448 research outputs found
Comparison of coherence times in three dc SQUID phase qubits
We report measurements of spectroscopic linewidth and Rabi oscillations in
three thin-film dc SQUID phase qubits. One device had a single-turn Al loop,
the second had a 6-turn Nb loop, and the third was a first order gradiometer
formed from 6-turn wound and counter-wound Nb coils to provide isolation from
spatially uniform flux noise. In the 6 - 7.2 GHz range, the spectroscopic
coherence times for the gradiometer varied from 4 ns to 8 ns, about the same as
for the other devices (4 to 10 ns). The time constant for decay of Rabi
oscillations was significantly longer in the single-turn Al device (20 to 30
ns) than either of the Nb devices (10 to 15 ns). These results imply that
spatially uniform flux noise is not the main source of decoherence or
inhomogenous broadening in these devices.Comment: 4 pages, 5 figures, accepted for publication in IEEE Trans. Appl.
Supercon
Strong-field effects in the Rabi oscillations of the superconducting phase qubit
Rabi oscillations have been observed in many superconducting devices, and
represent prototypical logic operations for quantum bits (qubits) in a quantum
computer. We use a three-level multiphoton analysis to understand the behavior
of the superconducting phase qubit (current-biased Josephson junction) at high
microwave drive power. Analytical and numerical results for the ac Stark shift,
single-photon Rabi frequency, and two-photon Rabi frequency are compared to
measurements made on a dc SQUID phase qubit with Nb/AlOx/Nb tunnel junctions.
Good agreement is found between theory and experiment.Comment: 4 pages, 4 figures, accepted for publication in IEEE Trans. Appl.
Supercon
Incorporating DNA Sequencing into Current Prenatal Screening Practice for Down's Syndrome
PMCID: PMC3604109This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Multilevel effects in the Rabi oscillations of a Josephson phase qubit
We present Rabi oscillation measurements of a Nb/AlOx/Nb dc superconducting
quantum interference device (SQUID) phase qubit with a 100 um^2 area junction
acquired over a range of microwave drive power and frequency detuning. Given
the slightly anharmonic level structure of the device, several excited states
play an important role in the qubit dynamics, particularly at high power. To
investigate the effects of these levels, multiphoton Rabi oscillations were
monitored by measuring the tunneling escape rate of the device to the voltage
state, which is particularly sensitive to excited state population. We compare
the observed oscillation frequencies with a simplified model constructed from
the full phase qubit Hamiltonian and also compare time-dependent escape rate
measurements with a more complete density-matrix simulation. Good quantitative
agreement is found between the data and simulations, allowing us to identify a
shift in resonance (analogous to the ac Stark effect), a suppression of the
Rabi frequency, and leakage to the higher excited states.Comment: 14 pages, 9 figures; minor corrections, updated reference
Switchable Coupling of Vibrations to Two-Electron Carbon-Nanotube Quantum Dot States
We report transport measurements on a quantum dot in a partly suspended
carbon nanotube. Electrostatic tuning allows us to modify and even switch 'on'
and 'off' the coupling to the quantized stretching vibration across several
charge states. The magnetic-field dependence indicates that only the
two-electron spin-triplet excited state couples to the mechanical motion,
indicating mechanical coupling to both the valley degree of freedom and the
exchange interaction, in contrast to standard models
State Transfer Between a Mechanical Oscillator and Microwave Fields in the Quantum Regime
Recently, macroscopic mechanical oscillators have been coaxed into a regime
of quantum behavior, by direct refrigeration [1] or a combination of
refrigeration and laser-like cooling [2, 3]. This exciting result has
encouraged notions that mechanical oscillators may perform useful functions in
the processing of quantum information with superconducting circuits [1, 4-7],
either by serving as a quantum memory for the ephemeral state of a microwave
field or by providing a quantum interface between otherwise incompatible
systems [8, 9]. As yet, the transfer of an itinerant state or propagating mode
of a microwave field to and from a mechanical oscillator has not been
demonstrated owing to the inability to agilely turn on and off the interaction
between microwave electricity and mechanical motion. Here we demonstrate that
the state of an itinerant microwave field can be coherently transferred into,
stored in, and retrieved from a mechanical oscillator with amplitudes at the
single quanta level. Crucially, the time to capture and to retrieve the
microwave state is shorter than the quantum state lifetime of the mechanical
oscillator. In this quantum regime, the mechanical oscillator can both store
and transduce quantum information
Chapter 11: Challenges in and Principles for Conducting Systematic Reviews of Genetic Tests used as Predictive Indicators
