395 research outputs found
Canonical form of master equations and characterization of non-Markovianity
Master equations govern the time evolution of a quantum system interacting
with an environment, and may be written in a variety of forms. Time-independent
or memoryless master equations, in particular, can be cast in the well-known
Lindblad form. Any time-local master equation, Markovian or non-Markovian, may
in fact also be written in a Lindblad-like form. A diagonalisation procedure
results in a unique, and in this sense canonical, representation of the
equation, which may be used to fully characterize the non-Markovianity of the
time evolution. Recently, several different measures of non-Markovianity have
been presented which reflect, to varying degrees, the appearance of negative
decoherence rates in the Lindblad-like form of the master equation. We
therefore propose using the negative decoherence rates themselves, as they
appear in the canonical form of the master equation, to completely characterize
non-Markovianity. The advantages of this are especially apparent when more than
one decoherence channel is present. We show that a measure proposed by Rivas et
al. is a surprisingly simple function of the canonical decoherence rates, and
give an example of a master equation that is non-Markovian for all times t>0,
but to which nearly all proposed measures are blind. We also give necessary and
sufficient conditions for trace distance and volume measures to witness
non-Markovianity, in terms of the Bloch damping matrix.Comment: v2: Significant update, with many new results and one new author. 12
pages; v3: Minor clarifications, to appear in PRA; v4: matches published
versio
Efficiency of quantum controlled non-Markovian thermalization
We study optimal control strategies to optimize the relaxation rate towards
the fixed point of a quantum system in the presence of a non-Markovian
dissipative bath. Contrary to naive expectations that suggest that memory
effects might be exploited to improve optimal control effectiveness,
non-Markovian effects influence the optimal strategy in a non trivial way: we
present a necessary condition to be satisfied so that the effectiveness of
optimal control is enhanced by non-Markovianity subject to suitable unitary
controls. For illustration, we specialize our findings for the case of the
dynamics of single qubit amplitude damping channels. The optimal control
strategy presented here can be used to implement optimal cooling processes in
quantum technologies and may have implications in quantum thermodynamics when
assessing the efficiency of thermal micro-machines.Comment: 7 pages, 3 figure
Thermodynamic fingerprints of non-Markovianity in a system of coupled superconducting qubits
The exploitation and characterization of memory effects arising from the
interaction between system and environment is a key prerequisite for quantum
reservoir engineering beyond the standard Markovian limit. In this paper we
investigate a prototype of non-Markovian dynamics experimentally implementable
with superconducting qubits. We rigorously quantify non-Markovianity
highlighting the effects of the environmental temperature on the Markovian to
non-Markovian crossover. We investigate how memory effects influence, and
specifically suppress, the ability to perform work on the driven qubit. We show
that the average work performed on the qubit can be used as a diagnostic tool
to detect the presence or absence of memory effects.Comment: 9 page
Degradation of Entanglement in Markovian Noise
The entanglement survival time is defined as the maximum time a system which
is evolving under the action of local Markovian, homogenous in time noise, is
capable to preserve the entanglement it had at the beginning of the temporal
evolution. In this paper we study how this quantity is affected by the
interplay between the coherent preserving and dissipative contributions of the
corresponding dynamical generator. We report the presence of a
counterintuitive, non-monotonic behaviour in such functional, capable of
inducing sudden death of entanglement in models which, in the absence of
unitary driving are capable to sustain entanglement for arbitrarily long times.Comment: 15 pages, 5 figure
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