3,595 research outputs found
Adaptive tracking of a time-varying field with a quantum sensor
Sensors based on single spins can enable magnetic field detection with very
high sensitivity and spatial resolution. Previous work has concentrated on
sensing of a constant magnetic field or a periodic signal. Here, we instead
investigate the problem of estimating a field with non-periodic variation
described by a Wiener process. We propose and study, by numerical simulations,
an adaptive tracking protocol based on Bayesian estimation. The tracking
protocol updates the probability distribution for the magnetic field, based on
measurement outcomes, and adapts the choice of sensing time and phase in real
time. By taking the statistical properties of the signal into account, our
protocol strongly reduces the required measurement time. This leads to a
reduction of the error in the estimation of a time-varying signal by up to a
factor 4 compared to protocols that do not take this information into account.Comment: 10 pages, 6 figure
On the Thermodynamics of NUT charged spaces
We discuss and compare at length the results of two methods used recently to
describe the thermodynamics of Taub-NUT solutions in a deSitter background. In
the first approach (\mathbb{% C}-approach), one deals with an analytically
continued version of the metric while in the second approach
(-approach), the discussion is carried out using the unmodified
metric with Lorentzian signature. No analytic continuation is performed on the
coordinates and/or the parameters that appear in the metric. We find that the
results of both these approaches are completely equivalent modulo analytic
continuation and we provide the exact prescription that relates the results in
both methods. The extension of these results to the AdS/flat cases aims to give
a physical interpretation of the thermodynamics of nut-charged spacetimes in
the Lorentzian sector. We also briefly discuss the higher dimensional spaces
and note that, analogous with the absence of hyperbolic nuts in AdS
backgrounds, there are no spherical Taub-Nut-dS solutions.Comment: 35pages, 4 figures. v.4 references added,few typos corrected, to
appear in Phys. Rev.
Null Polygonal Wilson Loops in Full N=4 Superspace
We compute the one-loop expectation value of light-like polygonal Wilson
loops in N=4 super-Yang-Mills theory in full superspace. When projecting to
chiral superspace we recover the known results for tree-level
next-to-maximally-helicity-violating (NMHV) scattering amplitude. The one-loop
MHV amplitude is also included in our result but there are additional terms
which do not immediately correspond to scattering amplitudes. We finally
discuss different regularizations and their Yangian anomalies.Comment: 55 pages, v2: reference adde
Note on counterterms in asymptotically flat spacetimes
We consider in more detail the covariant counterterm proposed by Mann and
Marolf in asymptotically flat spacetimes. With an eye to specific practical
computations using this counterterm, we present explicit expressions in general
dimensions that can be used in the so-called `cylindrical cut-off' to
compute the action and the associated conserved quantities for an
asymptotically flat spacetime. As applications, we show how to compute the
action and the conserved quantities for the NUT-charged spacetime and for the
Kerr black hole in four dimensions.Comment: 13 pages, v. 2 added reference
Hybrid MIMO Architectures for Millimeter Wave Communications: Phase Shifters or Switches?
Hybrid analog/digital MIMO architectures were recently proposed as an
alternative for fully-digitalprecoding in millimeter wave (mmWave) wireless
communication systems. This is motivated by the possible reduction in the
number of RF chains and analog-to-digital converters. In these architectures,
the analog processing network is usually based on variable phase shifters. In
this paper, we propose hybrid architectures based on switching networks to
reduce the complexity and the power consumption of the structures based on
phase shifters. We define a power consumption model and use it to evaluate the
energy efficiency of both structures. To estimate the complete MIMO channel, we
propose an open loop compressive channel estimation technique which is
independent of the hardware used in the analog processing stage. We analyze the
performance of the new estimation algorithm for hybrid architectures based on
phase shifters and switches. Using the estimated, we develop two algorithms for
the design of the hybrid combiner based on switches and analyze the achieved
spectral efficiency. Finally, we study the trade-offs between power
consumption, hardware complexity, and spectral efficiency for hybrid
architectures based on phase shifting networks and switching networks.
Numerical results show that architectures based on switches obtain equal or
better channel estimation performance to that obtained using phase shifters,
while reducing hardware complexity and power consumption. For equal power
consumption, all the hybrid architectures provide similar spectral
efficiencies.Comment: Submitted to IEEE Acces
Exploratory X-ray Monitoring of Luminous Radio-Quiet Quasars at High Redshift: No Evidence for Evolution in X-ray Variability
We report on the second installment of an X-ray monitoring project of seven
luminous radio-quiet quasars (RQQs). New {\sl Chandra} observations of four of
these, at , yield a total of six X-ray epochs, per source,
with temporal baselines of days in the rest frame. These data
provide the best X-ray light curves for RQQs at , to date, enabling
qualitative investigations of the X-ray variability behavior of such sources
for the first time. On average, these sources follow the trend of decreasing
variability amplitude with increasing luminosity, and there is no evidence for
X-ray variability increasing toward higher redshifts, in contrast with earlier
predictions of potential evolutionary scenarios. An ensemble variability
structure function reveals that their variability level remains relatively flat
across days in the rest frame and it is generally lower than
that of three similarly luminous RQQs at over the same
temporal range. We discuss possible explanations for the increased variability
of the lower-redshift subsample and, in particular, whether higher accretion
rates play a leading role. Near-simultaneous optical monitoring of the sources
at indicates that none is variable on -day
timescales, although flux variations of up to \% are observed on
-day timescales, typical of RQQs at similar redshifts. Significant
optical-X-ray spectral slope variations observed in two of these sources are
consistent with the levels observed in luminous RQQs and are dominated by X-ray
variations.Comment: 11 pages (emulateapj), 4 figures. Accepted for publication in Ap
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