167 research outputs found
Persisting correlations of a central spin coupled to large spin baths
The decohering environment of a quantum bit is often described by the
coupling to a large bath of spins. The quantum bit itself can be seen as a spin
which is commonly called the central spin. The resulting central spin
model describes an important mechanism of decoherence. We provide
mathematically rigorous bounds for a persisting magnetization of the central
spin in this model with and without magnetic field. In particular, we show that
there is a well defined limit of infinite number of bath spins. Only if the
fraction of very weakly coupled bath spins tends to 100\% does no magnetization
persist.Comment: 19 pages, 15 figures, rigorous bounds for the central spin mode
Nuclear magnetic resonance spectroscopy of nonequilibrium steady states in quantum dots
The optically induced polarization of localized electron spins in an ensemble
of quantum dots (QDs) dephases due to the interaction with the surrounding
nuclear spins. Despite this dephasing, the spins in the QDs can be controlled
to respond coherently by applying periodic laser pulses, leading to a revival
of the spin polarization before each pulse. This effect, known as spin mode
locking, strongly depends on an emerging selection of certain polarizations of
the nuclear spin bath which is driven to a steady state far from equilibrium.
We investigate the influence of the nuclear composition in InGaAs
QDs on this nonequilibrium behavior and demonstrate that nuclear magnetic
resonances (NMR) appear as very sharp dips in the revival signal. This suggests
a novel kind of NMR spectroscopy.Comment: 7 pages including 4 figures; 3 pages supplement including 3 figure
Evaluation of a general practice based Hepatitis C virus screening intervention
In 2003 an estimated 37,500 of Scotland's population was chronically infected with HCV; 44% were undiagnosed former injecting drug users (IDU) - a priority group for arrival therapy. Aims to evaluate a hepatitis C virus (HCV) screening intervention. Outcomes measures among two similar general practice populations in an area of high HCV and drug use prevalence, one of which was exposed to an HCV screening intervention, were compared. Thirty to fifty four year old attendees of the intervention practice were opportunistically offered testing and counselling, where clinically appropriate, (November 2003 - April 2004). Outcomes: HCV test uptake, case detection, referral and treatment administration rates. Of 584 eligible attendees, 421 (72%) were offered and 117 (28%) accepted testing in the intervention practice; no testing was undertaken in the comparison practice. Prevalences of HCV antibody were 13% (15/117), 75% (3/4) and 91% (10/11) among all tested persons, current IDUs and former IDUs respectively. For 4/15 (27%) evidence of binge drinking following the receipt of their positive result, was available. Of the 11 referred to specialist care because they were HCV RNA positive, nine attended at least one appointment. Two received treatment: one had achieved a sustained viral response as of February 2008. While non targeted HCV screening in the general practice setting can detect infected former IDU, the low diagnostic yield among non IDUs limited the effectiveness of the intervention. A more targeted approach for identifying former IDUs is recommended. Additionally, the low uptake of treatment among chronically infected persons four years after diagnosis demonstrates the difficulties in clinically managing such individuals. Strategies, including support for those with a history of problem alcohol use, to improve treatment uptake are required
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