478 research outputs found
Density of near-extreme events
We provide a quantitative analysis of the phenomenon of crowding of
near-extreme events by computing exactly the density of states (DOS) near the
maximum of a set of independent and identically distributed random variables.
We show that the mean DOS converges to three different limiting forms depending
on whether the tail of the distribution of the random variables decays slower
than, faster than, or as a pure exponential function. We argue that some of
these results would remain valid even for certain {\em correlated} cases and
verify it for power-law correlated stationary Gaussian sequences. Satisfactory
agreement is found between the near-maximum crowding in the summer temperature
reconstruction data of western Siberia and the theoretical prediction.Comment: 4 pages, 3 figures, revtex4. Minor corrections, references updated.
This is slightly extended version of the Published one (Phys. Rev. Lett.
Charge-based silicon quantum computer architectures using controlled single-ion implantation
We report a nanofabrication, control and measurement scheme for charge-based
silicon quantum computing which utilises a new technique of controlled single
ion implantation. Each qubit consists of two phosphorus dopant atoms ~50 nm
apart, one of which is singly ionized. The lowest two energy states of the
remaining electron form the logical states. Surface electrodes control the
qubit using voltage pulses and dual single electron transistors operating near
the quantum limit provide fast readout with spurious signal rejection. A low
energy (keV) ion beam is used to implant the phosphorus atoms in high-purity
Si. Single atom control during the implantation is achieved by monitoring
on-chip detector electrodes, integrated within the device structure, while
positional accuracy is provided by a nanomachined resist mask. We describe a
construction process for implanted single atom and atom cluster devices with
all components registered to better than 20 nm, together with electrical
characterisation of the readout circuitry. We also discuss universal one- and
two-qubit gate operations for this architecture, providing a possible path
towards quantum computing in silicon.Comment: 9 pages, 5 figure
Controlled single electron transfer between Si:P dots
We demonstrate electrical control of Si:P double dots in which the potential
is defined by nanoscale phosphorus doped regions. Each dot contains
approximately 600 phosphorus atoms and has a diameter close to 30 nm. On
application of a differential bias across the dots, electron transfer is
observed, using single electron transistors in both dc- and rf-mode as charge
detectors. With the possibility to scale the dots down to few and even single
atoms these results open the way to a new class of precision-doped quantum dots
in silicon.Comment: 3 figures, 3 page
Fosamprenavir or atazanavir once daily boosted with ritonavir 100 mg, plus tenofovir/emtricitabine, for the initial treatment of HIV infection: 48-week results of ALERT
<p>Abstract</p> <p>Background</p> <p>Once-daily (QD) ritonavir 100 mg-boosted fosamprenavir 1400 mg (FPV/r100) or atazanavir 300 mg (ATV/r100), plus tenofovir/emtricitabine (TDF/FTC) 300 mg/200 mg, have not been compared as initial antiretroviral treatment. To address this data gap, we conducted an open-label, multicenter 48-week study (ALERT) in 106 antiretroviral-naïve, HIV-infected patients (median HIV-1 RNA 4.9 log<sub>10 </sub>copies/mL; CD4+ count 191 cells/mm<sup>3</sup>) randomly assigned to the FPV/r100 or ATV/r100 regimens.</p> <p>Results</p> <p>At baseline, the FPV/r100 or ATV/r100 arms were well-matched for HIV-1 RNA (median, 4.9 log<sub>10 </sub>copies/mL [both]), CD4+ count (mean, 176 vs 205 cells/mm<sup>3</sup>). At week 48, intent-to-treat: missing/discontinuation = failure analysis showed similar responses to FPV/r100 and ATV/r100 (HIV-1 RNA < 50 copies/mL: 75% (40/53) vs 83% (44/53), p = 0.34 [Cochran-Mantel-Haenszel test]); mean CD4+ count change-from-baseline: +170 vs +183 cells/mm<sup>3</sup>, p = 0.398 [Wilcoxon rank sum test]). Fasting total/LDL/HDL-cholesterol changes-from-baseline were also similar, although week 48 median fasting triglycerides were higher with FPV/r100 (150 vs 131 mg/dL). FPV/r100-treated patients experienced fewer treatment-related grade 2–4 adverse events (15% vs 57%), with differences driven by ATV-related hyperbilirubinemia. Three patients discontinued TDF/FTC because their GFR decreased to <50 mL/min.</p> <p>Conclusion</p> <p>The all-QD regimens of FPV/r100 and ATV/r100, plus TDF/FTC, provided similar virologic, CD4+ response, and fasting total/LDL/HDL-cholesterol changes through 48 weeks. Fewer FPV/r100-treated patients experienced treatment-related grade 2–4 adverse events.</p
Charge-based quantum computing using single donors in semiconductors
Solid-state quantum computer architectures with qubits encoded using single
atoms are now feasible given recent advances in atomic doping of
semiconductors. Here we present a charge qubit consisting of two dopant atoms
in a semiconductor crystal, one of which is singly ionised. Surface electrodes
control the qubit and a radio-frequency single electron transistor provides
fast readout. The calculated single gate times, of order 50ps or less, are much
shorter than the expected decoherence time. We propose universal one- and
two-qubit gate operations for this system and discuss prospects for fabrication
and scale up.Comment: 5 pages, 4 figures, updated version submitted to Physical Review
Fixed Effect Estimation of Large T Panel Data Models
This article reviews recent advances in fixed effect estimation of panel data
models for long panels, where the number of time periods is relatively large.
