50,504 research outputs found
Instanton Approach to Josephson Tunneling between Trapped Condensates
An instanton method is proposed to investigate the quantum tunneling between
two weakly-linked Bose-Einstein condensates confined in double-well potential
traps. We point out some intrinsic pathologies in the earlier treatments of
other authors and make an effort to go beyond these very simple zero order
models. The tunneling amplitude may be calculated in the Thomas-Fermi
approximation and beyond it; we find it depends on the number of the trapped
atoms, through the chemical potential. Some suggestions are given for the
observation of the Josephson oscillation and the MQST.Comment: 20 pages, Revtex4, 6 figures. Abbreviated version accepted by Eur.
Phys. J
Observation of quantum spin noise in a 1D light-atoms quantum interface
We observe collective quantum spin states of an ensemble of atoms in a
one-dimensional light-atom interface. Strings of hundreds of cesium atoms
trapped in the evanescent fiel of a tapered nanofiber are prepared in a
coherent spin state, a superposition of the two clock states. A weak quantum
nondemolition measurement of one projection of the collective spin is performed
using a detuned probe dispersively coupled to the collective atomic observable,
followed by a strong destructive measurement of the same spin projection. For
the coherent spin state we achieve the value of the quantum projection noise 40
dB above the detection noise, well above the 3 dB required for reconstruction
of the negative Wigner function of nonclassical states. We analyze the effects
of strong spatial inhomogeneity inherent to atoms trapped and probed by the
evanescent waves. We furthermore study temporal dynamics of quantum
fluctuations relevant for measurement-induced spin squeezing and assess the
impact of thermal atomic motion. This work paves the road towards observation
of spin squeezed and entangled states and many-body interactions in 1D spin
ensembles
Once again: Instanton method vs. WKB
A recent analytic test of the instanton method performed by comparing the
exact spectrum of the Lam potential (derived from representations
of a finite dimensional matrix expressed in terms of generators) with
the results of the tight--binding and instanton approximations as well as the
standard WKB approximation is commented upon. It is pointed out that in the
case of the Lam potential as well as others the WKB--related method
of matched asymptotic expansions yields the exact instanton result as a result
of boundary conditions imposed on wave functions which are matched in domains
of overlap.Comment: 10 pages, no figures. References list revised according to JHE
Equation of state and critical behavior of polymer models: A quantitative comparison between Wertheim's thermodynamic perturbation theory and computer simulations
We present an application of Wertheim's Thermodynamic Perturbation Theory
(TPT1) to a simple coarse grained model made of flexibly bonded Lennard-Jones
monomers. We use both the Reference Hyper-Netted-Chain (RHNC) and Mean
Spherical approximation (MSA) integral equation theories to describe the
properties of the reference fluid. The equation of state, the density
dependence of the excess chemical potential, and the critical points of the
liquid--vapor transition are compared with simulation results and good
agreement is found. The RHNC version is somewhat more accurate, while the MSA
version has the advantage of being almost analytic. We analyze the scaling
behavior of the critical point of chain fluids according to TPT1 and find it to
reproduce the mean field exponents: The critical monomer density is predicted
to vanish as upon increasing the chain length while the critical
temperature is predicted to reach an asymptotic finite temperature that is
attained as . The predicted asymptotic finite critical temperature
obtained from the RHNC and MSA versions of TPT1 is found to be in good
agreement with the point of our polymer model as obtained from the
temperature dependence of the single chain conformations.Comment: to appear in J.Chem.Phy
Microscopic laser-driven high-energy colliders
The concept of a laser-guided collider in the high-energy regime is
presented and its feasibility discussed. Ultra-intense laser pulses and strong
static magnetic fields are employed to unite in one stage the electron and
positron acceleration and their head-on-head collision. We show that the
resulting coherent collisions in the GeV regime yield an enormous enhancement
of the luminosity with regard to conventional incoherent colliders
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