397 research outputs found
Protecting quantum entanglement from leakage and qubit errors via repetitive parity measurements
Protecting quantum information from errors is essential for large-scale
quantum computation. Quantum error correction (QEC) encodes information in
entangled states of many qubits, and performs parity measurements to identify
errors without destroying the encoded information. However, traditional QEC
cannot handle leakage from the qubit computational space. Leakage affects
leading experimental platforms, based on trapped ions and superconducting
circuits, which use effective qubits within many-level physical systems. We
investigate how two-transmon entangled states evolve under repeated parity
measurements, and demonstrate the use of hidden Markov models to detect leakage
using only the record of parity measurement outcomes required for QEC. We show
the stabilization of Bell states over up to 26 parity measurements by
mitigating leakage using postselection, and correcting qubit errors using
Pauli-frame transformations. Our leakage identification method is
computationally efficient and thus compatible with real-time leakage tracking
and correction in larger quantum processors.Comment: 22 pages, 15 figure
Testing the standard fireball model of GRBs using late X-ray afterglows measured by Swift
We show that all X-ray decay curves of GRBs measured by Swift can be fitted
using one or two components both of which have exactly the same functional form
comprised of an early falling exponential phase followed by a power law decay.
The 1st component contains the prompt gamma-ray emission and the initial X-ray
decay. The 2nd component appears later, has a much longer duration and is
present for ~80% of GRBs. It most likely arises from the external shock which
eventually develops into the X-ray afterglow. In the remaining ~20% of GRBs the
initial X-ray decay of the 1st component fades more slowly than the 2nd and
dominates at late times to form an afterglow but it is not clear what the
origin of this emission is.
The temporal decay parameters and gamma/X-ray spectral indices derived for
107 GRBs are compared to the expectations of the standard fireball model
including a search for possible "jet breaks". For ~50% of GRBs the observed
afterglow is in accord with the model but for the rest the temporal and
spectral indices do not conform to the expected closure relations and are
suggestive of continued, late, energy injection. We identify a few possible jet
breaks but there are many examples where such breaks are predicted but are
absent.
The time, T_a, at which the exponential phase of the 2nd component changes to
a final powerlaw decay afterglow is correlated with the peak of the gamma-ray
spectrum, E_peak. This is analogous to the Ghirlanda relation, indicating that
this time is in some way related to optically observed break times measured for
pre-Swift bursts.Comment: submitted to Ap
Probing exciton localization in non-polar GaN/AlN Quantum Dots by single dot optical spectroscopy
We present an optical spectroscopy study of non-polar GaN/AlN quantum dots by
time-resolved photoluminescence and by microphotoluminescence. Isolated quantum
dots exhibit sharp emission lines, with linewidths in the 0.5-2 meV range due
to spectral diffusion. Such linewidths are narrow enough to probe the inelastic
coupling of acoustic phonons to confined carriers as a function of temperature.
This study indicates that the carriers are laterally localized on a scale that
is much smaller than the quantum dot size. This conclusion is further confirmed
by the analysis of the decay time of the luminescence
Evidence for a supernova in reanalyzed optical and near-infrared images of GRB970228
We present B-, V-, R_c-, I_c-, J-, H-, K- and K'-band observations of the
optical transient (OT) associated with GRB970228, based on a reanalysis of
previously used images and unpublished data. In order to minimize calibration
differences we have collected and analyzed most of the photometry and
consistently determined the magnitude of the OT relative to a set of secondary
field stars. We confirm our earlier finding that the early decay of the light
curves (before March 6, 1997) was faster than that at intermediate times
(between March 6 and April 7, 1997). At late times the light curves resume a
fast decay (after April 7, 1997). The early-time observations of GRB970228 are
consistent with relativistic blast-wave models but the intermediate- and
late-time observations are hard to understand in this framework. The
observations are well explained by an initial power law decay with index -1.73
+0.09 -0.12 modified at later times by a type-I_c supernova light curve.
