754 research outputs found

    The Effect of absorbing sites on the one-dimensional cellular automaton traffic flow with open boundaries

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    The effect of the absorbing sites with an absorbing rate β0\beta_{0}, 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 ic1 i_{c1} below which the current is constant for β0\beta_{0}<<β0c2\beta_{0c2} and decreases when increasing β0\beta_{0} for β0\beta_{0}>>β0c2\beta_{0c2}. When the way out is located at a position greater than ic2 i_{c2}, the current increases with β0\beta_{0} for β0\beta_{0}<<β0c1\beta_{0c1} and becomes constant for any value of β0\beta_{0} greater than β0c1\beta_{0c1}. While, when the way out is located at any position between ic1 i_{c1} and ic2 i_{c2} (ic1 i_{c1}<<ic2 i_{c2}), the current increases, for β0\beta_{0}<<β0c1\beta_{0c1}, with β0\beta_{0} and becomes constant for β0c1\beta_{0c1}<<β0\beta_{0}<<β0c2\beta_{0c2} and decreases with β0\beta_{0} for β0\beta_{0}>>β0c2\beta_{0c2}. In the later case the density undergoes two successive first order transitions; from high density to maximal current phase at β0\beta_{0}==β0c1\beta_{0c1} and from intermediate density to the low one at β0\beta_{0}==β0c2\beta_{0c2}. In the case of two ways out located respectively at the positions i1 i_{1} and i2 i_{2}, the two successive transitions occur only when the distance i2i_{2}-i1i_{1} separating the two ways is smaller than a critical distance dcd_{c}. Phase diagrams in the (α,β0\alpha,\beta_{0}), (β,β0\beta,\beta_{0}) and (i1,β0i_{1},\beta_{0}) 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

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    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

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    A new possibility of laser cooling of alkaline-earth-metal and Ytterbium atoms using a two-photon transition is analyzed. We consider a 1S0^{1}S_{0} - 1S0^{1}S_{0} transition, with excitation in near resonance with the 1P1^{1}P_{1} 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 NN particles with 1/r21/r^2 interaction in two dimensions and an external magnetic field

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    The 2N2N-dimensional quantum problem of NN particles (e.g. electrons) with interaction β/r2\beta/r^2 in a two-dimensional parabolic potential ω0\omega_0 (e.g. quantum dot) and magnetic field BB, reduces exactly to solving a (2N4)(2N-4)-dimensional problem which is independent of BB and ω0\omega_0. An exact, infinite set of relative mode excitations are obtained for any NN. The N=3N=3 problem reduces to that of a ficticious particle in a two-dimensional, non-linear potential of strength β\beta, subject to a ficticious magnetic field BficJB_{\rm fic}\propto J, 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

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    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

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    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

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    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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