3,363 research outputs found
Entangling power of the quantum baker's map
We investigate entanglement production in a class of quantum baker's maps.
The dynamics of these maps is constructed using strings of qubits, providing a
natural tensor-product structure for application of various entanglement
measures. We find that, in general, the quantum baker's maps are good at
generating entanglement, producing multipartite entanglement amongst the qubits
close to that expected in random states. We investigate the evolution of
several entanglement measures: the subsystem linear entropy, the concurrence to
characterize entanglement between pairs of qubits, and two proposals for a
measure of multipartite entanglement. Also derived are some new analytical
formulae describing the levels of entanglement expected in random pure states.Comment: 22 pages, 11 figure
Fuel Retention Improvement at High Temperatures in Tungsten-Uranium Dioxide Dispersion Fuel Elements by Plasma-Spray Cladding
An investigation was undertaken to determine the feasibility of depositing integrally bonded plasma-sprayed tungsten coatings onto 80-volume-percent tungsten - 20-volume-percent uranium dioxide composites. These composites were face clad with thin tungsten foil to inhibit uranium dioxide loss at elevated temperatures, but loss at the unclad edges was still significant. By preheating the composite substrates to approximately 3700 degrees F in a nitrogen environment, metallurgically bonded tungsten coatings could be obtained directly by plasma spraying. Furthermore, even though these coatings were thin and somewhat porous, they greatly inhibited the loss of uranium dioxide. For example, a specimen that was face clad but had no edge cladding lost 5.8 percent uranium dioxide after 2 hours at 4750 dgrees F in flowing hydrogen. A similar specimen with plasma-spray-coated edges, however, lost only 0.75 percent uranium dioxide under the same testing conditions
Generalized uncertainty relations: Theory, examples, and Lorentz invariance
The quantum-mechanical framework in which observables are associated with
Hermitian operators is too narrow to discuss measurements of such important
physical quantities as elapsed time or harmonic-oscillator phase. We introduce
a broader framework that allows us to derive quantum-mechanical limits on the
precision to which a parameter---e.g., elapsed time---may be determined via
arbitrary data analysis of arbitrary measurements on identically prepared
quantum systems. The limits are expressed as generalized Mandelstam-Tamm
uncertainty relations, which involve the operator that generates displacements
of the parameter---e.g., the Hamiltonian operator in the case of elapsed time.
This approach avoids entirely the problem of associating a Hermitian operator
with the parameter. We illustrate the general formalism, first, with
nonrelativistic uncertainty relations for spatial displacement and momentum,
harmonic-oscillator phase and number of quanta, and time and energy and,
second, with Lorentz-invariant uncertainty relations involving the displacement
and Lorentz-rotation parameters of the Poincar\'e group.Comment: 39 pages of text plus one figure; text formatted in LaTe
Optical cavities as amplitude filters for squeezed fields
We explore the use of Fabry-P\'erot cavities as high-pass filters for
squeezed light, and show that they can increase the sensitivity of
interferometric gravitational-wave detectors without the need for long
(kilometer scale) filter cavities. We derive the parameters for the filters,
and analyze the performance of several possible cavity configurations in the
context of a future gravitational-wave interferometer with squeezed light
(vacuum) injected into the output port.Comment: 9 pages, 6 figure
Derivation of the Quantum Probability Rule without the Frequency Operator
We present an alternative frequencists' proof of the quantum probability rule
which does not make use of the frequency operator, with expectation that this
can circumvent the recent criticism against the previous proofs which use it.
We also argue that avoiding the frequency operator is not only for technical
merits for doing so but is closely related to what quantum mechanics is all
about from the viewpoint of many-world interpretation.Comment: 12 page
Universal state inversion and concurrence in arbitrary dimensions
Wootters [Phys. Rev. Lett. 80, 2245 (1998)] has given an explicit formula for
the entanglement of formation of two qubits in terms of what he calls the
concurrence of the joint density operator. Wootters's concurrence is defined
with the help of the superoperator that flips the spin of a qubit. We
generalize the spin-flip superoperator to a "universal inverter," which acts on
quantum systems of arbitrary dimension, and we introduce the corresponding
concurrence for joint pure states of (D1 X D2) bipartite quantum systems. The
universal inverter, which is a positive, but not completely positive
superoperator, is closely related to the completely positive universal-NOT
superoperator, the quantum analogue of a classical NOT gate. We present a
physical realization of the universal-NOT superoperator.Comment: Revtex, 25 page
Quantum Estimation of Parameters of Classical Spacetimes
We describe a quantum limit to measurement of classical spacetimes.
Specifically, we formulate a quantum Cramer-Rao lower bound for estimating the
single parameter in any one-parameter family of spacetime metrics. We employ
the locally covariant formulation of quantum field theory in curved spacetime,
which allows for a manifestly background-independent derivation. The result is
an uncertainty relation that applies to all globally hyperbolic spacetimes.
Among other examples, we apply our method to detection of gravitational waves
using the electromagnetic field as a probe, as in laser-interferometric
gravitational-wave detectors. Other applications are discussed, from
terrestrial gravimetry to cosmology.Comment: 23 pages. This article supersedes arXiv:1108.522
Evolution of Fock states in three mixed harmonic oscillators: quantum statistics
In this communication we investigate the quantum statistics of three harmonic
oscillators mutually interacting with each other considering the modes are
initially in Fock states. After solving the equations of motion, the squeezing
phenomenon, sub-Poissonian statistics and quasiprobability functions are
discussed. We demonstrate that the interaction is able to produce squeezing of
different types. We show also that certain types of Fock states can evolve in
this interaction into thermal state and squeezed thermal state governed by the
interaction parameters.Comment: 25 pages, 7 figure
Generalizing Quantum Mechanics for Quantum Gravity
`How do our ideas about quantum mechanics affect our understanding of
spacetime?' This familiar question leads to quantum gravity. The complementary
question is also important: `How do our ideas about spacetime affect our
understanding of quantum mechanics?' This short abstract of a talk given at the
Gafka2004 conference contains a very brief summary of some of the author's
papers on generalizations of quantum mechanics needed for quantum gravity. The
need for generalization is motivated. The generalized quantum theory framework
for such generalizations is described and illustrated for usual quantum
mechanics and a number of examples to which it does not apply. These include
spacetime alternatives extended over time, time-neutral quantum theory, quantum
field theory in fixed background spacetime not foliable by spacelike surfaces,
and systems with histories that move both forward and backward in time. A fully
four-dimensional, sum-over-histories generalized quantum theory of cosmological
geometries is briefly described. The usual formulation of quantum theory in
terms of states evolving unitarily through spacelike surfaces is an
approximation to this more general framework that is appropriate in the late
universe for coarse-grained descriptions of geometry in which spacetime behaves
classically. This abstract is unlikely to be clear on its own, but references
are provided to the author's works where the ideas can be followed up.Comment: 8 pages, LATEX, a very brief abstract of much wor
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