131 research outputs found
Robust and fragile Werner states in the collective dephasing
We investigate the concurrence and Bell violation of the standard Werner
state or Werner-like states in the presence of collective dephasing. It is
shown that the standard Werner state and certain kinds of Werner-like states
are robust against the collective dephasing, and some kinds of Werner-like
states is fragile and becomes completely disentangled in a finite-time. The
threshold time of complete disentanglement of the fragile Werner-like states is
given. The influence of external driving field on the finite-time
disentanglement of the standard Werner state or Werner-like states is
discussed. Furthermore, we present a simple method to control the stationary
state entanglement and Bell violation of two qubits. Finally, we show that the
theoretical calculations of fidelity based on the initial Werner state
assumption well agree with previous experimental results.Comment: 7 pages, 6 figures, 1 table, RevTex4, Accepted by EPJ
Chiral approach to nuclear matter: Role of explicit short-range NN-terms
We extend a recent chiral approach to nuclear matter by including the most
general (momentum-independent) NN-contact interaction. Iterating this
two-parameter contact-vertex with itself and with one-pion exchange the
emerging energy per particle exhausts all terms possible up-to-and-including
fourth order in the small momentum expansion. The equation of state of pure
neutron matter, , can be reproduced very well up to quite high
neutron densities of \rho_n=0.5\fmd by adjusting the strength of a repulsive
-contact interaction. Binding and saturation of isospin-symmetric nuclear
matter is a generic feature of our perturbative calculation. Fixing the maximum
binding energy per particle to MeV we find that any
possible equilibrium density lies below \rho_0^{\rm max}=0.191\fmd.
The additional constraint from the neutron matter equation of state leads
however to a somewhat too low saturation density of \rho_0 =0.134 \fmd. We
also investigate the effects of the NN-contact interaction on the complex
single-particle potential . We find that the effective
nucleon mass at the Fermi-surface is bounded from below by . This property keeps the critical temperature of the liquid-gas phase
transition at somewhat too high values MeV. The downward bending
of the asymmetry energy above nuclear matter saturation density is a
generic feature of the approximation to fourth order. Altogether, there is
within this complete fourth-order calculation no "magic" set of adjustable
short-range parameters with which one could reproduce simultaneously and
accurately all semi-empirical properties of nuclear matter.Comment: 24 pages, 12 figures, accepted for publication in: Eur. Phys. J.
Geometric Phase of a qubit interacting with a squeezed-thermal bath
We study the geometric phase of an open two-level quantum system under the
influence of a squeezed, thermal environment for both non-dissipative as well
as dissipative system-environment interactions. In the non-dissipative case,
squeezing is found to have a similar influence as temperature, of suppressing
geometric phase, while in the dissipative case, squeezing tends to counteract
the suppressive influence of temperature in certain regimes. Thus, an
interesting feature that emerges from our work is the contrast in the interplay
between squeezing and thermal effects in non-dissipative and dissipative
interactions. This can be useful for the practical implementation of geometric
quantum information processing. By interpreting the open quantum effects as
noisy channels, we make the connection between geometric phase and quantum
noise processes familiar from quantum information theory.Comment: Accepted for publication in Eur. Phys. J. D; slightly abridged
version of v2; 10 pages, 12 figure
Coherence length of an elongated condensate: a study by matter-wave interferometry
We measure the spatial correlation function of Bose-Einstein condensates in
the cross-over region between phase-coherent and strongly phase-fluctuating
condensates. We observe the continuous path from a gaussian-like shape to an
exponential-like shape characteristic of one-dimensional phase-fluctuations.
The width of the spatial correlation function as a function of the temperature
shows that the condensate coherence length undergoes no sharp transition
between these two regimes.Comment: 8 pages, 6 figure, submitted to EPJ
A new analysis of scattering from Roy and Steiner type equations
With the aim of generating new constraints on the OZI suppressed couplings of
chiral perturbation theory a set of six equations of the Roy and Steiner type
for the - and -waves of the scattering amplitudes is derived. The
range of validity and the multiplicity of the solutions are discussed. Precise
numerical solutions are obtained in the range E\lapprox 1 GeV which make use
as input, for the first time, of the most accurate experimental data available
at GeV for both and amplitudes.
