513 research outputs found
Operations and single particle interferometry
Interferometry of single particles with internal degrees of freedom is
investigated. We discuss the interference patterns obtained when an internal
state evolution device is inserted into one or both the paths of the
interferometer. The interference pattern obtained is not uniquely determined by
the completely positive maps (CPMs) that describe how the devices evolve the
internal state of a particle. By using the concept of gluing of CPMs, we
investigate the structure of all possible interference patterns obtainable for
given trace preserving internal state CPMs. We discuss what can be inferred
about the gluing, given a sufficiently rich set of interference experiments. It
is shown that the standard interferometric setup is limited in its abilities to
distinguish different gluings. A generalized interferometric setup is
introduced with the capacity to distinguish all gluings. We also connect to
another approach using the well known fact that channels can be realized using
a joint unitary evolution of the system and an ancillary system. We deduce the
set of all such unitary `representations' and relate the structure of this set
to gluings and interference phenomena.Comment: Journal reference added. Material adde
Collective motion in quantum diffusive environment
The general problem of dissipation in macroscopic large-amplitude collective
motion and its relation to energy diffusion of intrinsic degrees of freedom of
a nucleus is studied. By applying the cranking approach to the nuclear
many-body system, a set of coupled dynamical equations for the collective
classical variable and the quantum mechanical occupancies of the intrinsic
nuclear states is derived. Different dynamical regimes of the intrinsic nuclear
motion and its consequences on time properties of collective dissipation are
discussed.Comment: 15 pages, 5 figure
Semiclassical Theory of Bardeen-Cooper-Schrieffer Pairing-Gap Fluctuations
Superfluidity and superconductivity are genuine many-body manifestations of
quantum coherence. For finite-size systems the associated pairing gap
fluctuates as a function of size or shape. We provide a parameter free
theoretical description of pairing fluctuations in mesoscopic systems
characterized by order/chaos dynamics. The theory accurately describes
experimental observations of nuclear superfluidity (regular system), predicts
universal fluctuations of superconductivity in small chaotic metallic grains,
and provides a global analysis in ultracold Fermi gases.Comment: 4 pages, 2 figure
Exact Coupling Coefficient Distribution in the Doorway Mechanism
In many--body and other systems, the physics situation often allows one to
interpret certain, distinct states by means of a simple picture. In this
interpretation, the distinct states are not eigenstates of the full
Hamiltonian. Hence, there is an interaction which makes the distinct states act
as doorways into background states which are modeled statistically. The crucial
quantities are the overlaps between the eigenstates of the full Hamiltonian and
the doorway states, that is, the coupling coefficients occuring in the
expansion of true eigenstates in the simple model basis. Recently, the
distribution of the maximum coupling coefficients was introduced as a new,
highly sensitive statistical observable. In the particularly important regime
of weak interactions, this distribution is very well approximated by the
fidelity distribution, defined as the distribution of the overlap between the
doorway states with interaction and without interaction. Using a random matrix
model, we calculate the latter distribution exactly for regular and chaotic
background states in the cases of preserved and fully broken time--reversal
invariance. We also perform numerical simulations and find excellent agreement
with our analytical results.Comment: 22 pages, 4 figure
Survival Probability of a Doorway State in regular and chaotic environments
We calculate survival probability of a special state which couples randomly
to a regular or chaotic environment. The environment is modelled by a suitably
chosen random matrix ensemble. The exact results exhibit non--perturbative
features as revival of probability and non--ergodicity. The role of background
complexity and of coupling complexity is discussed as well.Comment: 19 pages 5 Figure
Chaoticity and Shell Effects in the Nearest-Neighbor Distributions
Statistics of the single-particle levels in a deformed Woods-Saxon potential
is analyzed in terms of the Poisson and Wigner nearest-neighbor distributions
for several deformations and multipolarities of its surface distortions. We
found the significant differences of all the distributions with a fixed value
of the angular momentum projection of the particle, more closely to the Wigner
distribution, in contrast to the full spectra with Poisson-like behavior.
Important shell effects are observed in the nearest neighbor spacing
distributions, the larger the smaller deformations of the surface
multipolarities.Comment: 10 pages and 9 figure
Toward physical realizations of thermodynamic resource theories
Conventional statistical mechanics describes large systems and averages over
many particles or over many trials. But work, heat, and entropy impact the
small scales that experimentalists can increasingly control, e.g., in
single-molecule experiments. The statistical mechanics of small scales has been
quantified with two toolkits developed in quantum information theory: resource
theories and one-shot information theory. The field has boomed recently, but
the theorems amassed have hardly impacted experiments. Can thermodynamic
resource theories be realized experimentally? Via what steps can we shift the
theory toward physical realizations? Should we care? I present eleven
opportunities in physically realizing thermodynamic resource theories.Comment: Publication information added. Cosmetic change
Cold Fermionic Atoms in Two-Dimensional Traps -- Pairing versus Hund's Rule
The microscopic properties of few interacting cold fermionic atoms confined
in a two-dimensional (2D) harmonic trap are studied by numerical
diagonalization. For repulsive interactions, a strong shell structure
dominates, with Hund's rule acting at its extreme for the mid-shell
configurations. In the attractive case, odd-even oscillations due to pairing
occur simultaneously with deformations in the internal structure of the ground
states, as seen from pair correlation functions.Comment: RevTeX 4.0, 4 pages, 5 colour postscript figure
Effective-interaction approach to the many-boson problem
We show that the convergence behavior of the many-body numerical
diagonalization scheme for strongly interacting bosons in a trap can be
significantly improved by the Lee-Suzuki method adapted from nuclear physics:
One can construct an effective interaction that acts in a space much smaller
than the original Hilbert space. In particular for short-ranged forces and
strong correlations, the method offers a good estimate of the energy and the
excitation spectrum, at a computational cost several orders of magnitude
smaller than that required by the standard method.Comment: 5 pages, 4 figure
- …