516 research outputs found
Noise sensitivity of sub- and supercritically bifurcating patterns with group velocities close to the convective-absolute instability
The influence of small additive noise on structure formation near a forwards
and near an inverted bifurcation as described by a cubic and quintic Ginzburg
Landau amplitude equation, respectively, is studied numerically for group
velocities in the vicinity of the convective-absolute instability where the
deterministic front dynamics would empty the system.Comment: 16 pages, 7 Postscript figure
Interaction-free measurements by quantum Zeno stabilisation of ultracold atoms
Quantum mechanics predicts that our physical reality is influenced by events
that can potentially happen but factually do not occur. Interaction-free
measurements (IFMs) exploit this counterintuitive influence to detect the
presence of an object without requiring any interaction with it. Here we
propose and realize an IFM concept based on an unstable many-particle system.
In our experiments, we employ an ultracold gas in an unstable spin
configuration which can undergo a rapid decay. The object - realized by a laser
beam - prevents this decay due to the indirect quantum Zeno effect and thus,
its presence can be detected without interacting with a single atom. Contrary
to existing proposals, our IFM does not require single-particle sources and is
only weakly affected by losses and decoherence. We demonstrate confidence
levels of 90%, well beyond previous optical experiments.Comment: manuscript with 5 figures, 3 supplementary figure, 1 supplementary
not
Pattern selection as a nonlinear eigenvalue problem
A unique pattern selection in the absolutely unstable regime of driven,
nonlinear, open-flow systems is reviewed. It has recently been found in
numerical simulations of propagating vortex structures occuring in
Taylor-Couette and Rayleigh-Benard systems subject to an externally imposed
through-flow. Unlike the stationary patterns in systems without through-flow
the spatiotemporal structures of propagating vortices are independent of
parameter history, initial conditions, and system length. They do, however,
depend on the boundary conditions in addition to the driving rate and the
through-flow rate. Our analysis of the Ginzburg-Landau amplitude equation
elucidates how the pattern selection can be described by a nonlinear eigenvalue
problem with the frequency being the eigenvalue. Approaching the border between
absolute and convective instability the eigenvalue problem becomes effectively
linear and the selection mechanism approaches that of linear front propagation.
PACS: 47.54.+r,47.20.Ky,47.32.-y,47.20.FtComment: 18 pages in Postsript format including 5 figures, to appear in:
Lecture Notes in Physics, "Nonlinear Physics of Complex Sytems -- Current
Status and Future Trends", Eds. J. Parisi, S. C. Mueller, and W. Zimmermann
(Springer, Berlin, 1996
Spontaneous breaking of spatial and spin symmetry in spinor condensates
Parametric amplification of quantum fluctuations constitutes a fundamental
mechanism for spontaneous symmetry breaking. In our experiments, a spinor
condensate acts as a parametric amplifier of spin modes, resulting in a twofold
spontaneous breaking of spatial and spin symmetry in the amplified clouds. Our
experiments permit a precise analysis of the amplification in specific spatial
Bessel-like modes, allowing for the detailed understanding of the double
symmetry breaking. On resonances that create vortex-antivortex superpositions,
we show that the cylindrical spatial symmetry is spontaneously broken, but
phase squeezing prevents spin-symmetry breaking. If, however, nondegenerate
spin modes contribute to the amplification, quantum interferences lead to
spin-dependent density profiles and hence spontaneously-formed patterns in the
longitudinal magnetization.Comment: 5 pages, 4 figure
PCT, spin and statistics, and analytic wave front set
A new, more general derivation of the spin-statistics and PCT theorems is
presented. It uses the notion of the analytic wave front set of
(ultra)distributions and, in contrast to the usual approach, covers nonlocal
quantum fields. The fields are defined as generalized functions with test
functions of compact support in momentum space. The vacuum expectation values
are thereby admitted to be arbitrarily singular in their space-time dependence.
The local commutativity condition is replaced by an asymptotic commutativity
condition, which develops generalizations of the microcausality axiom
previously proposed.Comment: LaTeX, 23 pages, no figures. This version is identical to the
original published paper, but with corrected typos and slight improvements in
the exposition. The proof of Theorem 5 stated in the paper has been published
in J. Math. Phys. 45 (2004) 1944-195
Satisfying the Einstein-Podolsky-Rosen criterion with massive particles
In 1935, Einstein, Podolsky and Rosen (EPR) questioned the completeness of
quantum mechanics by devising a quantum state of two massive particles with
maximally correlated space and momentum coordinates. The EPR criterion
qualifies such continuous-variable entangled states, where a measurement of one
subsystem seemingly allows for a prediction of the second subsystem beyond the
Heisenberg uncertainty relation. Up to now, continuous-variable EPR
correlations have only been created with photons, while the demonstration of
such strongly correlated states with massive particles is still outstanding.
Here, we report on the creation of an EPR-correlated two-mode squeezed state in
an ultracold atomic ensemble. The state shows an EPR entanglement parameter of
0.18(3), which is 2.4 standard deviations below the threshold 1/4 of the EPR
criterion. We also present a full tomographic reconstruction of the underlying
many-particle quantum state. The state presents a resource for tests of quantum
nonlocality and a wide variety of applications in the field of
continuous-variable quantum information and metrology.Comment: 8 pages, 7 figure
A New Derivation of the CPT and Spin-Statistics Theorems in Non-Commutative Field Theories
We propose an alternative axiomatic description for non-commutative field
theories (NCFT) based on some ideas by Soloviev to nonlocal quantum fields. The
local commutativity axiom is replaced by the weaker condition that the fields
commute at sufficiently large spatial separations, called asymptotic
commutativity, formulated in terms of the theory of analytic functionals. The
question of a possible violation of the CPT and Spin-Statistics theorems caused
by nonlocality of the commutation relations
is investigated. In spite of this
inherent nonlocality, we show that the modification aforementioned is
sufficient to ensure the validity of these theorems for NCFT. We restrict
ourselves to the simplest model of a scalar field in the case of only
space-space non-commutativity.Comment: The title is new, and the analysis in the manuscript has been made
more precise. This revised version is to be published in J.Math.Phy
Non-Localizability and Asymptotic Commutativity
The mathematical formalism commonly used in treating nonlocal highly singular
interactions is revised. The notion of support cone is introduced which
replaces that of support for nonlocalizable distributions. Such support cones
are proven to exist for distributions defined on the Gelfand-Shilov spaces
, where . This result leads to a refinement of previous
generalizations of the local commutativity condition to nonlocal quantum
fields. For string propagators, a new derivation of a representation similar to
that of K\"{a}llen-Lehmann is proposed. It is applicable to any initial and
final string configurations and manifests exponential growth of spectral
densities intrinsic in nonlocalizable theories.Comment: This version is identical to the initial one whose ps and pdf files
were unavailable, with few corrections of misprint
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