754 research outputs found
The Effect of absorbing sites on the one-dimensional cellular automaton traffic flow with open boundaries
The effect of the absorbing sites with an absorbing rate , in both
one absorbing site (one way out) and two absorbing sites (two ways out) in a
road, on the traffic flow phase transition is investigated using numerical
simulations in the one-dimensional cellular automaton traffic flow model with
open boundaries using parallel dynamics.In the case of one way out, there exist
a critical position of the way out below which the current is
constant for and decreases when increasing
for . When the way out is located at a
position greater than , the current increases with for
and becomes constant for any value of
greater than . While, when the way out is located at any position
between and (), the current increases,
for , with and becomes constant for
and decreases with for
. In the later case the density undergoes two
successive first order transitions; from high density to maximal current phase
at and from intermediate density to the low one at
. In the case of two ways out located respectively
at the positions and , the two successive transitions occur
only when the distance - separating the two ways is smaller than
a critical distance . Phase diagrams in the (),
() and () planes are established. It is found
that the transitions between Free traffic, Congested traffic and maximal
current phase are first order
Dark pair coherent states of the motion of a trapped ion
We propose a scheme for generating vibrational pair coherent states of the
motion of an ion in a two-dimensional trap. In our scheme, the trapped ion is
excited bichromatically by three laser beams along different directions in the
X-Y plane of the ion trap. We show that if the initial vibrational state is
given by a two-mode Fock state, the final steady state, indicated by the
extinction of the fluorescence emitted by the ion, is a pure state. The
motional state of the ion in the equilibrium realizes that of the
highly-correlated pair coherent state.Comment: 14 pages, 3 figure
Two-Photon Doppler cooling of alkaline-earth-metal and ytterbium atoms
A new possibility of laser cooling of alkaline-earth-metal and Ytterbium
atoms using a two-photon transition is analyzed. We consider a -
transition, with excitation in near resonance with the
level. This greatly increases the two-photon transition rate, allowing an
effective transfer of momentum. The experimental implementation of this
technique is discussed and we show that for Calcium, for example, two-photon
cooling can be used to achieve a Doppler limit of 123 microKelvin. The
efficiency of this cooling scheme and the main loss mechanisms are analyzed.Comment: 7 pages, 5 figure
Analytic results for particles with interaction in two dimensions and an external magnetic field
The -dimensional quantum problem of particles (e.g. electrons) with
interaction in a two-dimensional parabolic potential
(e.g. quantum dot) and magnetic field , reduces exactly to solving a
-dimensional problem which is independent of and . An
exact, infinite set of relative mode excitations are obtained for any . The
problem reduces to that of a ficticious particle in a two-dimensional,
non-linear potential of strength , subject to a ficticious magnetic
field , the relative angular momentum.Comment: To appear in Physical Review Letters (in press). RevTeX file. Two
figures available from [email protected] or
[email protected]
Quenched Narrow-Line Laser Cooling of 40Ca to Near the Photon Recoil Limit
We present a cooling method that should be generally applicable to atoms with
narrow optical transitions. This technique uses velocity-selective pulses to
drive atoms towards a zero-velocity dark state and then quenches the excited
state to increase the cooling rate. We demonstrate this technique of quenched
narrow-line cooling by reducing the 1-D temperature of a sample of neutral 40Ca
atoms. We velocity select and cool with the 1S0(4s2) to 3P1(4s4p) 657 nm
intercombination line and quench with the 3P1(4s4p) to 1S0(4s5s)
intercombination line at 553 nm, which increases the cooling rate eight-fold.
Limited only by available quenching laser power, we have transferred 18 % of
the atoms from our initial 2 mK velocity distribution and achieved temperatures
as low as 4 microK, corresponding to a vrms of 2.8 cm/s or 2 recoils at 657 nm.
This cooling technique, which is closely related to Raman cooling, can be
extended to three dimensions.Comment: 5 pages, 4 figures; Submitted to PRA Rapid Communication
Nonclassical Interference Effects In The Radiation From Coherently Driven Uncorrelated Atoms
We demonstrate the existence of new nonclassical correlations in the
radiation of two atoms, which are coherently driven by a continuous laser
source. The photon-photon-correlations of the fluorescence light show a spatial
interferene pattern not present in a classical treatment. A feature of the new
phenomenon is, that bunched and antibunched light is emitted in different
spatial directions. The calculations are performed analytically. It is pointed
out, that the correlations are induced by state reduction due to the
measurement process when the detection of the photons does not distinguish
between the atoms. It is interesting to note, that the phenomena show up even
without any interatomic interaction.Comment: 4 pages, 6 Figure
Single photon generation by pulsed excitation of a single dipole
The fluorescence of a single dipole excited by an intense light pulse can
lead to the generation of another light pulse containing a single photon. The
influence of the duration and energy of the excitation pulse on the number of
photons in the fluorescence pulse is studied. The case of a two-level dipole
with strongly damped coherences is considered. The presence of a metastable
state leading to shelving is also investigated.Comment: 17 pages, 4 figures, submitted to PR
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