3,587 research outputs found
Efficient C-Phase gate for single-spin qubits in quantum dots
Two-qubit interactions are at the heart of quantum information processing.
For single-spin qubits in semiconductor quantum dots, the exchange gate has
always been considered the natural two-qubit gate. The recent integration of
magnetic field or g-factor gradients in coupled quantum dot systems allows for
a one-step, robust realization of the controlled phase (C-Phase) gate instead.
We analyze the C-Phase gate durations and fidelities that can be obtained under
realistic conditions, including the effects of charge and nuclear field
fluctuations, and find gate error probabilities of below 10-4, possibly
allowing fault-tolerant quantum computation.Comment: 5 pages, 3 figure
The SPHERE data center: a reference for high contrast imaging processing
The objective of the SPHERE Data Center is to optimize the scientific return
of SPHERE at the VLT, by providing optimized reduction procedures, services to
users and publicly available reduced data. This paper describes our motivation,
the implementation of the service (partners, infrastructure and developments),
services, description of the on-line data, and future developments. The SPHERE
Data Center is operational and has already provided reduced data with a good
reactivity to many observers. The first public reduced data have been made
available in 2017. The SPHERE Data Center is gathering a strong expertise on
SPHERE data and is in a very good position to propose new reduced data in the
future, as well as improved reduction procedures.Comment: SF2A proceeding
The effect of activity-related meridional flow modulation on the strength of the solar polar magnetic field
We studied the effect of the perturbation of the meridional flow in the
activity belts detected by local helioseismology on the development and
strength of the surface magnetic field at the polar caps. We carried out
simulations of synthetic solar cycles with a flux transport model, which
follows the cyclic evolution of the surface field determined by flux emergence
and advective transport by near-surface flows. In each hemisphere, an
axisymmetric band of latitudinal flows converging towards the central latitude
of the activity belt was superposed onto the background poleward meridional
flow. The overall effect of the flow perturbation is to reduce the latitude
separation of the magnetic polarities of a bipolar magnetic region and thus
diminish its contribution to the polar field. As a result, the polar field
maximum reached around cycle activity minimum is weakened by the presence of
the meridional flow perturbation. For a flow perturbation consistent with
helioseismic observations, the polar field is reduced by about 18% compared to
the case without inflows. If the amplitude of the flow perturbation depends on
the cycle strength, its effect on the polar field provides a nonlinearity that
could contribute to limiting the amplitude of a Babcock-Leighton type dynamo.Comment: 16 pages, 5 figures, accepted by Ap
Scaling silicon-based quantum computing using CMOS technology: State-of-the-art, Challenges and Perspectives
Complementary metal-oxide semiconductor (CMOS) technology has radically
reshaped the world by taking humanity to the digital age. Cramming more
transistors into the same physical space has enabled an exponential increase in
computational performance, a strategy that has been recently hampered by the
increasing complexity and cost of miniaturization. To continue achieving
significant gains in computing performance, new computing paradigms, such as
quantum computing, must be developed. However, finding the optimal physical
system to process quantum information, and scale it up to the large number of
qubits necessary to build a general-purpose quantum computer, remains a
significant challenge. Recent breakthroughs in nanodevice engineering have
shown that qubits can now be manufactured in a similar fashion to silicon
field-effect transistors, opening an opportunity to leverage the know-how of
the CMOS industry to address the scaling challenge. In this article, we focus
on the analysis of the scaling prospects of quantum computing systems based on
CMOS technology.Comment: Comments welcom
Effects of zinc supplementation on cognitive function in healthy middle-aged and older adults: the ZENITH study
A randomised double-blind placebo-controlled design was employed to investigate the effects of Zn supplementation on cognitive function in 387 healthy adults aged 55–87 years. Several measures of visual memory, working memory, attention and reaction time were obtained using the Cambridge Automated Neuropsychological Test Battery at baseline and then after 3 and 6 months of 0 (placebo), 15 or 30 mg Zn/d. Younger adults (70 years), and performance improved with practice on some measures. For two out of eight dependent variables, there were significant interactions indicating a beneficial effect (at 3 months only) of both 15 and 30 mg/d on one measure of spatial working memory and a detrimental effect of 15 mg/d on one measure of attention. Further work is required to establish whether these findings generalise to older adults in poorer mental and physical health and with less adequate Zn intake and status than the present sample
Optimally adapted multi-state neural networks trained with noise
The principle of adaptation in a noisy retrieval environment is extended here
to a diluted attractor neural network of Q-state neurons trained with noisy
data. The network is adapted to an appropriate noisy training overlap and
training activity which are determined self-consistently by the optimized
retrieval attractor overlap and activity. The optimized storage capacity and
the corresponding retriever overlap are considerably enhanced by an adequate
threshold in the states. Explicit results for improved optimal performance and
new retriever phase diagrams are obtained for Q=3 and Q=4, with coexisting
phases over a wide range of thresholds. Most of the interesting results are
stable to replica-symmetry-breaking fluctuations.Comment: 22 pages, 5 figures, accepted for publication in PR
Solar Magnetic Field Studies Using the 12-Micron Emission Lines. IV. Observations of a Delta-Region Solar Flare
We have recently developed the capability to make solar vector (Stokes IQUV)
magnetograms using the infrared line of MgI at 12.32 microns. On 24 April 2001,
we obtained a vector magnetic map of solar active region NOAA 9433,
fortuitously just prior to the occurrence of an M2 flare. Examination of a
sequence of SOHO/MDI magnetograms, and comparison with ground-based H-alpha
images, shows that the flare was produced by the cancellation of newly emergent
magnetic flux outside of the main sunspot. The very high Zeeman-sensitivity of
the 12-micron data allowed us to measure field strengths on a spatial scale
which was not directly resolvable. At the flare trigger site, opposite polarity
fields of 2700 and 1000 Gauss occurred within a single 2 arc-sec resolution
element, as revealed by two resolved Zeeman splittings in a single spectrum.
Our results imply an extremely high horizontal field strength gradient (5 G/km)
prior to the flare, significantly greater than seen in previous studies. We
also find that the magnetic energy of the cancelling fields was more than
sufficient to account for the flare's X-ray luminosity.Comment: 14 pages, 5 figures, accepted for Ap.
Fifth Year Report: Institute of Making, UCL 2017-18
A report on the research activities of the Institute of Making covering the period from March 2017 to March 201
Single electron quantum tomography in quantum Hall edge channels
We propose a quantum tomography protocol to measure single electron coherence
in quantum Hall edge channels and therefore access for the first time the wave
function of single electron excitations propagating in ballistic quantum
conductors. Its implementation would open the way to quantitative studies of
single electron decoherence and would provide a quantitative tool for analyzing
single to few electron sources. We show how this protocol could be implemented
using ultrahigh sensitivity noise measurement schemes.Comment: Version 3: long version (7 figures): corrections performed and
references have been added. Figures reprocessed for better readabilit
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