63,899 research outputs found
Decoherence and dephasing in strongly driven colliding Bose-Einstein condensates
We report on a series of measurements of decoherence and wavepacket dephasing
between two colliding, strongly coupled, identical Bose-Einstein condensates.
We measure, in the strong excitation regime, a suppression of the mean-field
shift, compared to the shift which is observed for a weak excitation. This
suppression is explained by applying the Gross-Pitaevskii energy functional. By
selectively counting only the non-decohered fraction in a time of flight image
we observe oscillations for which both inhomogeneous and Doppler broadening are
suppressed, in quantitative agreement with a full Gross-Pitaevskii equation
simulation. If no post selection is used, the decoherence rate due to
collisions can be extracted, and is in agreement with the local density average
calculated rate.Comment: 4 pages, 5 figure
Toward a dynamical systems analysis of neuromodulation
This work presents some first steps toward a
more thorough understanding of the control systems
employed in evolutionary robotics. In order
to choose an appropriate architecture or to construct
an effective novel control system we need
insights into what makes control systems successful,
robust, evolvable, etc. Here we present analysis
intended to shed light on this type of question
as it applies to a novel class of artificial neural
networks that include a neuromodulatory mechanism:
GasNets.
We begin by instantiating a particular GasNet
subcircuit responsible for tuneable pattern generation
and thought to underpin the attractive
property of “temporal adaptivity”. Rather than
work within the GasNet formalism, we develop an
extension of the well-known FitzHugh-Nagumo
equations. The continuous nature of our model
allows us to conduct a thorough dynamical systems
analysis and to draw parallels between this
subcircuit and beating/bursting phenomena reported
in the neuroscience literature.
We then proceed to explore the effects of different
types of parameter modulation on the system
dynamics. We conclude that while there are
key differences between the gain modulation used
in the GasNet and alternative schemes (including
threshold modulation of more traditional synaptic
input), both approaches are able to produce
tuneable pattern generation. While it appears, at
least in this study, that the GasNet’s gain modulation
may not be crucial to pattern generation ,
we go on to suggest some possible advantages it
could confer
On a nonlinear theory of elastic shells
Nonlinear theory of elastic shells with deformation gradient
Bose-Einstein condensate in a rapidly rotating non-symmetric trap
A rapidly rotating Bose-Einstein condensate in a symmetric two-dimensional
harmonic trap can be described with the lowest Landau-level set of
single-particle states. The condensate wave function psi(x,y) is a Gaussian
exp(-r^2/2), multiplied by an analytic function f(z) of the complex variable z=
x+ i y. The criterion for a quantum phase transition to a non-superfluid
correlated many-body state is usually expressed in terms of the ratio of the
number of particles to the number of vortices. Here, a similar description
applies to a rapidly rotating non-symmetric two-dimensional trap with arbitrary
quadratic anisotropy (omega_x^2 < omega_y^2). The corresponding condensate wave
function psi(x,y) is a complex anisotropic Gaussian with a phase proportional
to xy, multiplied by an analytic function f(z), where z = x + i \beta_- y is a
stretched complex variable and 0< \beta_- <1 is a real parameter that depends
on the trap anisotropy and the rotation frequency. Both in the mean-field
Thomas-Fermi approximation and in the mean-field lowest Landau level
approximation with many visible vortices, an anisotropic parabolic density
profile minimizes the energy. An elongated condensate grows along the soft trap
direction yet ultimately shrinks along the tight trap direction. The criterion
for the quantum phase transition to a correlated state is generalized (1) in
terms of N/L_z, which suggests that a non-symmetric trap should make it easier
to observe this transition or (2) in terms of a "fragmented" correlated state,
which suggests that a non-symmetric trap should make it harder to observe this
transition. An alternative scenario involves a crossover to a quasi
one-dimensional condensate without visible vortices, as suggested by Aftalion
et al., Phys. Rev. A 79, 011603(R) (2009).Comment: 20 page
ac Stark shift and multiphoton-like resonances in low-frequency driven optical lattices
We suggest that Bose-Einstein condensates in optical lattices subjected to ac
forcing with a smooth envelope may provide detailed experimental access to
