1,206 research outputs found
Thermal Baths as Quantum Resources: More Friends than Foes?
In this article we argue that thermal reservoirs (baths) are potentially
useful resources in processes involving atoms interacting with quantized
electromagnetic fields and their applications to quantum technologies. One may
try to suppress the bath effects by means of dynamical control, but such
control does not always yield the desired results. We wish instead to take
advantage of bath effects, that do not obliterate "quantumness" in the
system-bath compound. To this end, three possible approaches have been pursued
by us: (i) Control of a quantum system faster than the correlation time of the
bath to which it couples: Such control allows us to reveal
quasi-reversible/coherent dynamical phenomena of quantum open systems, manifest
by the quantum Zeno or anti-Zeno effects (QZE or AZE, respectively). Dynamical
control methods based on the QZE are aimed not only at protecting the
quantumness of the system, but also diagnosing the bath spectra or transferring
quantum information via noisy media. By contrast, AZE-based control is useful
for fast cooling of thermalized quantum systems. (ii) Engineering the coupling
of quantum systems to selected bath modes: This approach, based on field -atom
coupling control in cavities, waveguides and photonic band structures, allows
to drastically enhance the strength and range of atom-atom coupling through the
mediation of the selected bath modes. More dramatically, it allows us to
achieve bath-induced entanglement that may appear paradoxical if one takes the
conventional view that coupling to baths destroys quantumness. (iii)
Engineering baths with appropriate non-flat spectra: This approach is a
prerequisite for the construction of the simplest and most efficient quantum
heat machines (engines and refrigerators). We may thus conclude that often
thermal baths are "more friends than foes" in quantum technologies.Comment: 27 pages, 17 figure
Instabilities, pattern formation, localized solutions, mode-locking and stochastic effects in nonlinear optical systems and beyond
In this thesis the results of scientific research about dierent nonlinear phenomena with particular emphasis to photonic systems are presented. Works about dissipation induced modulation instabilities with applications for signal amplification in nonlinear optics and mode-locking in lasers constitute the main part of the thesis. The dissipa-tive instabilities studied are of two kinds, parametric instabilities induced by a periodic variation of spectral losses and instabilities induced by non varying but spectrally asym-metric losses. Although the main achievements are theoretical successful collaboration with experimentalists are reported too. Other results presented in this thesis concern a new fundamental theory of active mode-locking in lasers having a more general validity than Haus’ one and hence useful for describing mode-locked lasers with a fast gain dynamics such as semiconductor or quantum cascade lasers; the prediction of the novel theoretical model have been successfully compared with experimental findings. Theo-retical studies are also presented about collective phenomena, such as synchronization and localization, in coupled excitable lasers with saturable absorber and localized so-lutions on the non-vanishing background of the two-dimensional nonlinear Schr¨odinger equation with periodic potential: the Bogoliubov-de Gennes bullets
Nonlinear brain dynamics as macroscopic manifestation of underlying many-body field dynamics
Neural activity patterns related to behavior occur at many scales in time and
space from the atomic and molecular to the whole brain. Here we explore the
feasibility of interpreting neurophysiological data in the context of many-body
physics by using tools that physicists have devised to analyze comparable
hierarchies in other fields of science. We focus on a mesoscopic level that
offers a multi-step pathway between the microscopic functions of neurons and
the macroscopic functions of brain systems revealed by hemodynamic imaging. We
use electroencephalographic (EEG) records collected from high-density electrode
arrays fixed on the epidural surfaces of primary sensory and limbic areas in
rabbits and cats trained to discriminate conditioned stimuli (CS) in the
various modalities. High temporal resolution of EEG signals with the Hilbert
transform gives evidence for diverse intermittent spatial patterns of amplitude
(AM) and phase modulations (PM) of carrier waves that repeatedly re-synchronize
in the beta and gamma ranges at near zero time lags over long distances. The
dominant mechanism for neural interactions by axodendritic synaptic
transmission should impose distance-dependent delays on the EEG oscillations
owing to finite propagation velocities. It does not. EEGs instead show evidence
for anomalous dispersion: the existence in neural populations of a low velocity
range of information and energy transfers, and a high velocity range of the
spread of phase transitions. This distinction labels the phenomenon but does
not explain it. In this report we explore the analysis of these phenomena using
concepts of energy dissipation, the maintenance by cortex of multiple ground
states corresponding to AM patterns, and the exclusive selection by spontaneous
breakdown of symmetry (SBS) of single states in sequences.Comment: 31 page
Cooperative surmounting of bottlenecks
The physics of activated escape of objects out of a metastable state plays a
key role in diverse scientific areas involving chemical kinetics, diffusion and
dislocation motion in solids, nucleation, electrical transport, motion of flux
lines superconductors, charge density waves, and transport processes of
macromolecules, to name but a few. The underlying activated processes present
the multidimensional extension of the Kramers problem of a single Brownian
particle. In comparison to the latter case, however, the dynamics ensuing from
the interactions of many coupled units can lead to intriguing novel phenomena
that are not present when only a single degree of freedom is involved. In this
review we report on a variety of such phenomena that are exhibited by systems
consisting of chains of interacting units in the presence of potential
barriers.
In the first part we consider recent developments in the case of a
deterministic dynamics driving cooperative escape processes of coupled
nonlinear units out of metastable states. The ability of chains of coupled
units to undergo spontaneous conformational transitions can lead to a
self-organised escape. The mechanism at work is that the energies of the units
become re-arranged, while keeping the total energy conserved, in forming
localised energy modes that in turn trigger the cooperative escape. We present
scenarios of significantly enhanced noise-free escape rates if compared to the
noise-assisted case.
The second part deals with the collective directed transport of systems of
interacting particles overcoming energetic barriers in periodic potential
landscapes. Escape processes in both time-homogeneous and time-dependent driven
systems are considered for the emergence of directed motion. It is shown that
ballistic channels immersed in the associated high-dimensional phase space are
the source for the directed long-range transport
Inhomogeneity growth in two-component fermionic systems
The dynamics of fermionic many-body systems is investigated in the framework
of Boltzmann-Langevin (BL) stochastic one-body approaches. Within the recently
introduced BLOB model, we examine the interplay between mean-field effects and
two-body correlations, of stochastic nature, for nuclear matter at moderate
temperature and in several density conditions, corresponding to stable or
mechanically unstable situations. Numerical results are compared to analytic
expectations for the fluctuation amplitude of isoscalar and isovector
densities, probing the link to the properties of the employed effective
interaction, namely symmetry energy (for isovector modes) and incompressibility
(for isoscalar modes). For unstable systems, clusterization is observed. The
associated features are compared to analytical results for the typical length
and time scales characterizing the growth of unstable modes in nuclear matter
and for the isotopic variance of the emerging fragments. We show that the BLOB
model is generally better suited than simplified approaches previously
introduced to solve the BL equation, and it is therefore more advantageous in
applications to open systems, like heavy ion collisions.Comment: 19 pages, 13 figure
Quantum optical memory protocols in atomic ensembles
We review a series of quantum memory protocols designed to store the quantum
information carried by light into atomic ensembles. In particular, we show how
a simple semiclassical formalism allows to gain insight into various memory
protocols and to highlight strong analogies between them. These analogies
naturally lead to a classification of light storage protocols into two
categories, namely photon echo and slow-light memories. We focus on the storage
and retrieval dynamics as a key step to map the optical information into the
atomic excitation. We finally review various criteria adapted for both
continuous variables and photon-counting measurement techniques to certify the
quantum nature of these memory protocols
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