892 research outputs found
Adiabatic geometric phases in hydrogenlike atoms
We examine the effect of spin-orbit coupling on geometric phases in
hydrogenlike atoms exposed to a slowly varying magnetic field. The marginal
geometric phases associated with the orbital angular momentum and the intrinsic
spin fulfill a sum rule that explicitly relates them to the corresponding
geometric phase of the whole system. The marginal geometric phases in the
Zeeman and Paschen-Back limit are analyzed. We point out the existence of nodal
points in the marginal phases that may be detected by topological means.Comment: Clarifying material added, one figure removed, journal reference
adde
The Coherent Crooks Equality
This chapter reviews an information theoretic approach to deriving quantum
fluctuation theorems. When a thermal system is driven from equilibrium, random
quantities of work are required or produced: the Crooks equality is a classical
fluctuation theorem that quantifies the probabilities of these work
fluctuations. The framework summarised here generalises the Crooks equality to
the quantum regime by modeling not only the driven system but also the control
system and energy supply that enables the system to be driven. As is reasonably
common within the information theoretic approach but high unusual for
fluctuation theorems, this framework explicitly accounts for the energy
conservation using only time independent Hamiltonians. We focus on explicating
a key result derived by Johan {\AA}berg: a Crooks-like equality for when the
energy supply is allowed to exist in a superposition of energy eigenstates
states.Comment: 11 pages, 3 figures; Chapter for the book "Thermodynamics in the
Quantum Regime - Recent Progress and Outlook", eds. F. Binder, L. A. Correa,
C. Gogolin, J. Anders and G. Adess
Correlation studies of fission fragment neutron multiplicities
We calculate neutron multiplicities from fission fragments with specified
mass numbers for events having a specified total fragment kinetic energy. The
shape evolution from the initial compound nucleus to the scission
configurations is obtained with the Metropolis walk method on the
five-dimensional potential-energy landscape, calculated with the
macroscopic-microscopic method for the three-quadratic-surface shape family.
Shape-dependent microscopic level densities are used to guide the random walk,
to partition the intrinsic excitation energy between the two proto-fragments at
scission, and to determine the spectrum of the neutrons evaporated from the
fragments. The contributions to the total excitation energy of the resulting
fragments from statistical excitation and shape distortion at scission is
studied. Good agreement is obtained with available experimental data on neutron
multiplicities in correlation with fission fragments from U(n,f). At higher neutron energies a superlong fission mode appears which
affects the dependence of the observables on the total fragment kinetic energy.Comment: 12 pages, 10 figure
Noncyclic geometric changes of quantum states
Non-Abelian quantum holonomies, i.e., unitary state changes solely induced by
geometric properties of a quantum system, have been much under focus in the
physics community as generalizations of the Abelian Berry phase. Apart from
being a general phenomenon displayed in various subfields of quantum physics,
the use of holonomies has lately been suggested as a robust technique to obtain
quantum gates; the building blocks of quantum computers. Non-Abelian holonomies
are usually associated with cyclic changes of quantum systems, but here we
consider a generalization to noncyclic evolutions. We argue that this open-path
holonomy can be used to construct quantum gates. We also show that a structure
of partially defined holonomies emerges from the open-path holonomy. This
structure has no counterpart in the Abelian setting. We illustrate the general
ideas using an example that may be accessible to tests in various physical
systems.Comment: Extended version, new title, journal reference adde
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
Propagation of transient electromagnetic waves in time-varying media - Direct and inverse scattering problems
Wave propagation of transient electromagnetic waves in time-varying media is considered. The medium, which is assumed to be inhomogeneous and dispersive, lacks invariance under time translations. The spatial variation of the medium is assumed to be in the depth coordinate, i.e., it is stratified. The constitutive relations of the medium is a time integral of a generalized susceptibility kernel and the field. The generalized susceptibility kernel depends on one spatial and two time coordinates. The concept of wave splitting is introduced. The direct and inverse scattering problems are solved by the use of an imbedding or a Green functions approach. The direct and the inverse scattering problems are solved for a homogeneous semi-infinite medium. Explicit algorithms are developed. In this inverse scattering problem, a function depending on two time coordinates is reconstructed. Several numerical computations illustrate the performance of the algorithms
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