216 research outputs found
Purity as a witness for initial system-environment correlations in open-system dynamics
We study the dynamics of a two-level atom interacting with a Lorentzian
structured reservoir considering initial system-environment correlations. It is
shown that under strong system-reservoir coupling the dynamics of purity can
determine whether there are initial correlations between system and
environment. Moreover, we investigate the interaction of two two-level atoms
with the same reservoir. In this case, we show that besides determining if
there are initial system-environment correlations, the dynamics of the purity
of the atomic system allows the identification of the distinct correlated
initial states. In addition, the dynamics of quantum and classical correlations
is analyzed.Comment: 6 pages, 3 figure
Protection of entanglement from sudden death using continuous dynamical decoupling
We show that continuous dynamical decoupling can protect a two-qubit
entangled state from sudden death at finite temperature due to uncorrelated
dephasing, bit flipping, and dissipation. We consider a situation where an
entangled state shared between two non-interacting qubits is initially prepared
and left evolve under the environmental perturbations and the protection of
external fields. To illustrate the protection of the entanglement, we solve
numerically a master equation in the Born approximation, considering
independent boson fields at the same temperature coupled to the different error
agents of each qubit
Entanglement versus Quantum Discord in Two Coupled Double Quantum Dots
We study the dynamics of quantum correlations of two coupled double quantum
dots containing two excess electrons. The dissipation is included through the
contact with an oscillator bath. We solve the Redfield master equation in order
to determine the dynamics of the quantum discord and the entanglement of
formation. Based on our results, we find that the quantum discord is more
resistant to dissipation than the entanglement of formation for such a system.
We observe that this characteristic is related to whether the oscillator bath
is common to both qubits or not and to the form of the interaction Hamiltonian.
Moreover, our results show that the quantum discord might be finite even for
higher temperatures in the asymptotic limit.Comment: 14 pages, 8 figures (new version is the final version to appear in
NJP
Emergence of the pointer basis through the dynamics of correlations
We use the classical correlation between a quantum system being measured and
its measurement apparatus to analyze the amount of information being retrieved
in a quantum measurement process. Accounting for decoherence of the apparatus,
we show that these correlations may have a sudden transition from a decay
regime to a constant level. This transition characterizes a non-asymptotic
emergence of the pointer basis, while the system-apparatus can still be quantum
correlated. We provide a formalization of the concept of emergence of a pointer
basis in an apparatus subject to decoherence. This contrast of the pointer
basis emergence to the quantum to classical transition is demonstrated in an
experiment with polarization entangled photon pairs.Comment: 4+2 pgs, 3 figures. Title changed. Revised version to appear on PR
Calculation of quantum discord for qubit-qudit or N qubits
Quantum discord, a kind of quantum correlation, is defined as the difference
between quantum mutual information and classical correlation in a bipartite
system. It has been discussed so far for small systems with only a few
independent parameters. We extend here to a much broader class of states when
the second party is of arbitrary dimension d, so long as the first, measured,
party is a qubit. We present two formulae to calculate quantum discord, the
first relating to the original entropic definition and the second to a recently
proposed geometric distance measure which leads to an analytical formulation.
The tracing over the qubit in the entropic calculation is reduced to a very
simple prescription. And, when the d-dimensional system is a so-called X state,
the density matrix having non-zero elements only along the diagonal and
anti-diagonal so as to appear visually like the letter X, the entropic
calculation can be carried out analytically. Such states of the full bipartite
qubit-qudit system may be named "extended X states", whose density matrix is
built of four block matrices, each visually appearing as an X. The optimization
involved in the entropic calculation is generally over two parameters, reducing
to one for many cases, and avoided altogether for an overwhelmingly large set
of density matrices as our numerical investigations demonstrate. Our results
also apply to states of a N-qubit system, where "extended X states" consist of
(2^(N+2) - 1) states, larger in number than the (2^(N+1) - 1) of X states of N
qubits. While these are still smaller than the total number (2^(2N) - 1) of
states of N qubits, the number of parameters involved is nevertheless large. In
the case of N = 2, they encompass the entire 15-dimensional parameter space,
that is, the extended X states for N = 2 represent the full qubit-qubit system.Comment: 6 pages, 1 figur
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