In this paper, we discuss common challenges in and principles for conducting systematic reviews of genetic tests. The types of genetic tests discussed are those used to 1). determine risk or susceptibility in asymptomatic individuals; 2). reveal prognostic information to guide clinical management in those with a condition; or 3). predict response to treatments or environmental factors. This paper is not intended to provide comprehensive guidance on evaluating all genetic tests. Rather, it focuses on issues that have been of particular concern to analysts and stakeholders and on areas that are of particular relevance for the evaluation of studies of genetic tests. The key points include:The general principles that apply in evaluating genetic tests are similar to those for other prognostic or predictive tests, but there are differences in how the principles need to be applied or the degree to which certain issues are relevant.A clear definition of the clinical scenario and an analytic framework is important when evaluating any test, including genetic tests.Organizing frameworks and analytic frameworks are useful constructs for approaching the evaluation of genetic tests.In constructing an analytic framework for evaluating a genetic test, analysts should consider preanalytic, analytic, and postanalytic factors; such factors are useful when assessing analytic validity.Predictive genetic tests are generally characterized by a delayed time between testing and clinically important events.Finding published information on the analytic validity of some genetic tests may be difficult. Web sites (FDA or diagnostic companies) and gray literature may be important sources.In situations where clinical factors associated with risk are well characterized, comparative effectiveness reviews should assess the added value of using genetic testing along with known factors compared with using the known factors alone.For genome-wide association studies, reviewers should determine whether the association has been validated in multiple studies to minimize both potential confounding and publication bias. In addition, reviewers should note whether appropriate adjustments for multiple comparisons were used
Decoherence in dc SQUID phase qubits
We report measurements of Rabi oscillations and spectroscopic coherence times
in an Al/AlOx/Al and three Nb/AlOx/Nb dc SQUID phase qubits. One junction of
the SQUID acts as a phase qubit and the other junction acts as a
current-controlled nonlinear isolating inductor, allowing us to change the
coupling to the current bias leads in situ by an order of magnitude. We found
that for the Al qubit a spectroscopic coherence time T2* varied from 3 to 7 ns
and the decay envelope of Rabi oscillations had a time constant T' = 25 ns on
average at 80 mK. The three Nb devices also showed T2* in the range of 4 to 6
ns, but T' was 9 to 15 ns, just about 1/2 the value we found in the Al device.
For all the devices, the time constants were roughly independent of the
isolation from the bias lines, implying that noise and dissipation from the
bias leads were not the principal sources of dephasing and inhomogeneous
broadening.Comment: 5 figures. Accepted to Physical Review
The emergency department arrival mode and its relations to ED management and 30-day mortality in acute heart failure: an ancillary analysis from the EURODEM study
Background Acute heart failure patients are often encountered in emergency departments (ED) from 11% to 57% using emergency medical services (EMS). Our aim was to evaluate the association of EMS use with acute heart failure patients' ED management and short-term outcomes. Methods This was a sub-analysis of a European EURODEM study. Data on patients presenting with dyspnoea were collected prospectively from European EDs. Patients with ED diagnosis of acute heart failure were categorized into two groups: those using EMS and those self-presenting (non- EMS). The independent association between EMS use and 30-day mortality was evaluated with logistic regression. Results Of the 500 acute heart failure patients, with information about the arrival mode to the ED, 309 (61.8%) arrived by EMS. These patients were older (median age 80 vs. 75 years, p 30/min in 17.1% patients vs. 7.5%, p = 0.005). The only difference in ED management appeared in the use of ventilatory support: 78.3% of EMS patients vs. 67.5% of non- EMS patients received supplementary oxygen (p = 0.007), and non-invasive ventilation was administered to 12.5% of EMS patients vs. 4.2% non- EMS patients (p = 0.002). EMS patients were more often hospitalized (82.4% vs. 65.9%, p < 0.001), had higher in-hospital mortality (8.7% vs. 3.1%, p = 0.014) and 30-day mortality (14.3% vs. 4.9%, p < 0.001). The use of EMS was an independent predictor of 30-day mortality (OR = 2.54, 95% CI 1.11-5.81, p = 0.027). Conclusion Most acute heart failure patients arrive at ED by EMS. These patients suffer from more severe respiratory distress and receive more often ventilatory support. EMS use is an independent predictor of 30-day mortality
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