Our main result is the determination of a narrow allowed region for the two
S-wave scattering lengths. Present experimental data below 1 GeV are found to
be in generally poor agreement with our results. A set of threshold expansion
parameters, as well as sub-threshold parameters are computed. For the latter,
matching with the SU(3) chiral expansion at NLO is performed.Comment: 45 pages, 17 figures. v2: New title, minor correction
Measurement of the Strong Coupling alpha s from Four-Jet Observables in e+e- Annihilation
Data from e+e- annihilation into hadrons at centre-of-mass energies between
91 GeV and 209 GeV collected with the OPAL detector at LEP, are used to study
the four-jet rate as a function of the Durham algorithm resolution parameter
ycut. The four-jet rate is compared to next-to-leading order calculations that
include the resummation of large logarithms. The strong coupling measured from
the four-jet rate is alphas(Mz0)=
0.1182+-0.0003(stat.)+-0.0015(exp.)+-0.0011(had.)+-0.0012(scale)+-0.0013(mass)
in agreement with the world average. Next-to-leading order fits to the
D-parameter and thrust minor event-shape observables are also performed for the
first time. We find consistent results, but with significantly larger
theoretical uncertainties.Comment: 25 pages, 15 figures, Submitted to Euro. Phys. J.
Evidence for an Excess of Soft Photons in Hadronic Decays of Z^0
Soft photons inside hadronic jets converted in front of the DELPHI main
tracker (TPC) in events of qqbar disintegrations of the Z^0 were studied in the
kinematic range 0.2 < E_gamma < 1 GeV and transverse momentum with respect to
the closest jet direction p_T < 80 MeV/c. A clear excess of photons in the
experimental data as compared to the Monte Carlo predictions is observed. This
excess (uncorrected for the photon detection efficiency) is (1.17 +/- 0.06 +/-
0.27) x 10^{-3} gamma/jet in the specified kinematic region, while the expected
level of the inner hadronic bremsstrahlung (which is not included in the Monte
Carlo) is (0.340 +/- 0.001 +/- 0.038) x 10^{-3} gamma/jet. The ratio of the
excess to the predicted bremsstrahlung rate is then (3.4 +/- 0.2 +/- 0.8),
which is similar in strength to the anomalous soft photon signal observed in
fixed target experiments with hadronic beams.Comment: 37 pages, 9 figures, Accepted by Eur. Phys. J.
Planck Intermediate Results II: Comparison of SunyaevâZeldovich measurements from Planck and from the Arcminute Microkelvin Imager for 11 galaxy clusters
A comparison is presented of SunyaevâZeldovich measurements for 11 galaxy clusters as obtained by Planck and by the ground-based interferom- eter, the Arcminute Microkelvin Imager. Assuming a universal spherically-symmetric Generalised Navarro, Frenk & White (GNFW) model for the cluster gas pressure profile, we jointly constrain the integrated Compton-Y parameter (Y500) and the scale radius (Ξ500) of each cluster. Our resulting constraints in the Y500 â Ξ500 2D parameter space derived from the two instruments overlap significantly for eight of the clusters, although, overall, there is a tendency for AMI to find the SunyaevâZeldovich signal to be smaller in angular size and fainter than Planck. Significant discrepancies exist for the three remaining clusters in the sample, namely A1413, A1914, and the newly-discovered Planck cluster PLCKESZ G139.59+24.18. The robustness of the analysis of both the Planck and AMI data is demonstrated through the use of detailed simulations, which also discount confusion from residual point (radio) sources and from diffuse astrophysical foregrounds as possible explanations for the discrepancies found. For a subset of our cluster sample, we have investigated the dependence of our results on the assumed pressure profile by repeating the analysis adopting the best-fitting GNFW profile shape which best matches X-ray observations. Adopting the best-fitting profile shape from the X-ray data does not, in general, resolve the discrepancies found in this subset of five clusters. Though based on a small sample, our results suggest that the adopted GNFW model may not be sufficiently flexible to describe clusters universally
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