393 research outputs found
Bound states near a moving charge in a quantum plasma
It is investigated how the shielding of a moving point charge in a
one-component fully degenerate fermion plasma affects the bound states near the
charge at velocities smaller than the Fermi one. The shielding is accounted for
by using the Lindhard dielectric function, and the resulting potential is
substituted into the Schr\"odinger equation in order to obtain the energy
levels. Their number and values are shown to be primarily determined by the
value of the charge and the quantum plasma coupling parameter, while the main
effect of the motion is to split certain energy levels. This provides a link
between quantum plasma theory and possible measurements of spectra of ions
passing through solids.Comment: Published in EPL, see
http://epljournal.edpsciences.org/articles/epl/abs/2011/09/epl13478/epl13478.htm
Shielding of a moving test charge in a quantum plasma
The linearized potential of a moving test charge in a one-component fully
degenerate fermion plasma is studied using the Lindhard dielectric function.
The motion is found to greatly enhance the Friedel oscillations behind the
charge, especially for velocities larger than a half of the Fermi velocity, in
which case the asymptotic behavior of their amplitude changes from 1/r^3 to
1/r^2.5. In the absence of the quantum recoil (tunneling) the potential reduces
to a form similar to that in a classical Maxwellian plasma, with a difference
being that the plasma oscillations behind the charge at velocities larger than
the Fermi velocity are not Landau-damped.Comment: 9 pages, 11 figures. v3: Fixed typo, updated abstrac
Symmetry-protected phases for measurement-based quantum computation
Ground states of spin lattices can serve as a resource for measurement-based
quantum computation. Ideally, the ability to perform quantum gates via
measurements on such states would be insensitive to small variations in the
Hamiltonian. Here, we describe a class of symmetry-protected topological orders
in one-dimensional systems, any one of which ensures the perfect operation of
the identity gate. As a result, measurement-based quantum gates can be a robust
property of an entire phase in a quantum spin lattice, when protected by an
appropriate symmetry.Comment: 5 pages, 1 figure, comments welcome; v2 fixed minor typographic
errors; v3 published versio
Therapeutic drug monitoring (TDM) of atazanavir in pregnancy
Purpose of the study: Pregnant women experience physiological changes during pregnancy resulting in clinically significant alterations in antiretroviral pharmacokinetics (PK). Therefore, achieving and maintaining optimal plasma concentrations of antiretroviral drugs is essential for maternal health and minimising the risk of mother-to-child transmission of HIV. The aim of this study is to describe atazanavir/ritonavir (ATV/r) PK during pregnancy. Methods: Pregnant HIV-positive women received ATV/r as part of their routine pre-natal care. Demographic and clinical data were collected, and ATV plasma concentrations [ATV] were determined in the first (T1), second (T2) and third (T3) trimester using HPLC-MS/MS (LLQ=0.05 µg/mL). Postpartum (PP) sampling was performed where applicable. Antepartum (AP) and PP PK parameters were compared using a one-way ANOVA. Summary of results: From January 2007, 44 women (37 black African) were enrolled in the study. All received ATV/r at a standard dose of 1 tablet once daily (300/100 mg od). 24 women were receiving ART prior to pregnancy, and 20 women initiated ATV/r during pregnancy. Median (range) gestation at treatment initiation in these patients was 23.5 weeks (7–35). At the time nearest to delivery 31 patients had an undetectable plasma viral load (pVL), 6 patients had detectable pVL and 2 were unavailable. [ATV] were determined in 11/44 (T1); 25/44 (T2); 35/44 (T3) and 28/44 (PP) patients. Time of TDM sampling, gestation time and [ATV] (geometric mean; 95% CI) are given in the Table. 6 patients were either below or approaching the ATV MEC (0.15 µg/mL) during pregnancy; of these, 4/6 achieved undetectable pVL at the time of delivery (1=pVL of 291 copies/mL; 1 unavailable). [ATV] were significantly lower at T2/T3 relative to T1/PP. Equally, in a paired analysis of 28 patients (T2/T3 vs. PP), [ATV] were significantly reduced at T2/T3 (P=0.003). Conclusions: This study represents one of the larger cohorts of women undergoing TDM for ATV in pregnancy. Lower [ATV] were seen in T2 and T3 when compared to T1. However, such findings were not associated with viral breakthrough or HIV transmissions. Nonetheless, careful monitoring of women in pregnancy is required, and if there is concern for inadequate levels, dose adjustment of ATV upward from 300 mg to 400 mg may be an option
Multi-label classification using ensembles of pruned sets
This paper presents a Pruned Sets method (PS) for multi-label classification. It is centred on the concept of treating sets of labels as single labels. This allows the classification process to inherently take into account correlations between labels. By pruning these sets, PS focuses only on the most important correlations, which reduces complexity and improves accuracy. By combining pruned sets in an ensemble scheme (EPS), new label sets can be formed to adapt to irregular or complex data. The results from experimental evaluation on a variety of multi-label datasets show that [E]PS can achieve better performance and train much faster than other multi-label methods
Symmetry protection of measurement-based quantum computation in ground states
The two-dimensional cluster state, a universal resource for measurement-based
quantum computation, is also the gapped ground state of a short-ranged
Hamiltonian. Here, we examine the effect of perturbations to this Hamiltonian.
We prove that, provided the perturbation is sufficiently small and respects a
certain symmetry, the perturbed ground state remains a universal resource. We
do this by characterising the operation of an adaptive measurement protocol
throughout a suitable symmetry-protected quantum phase, relying on generic
properties of the phase rather than any analytic control over the ground state.Comment: 20 pages plus appendices, 11 figures, comments very welcome; v2 minor
corrections and additional references; v3 published version with minor
correction
Observation of discrete time-crystalline order in a disordered dipolar many-body system
Understanding quantum dynamics away from equilibrium is an outstanding
challenge in the modern physical sciences. It is well known that
out-of-equilibrium systems can display a rich array of phenomena, ranging from
self-organized synchronization to dynamical phase transitions. More recently,
advances in the controlled manipulation of isolated many-body systems have
enabled detailed studies of non-equilibrium phases in strongly interacting
quantum matter. As a particularly striking example, the interplay of periodic
driving, disorder, and strong interactions has recently been predicted to
result in exotic "time-crystalline" phases, which spontaneously break the
discrete time-translation symmetry of the underlying drive. Here, we report the
experimental observation of such discrete time-crystalline order in a driven,
disordered ensemble of dipolar spin impurities in diamond at
room-temperature. We observe long-lived temporal correlations at integer
multiples of the fundamental driving period, experimentally identify the phase
boundary and find that the temporal order is protected by strong interactions;
this order is remarkably stable against perturbations, even in the presence of
slow thermalization. Our work opens the door to exploring dynamical phases of
matter and controlling interacting, disordered many-body systems.Comment: 6 + 3 pages, 4 figure
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