5,516 research outputs found
A New Form of Path Integral for the Coherent States Representation and its Semiclassical Limit
The overcompleteness of the coherent states basis leads to a multiplicity of
representations of Feynman's path integral. These different representations,
although equivalent quantum mechanically, lead to different semiclassical
limits. Two such semiclassical formulas were derived in \cite{Bar01} for the
two corresponding path integral forms suggested by Klauder and Skagerstan in
\cite{Klau85}. Each of these formulas involve trajectories governed by a
different classical representation of the Hamiltonian operator: the P
representation in one case and the Q representation in other. In this paper we
construct a third representation of the path integral whose semiclassical limit
involves directly the Weyl representation of the Hamiltonian operator, i.e.,
the classical Hamiltonian itself.Comment: 16 pages, no figure
Tourism in Azores Islands: Persistence in the Monthly Arrivals
This study analyses the persistence in the international monthly arrivals to the Azores Islands using a model based on fractional integration and seasonal autoregressions. The estimated fractional differencing parameter gives an indication of the long run evolution of the series. We use both aggregate data and disaggregate monthly data by location of origin and island destination. The results show that the aggregate series corresponding to the total number of arrivals is a nonstationary I(d) process with d above 1, and the most persistent ones are those travelling to Säo Miguel, especially from Holland, Finland, Norway, Germany, Denmark and the UK.Monthly arrivals; Seasonal fractional integration; Persistence; Azores Islands.
Quasi-relativistic behavior of cold atoms in light fields
We study the influence of three laser beams on the center of mass motion of
cold atoms with internal energy levels in a tripod configuration. We show that
similar to electrons in graphene the atomic motion can be equivalent to the
dynamics of ultra-relativistic two-component Dirac fermions. We propose and
analyze an experimental setup for observing such a quasi-relativistic motion of
ultracold atoms. We demonstrate that the atoms can experience negative
refraction and focussing by Veselago-type lenses. We also show how the chiral
nature of the atomic motion manifests itself as an oscillation of the atomic
internal state population which depends strongly on the direction of the center
of mass motion. For certain directions an atom remains in its initial state,
whereas for other directions the populations undergo oscillations between a
pair of internal states.Comment: 4 pages, updated version, Phys. Rev. A 77, (R)011802 (2008
Empirical Performance Models of MAC Protocols for Cooperative Platooning Applications
Vehicular ad-hoc networks (VANET) enable vehicles to exchange information on traffic
conditions, dynamic status and localization, to enhance road safety and transportation efficiency.
A typical VANET application is platooning, which can take advantage of exchanging information on
speed, heading and position to allow shorter inter-vehicle distances without compromising safety.
However, the platooning performance depends drastically on the quality of the communication
channel, which in turn is highly influenced by the medium access control protocol (MAC). Currently,
VANETs use the IEEE 802.11p MAC, which follows a carrier sense multiple access with collision
avoidance (CSMA/CA) policy that is prone to collisions and degrades significantly with network
load. This has led to recent proposals for a time-division multiple access (TDMA)-based MAC that
synchronize vehicles’ beacons to prevent or reduce collisions. In this paper, we take CSMA/CA
and two TDMA-based overlay protocols, i.e., deployed over CSMA/CA, namely PLEXE-slotted and
RA-TDMAp, and carry out extensive simulations with varying platoon sizes, number of occupied
lanes and transmit power to deduce empirical models that provide estimates of average number of
collisions per second and average busy time ratio. In particular, we show that these estimates can
be obtained from observing the number of radio-frequency (RF) neighbours, i.e., number of distinct
sources of the packets received by each vehicle per time unit. These estimates can enhance the online
adaptation of distributed applications, particularly platooning control, to varying conditions of the
communication channel.info:eu-repo/semantics/publishedVersio
Reversible quantum teleportation in an optical lattice
We propose a protocol, based on entanglement procedures recently suggested by
[D. Jaksch et al., Phys. Rev. Lett. 82, 1975 (1999)], which allows the
teleportation of an unknown state of a neutral atom in an optical lattice to
another atom in another site of the lattice, without any irreversible
detection.Comment: 8 pages, 3 figure
Antiferromagnetic Heisenberg Spin Chain of a Few Cold Atoms in a One-Dimensional Trap
We report on the deterministic preparation of antiferromagnetic Heisenberg
spin chains consisting of up to four fermionic atoms in a one-dimensional trap.
These chains are stabilized by strong repulsive interactions between the two
spin components without the need for an external periodic potential. We
independently characterize the spin configuration of the chains by measuring
the spin orientation of the outermost particle in the trap and by projecting
the spatial wave function of one spin component on single-particle trap levels.
Our results are in good agreement with a spin-chain model for fermionized
particles and with numerically exact diagonalizations of the full few-fermion
system
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Achieving accurate FTIR measurements on high performance bandpass filters
The sources of ordinate error in FTIR spectrometers are reviewed with reference to measuring small out-of-band features in the spectra of bandpass filters. Procedures for identifying instrumental artefacts are described. It is shown that features well below 0.01%T can be measured reliably
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