2,323 research outputs found

### Gravitational radiation and the ultimate speed in Rosen's Bimetric theory of gravity

Emission of gravitational radiation was shown to prevent particles of nonzero rest mass from exceeding the speed of gravitational radiation

### Explicit product ensembles for separable quantum states

We present a general method for constructing pure-product-state
representations for density operators of $N$ quantum bits. If such a
representation has nonnegative expansion coefficients, it provides an explicit
separable ensemble for the density operator. We derive the condition for
separability of a mixture of the Greenberger-Horne-Zeilinger state with the
maximally mixed state.Comment: 15 pages, no figure

### Theoretical frameworks for testing relativistic gravity. 5: Post-Newtonian limit of Rosen's theory

The post-Newtonian limit of Rosen's theory of gravity is evaluated and is shown to be identical to that of general relativity, except for the PPN parameter alpha sub 2, which is related to the difference in propagation speeds for gravitational and electromagnetic waves. Both the value of alpha sub 2 and the value of the Newtonian gravitational constant depend on the present cosmological structure of the Universe. If the cosmological structure has a specific but presumably special form, the Newtonian gravitational constant assumes its current value, alpha sub 2 is zero, the post-Newtonian limit of Rosen's theory is identical to that of general relativity--and standard solar system experiments cannot distinguish between the two theories

### Operational Discord Measure for Gaussian States with Gaussian Measurements

We introduce an operational discord-type measure for quantifying nonclassical
correlations in bipartite Gaussian states based on using Gaussian measurements.
We refer to this measure as operational Gaussian discord (OGD). It is defined
as the difference between the entropies of two conditional probability
distributions associated to one subsystem, which are obtained by performing
optimal local and joint Gaussian measurements. We demonstrate the operational
significance of this measure in terms of a Gaussian quantum protocol for
extracting maximal information about an encoded classical signal. As examples,
we calculate OGD for several Gaussian states in the standard form.Comment: 18 pages, 3 figure

### Separable balls around the maximally mixed multipartite quantum states

We show that for an m-partite quantum system, there is a ball of radius
2^{-(m/2-1)} in Frobenius norm, centered at the identity matrix, of separable
(unentangled) positive semidefinite matrices. This can be used to derive an
epsilon below which mixtures of epsilon of any density matrix with 1 - epsilon
of the maximally mixed state will be separable. The epsilon thus obtained is
exponentially better (in the number of systems) than existing results. This
gives a number of qubits below which NMR with standard pseudopure-state
preparation techniques can access only unentangled states; with parameters
realistic for current experiments, this is 23 qubits (compared to 13 qubits via
earlier results). A ball of radius 1 is obtained for multipartite states
separable over the reals.Comment: 8 pages, LaTe

### Sufficient Conditions for Efficient Classical Simulation of Quantum Optics

We provide general sufficient conditions for the efficient classical
simulation of quantum-optics experiments that involve inputting states to a
quantum process and making measurements at the output. The first condition is
based on the negativity of phase-space quasiprobability distributions (PQDs) of
the output state of the process and the output measurements; the second one is
based on the negativity of PQDs of the input states, the output measurements,
and the transition function associated with the process. We show that these
conditions provide useful practical tools for investigating the effects of
imperfections in implementations of boson sampling. In particular, we apply our
formalism to boson-sampling experiments that use single-photon or
spontaneous-parametric-down-conversion sources and on-off photodetectors.
Considering simple models for loss and noise, we show that above some threshold
for the probability of random counts in the photodetectors, these
boson-sampling experiments are classically simulatable. We identify mode
mismatching as the major source of error contributing to random counts and
suggest that this is the chief challenge for implementations of boson sampling
of interesting size.Comment: 12 pages, 1 figur

### Standard Quantum Limits for broadband position measurement

I utilize the Caves-Milburn model for continuous position measurements to
formulate a broadband version of the Standard Quantum Limit (SQL) for
monitoring the position of a free mass, and illustrate the use of Kalman
filtering to recover the SQL for estimating a weak classical force that acts on
a quantum-mechanical test particle under continuous observation. These
derivations are intended to clarify the interpretation of SQL's in the context
of continuous quantum measurement.Comment: Replaced version: changed title, fixed algebra error at the very end,
conclusions modified accordingly. Four pages, one eps figur

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