24,484 research outputs found
Looping on the Bloch sphere: Oscillatory effects in dephasing of qubits subject to broad-spectrum noise
For many implementations of quantum computing, 1/f and other types of
broad-spectrum noise are an important source of decoherence. An important step
forward would be the ability to back out the characteristics of this noise from
qubit measurements and to see if it leads to new physical effects. For certain
types of qubits, the working point of the qubit can be varied. Using a new
mathematical method that is suited to treat all working points, we present
theoretical results that show how this degree of freedom can be used to extract
noise parameters and to predict a new effect: noise-induced looping on the
Bloch sphere. We analyze data on superconducting qubits to show that they are
very near the parameter regime where this looping should be observed.Comment: 4 pages, 3 figure
Water resource problems of energy projects in the Colorado River Basin
The successful development of western coal and oil shale deposits
is dependent, to a significant degree, on the availability of adequate
water supplies. EQL is involved in a study of the aggregate effects
of various energy activities in the upper Colorado River Basin on
downstream water quantity and quality. These activities will tend
to reduce the available water in the river, and could increase its
salinity, which is already so high as to interfere with downstream
domestic and agricultural use
The X-ray Line Emission from the Supernova Remnant W49B
The Galactic supernova remnant W49B has one of the most impressive X-ray
emission line spectra obtained with the Advanced Satellite for Cosmology and
Astronomy (ASCA). We use both plasma line diagnostics and broadband model fits
to show that the Si and S emission lines require multiple spectral components.
The spectral data do not necessarily require individual elements to be
spatially stratified, as suggested by earlier work, although when ASCA line
images are considered, it is possible that Fe is stratified with respect to Si
and S. Most of the X-ray emitting gas is from ejecta, based on the element
abundances required, but is surprisingly close to being in collisional
ionization equilibrium. A high ionization age implies a high internal density
in a young remnant. The fitted emission measure for W49B indicates a minimum
density of 2 cm^-3, with the true density likely to be significantly higher.
W49B probably had a Type Ia progenitor, based on the relative element
abundances, although a low-mass Type II progenitor is still possible. We find
persuasive evidence for Cr and possibly Mn emission in the ASCA spectrum--the
first detection of these elements in X-rays from a cosmic source.Comment: 22 pages incl 8 postscript figures, to appear in Ap
Six-dimensional weak-strong simulations of head-on beam-beam compensation in RHIC
To compensate the large beam-beam tune spread and beam-beam resonance driving
terms in the polarized proton operation in the Relativistic Heavy Ion Collider
(RHIC), we will introduce a low-energy DC electron beam into each ring to
collide head-on with the opposing proton beam. The device to provide the
electron beam is called an electron lens. In this article, using a 6-D
weak-strong-beam-beam interaction simulation model, we investigate the effects
of head-on beam-beam compensation with electron lenses on the proton beam
dynamics in the RHIC 250 GeV polarized proton operation. This article is
abridged from the published article [1].Comment: 5 pages, contribution to the ICFA Mini-Workshop on Beam-Beam Effects
in Hadron Colliders, CERN, Geneva, Switzerland, 18-22 Mar 201
Coded Modulation for Satellite Broadcasting
In this paper, three-level block coded 8-PSK modulations, suitable for satellite broadcasting of digital TV signals, are presented. A design principle to achieve unequal error protection is introduced. The coding scheme is designed in such a way that the information bits carrying the basic definition TV signal have a lower error rate than the high definition information bits. The large error coefficients, formally associated with standard mapping by set partitioning, are reduced by considering a nonstandard partition of an 8-PSK signal set. The bits-to-signal mapping induced by this partition allows the use of suboptimal low-complexity soft-decision decoding of binary block codes. Parallel operation of the first and second stage decoders is possible, for high data rate transmission. Furthermore, there is no error propagation from the first-stage decoder to the second-stage decoder
