1,721,070 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
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
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
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
Non-markovian mesoscopic dissipative dynamics of open quantum spin chains
No full textWe study the dissipative dynamics of Nquantum spins with Lindblad generator consisting of operators scaling as fluctuations, namely with the inverse square-root of N. In the large Nlimit, the microscopic dissipative time-evolution converges to a non-Markovian unitary dynamics on strictly local operators, while at the mesoscopic level of fluctuations it gives rise to a dissipative non-Markovian dynamics. The mesoscopic time-evolution is Gaussian and exhibits either a stable or an unstable asymptotic character; furthermore, the mesoscopic dynamics builds correlations among fluctuations that survive in time even when the original microscopic dynamics is unable to correlate local observables
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
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
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Witnessing nonclassicality through large deviations in quantum optics
Get PDFNonclassical correlations in quantum optics as resources for quantum computation are important in the quest for highly specialized quantum devices. Here, we put forward a methodology to witness nonclassicality of the output field from a generic quantum optical setup via the statistics of time-integrated photocurrents. Specifically, exploiting the thermodynamics of quantum trajectories, we express a known nonclassicality witness for bosonic fields fully in terms of the source master equation, thus bypassing the explicit calculation of the output light state
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