7,114 research outputs found
Nonclassicality of noisy quantum states
Nonclassicality conditions for an oscillator-like system interacting with a
hot thermal bath are considered. Nonclassical properties of quantum states can
be conserved up to a certain temperature threshold only. In this case,
affection of the thermal noise can be compensated via transformation of an
observable, which tests the nonclassicality (witness function). Possibilities
for experimental implementations based on unbalanced homodyning are discussed.
At the same time, we demonstrate that the scheme based on balanced homodyning
cannot be improved for noisy states with proposed technique and should be
applied directly.Comment: 15 pages, 3 figure
Some exact properties of the gluon propagator
Recent numerical studies of the gluon propagator in the minimal Landau and
Coulomb gauges in space-time dimension 2, 3, and 4 pose a challenge to the
Gribov confinement scenario.
We prove, without approximation, that for these gauges, the continuum gluon
propagator in SU(N) gauge theory satisfies the bound . This holds for Landau
gauge, in which case is the dimension of space-time, and for Coulomb gauge,
in which case is the dimension of ordinary space and is the
instantaneous spatial gluon propagator. This bound implies that , where is the gluon propagator at momentum , and
consequently in Landau gauge in space-time , and in Coulomb
gauge in space dimension , but D(0) may be finite in higher dimension.
These results are compatible with numerical studies of the Landau-and
Coulomb-gauge propagator.
In 4-dimensional space-time a regularization is required, and we also prove
an analogous bound on the lattice gluon propagator, . Here we have taken the
infinite-volume limit of lattice gauge theory at fixed lattice spacing, and the
lattice momentum componant is a continuous angle . Unexpectedly, this implies a bound on the {\it high-momentum} behavior of
the continuum propagator in minimum Landau and Coulomb gauge in 4 space-time
dimensions which, moreover, is compatible with the perturbative renormalization
group when the theory is asymptotically free.Comment: 13 page
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