2,723 research outputs found
Thermal phase transitions for Dicke-type models in the ultra-strong coupling limit
We consider the Dicke model in the ultra-strong coupling limit to investigate
thermal phase transitions and their precursors at finite particle numbers
for bosonic and fermionic systems. We derive partition functions with
degeneracy factors that account for the number of configurations and derive
explicit expressions for the Landau free energy. This allows us to discuss the
difference between the original Dicke (fermionic) and the bosonic case. We find
a crossover between these two cases that shows up, e.g., in the specific heat.Comment: 4 pages Brief Report styl
Truncation method for Green's functions in time-dependent fields
We investigate the influence of a time dependent, homogeneous electric field
on scattering properties of non-interacting electrons in an arbitrary static
potential. We develop a method to calculate the (Keldysh) Green's function in
two complementary approaches. Starting from a plane wave basis, a formally
exact solution is given in terms of the inverse of a matrix containing
infinitely many 'photoblocks' which can be evaluated approximately by
truncation. In the exact eigenstate basis of the scattering potential, we
obtain a version of the Floquet state theory in the Green's functions language.
The formalism is checked for cases such as a simple model of a double barrier
in a strong electric field. Furthermore, an exact relation between the
inelastic scattering rate due to the microwave and the AC conductivity of the
system is derived which in particular holds near or at a metal-insulator
transition in disordered systems.Comment: to appear in Phys. Rev. B., 21 pages, 3 figures (ps-files
Shot noise spectrum of superradiant entangled excitons
The shot noise produced by tunneling of electrons and holes into a double dot
system incorporated inside a p-i-n junction is investigated theoretically. The
enhancement of the shot noise is shown to originate from the entangled
electron-hole pair created by superradiance. The analogy to the superconducting
cooper pair box is pointed out. A series of Zeno-like measurements is shown to
destroy the entanglement, except for the case of maximum entanglement.Comment: 5 pages, 3 figures, to appear in Phys. Rev. B (2004
Nonequilibrium Quantum Phase Transitions in the Dicke Model
We establish a set of nonequilibrium quantum phase transitions in the Dicke
model by considering a monochromatic nonadiabatic modulation of the atom-field
coupling. For weak driving the system exhibits a set of sidebands which allow
the circumvention of the no-go theorem which otherwise forbids the occurence of
superradiant phase transitions. At strong driving we show that the system
exhibits a rich multistable structure and exhibits both first- and second-order
nonequilibrium quantum phase transitions.Comment: 4 pages, 3 Figures, and supplementary material. This new version
contains corrected typos, new references and new versions of the figures.
Published by Physical Review Letter
Non-equilibrium Entanglement and Noise in Coupled Qubits
We study charge entanglement in two Coulomb-coupled double quantum dots in
thermal equilibrium and under stationary non-equilibrium transport conditions.
In the transport regime, the entanglement exhibits a clear switching threshold
and various limits due to suppression of tunneling by Quantum Zeno localisation
or by an interaction induced energy gap. We also calculate quantum noise
spectra and discuss the inter-dot current correlation as an indicator of the
entanglement in transport experiments.Comment: 4 pages, 4 figure
Decremental All-Pairs ALL Shortest Paths and Betweenness Centrality
We consider the all pairs all shortest paths (APASP) problem, which maintains
the shortest path dag rooted at every vertex in a directed graph G=(V,E) with
positive edge weights. For this problem we present a decremental algorithm
(that supports the deletion of a vertex, or weight increases on edges incident
to a vertex). Our algorithm runs in amortized O(\vstar^2 \cdot \log n) time per
update, where n=|V|, and \vstar bounds the number of edges that lie on shortest
paths through any given vertex. Our APASP algorithm can be used for the
decremental computation of betweenness centrality (BC), a graph parameter that
is widely used in the analysis of large complex networks. No nontrivial
decremental algorithm for either problem was known prior to our work. Our
method is a generalization of the decremental algorithm of Demetrescu and
Italiano [DI04] for unique shortest paths, and for graphs with \vstar =O(n), we
match the bound in [DI04]. Thus for graphs with a constant number of shortest
paths between any pair of vertices, our algorithm maintains APASP and BC scores
in amortized time O(n^2 \log n) under decremental updates, regardless of the
number of edges in the graph.Comment: An extended abstract of this paper will appear in Proc. ISAAC 201
Current Switch by Coherent Trapping of Electrons in Quantum Dots
We propose a new transport mechanism through tunnel-coupled quantum dots
based on the coherent population trapping effect. Coupling to an excited level
by the coherent radiation of two microwaves can lead to an extremely narrow
current antiresonance. The effect can be used to determine interdot dephasing
rates and is a mechanism for a very sensitive, optically controlled current
switch.Comment: to appear in Phys. Rev. Let
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