5,699 research outputs found
Beyond transcoherent states: Field states for effecting optimal coherent rotations on single or multiple qubits
Semiclassically, laser pulses can be used to implement arbitrary
transformations on atomic systems; quantum mechanically, residual atom-field
entanglement spoils this promise. Transcoherent states are field states that
fix this problem in the fully quantized regime by generating perfect coherence
in an atom initially in its ground or excited state. We extend this fully
quantized paradigm in four directions: First, we introduce field states that
transform an atom from its ground or excited state to any point on the Bloch
sphere without residual atom-field entanglement. The best strong pulses for
carrying out rotations by angle are are squeezed in photon-number
variance by a factor of . Next, we investigate implementing
rotation gates, showing that the optimal Gaussian field state for enacting a
pulse on an atom in an arbitrary, unknown initial state is number
squeezed by less: . Third, we extend these
investigations to fields interacting with multiple atoms simultaneously,
discovering once again that number squeezing by is optimal for
enacting pulses on all of the atoms simultaneously, with small
corrections on the order of the ratio of the number of atoms to the average
number of photons. Finally, we find field states that best perform arbitrary
rotations by through nonlinear interactions involving -photon
absorption, where the same optimal squeezing factor is found to be
. Backaction in a wide variety of atom-field interactions can
thus be mitigated by squeezing the control fields by optimal amounts.Comment: Updated formatting following acceptance in Quantu
Conditional probabilities in quantum theory, and the tunneling time controversy
It is argued that there is a sensible way to define conditional probabilities
in quantum mechanics, assuming only Bayes's theorem and standard quantum
theory. These probabilities are equivalent to the ``weak measurement''
predictions due to Aharonov {\it et al.}, and hence describe the outcomes of
real measurements made on subensembles. In particular, this approach is used to
address the question of the history of a particle which has tunnelled across a
barrier. A {\it gedankenexperiment} is presented to demonstrate the physically
testable implications of the results of these calculations, along with graphs
of the time-evolution of the conditional probability distribution for a
tunneling particle and for one undergoing allowed transmission. Numerical
results are also presented for the effects of loss in a bandgap medium on
transmission and on reflection, as a function of the position of the lossy
region; such loss should provide a feasible, though indirect, test of the
present conclusions. It is argued that the effects of loss on the pulse {\it
delay time} are related to the imaginary value of the momentum of a tunneling
particle, and it is suggested that this might help explain a small discrepancy
in an earlier experiment.Comment: 11 pages, latex, 4 postscript figures separate (one w/ 3 parts
Comment on ``Manipulating the frequency entangled states by an acoutic-optical modulator''
A recent theoretical paper [1] proposes a scheme for entanglement swapping
utilizing acousto-optic modulators without requiring a Bell-state measurement.
In this comment, we show that the proposal is flawed and no entanglement
swapping can occur without measurement.Comment: 6 pages, 2 figures submitted to Phys. Rev
Hotter, Denser, Faster, Smaller...and Nearly-Perfect: What's the matter at RHIC?
The experimental and theoretical status of the ``near perfect fluid'' at RHIC
is discussed. While the hydrodynamic paradigm for understanding collisions at
RHIC is well-established, there remain many important open questions to address
in order to understand its relevance and scope. It is also a crucial issue to
understand how the early equilibration is achieved, requiring insight into the
active degrees of freedom at early times.Comment: 10 Pages, 13 Figures, submitted to the proceedings of the Second
Meeting of the APS Topical Group on Hadronic Physics, Nashville, TN, October
22-24, 200
Efficient Mixing at low Reynolds numbers using polymer additives
Mixing in fluids is a rapidly developing field of fluid mechanics
\cite{Sreen,Shr,War}, being an important industrial and environmental problem.
The mixing of liquids at low Reynolds numbers is usually quite weak in simple
flows, and it requires special devices to be efficient. Recently, the problem
of mixing was solved analytically for a simple case of random flow, known as
the Batchelor regime \cite{Bat,Kraich,Fal,Sig,Fouxon}. Here we demonstrate
experimentally that very viscous liquids at low Reynolds number, . Here we
show that very viscous liquids containing a small amount of high molecular
weight polymers can be mixed quite efficiently at very low Reynolds numbers,
for a simple flow in a curved channel. A polymer concentration of only 0.001%
suffices. The presence of the polymers leads to an elastic instability
\cite{LMS} and to irregular flow \cite{Ours}, with velocity spectra
corresponding to the Batchelor regime \cite{Bat,Kraich,Fal,Sig,Fouxon}. Our
detailed observations of the mixing in this regime enable us to confirm sevearl
important theoretical predictions: the probability distributions of the
concentration exhibit exponential tails \cite{Fal,Fouxon}, moments of the
distribution decay exponentially along the flow \cite{Fouxon}, and the spatial
correlation function of concentration decays logarithmically.Comment: 11 pages, 5 figure
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