arXiv subjects

Italo Guarneri

Publications and source records attributed to Italo Guarneri.

30 records · Page 2Linked to original sources

arXiv2003

Decoherence as a Probe of Coherent Quantum Dynamics

The effect of decoherence, induced by spontaneous emission, on the dynamics of cold atoms periodically kicked by an optical lattice is experimentally and theoretically studied. Ideally, the mean energy growth is essentially unaffected by weak decoherence, but the resonant momentum distributions are fundamentally altered. It is shown that experiments are inevitably sensitive to certain nontrivial features of these distributions, in a way that explains the puzzle of the observed enhancement of resonances by decoherence [Phys. Rev. Lett. 87, 074102 (2001)]. This clarifies both the nature of the coherent evolution, and the way in which decoherence disrupts it.

quant-ph↗

arXiv2002

Quantum resonances and decoherence for delta-kicked atoms

The quantum resonances occurring with delta-kicked atoms when the kicking period is an integer multiple of the half-Talbot time are analyzed in detail. Exact results about the momentum distribution at exact resonance are established, both in the case of totally coherent dynamics and in the case when decoherence is induced by Spontaneous Emission. A description of the dynamics when the kicking period is close to, but not exactly at resonance, is derived by means of a quasi-classical approximation where the detuning from exact resonance plays the role of the Planck constant. In this way scaling laws describing the shape of the resonant peaks are obtained. Such analytical results are supported by extensive numerical simulations, and explain some recent surprising experimental observations.

nlin.CD↗

arXiv2002

Stable Quantum Resonances in Atom Optics

A theory for stabilization of quantum resonances by a mechanism similar to one leading to classical resonances in nonlinear systems is presented. It explains recent surprising experimental results, obtained for cold Cesium atoms when driven in the presence of gravity, and leads to further predictions. The theory makes use of invariance properties of the system, that are similar to those of solids, allowing for separation into independent kicked rotor problems. The analysis relies on a fictitious classical limit where the small parameter is {\em not} Planck's constant, but rather the detuning from the frequency that is resonant in absence of gravity.

nlin.CD↗

arXiv2002

A Theory for Quantum Accelerator Modes in Atom Optics

Unexpected accelerator modes were recently observed experimentally for cold cesium atoms when driven in the presence of gravity. A detailed theoretical explanation of this quantum effect is presented here. The theory makes use of invariance properties of the system, that are similar to the ones of solids, leading to a separation into independent kicked rotor problems. The analytical solution makes use of a limit that is very similar to the semiclassical limit, but with a small parameter that is not Planck's constant, but rather the detuning from the frequency that is resonant in absence of gravity.

nlin.CD↗

arXiv2001

Fractal fluctuations in quantum integrable scattering

We theoretically and numerically demonstrate that completely integrable scattering processes may exhibit fractal transmission fluctuations, due to typical spectral properties of integrable systems. Similar properties also occur with scattering processes in the presence of strong dynamical localization, thus explaining recent numerical observations of fractality in the latter class of systems.

cond-mat.mes-hall↗

arXiv2001

Quantum Fractal Fluctuations

We numerically analyse quantum survival probability fluctuations in an open, classically chaotic system. In a quasi-classical regime, and in the presence of classical mixed phase space, such fluctuations are believed to exhibit a fractal pattern, on the grounds of semiclassical arguments. In contrast, we work in a classical regime of complete chaoticity, and in a deep quantum regime of strong localization. We provide evidence that fluctuations are still fractal, due to the slow, purely quantum algebraic decay in time produced by dynamical localization. Such findings considerably enlarge the scope of the existing theory.

cond-mat↗

arXiv1999

Chaotic dynamics of a classical radiant cavity

The statistical properties of a classical electromagnetic field in interaction with matter are numerically investigated on a one-dimensional model of a radiant cavity, conservative and with finite total energy. Our results suggest a trend towards equipartition of energy, with the relaxation times of the normal modes of the cavity increasing with the mode frequency according to a law, the form of which depends on the shape of the charge distribution.

chao-dyn↗

arXiv1998

On the Dynamical Meaning of Spectral Dimensions

Time averaging over the trajectory of a wavepacket up to time T defines a statistical operator (density matrix). The corresponding (Von Neumann) entropy is proven to asymptotically increase with time like D.log T, with D the Hausdorff dimension of the Local Density of States, at least if the latter measure has good scaling properties. In more general cases, spectral upper and lower bounds for the increase of entropy are given, in terms of the Hausdorff and of the Fractal Dimension of the Local Density of States.

math-ph↗

arXiv1997

Quantum-Classical Correspondence in Energy Space: Two Interacting Spin-Particles

The Hamiltonian conservative system of two interacting particles has been considered both in classical and quantum description. The quantum model has been realized using a symmetrized two-particle basis reordered in the unperturbed energy. Main attention is paid to the structure of chaotic eigenfunctions (EF) and to the local spectral density of states (LDOS). A remarkable correspondence has been found for the shapes of EF and LDOS in the energy representation, to their classical counterparts. Comparison with the Band Random Matrix theory predictions has revealed quite significant differences which are due to dynamical nature of the model. On the other hand, a partial agreement is found by inserting randomness `` ad hoc '' in the dynamical model for two-body matrix elements. This shows that, at least for small number of particles, care must be taken when classical correlations are neglected. The question of quantum localization in the energy space is discussed both for dynamical and random model.

chao-dyn↗

arXiv1997

Landauer and Thouless Conductance: a Band Random Matrix Approach

We numerically analyze the transmission through a thin disordered wire of finite length attached to perfect leads, by making use of banded random Hamiltonian matrices. We compare the Landauer and the Thouless conductances, and find that they are proportional to each other in the diffusive regime, while in the localized regime the Landauer conductance is approximately proportional to the square of the Thouless one. Fluctuations of the Landauer conductance were also numerically computed; they are shown to slowly approach the theoretically predicted value.

cond-mat.mes-hall↗

arXiv1995

Fractal versus QuasiClassical Diffusive Transport in a class of quantum systems

We compare the properties of transmission across one-dimensional finite samples which are associated with two types of "quantum diffusion", one related to a classical chaotic dynamics, the other to a multifractal energy spectrum. We numerically investigate models exhibiting one or both of these features, and we find in all cases an inverse power law dependence of the average transmission on the sample length. Although in all the considered cases the quadratic spread of wave packets increases linearly (or very close to linearly) in time for both types of dynamics, a proper Ohmic dependence is always observed only in the case of quasi-classical diffusion. The analysis of the statistics of transmission fluctuations in the case of a fractal spectrum exposes some new features, which mark further differences from ordinary diffusion, and enforce the conclusion that the two types of transmission are intrinsically different.

cond-mat↗

arXiv1993

S-matrix Fluctuations in a model with Classical Diffusion and Quantum Localization

The statistics of S-matrix fluctuations are numerically investigated on a model for irregular quantum scattering in which a classical chaotic diffusion takes place within the interaction region. Agreement with various random-matrix theoretic predictions is discussed in the various regimes (ballistic, diffusive, localized).

chao-dyn↗