91 research outputs found
Double universality of a quantum phase transition in spinor condensates: the Kibble-\.Zurek mechanism and a conservation law
We consider a phase transition from antiferromagnetic to phase separated
ground state in a spin-1 Bose-Einstein condensate of ultracold atoms. We
demonstrate the occurrence of two scaling laws, for the number of spin
fluctuations just after the phase transition, and for the number of spin
domains in the final, stable configuration. Only the first scaling can be
explained by the standard Kibble-\.Zurek mechanism. We explain the occurrence
of two scaling laws by a model including post-selection of spin domains due to
the conservation of condensate magnetization
Classical Equations for Quantum Systems
The origin of the phenomenological deterministic laws that approximately
govern the quasiclassical domain of familiar experience is considered in the
context of the quantum mechanics of closed systems such as the universe as a
whole. We investigate the requirements for coarse grainings to yield decoherent
sets of histories that are quasiclassical, i.e. such that the individual
histories obey, with high probability, effective classical equations of motion
interrupted continually by small fluctuations and occasionally by large ones.
We discuss these requirements generally but study them specifically for coarse
grainings of the type that follows a distinguished subset of a complete set of
variables while ignoring the rest. More coarse graining is needed to achieve
decoherence than would be suggested by naive arguments based on the uncertainty
principle. Even coarser graining is required in the distinguished variables for
them to have the necessary inertia to approach classical predictability in the
presence of the noise consisting of the fluctuations that typical mechanisms of
decoherence produce. We describe the derivation of phenomenological equations
of motion explicitly for a particular class of models. Probabilities of the
correlations in time that define equations of motion are explicitly considered.
Fully non-linear cases are studied. Methods are exhibited for finding the form
of the phenomenological equations of motion even when these are only distantly
related to those of the fundamental action. The demonstration of the connection
between quantum-mechanical causality and causalty in classical phenomenological
equations of motion is generalized. The connections among decoherence, noise,
dissipation, and the amount of coarse graining necessary to achieve classical
predictability are investigated quantitatively.Comment: 100pages, 1 figur
A Thermodynamical Approach to Quantifying Quantum Correlations
We consider the amount of work which can be extracted from a heat bath using
a bipartite state shared by two parties. In general it is less then the amount
of work extractable when one party is in possession of the entire state. We
derive bounds for this "work deficit" and calculate it explicitly for a number
of different cases. For pure states the work deficit is exactly equal to the
distillable entanglement of the state, and this is also achievable for
maximally correlated states. In these cases a form of complementarity exists
between physical work which can be extracted and distillable entanglement. The
work deficit is a good measure of the quantum correlations in a state and
provides a new paradigm for understanding quantum non-locality.Comment: 4 pages, Revtex4, title changed, caveat added to theore
Quantum Superposition States of Bose-Einstein Condensates
We propose a scheme to create a macroscopic ``Sch\"odinger cat'' state formed
by two interacting Bose condensates. In analogy with quantum optics, where the
control and engineering of quantum states can be maintained to a large extend,
we consider the present scheme to be an example of quantum atom optics at work.Comment: 24 pages, 6 figure
Classicality of quantum information processing
The ultimate goal of the classicality programme is to quantify the amount of
quantumness of certain processes. Here, classicality is studied for a
restricted type of process: quantum information processing (QIP). Under special
conditions, one can force some qubits of a quantum computer into a classical
state without affecting the outcome of the computation. The minimal set of
conditions is described and its structure is studied. Some implications of this
formalism are the increase of noise robustness, a proof of the quantumness of
mixed state quantum computing and a step forward in understanding the very
foundation of QIP.Comment: Minor changes, published in Phys. Rev. A 65, 42319 (2002
Dynamics of quantum entanglement
A model of discrete dynamics of entanglement of bipartite quantum state is
considered. It involves a global unitary dynamics of the system and periodic
actions of local bistochastic or decaying channel. For initially pure states
the decay of entanglement is accompanied with an increase of von Neumann
entropy of the system. We observe and discuss revivals of entanglement due to
unitary interaction of both subsystems. For some mixed states having different
marginal entropies of both subsystems (one of them larger than the global
entropy and the other one one smaller) we find an asymmetry in speed of
entanglement decay. The entanglement of these states decreases faster, if the
depolarizing channel acts on the "classical" subsystem, characterized by
smaller marginal entropy.Comment: 10 pages, Revtex, 10 figures, refined versio
Measurement of the Reaction in Search for the Recently Observed Resonance Structure in and systems
Exclusive measurements of the quasi-free reaction have
been performed by means of collisions at = 1.2 GeV using the WASA
detector setup at COSY. Total and differential cross sections have been
obtained covering the energy region = (2.35 - 2.46) GeV, which
includes the region of the ABC effect and its associated resonance structure.
No ABC effect, {\it i.e.} low-mass enhancement is found in the
-invariant mass spectrum -- in agreement with the constraint from
Bose statistics that the isovector pion pair can not be in relative s-wave. At
the upper end of the covered energy region -channel processes for Roper,
and excitations provide a reasonable description
of the data, but at low energies the measured cross sections are much larger
than predicted by such processes. Adding a resonance amplitude for the
resonance at =~2.37 GeV with =~70 MeV and observed
recently in and reactions leads to an
agreement with the data also at low energies
Charge Symmetry Breaking in dd->4He{\pi}0 with WASA-at-COSY
Charge symmetry breaking (CSB) observables are a suitable experimental tool
to examine effects induced by quark masses on the nuclear level. Previous high
precision data from TRIUMF and IUCF are currently used to develop a consistent
description of CSB within the framework of chiral perturbation theory. In this
work the experimental studies on the reaction dd->4He{\pi}0 have been extended
towards higher excess energies in order to provide information on the
contribution of p-waves in the final state. For this, an exclusive measurement
has been carried out at a beam momentum of p=1.2 GeV/c using the WASA-at-COSY
facility. The total cross section amounts to sigma(tot) = (118 +- 18(stat) +-
13(sys) +- 8(ext)) pb and first data on the differential cross section are
consistent with s-wave pion production.Comment: 14 pages, 5 figure
Neutron-Proton Scattering in the Context of the (2380) Resonance
New data on quasifree polarized neutron-proton scattering, in the region of
the recently observed resonance structure, have been obtained by
exclusive and kinematically complete high-statistics measurements with WASA at
COSY. This paper details the determination of the beam polarization, checks of
the quasifree character of the scattering process, on all obtained
angular distributions and on the new partial-wave analysis, which includes the
new data producing a resonance pole in the - coupled partial
waves at () MeV -- in accordance with the dibaryon
resonance hypothesis. The effect of the new partial-wave solution on the
description of total and differential cross section data as well as specific
combinations of spin-correlation and spin-transfer observables available from
COSY-ANKE measurements at = 2.27 GeV is discussed
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