612 research outputs found
Quantum Bayesian methods and subsequent measurements
After a derivation of the quantum Bayes theorem, and a discussion of the
reconstruction of the unknown state of identical spin systems by repeated
measurements, the main part of this paper treats the problem of determining the
unknown phase difference of two coherent sources by photon measurements. While
the approach of this paper is based on computing correlations of actual
measurements (photon detections), it is possible to derive indirectly a
probability distribution for the phase difference. In this approach, the
quantum phase is not an observable, but a parameter of an unknown quantum
state. Photon measurements determine a probability distribution for the phase
difference. The approach used in this paper takes into account both photon
statistics and the finite efficiency of the detectors.Comment: Expanded and corrected version. 13 pages, 1 figur
Strongly focused light beams interacting with single atoms in free space
We construct 3-D solutions of Maxwell's equations that describe Gaussian
light beams focused by a strong lens. We investigate the interaction of such
beams with single atoms in free space and the interplay between angular and
quantum properties of the scattered radiation. We compare the exact results
with those obtained with paraxial light beams and from a standard input-output
formalism. We put our results in the context of quantum information processing
with single atoms.Comment: 9 pages, 9 figure
The Quantum State of an Ideal Propagating Laser Field
We give a quantum information-theoretic description of an ideal propagating
CW laser field and reinterpret typical quantum-optical experiments in light of
this. In particular we show that contrary to recent claims [T. Rudolph and B.
C. Sanders, Phys. Rev. Lett. 87, 077903 (2001)], a conventional laser can be
used for quantum teleportation with continuous variables and for generating
continuous-variable entanglement. Optical coherence is not required, but phase
coherence is. We also show that coherent states play a priveleged role in the
description of laser light.Comment: 4 pages RevTeX, to appear in PRL. For an extended version see
quant-ph/011115
Entanglement of photons
It is argued that the title of this paper represents a misconception.
Contrary to widespread beliefs it is electromagnetic field modes that are
``systems'' and can be entangled, not photons. The amount of entanglement in a
given state is shown to depend on redefinitions of the modes; we calculate the
minimum and maximum over all such redefinitions for several examples.Comment: 5 pages ReVTe
Decoherence of a two-state atom driven by coherent light
Recent studies of the decoherence induced by the quantum nature of the laser
field driving a two-state atom [J. Gea-Banacloche, Phys. Rev. A 65, 022308
(2002); S. J. van Enk and H. J. Kimble, Quantum Inf. and Comp. 2, 1 (2002)]
have been questioned by Itano [W. M. Itano, Phys. Rev. A 68, 046301 (2003)] and
the proposal made that all decoherence is due to spontaneous emission. We
analyze the problem within the formalism of cascaded open quantum systems. Our
conclusions agree with the Itano proposal. We show that the decoherence,
nevertheless, may be divided into two parts--that due to forwards scattering
and to scattering out of the laser mode. Previous authors attribute the former
to the quantum nature of the laser field.Comment: 6 pages, 2 figures, to appear in Phys. Rev.
Photons in polychromatic rotating modes
We propose a quantum theory of rotating light beams and study some of its
properties. Such beams are polychromatic and have either a slowly rotating
polarization or a slowly rotating transverse mode pattern. We show there are,
for both cases, three different natural types of modes that qualify as
rotating, one of which is a new type not previously considered. We discuss
differences between these three types of rotating modes on the one hand and
non-rotating modes as viewed from a rotating frame of reference on the other.
We present various examples illustrating the possible use of rotating photons,
mostly for quantum information processing purposes. We introduce in this
context a rotating version of the two-photon singlet state.Comment: enormously expanded: 12 pages, 3 figures; a new, more informative,
but less elegant title, especially designed for Phys. Rev.
The power of random measurements: measuring Tr(\rho^n) on single copies of \rho
While it is known that Tr(\rho^n) can be measured directly (i.e., without
first reconstructing the density matrix) by performing joint measurements on n
copies of the same state rho, it is shown here that random measurements on
single copies suffice, too. Averaging over the random measurements directly
yields estimates of Tr(\rho^n), even when it is not known what measurements
were actually performed (so that one cannot reconstruct \rho)
There is no unmet requirement of optical coherence for continuous-variable quantum teleportation
It has been argued [T. Rudolph and B.C. Sanders, Phys. Rev. Lett. 87, 077903
(2001)] that continuous-variable quantum teleportation at optical frequencies
has not been achieved because the source used (a laser) was not `truly
coherent'. Here I show that `true coherence' is always illusory, as the concept
of absolute time on a scale beyond direct human experience is meaningless. A
laser is as good a clock as any other, even in principle, and this objection to
teleportation experiments is baseless.Comment: 6 pages, no figures, no equations, to be published in Journal of
Modern Optics. This is a long version of quant-ph/0104004. I have not
replaced that paper with this one because some authors have referenced that
one approvingly who may feel differently about doing so to this versio
Mutual first order coherence of phase-locked lasers
We argue that (first-order) coherence is a relative, and not an absolute,
property. It is shown how feedforward or feedback can be employed to make two
(or more) lasers relatively coherent. We also show that after the relative
coherence is established, the two lasers will stay relatively coherent for some
time even if the feedforward or feedback loop has been turned off, enabling,
e.g., demonstration of unconditional quantum teleportation using lasers.Comment: 9 pages, 6 figure
Shotgun Mitogenomics Provides a Reference Phylogenetic Framework and Timescale for Living Xenarthrans
Xenarthra (armadillos, sloths, and anteaters) constitutes one of the four major clades of placental mammals. Despite their phylogenetic distinctiveness in mammals, a reference phylogeny is still lacking for the 31 described species. Here we used Illumina shotgun sequencing to assemble 33 new complete mitochondrial genomes, establishing Xenarthra as the first major placental clade to be fully sequenced at the species level for mitogenomes. The resulting data set allowed the reconstruction of a robust phylogenetic framework and timescale that are consistent with previous studies conducted at the genus level using nuclear genes. Incorporating the full species diversity of extant xenarthrans points to a number of inconsistencies in xenarthran systematics and species definition. We propose to split armadillos in two distinct families Dasypodidae (dasypodines) and Chlamyphoridae (euphractines, chlamyphorines, and tolypeutines) to better reflect their ancient divergence, estimated around 42 million years ago. Species delimitation within long-nosed armadillos (genus Dasypus) appeared more complex than anticipated, with the discovery of a divergent lineage in French Guiana. Diversification analyses showed Xenarthra to be an ancient clade with a constant diversification rate through time with a species turnover driven by high but constant extinction. We also detected a significant negative correlation between speciation rate and past temperature fluctuations with an increase in speciation rate corresponding to the general cooling observed during the last 15 million years. Biogeographic reconstructions identified the tropical rainforest biome of Amazonia and the Guianan shield as the cradle of xenarthran evolutionary history with subsequent dispersions into more open and dry habitats.Fil: Gibb, Gillian C.. Universite de Montpellier; Francia. Massey Universit; Nueva ZelandaFil: Condamine, Fabien L.. University of Gothenburg; Suecia. Universite de Montpellier; Francia. University of Alberta; CanadĂĄFil: Kuch, Melanie. McMaster University; CanadĂĄFil: Enk, Jacob. McMaster University; CanadĂĄFil: Moraes Barros, Nadia. Universidade Do Porto; Portugal. Universidade de Sao Paulo; BrasilFil: Superina, Mariella. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Mendoza. Instituto de Medicina y BiologĂa Experimental de Cuyo; ArgentinaFil: Poinar, Hendrik N.. McMaster University; CanadĂĄFil: Delsuc, Frederic. Universite de Montpellier; Franci
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