372 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
Giant Cross Kerr Effect for Propagating Microwaves Induced by an Artificial Atom
We have investigated the cross Kerr phase shift of propagating microwave
fields strongly coupled to an artificial atom. The artificial atom is a
superconducting transmon qubit in an open transmission line. We demonstrate
average phase shifts of 11 degrees per photon between two coherent microwave
fields both at the single-photon level. At high control power, we observe phase
shifts up to 30 degrees. Our results provide an important step towards quantum
gates with propagating photons in the microwave regime.Comment: 5 pages, 4 figure
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
Generation of nonclassical microwave states using an artificial atom in 1D open space
We have embedded an artificial atom, a superconducting transmon qubit, in a
1D open space and investigated the scattering properties of an incident
microwave coherent state. By studying the statistics of the reflected and
transmitted fields, we demonstrate that the scattered states can be
nonclassical. In particular, by measuring the second-order correlation
function, , we show photon antibunching in the reflected field and
superbunching in the transmitted field. We also compare the elastically and
inelastically scattered fields using both phase-sensitive and phase-insensitive
measurements.Comment: 5 pages, 3 figure
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
New Protocols and Negative Results for Textual Entailment Data Collection
Natural language inference (NLI) data has proven useful in benchmarking and,
especially, as pretraining data for tasks requiring language understanding.
However, the crowdsourcing protocol that was used to collect this data has
known issues and was not explicitly optimized for either of these purposes, so
it is likely far from ideal. We propose four alternative protocols, each aimed
at improving either the ease with which annotators can produce sound training
examples or the quality and diversity of those examples. Using these
alternatives and a fifth baseline protocol, we collect and compare five new
8.5k-example training sets. In evaluations focused on transfer learning
applications, our results are solidly negative, with models trained on our
baseline dataset yielding good transfer performance to downstream tasks, but
none of our four new methods (nor the recent ANLI) showing any improvements
over that baseline. In a small silver lining, we observe that all four new
protocols, especially those where annotators edit pre-filled text boxes, reduce
previously observed issues with annotation artifacts.Comment: To appear at EMNLP 202
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
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
A Rapid-ACCE review of CYP2C9 and VKORC1 alleles testing to informwarfarin dosing in adults at elevated risk for thrombotic events to avoid serious bleeding,”
Purpose: Summarize evidence regarding genetic testing in adults to inform warfarin dosing to reduce adverse drug events such as serious bleeding. Methods: Review published (and selected gray) literature using the Rapid-ACCE structure that addresses analytic validity, clinical validity, clinical utility, and ethical, legal, and social implications. Results: Preliminary data suggest overall analytic sensitivity and specificity will be 98% or higher for CYP2C9 genotyping, but strength of evidence for analytic validity is low, especially for VKORC1 testing. Strength of evidence is high for the clinical validity of both genes in predicting stable warfarin dose, an intermediate outcome, but is low for the association between CYP2C9 testing and severe bleeding events (clinical sensitivity 46% (95% CI 32-60%); specificity 69% (95% CI 62-75%) and absent for bleeding events associated with VKORC1 testing. No data are available to document clinical utility of genotyping before warfarin dosing. Conclusions: The most important gaps identified are: which variants should be included in a testing panel, lack of data from external proficiency testing, lack of validated dosing algorithm incorporating genetic and nongenetic factors, evidence of clinical utility, reliable economic analyses, and methods to address several ethical, legal, and social implications issues. Genet Med 2008:10(2):89 -98
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