447 research outputs found
Quantum decoherence reduction by increasing the thermal bath temperature
The well-known increase of the decoherence rate with the temperature, for a
quantum system coupled to a linear thermal bath, holds no longer for a
different bath dynamics. This is shown by means of a simple classical
non-linear bath, as well as a quantum spin-boson model. The anomalous effect is
due to the temperature dependence of the bath spectral profile. The decoherence
reduction via the temperature increase can be relevant for the design of
quantum computers
Quantum effects in linguistic endeavors
Classifying the information content of neural spike trains in a linguistic
endeavor, an uncertainty relation emerges between the bit size of a word and
its duration. This uncertainty is associated with the task of synchronizing the
spike trains of different duration representing different words. The
uncertainty involves peculiar quantum features, so that word comparison amounts
to measurement-based-quantum computation. Such a quantum behavior explains the
onset and decay of the memory window connecting successive pieces of a
linguistic text. The behavior here discussed is applicable to other reported
evidences of quantum effects in human linguistic processes, so far lacking a
plausible framework, since either no efforts to assign an appropriate quantum
constant had been associated or speculating on microscopic processes dependent
on Planck's constant resulted in unrealistic decoherence times
Two-dimensional solitary pulses in driven diffractive-diffusive complex Ginzburg-Landau equations
Two models of driven optical cavities, based on two-dimensional
Ginzburg-Landau equations, are introduced. The models include loss, the Kerr
nonlinearity, diffraction in one transverse direction, and a combination of
diffusion and dispersion in the other one (which is, actually, a temporal
direction). Each model is driven either parametrically or directly by an
external field. By means of direct simulations, stable completely localized
pulses are found (in the directly driven model, they are built on top of a
nonzero flat background). These solitary pulses correspond to spatio-temporal
solitons in the optical cavities. Basic results are presented in a compact form
as stability regions for the solitons in a full three-dimensional parameter
space of either model. The stability region is bounded by two surfaces; beyond
the left one, any two-dimensional (2D) pulse decays to zero, while quasi-1D
pulses, representing spatial solitons in the optical cavity, are found beyond
the right boundary. The spatial solitons are found to be stable both inside the
stability region of the 2D pulses (hence, bistability takes place in this
region) and beyond the right boundary of this region (although they are not
stable everywhere). Unlike the spatial solitons, their quasi-1D counterparts in
the form of purely temporal solitons are always subject to modulational
instability, which splits them into an array of 2D pulses, that further
coalesce into two final pulses. A uniform nonzero state in the parametrically
driven model is also modulationally unstable, which leads to formation of many
2D pulses that subsequently merge into few ones.Comment: a latex text file and 11 eps files with figures. Physica D, in pres
Non-Gaussian statistics and extreme waves in a nonlinear optical cavity
A unidirectional optical oscillator is built by using a liquid crystal
light-valve that couples a pump beam with the modes of a nearly spherical
cavity. For sufficiently high pump intensity, the cavity field presents a
complex spatio-temporal dynamics, accompanied by the emission of extreme waves
and large deviations from the Gaussian statistics. We identify a mechanism of
spatial symmetry breaking, due to a hypercycle-type amplification through the
nonlocal coupling of the cavity field
Contracting the Wigner kernel of a spin to the Wigner kernel of a particle
A general relation between the Moyal formalisms for a spin and a particle is established. Once the formalism has been set up for a spin, the phase-space description of a particle is obtained from contracting the group of rotations to the oscillator group. In this process, turn into a spin Wigner kernel turns into the Wigner kernel of a particle. In fact, only one out of 22s different possible kernels for a spin shows this behavior
Long-lived Quantum Coherence between Macroscopically Distinct States in Superradiance
The dephasing influence of a dissipative environment reduces linear
superpositions of macroscopically distinct quantum states (sometimes also
called Schr\"odinger cat states) usually almost immediately to a statistical
mixture. This process is called decoherence. Couplings to the environment with
a certain symmetry can lead to slow decoherence. In this Letter we show that
the collective coupling of a large number of two-level atoms to an
electromagnetic field mode in a cavity that leads to the phenomena of
superradiance has such a symmetry, at least approximately. We construct
superpositions of macroscopically distinct quantum states decohering only on a
classical time scale and propose an experiment in which the extraordinarily
slow decoherence should be observable.Comment: 4 pages of revte
Time interval distributions of atoms in atomic beams
We report on the experimental investigation of two-particle correlations
between neutral atoms in a Hanbury Brown and Twiss experiment. Both an atom
laser beam and a pseudo-thermal atomic beam are extracted from a Bose-Einstein
condensate and the atom flux is measured with a single atom counter. We
determine the conditional and the unconditional detection probabilities for the
atoms in the beam and find good agreement with the theoretical predictions.Comment: 4 pages, 3 figure
Dependence of transient dynamics in a class-C laser upon variation of inversion with time
The transient statistics of a gain-switched coherently pumped class-C laser displays a linear correlation between the first passage time and subsequent peak intensity. Measurements are reported showing a positive or negative sign of this linear correlation, controlled through the switching time and the laser detuning. Further measurements of the small-signal laser gain combined with calculations involving a three-level laser model indicate that this sign fundamentally depends upon the way the laser inversion varies during the gain switching, despite the added dynamics of the laser polarization in the class-C laser. [S1050-2947(97)07112-6]
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