10,503 research outputs found
Giant optical Faraday rotation induced by a single electron spin in a quantum dot: Applications to entangling remote spins via a single photon
We propose a quantum non-demolition method - giant Faraday rotation - to
detect a single electron spin in a quantum dot inside a microcavity where
negatively-charged exciton strongly couples to the cavity mode. Left- and
right-circularly polarized light reflected from the cavity feels different
phase shifts due to cavity quantum electrodynamics and the optical spin
selection rule. This yields giant and tunable Faraday rotation which can be
easily detected experimentally. Based on this spin-detection technique, a
scalable scheme to create an arbitrary amount of entanglement between two or
more remote spins via a single photon is proposed.Comment: 5 pages, 3 figure
Water Scarcity and Water Markets: A Comparison of Institutions and Practices in the Murray-Darling Basin of Australia and the Western US
Water markets in Australia’s Murray-Darling Basin (MDB) and the US west are compared in terms of their ability to allocate scarce water resources among competing uses. Both locations have been in the forefront of the development of water markets with defined water rights and conveyance structures to assist in the reallocation of water across competing demands. They also share the challenge of managing water with climate variability and climate change. As these two markets occur in developed, wealthy countries, their experiences in water markets with different water rights (appropriative, riparian and statutory rights) provide ‘best-case’ scenarios of what institutional arrangements work best, indicate which are less effective, and demonstrate what might be possible for greater use of water markets elsewhere in the world. The paper finds that the gains from trade in the MDB is worth hundreds of millions of dollars in per year, total turnover in water rights exceeds 4.3 billion (2008 $) spent or committed by urban buyers between 1987 and 2008. Despite the clear benefits of water markets in both locations, there are on-going restrictions to trade that limit the potential gains and also third-party effects from use that require resolution.
Finite energy/action solutions of Yang-Mills equations on Schwarzschild and deSitter backgrounds for dimension
Physically relevant gauge and gravitational theories can be seen as special
members of hierarchies of more elaborate systems. The Yang-Mills (YM) system is
the first member of a hierarchy of Lagrangians which we will index by ,
and the Einstein-Hilbert (EH) system of general relativity is the first member
of another hierarchy which we index by . In this paper, we study the
classical equations of the YM hierarchy considered in the
background of special geometries (Schwarzschild, deSitter,anti-deSitter) of the
EH hierarchy. Solutions are obtained in various dimensions and lead
to several examples of non-self-dual YM fields. When self-dual
solutions exist in addition. Their action is equal to the Chern-Pontryagin
charge and can be compared with that of the non-self-dual solutions.Comment: LaTeX, 25 pages, 2 figures, new title, minor change
Detection of Coronal Mass Ejections in V471 Tauri with the Hubble Space Telescope
V471 Tauri, an eclipsing system consisting of a hot DA white dwarf (WD) and a
dK2 companion in a 12.5-hour orbit, is the prototype of the pre-cataclysmic
binaries. The late-type component is magnetically active, due to its being
constrained to rotate synchronously with the short orbital period. During a
program of UV spectroscopy of V471 Tau, carried out with the Goddard High
Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope, we
serendipitously detected two episodes in which transient absorptions in the Si
III 1206 A resonance line appeared suddenly, on a timescale of <2 min. The
observations were taken in a narrow spectral region around Ly-alpha, and were
all obtained near the two quadratures of the binary orbit, i.e., at maximum
projected separation (~3.3 Rsun) of the WD and K star.
