22 research outputs found
Memory Effects in the Dynamics of Open Quantum Systems
In this Thesis various aspects of memory effects in the dynamics of open quantum systems are studied. We develop a general theoretical framework for open quantum systems beyond the Markov approximation which allows us to investigate different sources of memory effects and to develop methods for harnessing them in order to realise controllable open quantum systems.
In the first part of the Thesis a characterisation of non-Markovian dynamics in terms of information flow is developed and applied to study different sources of memory effects. Namely, we study nonlocal memory effects which arise due to initial correlations between two local environments and further the memory effects induced by initial correlations between the open system and the environment.
The last part focuses on describing two all-optical experiment in which through selective preparation of the initial environment states the information flow between the system and the environment can be controlled. In the first experiment the system is driven from the Markovian to the non- Markovian regime and the degree of non-Markovianity is determined. In the second experiment we observe the nonlocal nature of the memory effects and provide a novel method to experimentally quantify frequency correlations in photonic environments via polarisation measurements.Siirretty Doriast
Non-Markovian dynamics in open quantum systems
The dynamical behavior of open quantum systems plays a key role in many
applications of quantum mechanics, examples ranging from fundamental problems,
such as the environment-induced decay of quantum coherence and relaxation in
many-body systems, to applications in condensed matter theory, quantum
transport, quantum chemistry and quantum information. In close analogy to a
classical Markov process, the interaction of an open quantum system with a
noisy environment is often modelled by a dynamical semigroup with a generator
in Lindblad form, which describes a memoryless dynamics leading to an
irreversible loss of characteristic quantum features. However, in many
applications open systems exhibit pronounced memory effects and a revival of
genuine quantum properties such as quantum coherence and correlations. Here,
recent results on the rich non-Markovian quantum dynamics of open systems are
discussed, paying particular attention to the rigorous mathematical definition,
to the physical interpretation and classification, as well as to the
quantification of memory effects. The general theory is illustrated by a series
of examples. The analysis reveals that memory effects of the open system
dynamics reflect characteristic features of the environment which opens a new
perspective for applications, namely to exploit a small open system as a
quantum probe signifying nontrivial features of the environment it is
interacting with. This article further explores the various physical sources of
non-Markovian quantum dynamics, such as structured spectral densities, nonlocal
correlations between environmental degrees of freedom and correlations in the
initial system-environment state, in addition to developing schemes for their
local detection. Recent experiments on the detection, quantification and
control of non-Markovian quantum dynamics are also discussed.Comment: 26 pages, 10 figure
Problem of coherent control in non-Markovian open quantum systems
We critically evaluate the most widespread assumption in the theoretical description of coherent control strategies for open quantum systems. We show that, for non-Markovian open systems dynamics, this fixed-dissipator assumption leads to a serious pitfall generally causing difficulties in the effective modeling of the controlled system. We show that at present, to avoid these problems, a full microscopic description of the controlled system in the presence of noise may often be necessary. We illustrate our findings with a paradigmatic example.</p
Measure for the Degree of Non-Markovian Behavior of Quantum Processes in Open Systems
We construct a general measure for the degree of non-Markovian behavior in
open quantum systems. This measure is based on the trace distance which
quantifies the distinguishability of quantum states. It represents a functional
of the dynamical map describing the time evolution of physical states, and can
be interpreted in terms of the information flow between the open system and its
environment. The measure takes on nonzero values whenever there is a flow of
information from the environment back to the open system, which is the key
feature of non-Markovian dynamics.Comment: 4 pages, 2 figures, published versio
Dynamics of incompatibility of quantum measurements in open systems
The non-classical nature of quantum states, often illustrated using
entanglement measures or quantum discord, constitutes a resource for quantum
information protocols. However, the non-classicality of a quantum system cannot
be encapsulated as a property of the state alone, as the set of available
measurements used to extract information on the system is typically restricted.
In this work we study how the non-classicality of quantum measurements,
quantified via their incompatibility, is influenced by quantum noise and,
further, how a non-Markovian environment may help us in maintaining the
measurement resources.Comment: v2: presentation improved and typos correcte
Nonlocal memory assisted entanglement distribution in optical fibers
Successful implementation of several quantum information and communication
protocols require distributing entangled pairs of quantum bits in reliable
manner. While there exists a substantial amount of recent theoretical and
experimental activities dealing with non-Markovian quantum dynamics,
experimental application and verification of the usefulness of memory-effects
for quantum information tasks is still missing. We combine these two aspects
and show experimentally that a recently introduced concept of nonlocal memory
effects allows to protect and distribute polarization entangled pairs of
photons in efficient manner within polarization-maintaining (PM) optical
fibers. The introduced scheme is based on correlating the environments, i.e.
frequencies of the polarization entangled photons, before their physical
distribution. When comparing to the case without nonlocal memory effects, we
demonstrate at least 12-fold improvement in the channel, or fiber length, for
preserving the highly-entangled initial polarization states of photons against
dephasing