1,851,769 research outputs found
A quantum neural network computes its own relative phase
Complete characterization of the state of a quantum system made up of
subsystems requires determination of relative phase, because of interference
effects between the subsystems. For a system of qubits used as a quantum
computer this is especially vital, because the entanglement, which is the basis
for the quantum advantage in computing, depends intricately on phase. We
present here a first step towards that determination, in which we use a
two-qubit quantum system as a quantum neural network, which is trained to
compute and output its own relative phase
Instability and control of a periodically-driven Bose-Einstein condensate
We investigate the dynamics of a Bose-Einstein condensate held in an optical
lattice under the influence of a strong periodic driving potential. Studying
the mean-field version of the Bose-Hubbard model reveals that the condensate
becomes highly unstable when the effective intersite tunneling becomes
negative. We further show how controlling the sign of the tunneling can be used
as a powerful tool to manage the dispersion of an atomic wavepacket, and thus
to create a pulsed atomic soliton laser.Comment: 4 pages, 3 eps figure
Abiotic O Levels on Planets around F, G, K, and M Stars: Possible False Positives for Life?
In the search for life on Earth-like planets around other stars, the first
(and likely only) information will come from the spectroscopic characterization
of the planet's atmosphere. Of the countless number of chemical species
terrestrial life produces, only a few have the distinct spectral features and
the necessary atmospheric abundance to be detectable. The easiest of these
species to observe in Earth's atmosphere is O (and its photochemical
byproduct, O). But O can also be produced abiotically by photolysis
of CO, followed by recombination of O atoms with each other. CO is
produced in stoichiometric proportions. Whether O and CO can accumulate
to appreciable concentrations depends on the ratio of far-UV to near-UV
radiation coming from the planet's parent star and on what happens to these
gases when they dissolve in a planet's oceans. Using a one-dimensional
photochemical model, we demonstrate that O derived from CO
photolysis should not accumulate to measurable concentrations on planets around
F- and G-type stars. K-star, and especially M-star planets, however, may build
up O because of the low near-UV flux from their parent stars, in
agreement with some previous studies. On such planets, a 'false positive' for
life is possible if recombination of dissolved CO and O in the oceans is
slow and if other O sinks (e.g., reduced volcanic gases or dissolved
ferrous iron) are small. O, on the other hand, could be detectable at UV
wavelengths ( < 300 nm) for a much broader range of boundary
conditions and stellar types.Comment: 20 pages text, 9 figure
Selective Control of the Symmetric Dicke Subspace in Trapped Ions
We propose a method of manipulating selectively the symmetric Dicke subspace
in the internal degrees of freedom of N trapped ions. We show that the direct
access to ionic-motional subspaces, based on a suitable tuning of
motion-dependent AC Stark shifts, induces a two-level dynamics involving
previously selected ionic Dicke states. In this manner, it is possible to
produce, sequentially and unitarily, ionic Dicke states with increasing
excitation number. Moreover, we propose a probabilistic technique to produce
directly any ionic Dicke state assuming suitable initial conditions.Comment: 5 pages and 1 figure. New version with minor changes and added
references. Accepted in Physical Review
The surprising lability of bis(2,2’:6’,2’’-terpyridine)- chromium(III) complexes
The complex [Cr(tpy)(O3SCF3)3] (tpy = 2,2′:6′,2′′-terpyridine) is readily made from [Cr(tpy)Cl3] and is a convenient precursor to [Cr(tpy)2][PF6]3 and to [Cr(tpy)(4′-(4-tolyl)tpy)][PF6]3 and [Cr(tpy)(5,5′′-Me2tpy)][PF6]3 (4′-(4-tolyl)tpy = 4′-(4-tolyl)-2,2′:6′,2′′-terpyridine; 5,5′′-Me2tpy = 5,5′′-dimethyl-2,2′:6′,2′′-terpyridine); these are the first examples of heteroleptic bis(tpy) chromium(III) complexes. The single crystal structures of 2{[Cr(tpy)2][PF6]3}·5MeCN, [Cr(tpy)(4′-(4-tolyl)tpy)][PF6]3·3MeCN and [Cr(tpy)(5,5′′-Me2tpy)][PF6]3·3MeCN have been determined. Each cation contains the expected octahedral {Cr(tpy)2}3+ unit; in all three structures, the need to accommodate three anions per cation and the solvent molecules prevents the formation of a grid-like array of cations that is typical of many lattices containing {M(tpy)2}2+ motifs. Three reversible electrochemical processes are observed for [Cr(tpy)(4′-(4-tolyl)tpy)][PF6]3 and [Cr(tpy)(5,5′′-Me2tpy)][PF6]3, consistent with those documented for [Cr(tpy)2]3+. At pH 6.36, aqueous solutions of [Cr(tpy)2][PF6]3 are stable for at least two months. However, contrary to the expectations of the d3 Cr3+ ion being a kinetically inert metal centre, the tpy ligands in [Cr(tpy)2]3+are labile in the presence of base; absorption and 1H NMR spectroscopies have been used to monitor the effects of adding NaOH to aqueous and CD3OD solutions, respectively, of the homo- and heteroleptic complexes. Ligand dissociation is also observed when [Bu4N]F is added to CD3OD solutions of the complexes, but in aqueous solution, [Cr(tpy)2][PF6]3 is stable in the presence of fluoride ion
An Investigation into the Geometry of Seyfert Galaxies
We present a new method for the statistical investigation into the
distributions of the angle beta between the radio axis and the normal to the
galactic disk for a sample of Seyfert galaxies. We discuss how further
observations of the sample galaxies can strengthen the conclusions. Our data
are consistent with the hypothesis that AGN jets are oriented randomly in
space, independent of the position of the plane of the galaxy. By making the
simple assumption that the Standard Model of AGN holds, with a universal
opening angle of the thick torus of phi_c, we demonstrate a statistical method
to obtain an estimate of phi_c. Our data are not consistent with the
simple-minded idea that Seyfert 1s and Seyfert 2s are differentiated solely by
whether or not our line of sight lies within some fixed angle of the jet axis.
Our result is significant on the 2 sigma level and can thus be considered only
suggestive, not conclusive. A complete sample of Seyfert galaxies selected on
an isotropic property is required to obtain a conclusive result.Comment: 13 pages, Tex, 5 Postscript figures. Accepted Ap
Assessment of water pollution by airborne measurement of chlorophyll
Remote measurement of chlorophyll concentrations to determine extent of water pollution is discussed. Construction and operation of radiometer to provide measurement capability are explained. Diagram of equipment is provided
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