We focus on semiparametric models with unobserved individual and time effects,
where the distribution of the outcome variable conditional on covariates and
unobserved effects is specified parametrically, while the distribution of the
unobserved effects is left unrestricted. Compared to existing reviews on long
panels (Arellano and Hahn 2007; a section in Arellano and Bonhomme 2011) we
discuss models with both individual and time effects, split-panel Jackknife
bias corrections, unbalanced panels, distribution and quantile effects, and
other extensions. Understanding and correcting the incidental parameter bias
caused by the estimation of many fixed effects is our main focus, and the
unifying theme is that the order of this bias is given by the simple formula
p/n for all models discussed, with p the number of estimated parameters and n
the total sample size.Comment: 40 pages, 1 tabl
Predictive Value of Pharmacokinetics-Adjusted Phenotypic Susceptibility on Response to Ritonavir-Enhanced Protease Inhibitors (PIs) in Human Immunodeficiency Virus-Infected Subjects Failing Prior PI Therapy
The activities of protease inhibitors in vivo may depend on plasma concentrations and viral susceptibility. This nonrandomized, open-label study evaluated the relationship of the inhibitory quotient (IQ [the ratio of drug exposure to viral phenotypic susceptibility]) to the human immunodeficiency virus type 1 (HIV-1) viral load (VL) change for ritonavir-enhanced protease inhibitors (PIs). Subjects on PI-based regimens replaced their PIs with ritonavir-enhanced indinavir (IDV/r) 800/200 mg, fosamprenavir (FPV/r) 700/100 mg, or lopinavir (LPV/r) 400/200 mg twice daily. Pharmacokinetics were assessed at day 14; follow-up lasted 24 weeks. Associations between IQ and VL changes were examined. Fifty-three subjects enrolled, 12 on IDV/r, 33 on FPV/r, and 8 on LPV/r. Median changes (n-fold) (FC) of 50% inhibitory concentrations (IC50s) to the study PI were high. Median 2-week VL changes were −0.7, −0.1, and −1.0 log10 for IDV/r, FPV/r, and LPV/r. With FPV/r, correlations between the IQ and the 2-week change in VL were significant (Spearman's r range, −0.39 to −0.50; P ≤ 0.029). The strongest correlation with response to FPV/r was the IC50 FC (r = 0.57; P = 0.001), which improved when only adherent subjects were included (r = 0.68; P = 0.001). In multivariable analyses of the FPV/r arm that included FC, one measure of the drug concentration, corresponding IQ, baseline VL, and CD4, the FC to FPV was the only significant predictor of VL decline (P < 0.001). In exploratory analyses of all arms, the area under the concentration-time curve IQ was correlated with the week 2 VL change (r = −0.72; P < 0.001). In conclusion, in PI-experienced subjects with highly resistant HIV-1, short-term VL responses to RTV-enhanced FPV/r correlated best with baseline susceptibility. The IQ improved correlation in analyses of all arms where a greater range of virologic responses was observed
Collusion through Joint R&D: An Empirical Assessment
This paper tests whether upstream R&D cooperation leads to downstream collusion. We consider an oligopolistic setting where firms enter in research joint ventures (RJVs) to lower production costs or coordinate on collusion in the product market. We show that a sufficient condition for identifying collusive behavior is a decline in the market share of RJV-participating firms, which is also necessary and sufficient for a decrease in consumer welfare. Using information from the US National Cooperation Research Act, we estimate a market share equation correcting for the endogeneity of RJV participation and R&D expenditures. We find robust evidence that large networks between direct competitors – created through firms being members in several RJVs at the same time – are conducive to collusive outcomes in the product market which reduce consumer welfare. By contrast, RJVs among non-competitors are efficiency enhancing
Single-spin readout for buried dopant semiconductor qubits
In the design of quantum computer architectures that take advantage of the
long coherence times of dopant nuclear and electron spins in the solid-state,
single-spin detection for readout remains a crucial unsolved problem. Schemes
based on adiabatically induced spin-dependent electron tunnelling between
individual donor atoms, detected using a single electron transistor (SET) as an
ultra-sensitive electrometer, are thought to be problematic because of the low
ionisaton energy of the final D- state. In this paper we analyse the adiabatic
scheme in detail. We find that despite significant stabilization due to the
presence of the D+, the field strengths required for the transition lead to a
shortened dwell-time placing severe constraints on the SET measurement time. We
therefore investigate a new method based on resonant electron transfer, which
operates with much reduced field strengths. Various issues in the
implementation of this method are also discussed.Comment: 12 pages, 5 figures, 1 tabl
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