5,691 research outputs found
Multichannel operation of an integrated acousto-optic wavelength routing switch for WDM systems
Polarization independent acousto-optic tunable filters (PIAOTF's) can operate as transparent wavelength-selective crossconnects to route signals in wavelength division multiplexed optical networks. In this paper, a new low power PIAOTF is characterized as a switch in multiwavelength operation, using four equally spaced lightwave signals with wavelengths between 1546 nm and 1558 nm. Interchannel interference due to sidelobe excitation is lower than -11 dB for single wavelength switching and is equal to -6 dB in the extreme case of simultaneous switching of all wavelength channels. Sources of interport and interchannel crosstalk for single and multiple wavelength switching are identified
Observations of microglitches in HartRAO radio pulsars
A detailed observation of microglitch phenomenon in relatively slow radio
pulsars is presented. Our analyses for these small amplitude jumps in pulse
rotation frequency () and/or spin down rate () combine the
traditional manual detection method (which hinges on careful visual inspections
of the residuals of pulse phase residuals) and a new, and perhaps more
objective, automated search technique (which exploits the power of the
computer, rather than the eyes, for resolving discrete events in pulsar spin
parameters). The results of the analyses of a sample of 26 radio pulsars reveal
that: (i) only 20 pulsars exhibit significant fluctuations in their arrival
times to be considered suitable for meaningful microglitch analyses; (ii) a
phenomenal 299 microglitch events were identified in and/or :
266 of these events were found to be simultaneously significant in and
, while 19 and 14 were noticeable only in and ,
respectively; (iii) irrespective of sign, the microglitches have fractional
sizes which cover about 3 orders of magnitude in and
( and ) with median values as
and , respectively.Comment: 12 pages, 3 figures, 2 Tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Society Main Journa
Degrees of controllability for quantum systems and applications to atomic systems
Precise definitions for different degrees of controllability for quantum
systems are given, and necessary and sufficient conditions are discussed. The
results are applied to determine the degree of controllability for various
atomic systems with degenerate energy levels and transition frequencies.Comment: 20 pages, IoP LaTeX, revised and expanded versio
Quantum Machines
We discuss quantum information processing machines. We start with single
purpose machines that either redistribute quantum information or identify
quantum states. We then move on to machines that can perform a number of
functions, with the function they perform being determined by a program, which
is itself a quantum state. Examples of both deterministic and probabilistic
programmable machines are given, and we conclude with a discussion of the
utility of quantum programs.Comment: To appear in Contemporary Physic
Explanation in mathematics: Proofs and practice
Mathematicians distinguish between proofs that explain their results and those that merely prove. This paper explores the nature of explanatory proofs, their role in mathematical practice, and some of the reasons why philosophers should care about them. Among the questions addressed are the following: what kinds of proofs are generally explanatory (or not)? What makes a proof explanatory? Do all mathematical explanations involve proof in an essential way? Are there really such things as explanatory proofs, and if so, how do they relate to the sorts of explanation encountered in philosophy of science and metaphysics
General methods to control right-invariant systems on compact Lie groups and multilevel quantum systems
For a right-invariant system on a compact Lie group G, I present two methods
to design a control to drive the state from the identity to any element of the
group. The first method, under appropriate assumptions, achieves exact control
to the target but requires estimation of the `size' of a neighborhood of the
identity in G. The second method, does not involve any mathematical difficulty,
and obtains control to a desired target with arbitrary accuracy. A third method
is then given combining the main ideas of the previous methods. This is also
very simple in its formulation and turns out to be generically more efficient
as illustrated by one of the examples we consider.
