622 research outputs found
Electrometry Using Coherent Exchange Oscillations in a Singlet-Triplet-Qubit
Two level systems that can be reliably controlled and measured hold promise
in both metrology and as qubits for quantum information science (QIS). When
prepared in a superposition of two states and allowed to evolve freely, the
state of the system precesses with a frequency proportional to the splitting
between the states. In QIS,this precession forms the basis for universal
control of the qubit,and in metrology the frequency of the precession provides
a sensitive measurement of the splitting. However, on a timescale of the
coherence time, , the qubit loses its quantum information due to
interactions with its noisy environment, causing qubit oscillations to decay
and setting a limit on the fidelity of quantum control and the precision of
qubit-based measurements. Understanding how the qubit couples to its
environment and the dynamics of the noise in the environment are therefore key
to effective QIS experiments and metrology. Here we show measurements of the
level splitting and dephasing due to voltage noise of a GaAs singlet-triplet
qubit during exchange oscillations. Using free evolution and Hahn echo
experiments we probe the low frequency and high frequency environmental
fluctuations, respectively. The measured fluctuations at high frequencies are
small, allowing the qubit to be used as a charge sensor with a sensitivity of
, two orders of magnitude better than
the quantum limit for an RF single electron transistor (RF-SET). We find that
the dephasing is due to non-Markovian voltage fluctuations in both regimes and
exhibits an unexpected temperature dependence. Based on these measurements we
provide recommendations for improving in future experiments, allowing for
higher fidelity operations and improved charge sensitivity
The association between histamine 2 receptor antagonist use and Clostridium difficile infection: a systematic review and meta-analysis.
Background
Clostridium difficile infection (CDI) is a major health problem. Epidemiological evidence suggests that there is an association between acid suppression therapy and development of CDI.
Purpose
We sought to systematically review the literature that examined the association between histamine 2 receptor antagonists (H2RAs) and CDI.
Data source
We searched Medline, Current Contents, Embase, ISI Web of Science and Elsevier Scopus from 1990 to 2012 for all analytical studies that examined the association between H2RAs and CDI.
Study selection
Two authors independently reviewed the studies for eligibility.
Data extraction
Data about studies characteristics, adjusted effect estimates and quality were extracted.
Data synthesis
Thirty-five observations from 33 eligible studies that included 201834 participants were analyzed. Studies were performed in 6 countries and nine of them were multicenter. Most studies did not specify the type or duration of H2RAs therapy. The pooled effect estimate was 1.44, 95% CI (1.22–1.7), I2 = 70.5%. This association was consistent across different subgroups (by study design and country) and there was no evidence of publication bias. The pooled effect estimate for high quality studies was 1.39 (1.15–1.68), I2 = 72.3%. Meta-regression analysis of 10 study-level variables did not identify sources of heterogeneity. In a speculative analysis, the number needed to harm (NNH) with H2RAs at 14 days after hospital admission in patients receiving antibiotics or not was 58, 95% CI (37, 115) and 425, 95% CI (267, 848), respectively. For the general population, the NNH at 1 year was 4549, 95% CI (2860, 9097).
Conclusion
In this rigorous systematic review and meta-analysis, we observed an association between H2RAs and CDI. The absolute risk of CDI associated with H2RAs is highest in hospitalized patients receiving antibiotics
Demonstration of Entanglement of Electrostatically Coupled Singlet-Triplet Qubits
Quantum computers have the potential to solve certain interesting problems
significantly faster than classical computers. To exploit the power of a
quantum computation it is necessary to perform inter-qubit operations and
generate entangled states. Spin qubits are a promising candidate for
implementing a quantum processor due to their potential for scalability and
miniaturization. However, their weak interactions with the environment, which
leads to their long coherence times, makes inter-qubit operations challenging.
We perform a controlled two-qubit operation between singlet-triplet qubits
using a dynamically decoupled sequence that maintains the two-qubit coupling
while decoupling each qubit from its fluctuating environment. Using state
tomography we measure the full density matrix of the system and determine the
concurrence and the fidelity of the generated state, providing proof of
entanglement
Anomalous structure in the single particle spectrum of the fractional quantum Hall effect
The two-dimensional electron system (2DES) is a unique laboratory for the
physics of interacting particles. Application of a large magnetic field
produces massively degenerate quantum levels known as Landau levels. Within a
Landau level the kinetic energy of the electrons is suppressed, and
electron-electron interactions set the only energy scale. Coulomb interactions
break the degeneracy of the Landau levels and can cause the electrons to order
into complex ground states. In the high energy single particle spectrum of this
system, we observe salient and unexpected structure that extends across a wide
range of Landau level filling fractions. The structure appears only when the
2DES is cooled to very low temperature, indicating that it arises from delicate
ground state correlations. We characterize this structure by its evolution with
changing electron density and applied magnetic field. We present two possible
models for understanding these observations. Some of the energies of the
features agree qualitatively with what might be expected for composite
Fermions, which have proven effective for interpreting other experiments in
this regime. At the same time, a simple model with electrons localized on
ordered lattice sites also generates structure similar to those observed in the
experiment. Neither of these models alone is sufficient to explain the
observations across the entire range of densities measured. The discovery of
this unexpected prominent structure in the single particle spectrum of an
otherwise thoroughly studied system suggests that there exist core features of
