40 research outputs found
Non-Markovian Dynamics in Continuous Variable Quantum Systems
The present manuscript represents the completion of a research path carried forward during my doctoral studies in the University of Turku. It contains information regarding my scientific contribution to the field of open quantum systems, accomplished in collaboration with other scientists.
The main subject investigated in the thesis is the non-Markovian dynamics of open quantum systems with focus on continuous variable quantum channels, e.g. quantum Brownian motion models. Non-Markovianity is here interpreted as a manifestation of the existence of a flow of information exchanged by the system and environment during the dynamical evolution. While in Markovian systems the flow is unidirectional, i.e. from the system to the environment, in non-Markovian systems there are time windows in which the flow is reversed and the quantum state of the system may regain coherence and correlations previously lost.
Signatures of a non-Markovian behavior have been studied in connection with the dynamics of quantum correlations like entanglement or quantum discord. Moreover, in the attempt to recognisee non-Markovianity as a resource for quantum technologies, it is proposed, for the first time, to consider its effects in practical quantum key distribution protocols. It has been proven that security of coherent state protocols can be enhanced using non-Markovian properties of the transmission channels.
The thesis is divided in two parts: in the first part I introduce the reader to the world of continuous variable open quantum systems and non-Markovian dynamics. The second part instead consists of a collection of five publications inherent to the topic.Siirretty Doriast
Entanglement dynamics for two harmonic oscillators coupled to independent environments
We study the entanglement evolution between two harmonic oscillators having
different free frequencies each leaking into an independent bath. We use an
exact solution valid in the weak coupling limit and in the short time
non-Markovian regime. The reservoirs are identical and characterized by an
Ohmic spectral distribution with Lorents-Drude cut-off. This work is an
extension of the case reported in [Phys. Rev. A 80, 062324 (2009)] where the
oscillators have the same free frequency.Comment: 8 pages, 3 figures, submitted to Physica Script
Most probable paths in temporal weighted networks: An application to ocean transport
We consider paths in weighted and directed temporal networks, introducing
tools to compute sets of paths of high probability. We quantify the relative
importance of the most probable path between two nodes with respect to the
whole set of paths, and to a subset of highly probable paths which incorporate
most of the connection probability. These concepts are used to provide
alternative definitions of betweenness centrality. We apply our formalism to a
transport network describing surface flow in the Mediterranean sea. Despite the
full transport dynamics is described by a very large number of paths we find
that, for realistic time scales, only a very small subset of high probability
paths (or even a single most probable one) is enough to characterize global
connectivity properties of the network
The geometric approach to quantum correlations: Computability versus reliability
We propose a modified metric based on the Hilbert-Schmidt norm and adopt it
to define a rescaled version of the geometric measure of quantum discord. Such
a measure is found not to suffer from the pathological dependence on state
purity. Although the employed metric is still noncontractive under quantum
operations, we show that the resulting indicator of quantum correlations is in
agreement with other bona fide discord measures in a number of physical
examples. We present a critical assessment of the requirements of reliability
versus computability when approaching the task of quantifying, or measuring,
general quantum correlations in a bipartite state.Comment: 14 pages, 5 figures; presentation improved; to appear in J. Phys.
Continuous-variable quantum key distribution in non-Markovian channels
We address continuous-variable quantum key distribution (QKD) in non-Markovian lossy channels and show how the non-Markovian features may be exploited to enhance security and/or to detect the presence and the position of an eavesdropper along the transmission line. In particular, we suggest a coherent-state QKD protocol which is secure against Gaussian individual attacks based on optimal 1 ->2 asymmetric cloning machines for arbitrarily low values of the overall transmission line. The scheme relies on specific non-Markovian properties, and cannot be implemented in ordinary Markovian channels characterized by uniform losses. Our results give a clear indication of the potential impact of non-Markovian effects in QKD
Dynamical Casimir-Polder potentials in non-adiabatic conditions
In this paper we review different aspects of the dynamical Casimir- Polder
potential between a neutral atom and a perfectly conducting plate under
nonequilibrium conditions. In order to calculate the time evolution of the
atom-wall Casimir-Polder potential, we solve the Heisenberg equations
describing the dynamics of the coupled system using an iterative technique.
Different nonequilibrium initial states are considered, such as bare and
partially dressed states. The partially dressed states considered are obtained
by a sudden change of a physical parameter of the atom or of its position
relative to the conducting plate. Experimental feasibility of detecting the
considered dynamical effects is also discussed.Comment: 6 pages; Special Issue: 20th Central European Workshop on Quantum
Optics - Stockholm - June 201
Dominant transport pathways in an atmospheric blocking event
A Lagrangian flow network is constructed for the atmospheric blocking of
eastern Europe and western Russia in summer 2010. We compute the most probable
paths followed by fluid particles which reveal the {\it Omega}-block skeleton
of the event. A hierarchy of sets of highly probable paths is introduced to
describe transport pathways when the most probable path alone is not
representative enough. These sets of paths have the shape of narrow coherent
tubes flowing close to the most probable one. Thus, even when the most probable
path is not very significant in terms of its probability, it still identifies
the geometry of the transport pathways.Comment: Appendix added with path calculations for a simple kinematic model
flo
Quantifying non-Markovianity of continuous-variable Gaussian dynamical maps
We introduce a non-Markovianity measure for continuous-variable open quantum systems based on the idea put forward in H.-P. Breuer, that is, by quantifying the flow of information from the environment back to the open system. Instead of the trace distance we use here the fidelity to assess distinguishability of quantum states. We employ our measure to evaluate non-Markovianity of two paradigmatic Gaussian channels: the purely damping channel and the quantum Brownian motion channel with Ohmic environment. We consider different classes of Gaussian states and look for pairs of states maximizing the backflow of information. For coherent states we find simple analytical solutions, whereas for squeezed states we provide both exact numerical and approximate analytical solutions in the weak coupling limit