20 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
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
Invariant quantum discord in qubit-qutrit systems under local dephasing
We investigate the dynamics of quantum discord and entanglement for a
class of mixed qubit-qutrit states assuming that only the qutrit is under the action of a dephasing channel. We demonstrate that even though the entanglement in the qubit-qutrit state disappears in a finite time interval, partial coherence left in the system enables quantum discord to remain invariant throughout the whole time evolution
Robust non-Markovianity in ultracold gases
We study the effect of thermal fluctuations on a probe qubit interacting with
a Bose-Einstein condensed (BEC) reservoir. The zero-temperature case was
studied in [Haikka P et al 2011 Phys. Rev. A 84 031602], where we proposed a
method to probe the effects of dimensionality and scattering length of a BEC
based on its behavior as an environment. Here we show that the sensitivity of
the probe qubit is remarkably robust against thermal noise. We give an
intuitive explanation for the thermal resilience, showing that it is due to the
unique choice of the probe qubit architecture of our model.Comment: Submitted to FQMT11 topical issue of the Physica Script
Negativity and quantum discord in Davies environments
We investigate the time evolution of negativity and quantum discord for a
pair of non-interacting qubits with one being weakly coupled to a decohering
Davies--type Markovian environment. At initial time of preparation, the qubits
are prepared in one of the maximally entangled pure Bell states. In the
limiting case of pure decoherence (i.e. pure dephasing), both, the quantum
discord and negativity decay to zero in the long time limit. In presence of a
manifest dissipative dynamics, the entanglement negativity undergoes a sudden
death at finite time while the quantum discord relaxes continuously to zero
with increasing time. We find that in dephasing environments the decay of the
negativity is more propitious with increasing time; in contrast, the evolving
decay of the quantum discord proceeds weaker for dissipative environments.
Particularly, the slowest decay of the quantum discord emerges when the energy
relaxation time matches the dephasing time.Comment: submitted for publicatio
The Rotating-Wave Approximation: Consistency and Applicability from an Open Quantum System Analysis
We provide an in-depth and thorough treatment of the validity of the
rotating-wave approximation (RWA) in an open quantum system. We find that when
it is introduced after tracing out the environment, all timescales of the open
system are correctly reproduced, but the details of the quantum state may not
be. The RWA made before the trace is more problematic: it results in incorrect
values for environmentally-induced shifts to system frequencies, and the
resulting theory has no Markovian limit. We point out that great care must be
taken when coupling two open systems together under the RWA. Though the RWA can
yield a master equation of Lindblad form similar to what one might get in the
Markovian limit with white noise, the master equation for the two coupled
systems is not a simple combination of the master equation for each system, as
is possible in the Markovian limit. Such a naive combination yields inaccurate
dynamics. To obtain the correct master equation for the composite system a
proper consideration of the non-Markovian dynamics is required.Comment: 17 pages, 0 figures
Non-Markovian dynamics for an open two-level system without rotating wave approximation: Indivisibility versus backflow of information
By use of the two measures presented recently, the indivisibility and the
backflow of information, we study the non-Markovianity of the dynamics for a
two-level system interacting with a zero-temperature structured environment
without using rotating wave approximation (RWA). In the limit of weak coupling
between the system and the reservoir, and by expanding the time-convolutionless
(TCL) generator to the forth order with respect to the coupling strength, the
time-local non-Markovian master equation for the reduced state of the system is
derived. Under the secular approximation, the exact analytic solution is
obtained and the sufficient and necessary conditions for the indivisibility and
the backflow of information for the system dynamics are presented. In the more
general case, we investigate numerically the properties of the two measures for
the case of Lorentzian reservoir. Our results show the importance of the
counter-rotating terms to the short-time-scale non-Markovian behavior of the
system dynamics, further expose the relations between the two measures and
their rationality as non-Markovian measures. Finally, the complete positivity
of the dynamics of the considered system is discussed