The methods described in the paper provide arbitrary constructive control for
any right-invariant system on a compact Lie group. I give examples including
closed multilevel quantum systems and lossless electrical networks. In
particular, the results can be applied to the coherent control of general
multilevel quantum systems
Magnetic Component of Quark-Gluon Plasma
We describe recent developments of the "magnetic scenario" of sQGP. We show
that at there is a dense plasma of monopoles, capable of
supporting metastable flux tubes. Their existence allows to quantitatively
explained the non-trivial -dependence of the static potential
energy calculated on the lattice. By molecular dynamics simulation we derived
transport properties (shear viscosity and diffusion constant) and showed that
the best liquid is given by most symmetric plasma, with 50%-50% of electric and
magnetic charges. The results are close to those of the ``perfect liquid''
observed at RHIC.Comment: Contribution to the 20th International Conference on Nucleus Nucleus
Collisions (Quark Matter 2008
The role of leadership in people-centred health systems: a sub-national study in The Gambia
Recently, increasing attention has been given to behavioural and relational aspects of the people who both define and shape health systems, placing them at the core. A growing refrain includes the assertion that important decisions determining health system performance, including agenda setting, policy formulation and policy implementation, are made by people. Within this actor-oriented approach, good leadership has been identified as a key contributing factor in health systems strengthening. However, leadership remains ill-defined and under-researched, especially in resource-limited settings, and understanding the links between leadership and health outcomes remains a challenge. We explore the concept and practice of healthcare leadership at sub-national level in a low-income country setting, using a people-centric research methodology. In June and July 2013, 15 in-depth interviews were conducted with key informants in formal healthcare leadership roles across urban, peri-urban and rural settings of The Gambia, West Africa. Participants included the entire spectrum of Regional Health Team (RHT) Directors and Chief Executive Officers of all government hospitals, as well as one clinical officer-in-charge in a secondary-level major health centre. We found reference to several important aspects of, and approaches to, leadership, including (i) setting a clear vision; (ii) engendering shared leadership; and (iii) paying attention to human relations in management. Participants described attending to constituencies in government, international development agencies and civil society, as well as to the populations they serve. By illuminating the multi-polar networks within which these leaders are embedded, and through which they operate, we provide insight into the complex ‘organizational ecology’ of the Gambian health system. There is a need to further research and develop healthcare leadership across all levels, within various political, socio-economic and cultural contexts, in order to better work with a range of health actors and to engage them in identifying and acting upon opportunities for health systems strengthening
Avalanche dynamics of radio pulsar glitches
We test statistically the hypothesis that radio pulsar glitches result from
an avalanche process, in which angular momentum is transferred erratically from
the flywheel-like superfluid in the star to the slowly decelerating, solid
crust via spatially connected chains of local, impulsive, threshold-activated
events, so that the system fluctuates around a self-organised critical state.
Analysis of the glitch population (currently 285 events from 101 pulsars)
demonstrates that the size distribution in individual pulsars is consistent
with being scale invariant, as expected for an avalanche process. The
waiting-time distribution is consistent with being exponential in seven out of
nine pulsars where it can be measured reliably, after adjusting for
observational limits on the minimum waiting time, as for a constant-rate
Poisson process. PSR J05376910 and PSR J08354510 are the exceptions;
their waiting-time distributions show evidence of quasiperiodicity. In each
object, stationarity requires that the rate equals , where is the angular acceleration of the
crust, is the mean glitch size, and is the
relative angular acceleration of the crust and superfluid. There is no evidence
that changes monotonically with spin-down age. The rate distribution
itself is fitted reasonably well by an exponential for . For , its exact form is unknown; the
exponential overestimates the number of glitching pulsars observed at low
, where the limited total observation time exercises a selection bias.Comment: Accepted for publication in the Astrophysical Journa
Characterization of the Crab Pulsar's Timing Noise
We present a power spectral analysis of the Crab pulsar's timing noise,
mainly using radio measurements from Jodrell Bank taken over the period
1982-1989. The power spectral analysis is complicated by nonuniform data
sampling and the presence of a steep red power spectrum that can distort power
spectra measurement by causing severe power ``leakage''. We develop a simple
windowing method for computing red noise power spectra of uniformly sampled
data sets and test it on Monte Carlo generated sample realizations of red
power-law noise. We generalize time-domain methods of generating power-law red
noise with even integer spectral indices to the case of noninteger spectral
indices. The Jodrell Bank pulse phase residuals are dense and smooth enough
that an interpolation onto a uniform time series is possible. A windowed power
spectrum is computed revealing a periodic or nearly periodic component with a
period of about 568 days and a 1/f^3 power-law noise component with a noise
strength of 1.24 +/- 0.067 10^{-16} cycles^2/sec^2 over the analysis frequency
range 0.003 - 0.1 cycles/day. This result deviates from past analyses which
characterized the pulse phase timing residuals as either 1/f^4 power-law noise
or a quasiperiodic process. The analysis was checked using the Deeter
polynomial method of power spectrum estimation that was developed for the case
of nonuniform sampling, but has lower spectral resolution. The timing noise is
consistent with a torque noise spectrum rising with analysis frequency as f
implying blue torque noise, a result not predicted by current models of pulsar
timing noise. If the periodic or nearly periodic component is due to a binary
companion, we find a companion mass > 3.2 Earth masses.Comment: 53 pages, 9 figures, submitted to MNRAS, abstract condense
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