observables. Further, we extend this exact solution to the
two-particle problem and find the dynamics of entanglement in a subspace. We
find the phenomena of `sudden death' and `rebirth' of entanglement.
Interestingly, all memory effects enter via the functional form of the energy
and hence the time of death and rebirth is controlled by the amount of noisy
energy added into each oscillator. If this energy increases above (decreases
below) a threshold, we obtain sudden death (rebirth) of entanglement.Comment: 11 pages, 4 figures; revision for PR
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