Optimal time decay of the non cut-off Boltzmann equation in the whole space
In this paper we study the large-time behavior of perturbative classical
solutions to the hard and soft potential Boltzmann equation without the angular
cut-off assumption in the whole space \threed_x with \DgE. We use the
existence theory of global in time nearby Maxwellian solutions from
\cite{gsNonCutA,gsNonCut0}. It has been a longstanding open problem to
determine the large time decay rates for the soft potential Boltzmann equation
in the whole space, with or without the angular cut-off assumption
\cite{MR677262,MR2847536}. For perturbative initial data, we prove that
solutions converge to the global Maxwellian with the optimal large-time decay
rate of O(t^{-\frac{\Ndim}{2}+\frac{\Ndim}{2r}}) in the
L^2_\vel(L^r_x)-norm for any .Comment: 31 pages, final version to appear in KR
Concave Switching in Single and Multihop Networks
Switched queueing networks model wireless networks, input queued switches and
numerous other networked communications systems. For single-hop networks, we
consider a {()-switch policy} which combines the MaxWeight policies
with bandwidth sharing networks -- a further well studied model of Internet
congestion. We prove the maximum stability property for this class of
randomized policies. Thus these policies have the same first order behavior as
the MaxWeight policies. However, for multihop networks some of these
generalized polices address a number of critical weakness of the
MaxWeight/BackPressure policies.
For multihop networks with fixed routing, we consider the Proportional
Scheduler (or (1,log)-policy). In this setting, the BackPressure policy is
maximum stable, but must maintain a queue for every route-destination, which
typically grows rapidly with a network's size. However, this proportionally
fair policy only needs to maintain a queue for each outgoing link, which is
typically bounded in number. As is common with Internet routing, by maintaining
per-link queueing each node only needs to know the next hop for each packet and
not its entire route. Further, in contrast to BackPressure, the Proportional
Scheduler does not compare downstream queue lengths to determine weights, only
local link information is required. This leads to greater potential for
decomposed implementations of the policy. Through a reduction argument and an
entropy argument, we demonstrate that, whilst maintaining substantially less
queueing overhead, the Proportional Scheduler achieves maximum throughput
stability.Comment: 28 page
Universal Heat Conduction in YBa_2Cu_3O_6.9
The thermal conductivity of YBa_2Cu_3O_6.9 was measured at low temperatures
in untwinned single crystals with concentrations of Zn impurities from 0 to 3%
of Cu. A linear term kappa_0/T = 0.19 mW/K^2.cm is clearly resolved as T -> 0,
and found to be virtually independent of Zn concentration. The existence of
this residual normal fluid strongly validates the basic theory of transport in
unconventional superconductors. Moreover, the observed universal behavior is in
quantitative agreement with calculations for a gap function of d-wave symmetry.Comment: Latex file, 4 pages, 3 EPS figures, to appear in Physical Review
Letter
Exact solution of a model of qubit dephasing due to telegraph noise
We present a general and exact formalism for finding the evolution of a
quantum system subject to external telegraph noise. The various qubit
decoherence rates are determined by the eigenvalues of a transfer matrix. The
formalism can be applied to a qubit subject to an arbitrary combination of
dephasing and relaxational telegraph noise, in contrast to existing
non-perturbative methods that treat only one or the other of these limits. We
present 3 applications: 1) We obtain the full qubit dynamics on time scales
short compared with the enviromental correlation times. In the strong coupling
cases this reveals unexpected oscillations and induced magnetization
components; 2) We find in strong coupling case strong violations of the widely
used relation 1/T = 1/2T + 1/T, which is a result of
perturbation theory; 3) We discuss the effects of bang-bang and spin-echo
controls of the qubit dynamics in general settings of the telegraph noises.
%The result shows that these methods are not very effective in %reducing
decoherence arising from a single telegraph noise. Finally, we discuss the
extension of the method to the cases of many telegraph noise sources and
multiple qubits. The method still works when white noise is also present.Comment: 7 pages, 6 figures, revised and extende
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