3,800 research outputs found
Quantum fluctuations and entanglement in mesoscopic systems
No full textDue to the large amount of microscopic constituents, sensible information that can be gathered about many-
body systems concerns usually the behaviour of collective observables; among them, surely average observables, like the mean magnetization in quantum spin chains, but also fluctuations around
mean-values. Average operators over all particles are defined with a scaling proportional to the inverse number N of considered particles; in the large N limit, the emergent collective operators form a classical algebra, with no footprints of the
microscopic quantum structure they result from. On the contrary, another class of collective observables, the so-called fluctuation operators defined with a scaling proportional to square root of N , has been
proved, by means of quantum central limit theorems, to retain quantum properties, giving rise to a Gaussian Bosonic system. These collective observables may thus be
interpreted as witnesses of a mesoscopic behaviour positioned at the interface between macroscopic,
classical behaviours and microscopic quantum ones, providing a suitable framework where to look
for collective quantum phenomena in many-body systems.
In this thesis we studied the dynamical behaviour of these fluctuation operators, when the many-body mesoscopic system is considered not to be isolated, but in a weak interaction with a larger environment; this is the most common situation encountered in actual experiments, where these systems can never be thought of as completely isolated from their thermal surroundings. Under some conditions on the dynamical generator, we showed that such dissipative evolution of fluctuations exists and is such that it preserves their Gaussian character. By means of a particular example, we also demonstrated that two non-interacting many-body systems can become entangled, at the level of their fluctuation operators, through the presence of
a common environment usually responsible for decoherence and emergence of classical behaviours. Furthermore, the behaviour of such correlations has a neat
dependence on the temperature of the heat bath, displaying a sort of phase transition, witnessed
by the existence of a finite critical temperature above which entanglement is not possible
Emergent dissipative quasi-particle picture in noninteracting Markovian open quantum systems
No full textCorrelations between different regions of a quantum many-body system can be
quantified through measures based on entropies of (reduced) subsystem states.
For closed systems, several analytical and numerical tools, e.g., hydrodynamic
theories or tensor networks, can accurately capture the time-evolution of
subsystem entropies, thus allowing for a profound understanding of the unitary
dynamics of quantum correlations. However, so far, these methods either cannot
be applied to open quantum systems or do not permit an efficient computation of
quantum entropies for mixed states. Here, we make progress in solving this
issue by formulating a dissipative quasi-particle picture -- describing the
dynamics of quantum entropies in the hydrodynamic limit -- for a general class
of noninteracting open quantum systems. Our results show that also in
dissipative many-body systems, correlations are generically established through
the propagation of quasi-particles.Comment: 14 pages, 3 figure
A non-Markovian dissipative Maryland model
No full textThe so-called Maryland model is a linear version of the quantum kicked rotor; it exhibits Anderson localization in momentum space. By turning the kicks into a Markovian stochastic process, the dynamics becomes a dissipative quantum process described by a discrete family of completely positive maps that allows to explicitly study the relation between divisibility of the maps and the degree of memory of the process
Logarithmic negativity in out-of-equilibrium open free-fermion chains: An exactly solvable case
No full textEntangled multiplets and spreading of quantum correlations in a continuously monitored tight-binding chain
No full textWe analyze the dynamics of entanglement in a paradigmatic noninteracting system subject to continuous monitoring of the local excitation densities. Recently, it was conjectured that the evolution of quantum correlations in such system is described by a semiclassical theory, based on entangled pairs of ballistically propagating quasiparticles and inspired by the hydrodynamic approach to unitary (integrable) quantum systems. Here, however, we show that this conjecture does not fully capture the complex behavior of quantum correlations emerging from the interplay between coherent dynamics and continuous monitoring. We unveil the existence of multipartite quantum correlations which are inconsistent with an entangled-pair structure and which, within a quasiparticle picture, would require the presence of larger multiplets. We also observe that quantum information is highly delocalized, as it is shared in a collective nonredundant way among adjacent regions of the many-body system. Our results shed light onto the behavior of correlations in quantum stochastic dynamics and further show that these may be enhanced by a (weak) continuous monitoring process
Thermalization with a multibath: an investigation in simple models
Get PDFWe study analytically and numerically a couple of paradigmatic spin models,
each described in terms of two sets of variables attached to two different
thermal baths with characteristic timescales and and inverse
temperatures and . In the limit in which one bath becomes extremely
slow (), such models amount to a paramagnet and to a
one-dimensional ferromagnet, in contact with a single bath. We show that these
systems reach a stationary state in a finite time for any choice of and
. We determine the non-equilibrium fluctuation-dissipation relation
between the autocorrelation and the response function in such state and, from
that, we discuss if and how thermalization with the two baths occurs and the
emergence of a non-trivial fluctuation-dissipation ratio.Comment: 15 pages, 6 figure
Dissipative Dynamics of Quantum Fluctuations
No full textOne way to look for complex behaviours in many-body quantum systems is to let the number N of degrees of freedom become large and focus upon collective observables. Mean-field quantities scaling as inline image tend to commute, whence complexity at the quantum level can only be inherited from complexity at the classical level. Instead, fluctuations of microscopic observables scale as inline image and exhibit collective Bosonic features, typical of a mesoscopic regime half-way between the quantum one at the microscopic level and the classical one at the level of macroscopic averages. Here, we consider the mesoscopic behaviour emerging from an infinite quantum spin chain undergoing a microscopic dissipative, irreversible dynamics and from global states without long-range correlations and invariant under lattice translations and dynamics. We show that, from the fluctuations of one site spin observables whose linear span is mapped into itself by the dynamics, there emerge bosonic operators obeying a mesoscopic dissipative dynamics mapping Gaussian states into Gaussian states. Instead of just depleting quantum correlations because of decoherence effects, these maps can generate entanglement at the collective, mesoscopic level, a phenomenon with no classical analogue that embodies a peculiar complex behaviour at the interface between micro and macro regimes
Environment induced entanglement in many-body mesoscopic systems
No full textWe show that two, non-interacting, infinitely long spin chains can become globally entangled at the mesoscopic level of their fluctuation operators through a purely noisy microscopic mechanism induced by the presence of a common heat bath. By focusing on a suitable class of mesoscopic observables, the behaviour of the dissipatively generated quantum correlations between the two chains is studied as a function of the dissipation strength and bath temperature
Dissipative entanglement of quantum spin fluctuations
No full textWe consider two non-interacting infinite quantum spin chains immersed in a common thermal environment and undergoing a local dissipative dynamics of Lindblad type. We study the time evolution of collective mesoscopic quantum spin fluctuations that, unlike macroscopic mean-field observables, retain a quantum character in the thermodynamical limit. We show that the microscopic dissipative dynamics is able to entangle these mesoscopic degrees of freedom, through a purely mixing mech- anism. Further, the behaviour of the dissipatively generated quantum correlations between the two chains is studied as a function of temperature and dissipation strength
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