2,654 research outputs found
Electron barrier interaction in a vacuum tunneling probe
A model for dealing with energy and momentum exchanges between ballistic
electrons in a vacuum barrier in a tunneling probe used as an electromechanical
transducer is studied and its physical significance in devices of size
comparable to the mean free path of the tunneling electrons is discusse
Ultracold atomic Fermi-Bose mixtures in bichromatic optical dipole traps: a novel route to study fermion superfluidity
The study of low density, ultracold atomic Fermi gases is a promising avenue
to understand fermion superfluidity from first principles. One technique
currently used to bring Fermi gases in the degenerate regime is sympathetic
cooling through a reservoir made of an ultracold Bose gas. We discuss a
proposal for trapping and cooling of two-species Fermi-Bose mixtures into
optical dipole traps made from combinations of laser beams having two different
wavelengths. In these bichromatic traps it is possible, by a proper choice of
the relative laser powers, to selectively trap the two species in such a way
that fermions experience a stronger confinement than bosons. As a consequence,
a deep Fermi degeneracy can be reached having at the same time a softer
degenerate regime for the Bose gas. This leads to an increase in the
sympathetic cooling efficiency and allows for higher precision thermometry of
the Fermi-Bose mixture
Ground state of many-body lattice systems via a central limit theorem
We review a novel approach to evaluate the ground-state properties of
many-body lattice systems based on an exact probabilistic representation of the
dynamics and its long time approximation via a central limit theorem. The
choice of the asymptotic density probability used in the calculation is
discussed in detail.Comment: 9 pages, contribution to the proceedings of 12th International
Conference on Recent Progress in Many-Body Theories, Santa Fe, New Mexico,
August 23-27, 200
Asymptotic lower bound for the gap of Hermitian matrices having ergodic ground states and infinitesimal off-diagonal elements
Given a Hermitian matrix with possibly degenerate
eigenvalues , we provide,
in the limit , a lower bound for the gap assuming that (i) the eigenvector (eigenvectors) associated to
is ergodic (are all ergodic) and (ii) the off-diagonal terms of
vanish for more slowly than . Under these
hypotheses, we find . This general result turns out to be important for
upper bounding the relaxation time of linear master equations characterized by
a matrix equal, or isospectral, to . As an application, we
consider symmetric random walks with infinitesimal jump rates and show that the
relaxation time is upper bounded by the configurations (or nodes) with minimal
degree.Comment: 5 page
Obtaining pure steady states in nonequilibrium quantum systems with strong dissipative couplings
Dissipative preparation of a pure steady state usually involves a commutative
action of a coherent and a dissipative dynamics on the target state. Namely,
the target pure state is an eigenstate of both the coherent and dissipative
parts of the dynamics. We show that working in the Zeno regime, i.e. for
infinitely large dissipative coupling, one can generate a pure state by a non
commutative action, in the above sense, of the coherent and dissipative
dynamics. A corresponding Zeno regime pureness criterion is derived. We
illustrate the approach, looking at both its theoretical and applicative
aspects, in the example case of an open spin- chain, driven out of
equilibrium by boundary reservoirs targeting different spin orientations. Using
our criterion, we find two families of pure nonequilibrium steady states, in
the Zeno regime, and calculate the dissipative strengths effectively needed to
generate steady states which are almost indistinguishable from the target pure
states.Comment: 8 pages, 6 figure
A Selective Relaxation Method for Numerical Solution of Schr\"odinger Problems
We propose a numerical method for evaluating eigenvalues and eigenfunctions
of Schr\"odinger operators with general confining potentials. The method is
selective in the sense that only the eigenvalue closest to a chosen input
energy is found through an absolutely-stable relaxation algorithm which has
rate of convergence infinite. In the case of bistable potentials the method
allows one to evaluate the fundamental energy splitting for a wide range of
tunneling rates.Comment: 4 pages with figures, uuencoded Z-compressed ps fil
Phase transitions and gaps in quantum random energy models
By using a previously established exact characterization of the ground state
of random potential systems in the thermodynamic limit, we determine the ground
and first excited energy levels of quantum random energy models, discrete and
continuous. We rigorously establish the existence of a universal first order
quantum phase transition, obeyed by both the ground and the first excited
states. The presence of an exponentially vanishing minimal gap at the
transition is general but, quite interestingly, the gap averaged over the
realizations of the random potential is finite. This fact leaves still open the
chance for some effective quantum annealing algorithm, not necessarily based on
a quantum adiabatic scheme.Comment: 8 pages, 4 figure
Thermalization of noninteracting quantum systems coupled to blackbody radiation: A Lindblad-based analysis
We study the thermalization of an ensemble of elementary,
arbitrarily-complex, quantum systems, mutually noninteracting but coupled as
electric or magnetic dipoles to a blackbody radiation. The elementary systems
can be all the same or belong to different species, distinguishable or
indistinguishable, located at fixed positions or having translational degrees
of freedom. Even if the energy spectra of the constituent systems are
nondegenerate, as we suppose, the ensemble unavoidably presents degeneracies of
the energy levels and/or of the energy gaps. We show that, due to these
degeneracies, a thermalization analysis performed by the popular quantum
optical master equation reveals a number of serious pathologies, possibly
including a lack of ergodicity. On the other hand, a consistent thermalization
scenario is obtained by introducing a Lindblad-based approach, in which the
Lindblad operators, instead of being derived from a microscopic calculation,
are established as the elements of an operatorial basis with squared amplitudes
fixed by imposing a detailed balance condition and requiring their
correspondence with the dipole transition rates evaluated under the first-order
perturbation theory. Due to the above-mentioned degeneracies, this procedure
suffers a basis arbitrariness which, however, can be removed by exploiting the
fact that the thermalization of an ensemble of noninteracting systems cannot
depend on the ensemble size. As a result, we provide a clear-cut partitioning
of the thermalization time into dissipation and decoherence times, for which we
derive formulas giving the dependence on the energy levels of the elementary
systems, the size of the ensemble, and the temperature of the blackbody
radiation.Comment: 9 pages, 1 figur
- …