Together with the evidence for GRB980326 and GRB980425 this gives further
support for the idea that at least some GRBs are associated with a possibly
rare type of supernova.Comment: Submitted to the Astrophysical Journal, 9 pages including 3 figures,
uses emulateapj.st
The first Swift X-ray Flash: The faint afterglow of XRF 050215B
We present the discovery of XRF 050215B and its afterglow. The burst was
detected by the Swift BAT during the check-out phase and observations with the
X-ray telescope began approximately 30 minutes after the burst. These
observations found a faint, slowly fading X-ray afterglow near the centre of
the error box as reported by the BAT. Infrared data, obtained at UKIRT after 10
hours also revealed a very faint K-band afterglow. The afterglow appear unusual
since it is very faint, especially in the infrared with K>20 only 9 hours post
burst. The X-ray and infrared lightcurves exhibit a slow, monotonic decay with
alpha=0.8 and no evidence for steepening associated with the jet break to 10
days post burst. We discuss possible explanations for the faintness and slow
decay in the context of present models for the production of X-ray Flashes.Comment: 8 pages, 5 figures, accepted for publication in Ap
Association between pain outcomes and race and opioid treatment: Retrospective cohort study of Veterans
We examined whether pain outcomes (pain interference, perceived pain treatment effectiveness) vary by race and then whether opioid use moderates these associations.
These analyses are part of a retrospective cohort study among 3,505 black and 46,203 non-Hispanic, white Department of Veterans Affairs (VA) patients with diagnoses of chronic musculoskeletal pain who responded to the 2007 VA Survey of Healthcare Experiences of Patients (SHEP). We used electronic medical record data to identify prescriptions for pharmacologic pain treatments in the year after diagnosis (Pain Diagnosis index visit) and before the SHEP index visit (the visit that made one eligible to complete the SHEP); pain outcomes came from the SHEP. We found no significant associations between race and pain interference or perceived effectiveness of pain treatment. VA patients with opioid prescriptions between the Pain Diagnosis index visit and the SHEP index visit reported greater pain interference on the SHEP than those without opioid prescriptions during that period. Opioid prescriptions were not associated with perceived treatment effectiveness for most patients. Findings raise questions about benefits of opioids for musculoskeletal pain and point to the need for alternative treatments for addressing chronic noncancer pain
A New Constraint on the Escape Fraction in Distant Galaxies Using Gamma-ray Burst Afterglow Spectroscopy
We describe a new method to measure the escape fraction fesc of ionizing
radiation from distant star-forming galaxies using the afterglow spectra of
long-duration gamma-ray bursts (GRBs). Optical spectra of GRB afterglows allow
us to evaluate the optical depth of the host ISM, according to the neutral
hydrogen column density N(HI) observed along the sightlines toward the
star-forming regions where the GRBs are found. Different from previous effort
in searching for faint, transmitted Lyman continuum photons, our method is not
subject to background subtraction uncertainties and does not require prior
knowledge of either the spectral shape of the host galaxy population or the IGM
Lya forest absorption along these GRB sightlines. Because most GRBs occur in
sub-L_* galaxies, our study also offers the first constraint on fesc for
distant low-mass galaxies that dominate the cosmic luminosity density. We have
compiled a sample of 27 GRBs at redshift z>2 for which the underlying N(HI) in
the host ISM are known. These GRBs together offer a statistical sampling of the
integrated optical depth to ionizing photons along random sightlines from
star-forming regions in the host galaxies, and allow us to estimate the mean
escape fraction averaged over different viewing angles. We find
=0.02\pm 0.02 and place a 95% c.l. upper limit <= 0.075 for these
hosts. We discuss possible biases of our approach and implications of the
result. Finally, we propose to extend this technique for measuring at
z~0.2 using spectra of core-collapse supernovae.Comment: Five journal pages, including one figure; ApJL in pres
Time-domain characterization and correction of on-chip distortion of control pulses in a quantum processor
We introduce Cryoscope, a method for sampling on-chip baseband pulses used to
dynamically control qubit frequency in a quantum processor. We specifically use
Cryoscope to measure the step response of the dedicated flux control lines of
two-junction transmon qubits in circuit QED processors with the temporal
resolution of the room-temperature arbitrary waveform generator producing the
control pulses. As a first application, we iteratively improve this step
response using optimized real-time digital filters to counter the
linear-dynamical distortion in the control line, as needed for high-fidelity,
repeatable one- and two-qubit gates based on dynamical control of qubit
frequency
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