9 research outputs found
Sudden transition between classical and quantum decoherence
We study the dynamics of quantum and classical correlations in the presence
of nondissipative decoherence. We discover a class of initial states for which
the quantum correlations, quantified by the quantum discord, are not destroyed
by decoherence for times t < \bar{t}. In this initial time interval classical
correlations decay. For t > \bar{t}, on the other hand, classical correlations
do not change in time and only quantum correlations are lost due to the
interaction with the environment. Therefore, at the transition time \bar{t} the
open system dynamics exhibits a sudden transition from classical to quantum
decoherence regime.Comment: version accepted for publication by Physical Review Letter
Sudden death and sudden birth of entanglement in common structured reservoirs
We study the exact entanglement dynamics of two qubits in a common structured
reservoir. We demonstrate that, for certain classes of entangled states,
entanglement sudden death occurs, while for certain initially factorized
states, entanglement sudden birth takes place. The backaction of the
non-Markovian reservoir is responsible for revivals of entanglement after
sudden death has occurred, and also for periods of disentanglement following
entanglement sudden birth.Comment: 4 pages, 2 figure
Quantum discord dynamical behaviors due to initial system-cavity correlations
We analyze the roles of initial correlations between the two-qubit system and
a dissipative cavity on quantum discord dynamics of two qubits. Considering two
initial system-cavity states, we show that the initial system-cavity
correlations not only can initially increase the two-qubit quantum discord but
also would lead to a larger long-time quantum discord asymptotic value.
Moreover, quantum discord due to initial correlations is more robust than the
case of the initial factorized state. Finally, we show the initial
correlations' importance for dynamics behaviors of mutual information and
classical correlation
Phenomenological memory-kernel master equations and time-dependent Markovian processes
Do phenomenological master equations with memory kernel always describe a
non-Markovian quantum dynamics characterized by reverse flow of information? Is
the integration over the past states of the system an unmistakable signature of
non-Markovianity? We show by a counterexample that this is not always the case.
We consider two commonly used phenomenological integro-differential master
equations describing the dynamics of a spin 1/2 in a thermal bath. By using a
recently introduced measure to quantify non-Markovianity [H.-P. Breuer, E.-M.
Laine, and J. Piilo, Phys. Rev. Lett. 103, 210401 (2009)] we demonstrate that
as far as the equations retain their physical sense, the key feature of
non-Markovian behavior does not appear in the considered memory kernel master
equations. Namely, there is no reverse flow of information from the environment
to the open system. Therefore, the assumption that the integration over a
memory kernel always leads to a non-Markovian dynamics turns out to be
vulnerable to phenomenological approximations. Instead, the considered
phenomenological equations are able to describe time-dependent and
uni-directional information flow from the system to the reservoir associated to
time-dependent Markovian processes.Comment: 5 pages, no figure
Pseudomodes as an effective description of memory: Non-Markovian dynamics of two-state systems in structured reservoirs
We investigate the non-Markovian dynamics of two-state systems in structured
reservoirs. We establish a connection between two theoretical quantum
approaches, the pseudomodes [B. M. Garraway, Phys. Rev. A 55, 2290 (1997)] and
the recently developed non-Markovian quantum jump method [J. Piilo et al.,
Phys. Rev. Lett. 100, 180402 (2008)]. This connection provides a clear physical
picture of how the structured reservoir affects the system dynamics, indicating
the role of the pseudomodes as an effective description of the environmental
memory.Comment: 5 pages, 2 figures. V2: minor changes, published versio
Quantum Discord in the Ground and Thermal States of Spin Clusters
Quantum discord is a general measure of bipartite quantum correlations with a
potential role in quantum information processing tasks. Spin clusters serve as
ideal candidates for the implementation of some of the associated protocols. In
this paper, we consider a symmetric spin trimer and a tetramer, which describe
a number of known molecular magnets, and compute the quantum discord in the
ground and thermal states of the clusters. The variations of the quantum
discord as a function of anisotropy parameter, magnetic field and temperature
are investigated. We obtain a number of interesting results such as a finite
value of the quantum discord in the trimer ground state for which the pairwise
entanglement is known to be zero, differences in the nature of some of the
variations in the ferromagnetic and antiferromagnetic cases and discontinuous
jumps in the magnitude of the quantum discord at first order quantum phase
transition points. A remarkable feature that is observed is that the quantum
discord completely vanishes only in the asymptotic limit of temperature
. We further study the dynamics of the quantum discord and
the pairwise entanglement at T=0 under the effect of a dephasing channel
describing the interaction of the reduced spin cluster state with independent
local environments. The QD is found to vanish asymptotically as
. In the case of the spin trimer, the pairwise entanglement
has a zero value at all times and reaches a zero value in a finite time in the
case of the tetramer.Comment: Article, 16 pages, 9 figure