662 research outputs found
Interference in dielectrics and pseudo-measurements
Inserting a lossy dielectric into one arm of an interference experiment acts
in many ways like a measurement. If two entangled photons are passed through
the interferometer, a certain amount of information is gained about which path
they took, and the interference pattern in a coincidence count measurement is
suppressed. However, by inserting a second dielectric into the other arm of the
interferometer, one can restore the interference pattern. Two of these
pseudo-measurements can thus cancel each other out. This is somewhat analogous
to the proposed quantum eraser experiments.Comment: 7 pages RevTeX 3.0 + 2 figures (postscript). Submitted to Phys. Rev.
J-band variability of M dwarfs in the WFCAM Transit Survey
We present an analysis of the photometric variability of M dwarfs in the Wide Field Camera (WFCAM) Transit Survey. Although periodic light-curve variability in low mass stars is generally dominated by photospheric star spot activity, M dwarf variability in the J band has not been as thoroughly investigated as at visible wavelengths. Spectral type estimates for a sample of over 200 000 objects are made using spectral type-colour relations, and over 9600 dwarfs (J 0.2 mag flaring event from an M4V star in our sample.Peer reviewe
Periodic variability of spotted M dwarfs in WTS
This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.We present an analysis of the photometric variability of M dwarfs in the WFCAM Transit Survey, selected from spectral types inferred by their WTS and SDSS colours, with periods detected using a Lomb-Scargle Periodogram Analisys. We estimate population membership of these objects from their tangential velocities and photometric parralaxes. Examples of M dwarfs with variable light curve morphologuies are found. We discuss possible causes for this and make use of models of spotted stars in our interpretation of the results
Entanglement transformation at absorbing and amplifying four-port devices
Dielectric four-port devices play an important role in optical quantum
information processing. Since for causality reasons the permittivity is a
complex function of frequency, dielectrics are typical examples of noisy
quantum channels, which cannot preserve quantum coherence. To study the effects
of quantum decoherence, we start from the quantized electromagnetic field in an
arbitrary Kramers--Kronig dielectric of given complex permittivity and
construct the transformation relating the output quantum state to the input
quantum state, without placing restrictions on the frequency. We apply the
formalism to some typical examples in quantum communication. In particular we
show that for entangled qubits the Bell-basis states are more
robust against decoherence than the states .Comment: 12 pages, revtex, 10 eps figures, minor corrections in Appendi
On the equivalence of the Langevin and auxiliary field quantization methods for absorbing dielectrics
Recently two methods have been developed for the quantization of the
electromagnetic field in general dispersing and absorbing linear dielectrics.
The first is based upon the introduction of a quantum Langevin current in
Maxwell's equations [T. Gruner and D.-G. Welsch, Phys. Rev. A 53, 1818 (1996);
Ho Trung Dung, L. Kn\"{o}ll, and D.-G. Welsch, Phys. Rev. A 57, 3931 (1998); S.
Scheel, L. Kn\"{o}ll, and D.-G. Welsch, Phys. Rev. A 58, 700 (1998)], whereas
the second makes use of a set of auxiliary fields, followed by a canonical
quantization procedure [A. Tip, Phys. Rev. A 57, 4818 (1998)]. We show that
both approaches are equivalent.Comment: 7 pages, RevTeX, no figure
Effects of interatomic collisions on atom laser outcoupling
We present a computational approach to the outcoupling in a simple
one-dimensional atom laser model, the objective being to circumvent
mathematical difficulties arising from the breakdown of the Born and Markov
approximations. The approach relies on the discretization of the continuum
representing the reservoir of output modes, which allows the treatment of
arbitrary forms of outcoupling as well as the incorporation of non-linear terms
in the Hamiltonian, associated with interatomic collisions. By considering a
single-mode trapped condensate, we study the influence of elastic collisions
between trapped and free atoms on the quasi steady-state population of the
trap, as well as the energy distribution and the coherence of the outcoupled
atoms.Comment: 25 pages, 11 figures, to appear in J. Phys.
Quantum-optical communication rates through an amplifying random medium
We study the competing effects of stimulated and spontaneous emission on the
information capacity of an amplifying disordered waveguide. At the laser
threshold the capacity reaches a "universal" limit, independent of the degree
of disorder. Whether or not this limit is larger or smaller than the capacity
without amplification depends on the disorder, as well as on the input power.
Explicit expressions are obtained for heterodyne detection of coherent states,
and generalized for arbitrary detection scheme.Comment: 4 pages, 4 Postscript figure
Theory of Pseudomodes in Quantum Optical Processes
This paper deals with non-Markovian behaviour in atomic systems coupled to a
structured reservoir of quantum EM field modes, with particular relevance to
atoms interacting with the field in high Q cavities or photonic band gap
materials. In cases such as the former, we show that the pseudo mode theory for
single quantum reservoir excitations can be obtained by applying the Fano
diagonalisation method to a system in which the atomic transitions are coupled
to a discrete set of (cavity) quasimodes, which in turn are coupled to a
continuum set of (external) quasimodes with slowly varying coupling constants
and continuum mode density. Each pseudomode can be identified with a discrete
quasimode, which gives structure to the actual reservoir of true modes via the
expressions for the equivalent atom-true mode coupling constants. The quasimode
theory enables cases of multiple excitation of the reservoir to now be treated
via Markovian master equations for the atom-discrete quasimode system.
Applications of the theory to one, two and many discrete quasimodes are made.
For a simple photonic band gap model, where the reservoir structure is
associated with the true mode density rather than the coupling constants, the
single quantum excitation case appears to be equivalent to a case with two
discrete quasimodes
- …