2,686 research outputs found
Coherent states superpositions in cavity quantum electrodynamics with trapped ions
We investigate how superpositions of motional coherent states naturally arise
in the dynamics of a two-level trapped ion coupled to the quantized field
inside a cavity. We extend our considerations including a more realistic set up
where the cavity is not ideal and photons may leak through its mirrors. We
found that a detection of a photon outside the cavity would leave the ion in a
pure state. The statistics of the ionic state still keeps some interference
effects that might be observed in the weak coupling regime.Comment: Figure and typos correcte
A novel CMOS analog neural oscillator cell
A very flexible programmable CMOS analog neural oscillator cell architecture is presented. The proposed neuron circuit architecture is a hysteretic neural-type pulse oscillator. Its implementation consists of a transconductance comparator, a capacitor, and two nonlinear resistors. It has over nine decades of oscillation frequency range, i.e., from 10/sup -2/ Hz to 20 MHz. This range has been experimentally verified. The oscillator cell in the test chip was implemented in a standard 3- mu m (p-well), double-metal CMOS technology and has a dimension of about 44000 mu m/sup 2/ (without the capacitor). Preliminary measurements and simulated results agree very well
Collimation Considerations For PS2
A main concern in high intensity rings is the evaluation of uncontrolled losses and their minimization using collimation systems. A two-stage system is foreseen for the PS2. The fundamental design strategy for the collimation design is presented, including machine apertures and collimator materials. The dependence of the collimator system efficiency on the primary scraper length and the impact parameter of the particle is evaluated for different collimator locations. Beam loss maps are finally produced displaying the detailed power load deposited around the ring
Beam Losses and Collimation Considerations for PS2
The high intensity beams with different emittances foreseen to be delivered by the PS2, an upgraded version of the actual CERN Proton Synchrotron, require strict control of beam losses in order to protect the machine components and enable their hands-on maintenance. Beam loss simulations based on dedicated numerical tools are undertaken for a variety of PS2 beams and for different loss mechanisms, along the whole accelerating cycle. In this respect, a first iteration of the collimation system is presented
Ambipolar Diffusion in the Magnetorotational Instability
The effects of ambipolar diffusion on the linear stability of weakly ionised
accretion discs are examined. Earlier work on this topic has focused on axial
magnetic fields and perturbation wavenumbers. We consider here more general
field and wavenumber geometries, and find that qualitatively new results are
obtained. Provided a radial wavenumber and azimuthal field are present along
with their axial counterparts, ambipolar diffusion will always be
destabilising, with unstable local modes appearing at well-defined wavenumber
bands. The wavenumber corresponding to the maximum growth rate need not, in
general, lie along the vertical axis. Growth rates become small relative to the
local angular velocity when the ion-neutral collision time exceeds the orbital
time. In common with Hall electromotive forces, ambipolar diffusion
destabilises both positive and negative angular velocity gradients. In at least
some cases, therefore, uniformly rotating molecular cloud cores may reflect the
marginally stable state of the ambipolar magnetorotational instability.Comment: Submitted to MN, 6 pages, 3 figs, MN style file v2.
Critical frequency for vortex nucleation in Bose-Fermi mixtures in optical lattices
We investigate within mean-field theory the influence of a one-dimensional
optical lattice and of trapped degenerate fermions on the critical rotational
frequency for vortex line creation in a Bose-Einstein condensate. We consider
laser intensities of the lattice such that quantum coherence across the
condensate is ensured. We find a sizable decrease of the thermodynamic critical
frequency for vortex nucleation with increasing applied laser strength and
suggest suitable parameters for experimental observation. Since 87Rb-40K
mixtures may undergo collapse, we analyze the related question of how the
optical lattice affects the mechanical stability of the system.Comment: 5 pages, 4 figures, revtex
Freezing of He-4 and its liquid-solid interface from Density Functional Theory
We show that, at high densities, fully variational solutions of solid-like
type can be obtained from a density functional formalism originally designed
for liquid 4He. Motivated by this finding, we propose an extension of the
method that accurately describes the solid phase and the freezing transition of
liquid 4He at zero temperature. The density profile of the interface between
liquid and the (0001) surface of the 4He crystal is also investigated, and its
surface energy evaluated. The interfacial tension is found to be in
semiquantitative agreement with experiments and with other microscopic
calculations. This opens the possibility to use unbiased DF methods to study
highly non-homogeneous systems, like 4He interacting with strongly attractive
impurities/substrates, or the nucleation of the solid phase in the metastable
liquid.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
On the nonlocal predictions of quantum optics
We give a definition of locality in quantum optics based upon Bell's work, and show that locality has been violated in no experiment performed up to now. We argue that the interpretation of the Wigner function as a probability density gives a very attractive local realistic picture of quantum optics provided that this function is nonnegative. We conjecture that this is the case for all states which can be realized in the laboratory. In particular, we believe that the usual representation of 'single photon states' by a Fock state of the Hilbert space is not correct and that a more physical, although less simple mathematically, representation involves density matrices. We study in some detail the experiment showing anticorrelation after a beam splitter and prove that it naturally involves a positive Wigner function. Our (quantum) predictions for this experiment disagree with the ones reported in the literature
Spontaneous emission and teleportation in cavity QED
In this work, we consider atomic spontaneous emission in a system consisting
of two identical two-level atoms interacting dispersively with the quantized
electromagnetic field in a high-Q cavity. We investigate the destructive effect
of the atomic decay on the generation of maximally entangled states, following
the proposal by Zheng S B and Guo G C (2000 Phys. Rev. Lett. 85 2392). In
particular, we analyze the fidelity of teleportation performed using such a
noisy channel and calculatethe maximum spontaneous decay rate we may have in
order to realize teleportation.Comment: 11 pages, 6 figures, LaTe
Constraining Nonstandard Neutrino-Electron Interactions
We present a detailed analysis on nonstandard neutrino interactions (NSI)
with electrons including all muon and electron (anti)-neutrino data from
existing accelerators and reactors, in conjunction with the ``neutrino
counting'' data (e- e+ -> nu nu gamma) from the four LEP collaborations. First
we perform a one-parameter-at-a-time analysis, showing how most constraints
improve with respect to previous results reported in the literature. We also
present more robust results where the NSI parameters are allowed to vary freely
in the analysis. We show the importance of combining LEP data with the other
experiments in removing degeneracies in the global analysis constraining
flavor-conserving NSI parameters which, at 90 % and 95 % C.L., must lie within
unique allowed regions. Despite such improved constraints, there is still
substantial room for improvement, posing a big challenge for upcoming
experiments.Comment: 19 pages, 4 figures. Final version to appear in Phys. Rev.
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