788 research outputs found
Two-qubit non-Markovianity induced by a common environment
We study non-Markovianity as backflow of information in two-qubit systems. We
consider a setting where, by changing the distance between the qubits, one can
interpolate between independent reservoir and common reservoir scenarios. We
demonstrate that non-Markovianity can be induced by the common reservoir and
single out the physical origin of this phenomenon. We show that two-qubit
non-Markovianity coincides with instances of non-divisibility of the
corresponding dynamical map, and we discuss the pair of states maximizing
information flowback. We also discuss the issue of additivity for the measure
we use and in doing so, give an indication of its usefulness as a resource for
multipartite quantum systems.Comment: 9 pages, 5 figures, Published version with minor modification
Entanglement trapping in a non-stationary structured reservoir
We study a single two-level atom interacting with a reservoir of modes
defined by a reservoir structure function with a frequency gap. Using the
pseudomodes technique, we derive the main features of a trapping state formed
in the weak coupling regime. Utilising different entanglement measures we show
that strong correlations and entanglement between the atom and the modes are in
existence when this state is formed. Furthermore, an unexpected feature for the
reservoir is revealed. In the long time limit and for weak coupling the
reservoir spectrum is not constant in time.Comment: 10 pages, 16 figure
Remote polarization entanglement generation by local dephasing and frequency upconversion
We introduce a scheme for remote entanglement generation for the photon
polarization. The technique is based on transferring the initial frequency
correlations to specific polarization-frequency correlations by local dephasing
and their subsequent removal by frequency up-conversion. On fundamental level,
our theoretical results show how to create and transfer entanglement, to
particles which never interact, by means of local operations. This possibility
stems from the multi-path interference and its control in frequency space. For
applications, the developed techniques and results allow for the remote
generation of entanglement with distant parties without Bell state measurements
and opens the perspective to probe frequency-frequency entanglement by
measuring the polarization state of the photons.Comment: 8 page
Dynamical memory effects in correlated quantum channels
Memory effects play a fundamental role in the study of the dynamics of open quantum systems. There exist two conceptually distinct notions of memory discussed for quantum channels in the literature. In quantum information theory quantum channels with memory are characterized by the existence of correlations between successive applications of the channel on a sequence of quantum systems. In open quantum systems theory memory effects arise dynamically during the time evolution of quantum systems and define non-Markovian dynamics. Here we relate and combine these two different concepts of memory. In particular, we study the interplay between correlations between multiple uses of quantum channels and non-Markovianity as nondivisibility of the t-parametrized family of channels defining the dynamical map
Decoherence and robustness of parity-dependent entanglement in the dynamics of a trapped ion
We study the entanglement between the 2D vibrational motion and two ground
state hyperfine levels of a trapped ion, Under particular conditions this
entanglement depends on the parity of the total initial vibrational quanta. We
study the robustness of this quantum coherence effect with respect to the
presence of non-dissipative sources of decoherence, and of an imperfect initial
state preparation.Comment: 13 pages, 5 figure
Non-Markovian dynamics of a qubit
In this paper we investigate the non-Markovian dynamics of a qubit by
comparing two generalized master equations with memory. In the case of a
thermal bath, we derive the solution of the post-Markovian master equation
recently proposed in Ref. [A. Shabani and D.A. Lidar, Phys. Rev. A {\bf 71},
020101(R) (2005)] and we study the dynamics for an exponentially decaying
memory kernel. We compare the solution of the post-Markovian master equation
with the solution of the typical memory kernel master equation. Our results
lead to a new physical interpretation of the reservoir correlation function and
bring to light the limits of usability of master equations with memory for the
system under consideration.Comment: Replaced with published version (minor changes
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
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