We suggest that these transient features arise when coronal mass ejections
(CME's) from the K2 dwarf pass across the line of sight to the WD. Estimates of
the velocities, densities, and masses of the events in V471 Tau are generally
consistent with the properties of solar CME's. Given our detection of 2 events
during 6.8 hr of GHRS observing, along with a consideration of the restricted
range of latitudes and longitudes on the K star's surface that can give rise to
trajectories passing in front of the WD as seen from Earth, we estimate that
the active V471 Tau dK star emits some 100-500 CME's per day, as compared to
1-3 per day for the Sun. The K dwarf's mass-loss rate associated with CME's is
at least (5-25) x 10^{-14} Msun/yr, but it may well be orders of magnitude
higher if most of the silicon is in ionization states other than Si III.Comment: 24 pages AASTeX, 4 figures. Accepted by Astrophysical Journa
Measuring a photonic qubit without destroying it
Measuring the polarisation of a single photon typically results in its
destruction. We propose, demonstrate, and completely characterise a
\emph{quantum non-demolition} (QND) scheme for realising such a measurement
non-destructively. This scheme uses only linear optics and photo-detection of
ancillary modes to induce a strong non-linearity at the single photon level,
non-deterministically. We vary this QND measurement continuously into the weak
regime, and use it to perform a non-destructive test of complementarity in
quantum mechanics. Our scheme realises the most advanced general measurement of
a qubit: it is non-destructive, can be made in any basis, and with arbitrary
strength.Comment: 4 pages, 3 figure
Quantum Non-demolition Measurements on Qubits
We discuss the characterization and properties of quantum non-demolition
(QND) measurements on qubit systems. We introduce figures of merit which can be
applied to systems of any Hilbert space dimension thus providing universal
criteria for characterizing QND measurements. We discuss the controlled-NOT
gate and an optical implementation as examples of QND devices for qubits. We
also discuss the QND measurement of weak values
Measuring the Impact of Nextstep Career Advice in the North East of England
In September, 2007, the main contractors of the four nextstep networks in the North East (referred to in this paper as the ‘consortium’) successfully submitted a proposal to the Learning & Skills Council (LSC) for funding to develop a
framework for measuring the impact of career advice. It was agreed that the project would result in a practice-based ‘Impact Toolkit’ that would include:-
• a set of impact measures that should be used in the new adult careers service with a rationale for their inclusion;
• methodologies for each of the impact measures identified;
• approaches to obtaining information on progression in learning and work – with templates and case study material;
• approaches to measuring soft outcomes; and
• suggested Key Performance Indicators (KPIs)
Molecular Line Observations of Infrared Dark Clouds: Seeking the Precursors to Intermediate and Massive Star Formation
We have identified 41 infrared dark clouds from the 8 micron maps of the
Midcourse Space Experiment (MSX), selected to be found within one square degree
areas centered on known ultracompact HII regions. We have mapped these infrared
dark clouds in N2H+(1-0), CS(2-1) and C18O(1-0) emission using the Five College
Radio Astronomy Observatory. The maps of the different species often show
striking differences in morphologies, indicating differences in evolutionary
state and/or the presence of undetected, deeply embedded protostars. We derive
an average mass for these clouds using N2H+ column densities of ~2500 solar
masses, a value comparable to that found in previous studies of high mass star
forming cores using other mass tracers. The linewidths of these clouds are
typically ~2.0 - 2.9 km/s. Based on the fact that they are dark at 8 micron,
compact, massive, and have large velocity dispersions, we suggest that these
clouds may be the precursor sites of intermediate and high mass star formation.Comment: Accepted to ApJS, 22 pages, 10 pages of figures. For full-resolution
images, see http://www.astro.lsa.umich.edu/~seragan/pubs/fcrao/figures.tar.g
Optimal search strategies for hidden targets
What is the fastest way of finding a randomly hidden target? This question of
general relevance is of vital importance for foraging animals. Experimental
observations reveal that the search behaviour of foragers is generally
intermittent: active search phases randomly alternate with phases of fast
ballistic motion. In this letter, we study the efficiency of this type of two
states search strategies, by calculating analytically the mean first passage
time at the target. We model the perception mecanism involved in the active
search phase by a diffusive process. In this framework, we show that the search
strategy is optimal when the average duration of "motion phases" varies like
the power either 3/5 or 2/3 of the average duration of "search phases",
depending on the regime. This scaling accounts for experimental data over a
wide range of species, which suggests that the kinetics of search trajectories
is a determining factor optimized by foragers and that the perception activity
is adequately described by a diffusion process.Comment: 4 pages, 5 figures. to appear in Phys. Rev. Let
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind
RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts
have indicated that high velocity ejecta interact with a pre-existing red giant
wind, setting up shock systems analogous to those seen in Supernova Remnants.
However, in the previous outburst in 1985, X-ray observations did not commence
until 55 days after the initial explosion. Here we report on Swift observations
covering the first month of the 2006 outburst with the Burst Alert (BAT) and
X-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25
keV band from t=0 to days. XRT observationsfrom 0.3-10 keV, started at
3.17 days after outburst. The rapidly evolving XRT spectra clearly show the
presence of both line and continuum emission which can be fitted by thermal
emission from hot gas whose characteristic temperature, overlying absorbing
column, , and resulting unabsorbed total flux decline monotonically
after the first few days. Derived shock velocities are in good agreement with
those found from observations at other wavelengths. Similarly, is in
accord with that expected from the red giant wind ahead of the forward shock.
We confirm the basic models of the 1985 outburst and conclude that standard
Phase I remnant evolution terminated by days and the remnant then
rapidly evolved to display behaviour characteristic of Phase III. Around t=26
days however, a new, luminous and highly variable soft X-ray source began to
appear whose origin will be explored in a subsequent paper.Comment: 20 pages, 4 figures (2 updated), accepted by Ap
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