15,460 research outputs found
Gaussian quantum fluctuations in interacting many particle systems
We consider a many particle quantum system, in which each particle interacts
only with its nearest neighbours. Provided that the energy per particle has an
upper bound, we show, that the energy distribution of almost every product
state becomes a Gaussian normal distribution in the limit of infinite number of
particles. We indicate some possible applications.Comment: 10 pages, formulation made mathematically more precise, two examples
added, accepted for publication in Letters in Mathematical Physic
Scaling behavior of interactions in a modular quantum system and the existence of local temperature
We consider a quantum system of fixed size consisting of a regular chain of
-level subsystems, where is finite. Forming groups of subsystems
each, we show that the strength of interaction between the groups scales with
. As a consequence, if the total system is in a thermal state with
inverse temperature , a sufficient condition for subgroups of size
to be approximately in a thermal state with the same temperature is , where is the width of the occupied
level spectrum of the total system. These scaling properties indicate on what
scale local temperatures may be meaningfully defined as intensive variables.
This question is particularly relevant for non-equilibrium scenarios such as
heat conduction etc.Comment: 7 pages, accepted for publication in Europhysics Letter
Spontaneous Conversion from Virtual to Real Photons in the Ultrastrong Coupling Regime
We show that a spontaneous release of virtual photon pairs can occur in a
quantum optical system in the ultrastrong coupling regime. In this regime,
which is attracting interest both in semiconductor and superconducting systems,
the light-matter coupling rate {\Omega}R becomes comparable to the bare
resonance frequency of photons {\omega}0. In contrast to the dynamical Casimir
effect and other pair creation mechanisms, this phenomenon does not require
external forces or time dependent parameters in the Hamiltonian.Comment: To appear on Phys. Rev. Let
Coherent control of photon transmission : slowing light in coupled resonator waveguide doped with Atoms
In this paper, we propose and study a hybrid mechanism for coherent
transmission of photons in the coupled resonator optical waveguide (CROW) by
incorporating the electromagnetically induced transparency (EIT) effect into
the controllable band gap structure of the CROW. Here, the configuration setup
of system consists of a CROW with homogeneous couplings and the artificial
atoms with -type three levels doped in each cavity. The roles of three
levels are completely considered based on a mean field approach where the
collection of three-level atoms collectively behave as two-mode spin waves. We
show that the dynamics of low excitations of atomic ensemble can be effectively
described by an coupling boson model. The exactly solutions show that the light
pulses can be stopped and stored coherently by adiabatically controlling the
classical field.Comment: 10 pages, 6 figure
On the merit of a Central Limit Theorem-based approximation in statistical physics
The applicability conditions of a recently reported Central Limit
Theorem-based approximation method in statistical physics are investigated and
rigorously determined. The failure of this method at low and intermediate
temperature is proved as well as its inadequacy to disclose quantum
criticalities at fixed temperatures. Its high temperature predictions are in
addition shown to coincide with those stemming from straightforward appropriate
expansions up to (k_B T)^(-2). Our results are clearly illustrated by comparing
the exact and approximate temperature dependence of the free energy of some
exemplary physical systems.Comment: 12 pages, 1 figur
Fractal dimension of domain walls in two-dimensional Ising spin glasses
We study domain walls in 2d Ising spin glasses in terms of a minimum-weight
path problem. Using this approach, large systems can be treated exactly. Our
focus is on the fractal dimension of domain walls, which describes via
\simL^{d_f} the growth of the average domain-wall length with %%
systems size . %% 20.07.07 OM %% Exploring systems up to L=320 we
yield for the case of Gaussian disorder, i.e. a much higher
accuracy compared to previous studies. For the case of bimodal disorder, where
many equivalent domain walls exist due to the degeneracy of this model, we
obtain a true lower bound and a (lower) estimate
as upper bound. Furthermore, we study the distributions of the domain-wall
lengths. Their scaling with system size can be described also only by the
exponent , i.e. the distributions are monofractal. Finally, we investigate
the growth of the domain-wall width with system size (``roughness'') and find a
linear behavior.Comment: 8 pages, 8 figures, submitted to Phys. Rev. B; v2: shortened versio
Discrete energy landscapes and replica symmetry breaking at zero temperature
The order parameter P(q) for disordered systems with degenerate ground-states
is reconsidered. We propose that entropy fluctuations lead to a trivial P(q) at
zero temperature as in the non-degenerate case, even if there are zero-energy
large-scale excitations (complex energy landscape). Such a situation should
arise in the 3-dimensional +-J Ising spin glass and in MAX-SAT. Also, we argue
that if the energy landscape is complex with a finite number of ground-state
families, then replica symmetry breaking reappears at positive temperature.Comment: 7 pages; clarifications on valley definition
Coupled cavity QED for coherent control of photon transmission (I): Green function approach for hybrid systems with two-level doping
This is the first one of a series of our papers theoretically studying the
coherent control of photon transmission along the coupled resonator optical
waveguide (CROW) by doping artificial atoms for various hybrid structures. We
will provide the several approaches correspondingly based on Green function,
the mean field method and spin wave theory et al. In the present paper we adopt
the two-time Green function approach to study the coherent transmission photon
in a CROW with homogeneous couplings, each cavity of which is doped by a
two-level artificial atom. We calculate the two-time correlation function for
photon in the weak-coupling case. Its poles predict the exact dispersion
relation, which results in the group velocity coherently controlled by the
collective excitation of the doping atoms. We emphasize the role of the
population inversion of doping atoms induced by some polarization mechanism.Comment: 11 pages, 9 figure
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