the 2DES that have yet to be understood.Comment: 15 pages, 10 figure
Clinical and pathological kidney aspects of sickle cell anemia at Dakar: study of 11 cases of renal biopsies
Few studies are devoted to the practice of renal biopsy in sickle cell nephropathy; our objective was to determine the histological and evolutionary patterns of renal lesions in sickle cell patients who underwent renal biopsy in Dakar.Methods:This was a retrospective multicentric study (conducted from December 2009 to August 2011) on renal biopsies performed on sickle cell anaemic patients at the Nephrology Department of Teaching Hospital Aristide Le Dantec and the Albert Royer Childrens Hospital. The histological, therapeutic and evolutionary data were analysed.From the 292 total renal biopsies, 11 (3.80%) were performed on sickle cell patients (6SS, 1SBth + 4 AS) with a mean age of 23.1 [13-51 years]. Nephrotic syndrome was the indication of renal biopsy in all cases. Focal segmental glomerulosclerosis was the most frequent histological finding (five cases), followed by a combination of various specific lesions (hypertrophy of glomerular and peritubular capillaries), minimal glomerular lesions (three cases), membranoproliferative glomerulonephritis (two cases) and extra-membranous glomerulonephritis (one case). Complete remission after treatment was achieved in seven cases and one patient expired. Three patients did not continue with follow-up appointments.Conclusions:Renal biopsy is not very frequent in the course of sickle cell anaemia and in most cases it is performed because of nephrotic syndrome. The histological findings are diverse with a predominance of focal segmental glomerulosclerosis
A valley-spin qubit in a carbon nanotube
Although electron spins in III-V semiconductor quantum dots have shown great
promise as qubits, a major challenge is the unavoidable hyperfine decoherence
in these materials. In group IV semiconductors, the dominant nuclear species
are spinless, allowing for qubit coherence times that have been extended up to
seconds in diamond and silicon. Carbon nanotubes are a particularly attractive
host material, because the spin-orbit interaction with the valley degree of
freedom allows for electrical manipulation of the qubit. In this work, we
realise such a qubit in a nanotube double quantum dot. The qubit is encoded in
two valley-spin states, with coherent manipulation via electrically driven spin
resonance (EDSR) mediated by a bend in the nanotube. Readout is performed by
measuring the current in Pauli blockade. Arbitrary qubit rotations are
demonstrated, and the coherence time is measured via Hahn echo. Although the
measured decoherence time is only 65 ns in our current device, this work offers
the possibility of creating a qubit for which hyperfine interaction can be
virtually eliminated
Association between proton pump inhibitor therapy and clostridium difficile infection: a contemporary systematic review and meta-analysis.
Abstract
Introduction
Emerging epidemiological evidence suggests that proton pump inhibitor (PPI) acid-suppression therapy is associated with an increased risk of Clostridium difficile infection (CDI).
Methods
Ovid MEDLINE, EMBASE, ISI Web of Science, and Scopus were searched from 1990 to January 2012 for analytical studies that reported an adjusted effect estimate of the association between PPI use and CDI. We performed random-effect meta-analyses. We used the GRADE framework to interpret the findings.
Results
We identified 47 eligible citations (37 case-control and 14 cohort studies) with corresponding 51 effect estimates. The pooled OR was 1.65, 95% CI (1.47, 1.85), I2 = 89.9%, with evidence of publication bias suggested by a contour funnel plot. A novel regression based method was used to adjust for publication bias and resulted in an adjusted pooled OR of 1.51 (95% CI, 1.26–1.83). In a speculative analysis that assumes that this association is based on causality, and based on published baseline CDI incidence, the risk of CDI would be very low in the general population taking PPIs with an estimated NNH of 3925 at 1 year.
Conclusions
In this rigorously conducted systemic review and meta-analysis, we found very low quality evidence (GRADE class) for an association between PPI use and CDI that does not support a cause-effect relationship
Microscopic Polarization in Bilayer Graphene
Bilayer graphene has drawn significant attention due to the opening of a band
gap in its low energy electronic spectrum, which offers a promising route to
electronic applications. The gap can be either tunable through an external
electric field or spontaneously formed through an interaction-induced symmetry
breaking. Our scanning tunneling measurements reveal the microscopic nature of
the bilayer gap to be very different from what is observed in previous
macroscopic measurements or expected from current theoretical models. The
potential difference between the layers, which is proportional to charge
imbalance and determines the gap value, shows strong dependence on the disorder
potential, varying spatially in both magnitude and sign on a microscopic level.
Furthermore, the gap does not vanish at small charge densities. Additional
interaction-induced effects are observed in a magnetic field with the opening
of a subgap when the zero orbital Landau level is placed at the Fermi energy
Imaging Electronic Correlations in Twisted Bilayer Graphene near the Magic Angle
Twisted bilayer graphene with a twist angle of around 1.1{\deg} features a
pair of isolated flat electronic bands and forms a strongly correlated
electronic platform. Here, we use scanning tunneling microscopy to probe local
properties of highly tunable twisted bilayer graphene devices and show that the
flat bands strongly deform when aligned with the Fermi level. At half filling
of the bands, we observe the development of gaps originating from correlated
insulating states. Near charge neutrality, we find a previously unidentified
correlated regime featuring a substantially enhanced flat band splitting that
we describe within a microscopic model predicting a strong tendency towards
nematic ordering. Our results provide insights into symmetry breaking
correlation effects and highlight the importance of electronic interactions for
all filling factors in twisted bilayer graphene.Comment: Main text 9 pages, 4 figures; Supplementary Information 25 page
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