18 research outputs found
Comparing different non-Markovianity measures: A case study
We consider two recently proposed measures of non-Markovianity applied to a
particular quantum process describing the dynamics of a driven qubit in a
structured reservoir. The motivation of this study is twofold: on one hand, we
study the differences and analogies of the non-Markovianity measures and on the
other hand, we investigate the effect of the driving force on the dissipative
dynamics of the qubit. In particular we ask if the drive introduces new
channels for energy and/or information transfer between the system and the
environment, or amplifies existing ones. We show under which conditions the
presence of the drive slows down the inevitable loss of quantum properties of
the qubit.Comment: 5 pages, no figures. Published version with minor modification
Markovian evolution of strongly coupled harmonic oscillators
We investigate how to model Markovian evolution of coupled harmonic
oscillators, each of them interacting with a local environment. When the
coupling between the oscillators is weak, dissipation may be modeled using
local Lindblad terms for each of the oscillators in the master equation, as is
commonly done. When the coupling between oscillators is strong, this model may
become invalid. We derive a master equation for two coupled harmonic
oscillators which are subject to individual heat baths modeled by a collection
of harmonic oscillators, and show that this master equation in general contains
non-local Lindblad terms. We compare the resulting time evolution with that
obtained for dissipation through local Lindblad terms for each individual
oscillator, and show that the evolution is different in the two cases. In
particular, the two descriptions give different predictions for the steady
state and for the entanglement between strongly coupled oscillators. This shows
that when describing strongly coupled harmonic oscillators, one must take great
care in how dissipation is modeled, and that a description using local Lindblad
terms may fail. This may be particularly relevant when attempting to generate
entangled states of strongly coupled quantum systems.Comment: 11 pages, 4 figures, significantly revised and close to the published
versio
Geometric phase for an adiabatically evolving open quantum system
We derive an elegant solution for a two-level system evolving adiabatically
under the influence of a driving field with a time-dependent phase, which
includes open system effects such as dephasing and spontaneous emission. This
solution, which is obtained by working in the representation corresponding to
the eigenstates of the time-dependent Hermitian Hamiltonian, enables the
dynamic and geometric phases of the evolving density matrix to be separated and
relatively easily calculated.Comment: 10 pages, 0 figure
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
Quantum probability rule : a generalization of the theorems of Gleason and Busch
Buschs theorem deriving the standard quantum probability rule can be regarded as a more general form of Gleasons theorem. Here we show that a further generalization is possible by reducing the number of quantum postulates used by Busch. We do not assume that the positive measurement outcome operators are effects or that they form a probability operator measure. We derive a more general probability rule from which the standard rule can be obtained from the normal laws of probability when there is no measurement outcome information available, without the need for further quantum postulates. Our general probability rule has prediction-retrodiction symmetry and we show how it may be applied in quantum communications and in retrodictive quantum theory
Revisiting the damped quantum harmonic oscillator
We reanalyse the quantum damped harmonic oscillator, introducing three less than common features. These are (i) the use of a continuum model of the reservoir rather than an ensemble of discrete oscillators, (ii) an exact diagonalisation of the Hamiltonian by adapting a technique pioneered by Fano, and (iii) the use of the thermofield technique for describing a finite temperature reservoir. We recover in this way a number of well-known and some, perhaps, less familiar results. An example of the latter is an ab initio proof that the oscillator relaxes to the mean-force Gibbs state. We find that special care is necessary when comparing the damped oscillator with its undamped counterpart as the former has two distinct natural frequencies, one associated with short time evolution and the other with longer times
Quantum-classical correspondence in spin-boson equilibrium states at arbitrary coupling
It is known that the equilibrium properties of nanoscale systems can deviate
significantly from standard thermodynamics due to their coupling to an
environment. For the generalized -angled spin-boson model, here we
derive an explicit form of the classical mean force equilibrium state. Taking
the large spin limit of the quantum spin-boson model, we demonstrate that the
quantum-classical correspondence is maintained at arbitrary coupling strength.
This correspondence gives insight into the conditions for a quantum system to
be well-approximated by its classical counterpart. We further demonstrate that,
counterintuitively, previously identified environment-induced 'coherences' in
the equilibrium state of weakly coupled quantum spins, do not disappear in the
classical case. Finally, we categorise various coupling regimes, from
ultra-weak to ultra-strong, and find that the same value of coupling strength
can either be 'weak' or 'strong', depending on whether the system is quantum or
classical. Our results shed light on the interplay of quantum and mean force
corrections in equilibrium states of the spin-boson model, and will help draw
the quantum to classical boundary in a range of fields, such as magnetism and
exciton dynamics
Completely positive maps with memory
The prevailing description for dissipative quantum dynamics is given by the
Lindblad form of a Markovian master equation, used under the assumption that
memory effects are negligible. However, in certain physical situations, the
master equation is essentially of a non-Markovian nature. This paper examines
master equations that possess a memory kernel, leading to a replacement of
white noise by colored noise. The conditions under which this leads to a
completely positive, trace-preserving map are discussed for an exponential
memory kernel. A physical model that possesses such an exponential memory
kernel is presented. This model contains a classical, fluctuating environment
based on random telegraph signal stochastic variables.Comment: 4 page
Measurement master equation
We derive a master equation describing the evolution of a quantum system
subjected to a sequence of observations. These measurements occur randomly at a
given rate and can be of a very general form. As an example, we analyse the
effects of these measurements on the evolution of a two-level atom driven by an
electromagnetic field. For the associated quantum trajectories we find Rabi
oscillations, Zeno-effect type behaviour and random telegraph evolution spawned
by mini quantum jumps as we change the rates and strengths of measurement.Comment: 14 pages and 8 figures, Optics Communications in pres