631 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
Fidelity and coherence measures from interference
By utilizing single particle interferometry, the fidelity or coherence of a pair of quantum states is identified with their capacity for interference. We consider processes acting on the internal degree of freedom (e.g., spin or polarization) of the interfering particle, preparing it in states ρA or ρB in the respective path of the interferometer. The maximal visibility depends on the choice of interferometer, as well as the locality or nonlocality of the preparations, but otherwise depends only on the states ρA and ρB and not the individual preparation processes themselves. This allows us to define interferometric measures which probe locality and correlation properties of spatially or temporally separated processes, and can be used to differentiate between processes that cannot be distinguished by direct process tomography using only the internal state of the particle
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
Operational approach to the Uhlmann holonomy
We suggest a physical interpretation of the Uhlmann amplitude of a density
operator. Given this interpretation we propose an operational approach to
obtain the Uhlmann condition for parallelity. This allows us to realize
parallel transport along a sequence of density operators by an iterative
preparation procedure. At the final step the resulting Uhlmann holonomy can be
determined via interferometric measurements.Comment: Added material, references, and journal reference
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
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
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
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