multiphoton-like transitions between ac-Stark-shifted Bloch bands. Such
transitions correspond to resonances described theoretically by avoided
quasienergy crossings. We show that the width of such anticrossings can be
inferred from measurements involving asymmetric pulses. We also introduce a
pulse tracking strategy for locating the particular driving amplitudes for
which resonances occur. Our numerical calculations refer to a currently
existing experimental set-up [Haller et al., PRL 104, 200403 (2010)].Comment: 5 pages, 6 figure
Regions of beta 2 and beta 4 responsible for differences between the steady state dose-response relationships of the alpha 3 beta 2 and alpha 3 beta 4 neuronal nicotinic receptors
We constructed chimeras of the rat beta 2 and beta 4 neuronal nicotinic subunits to locate the regions that contribute to differences between the acetylcholine (ACh) dose-response relationships of the alpha 3 beta 2 and alpha 3 beta 4 receptors. Expressed in Xenopus oocytes, the alpha 3 beta 2 receptor displays an EC50 for ACh approximately 20-fold less than the EC50 of the alpha 3 beta 4 receptor. The apparent Hill slope (n(app)) of alpha 3 beta 2 is near one whereas the alpha 3 beta 4 receptor displays an n(app) near two. Substitutions within the first 120 residues convert the EC50 for ACh from one wild-type value to the other. Exchanging just beta 2:104-120 for the corresponding region of beta 4 shifts the EC50 of ACh dose-response relationship in the expected direction but does not completely convert the EC50 of the dose- response relationship from one wild-type value to the other. However, substitutions in the beta 2:104-120 region do account for the relative sensitivity of the alpha 3 beta 2 receptor to cytisine, tetramethylammonium, and ACh. The expression of beta 4-like (strong) cooperativity requires an extensive region of beta 4 (beta 4:1-301). Relatively short beta 2 substitutions (beta 2:104-120) can reduce cooperativity to beta 2-like values. The results suggest that amino acids within the first 120 residues of beta 2 and the corresponding region of beta 4 contribute to an agonist binding site that bridges the alpha and beta subunits in neuronal nicotinic receptors
Harmonic oscillators coupled by springs: discrete solutions as a Wigner Quantum System
We consider a quantum system consisting of a one-dimensional chain of M
identical harmonic oscillators with natural frequency , coupled by
means of springs. Such systems have been studied before, and appear in various
models. In this paper, we approach the system as a Wigner Quantum System, not
imposing the canonical commutation relations, but using instead weaker
relations following from the compatibility of Hamilton's equations and the
Heisenberg equations. In such a setting, the quantum system allows solutions in
a finite-dimensional Hilbert space, with a discrete spectrum for all physical
operators. We show that a class of solutions can be obtained using generators
of the Lie superalgebra gl(1|M). Then we study the properties and spectra of
the physical operators in a class of unitary representations of gl(1|M). These
properties are both interesting and intriguing. In particular, we can give a
complete analysis of the eigenvalues of the Hamiltonian and of the position and
momentum operators (including multiplicities). We also study probability
distributions of position operators when the quantum system is in a stationary
state, and the effect of the position of one oscillator on the positions of the
remaining oscillators in the chain
Splitting in the Excitation Spectrum of A Bose-Einstein Condensate Undergoing Strong Rabi Oscillations
We report on a measurement of splitting in the excitation spectrum of a
condensate driven by an optical travelling wave. Experimental results are
compared to a numerical solution of the Gross Pitaevskii equation, and analyzed
by a simple two level model and by the more complete band theory, treating the
driving beams as an optical lattice. In this picture, the splitting is a
manifestation of the energy gap between neighboring bands that opens on the
boundary of the Brillouin zone.Comment: 5 pages, 5 figure
On Markovian solutions to Markov Chain BSDEs
We study (backward) stochastic differential equations with noise coming from
a finite state Markov chain. We show that, for the solutions of these equations
to be `Markovian', in the sense that they are deterministic functions of the
state of the underlying chain, the integrand must be of a specific form. This
allows us to connect these equations to coupled systems of ODEs, and hence to
give fast numerical methods for the evaluation of Markov-Chain BSDEs
- …