3,234 research outputs found
Feed-forward and its role in conditional linear optical quantum dynamics
Nonlinear optical quantum gates can be created probabilistically using only
single photon sources, linear optical elements and photon-number resolving
detectors. These gates are heralded but operate with probabilities much less
than one. There is currently a large gap between the performance of the known
circuits and the established upper bounds on their success probabilities. One
possibility for increasing the probability of success of such gates is
feed-forward, where one attempts to correct certain failure events that
occurred in the gate's operation. In this brief report we examine the role of
feed-forward in improving the success probability. In particular, for the
non-linear sign shift gate, we find that in a three-mode implementation with a
single round of feed-forward the optimal average probability of success is
approximately given by p= 0.272. This value is only slightly larger than the
general optimal success probability without feed-forward, P= 0.25.Comment: 4 pages, 3 eps figures, typeset using RevTex4, problems with figures
resolve
Resolving the fine-scale structure in turbulent Rayleigh-Benard convection
We present high-resolution direct numerical simulation studies of turbulent
Rayleigh-Benard convection in a closed cylindrical cell with an aspect ratio of
one. The focus of our analysis is on the finest scales of convective
turbulence, in particular the statistics of the kinetic energy and thermal
dissipation rates in the bulk and the whole cell. The fluctuations of the
energy dissipation field can directly be translated into a fluctuating local
dissipation scale which is found to develop ever finer fluctuations with
increasing Rayleigh number. The range of these scales as well as the
probability of high-amplitude dissipation events decreases with increasing
Prandtl number. In addition, we examine the joint statistics of the two
dissipation fields and the consequences of high-amplitude events. We also have
investigated the convergence properties of our spectral element method and have
found that both dissipation fields are very sensitive to insufficient
resolution. We demonstrate that global transport properties, such as the
Nusselt number, and the energy balances are partly insensitive to insufficient
resolution and yield correct results even when the dissipation fields are
under-resolved. Our present numerical framework is also compared with
high-resolution simulations which use a finite difference method. For most of
the compared quantities the agreement is found to be satisfactory.Comment: 33 pages, 24 figure
Quantum local-field corrections and spontaneous decay
A recently developed scheme [S. Scheel, L. Knoll, and D.-G. Welsch, Phys.
Rev. A 58, 700 (1998)] for quantizing the macroscopic electromagnetic field in
linear dispersive and absorbing dielectrics satisfying the Kramers-Kronig
relations is used to derive the quantum local-field correction for the standard
virtual-sphere-cavity model. The electric and magnetic local-field operators
are shown to be consistent with QED only if the polarization noise is fully
taken into account. It is shown that the polarization fluctuations in the local
field can dramatically change the spontaneous decay rate, compared with the
familiar result obtained from the classical local-field correction. In
particular, the spontaneous emission rate strongly depends on the radius of the
local-field virtual cavity.Comment: 7 pages, using RevTeX, 4 figure
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