174,734 research outputs found
Coherent manipulation of cold Rydberg atoms near the surface of an atom chip
Coherent superpositions of the 49s and 48s Rydberg states of cold Rb atoms
were studied near the surface of an atom chip. The superpositions were created
and manipulated using microwaves resonant with the two-photon 49s-48s
transition. Coherent behavior was observed using Rabi flopping, Ramsey
sequences, spin-echo and spin-locking. These results are discussed in the
context of Rydberg atoms as electric field noise sensors. We consider the
coherence of systems quadratically coupled to noise fields with 1/f^k power
spectral densities (k \approx 1).Comment: 11 pages, 7 figure
Anticoherence measures for pure spin states
The set of pure spin states with vanishing spin expectation value can be
regarded as the set of the less coherent pure spin states. This set can be
divided into a finite number of nested subsets on the basis of higher order
moments of the spin operators. This subdivision relies on the notion of
anticoherent spin state to order : A spin state is said to be anticoherent
to order if the moment of order of the spin components along any
directions are equal for . Most spin states are neither
coherent nor anticoherent, but can be arbitrary close to one or the other. In
order to quantify the degree of anticoherence of pure spin states, we introduce
the notion of anticoherence measures. By relying on the mapping between
spin- states and symmetric states of spin- (Majorana
representation), we present a systematic way of constructing anticoherence
measures to any order. We briefly discuss their connection with measures of
quantum coherence. Finally, we illustrate our measures on various spin states
and use them to investigate the problem of the existence of anticoherent spin
states with degenerated Majorana points
Using large galaxy surveys to distinguish z~0.5 quiescent galaxy models
One of the most striking properties of galaxies is the bimodality in their
star-formation rates. A major puzzle is why any given galaxy is star-forming or
quiescent, and a wide range of physical mechanisms have been proposed as
solutions. We consider how observations, such as might be available in upcoming
large galaxy surveys, might distinguish different galaxy quenching scenarios.
To do this, we combine an N-body simulation and multiple prescriptions from the
literature to create several quiescent galaxy mock catalogues. Each
prescription uses a different set of galaxy properties (such as history,
environment, centrality) to assign individual simulation galaxies as quiescent.
We find how and how much the resulting quiescent galaxy distributions differ
from each other, both intrinsically and observationally. In addition to tracing
observational consequences of different quenching mechanisms, our results
indicate which sorts of quenching models might be most readily disentangled by
upcoming observations and which combinations of observational quantities might
provide the most discriminatory power.
Our observational measures are auto, cross, and marked correlation functions,
pro- jected density distributions, and group multiplicity functions, which rely
upon galaxy positions, stellar masses and of course quiescence. Although
degeneracies between models are present for individual observations, using
multiple observations in concert allows us to distinguish between all ten
models we consider. In addition to identifying intrinsic and observational
consequences of quiescence prescriptions and testing these quiescence models
against each other and observations, these methods can also be used to validate
colors (or other history and environment dependent properties) in simulated
mock catalogues.Comment: 20 pages, 14 figures. Version to appear in MNRAS, incorporating
helpful suggestions from referee and other
Cooperative spontaneous emission from indistinguishable atoms in arbitrary motional quantum states
We investigate superradiance and subradiance of indistinguishable atoms with
quantized motional states, starting with an initial total state that factorizes
over the internal and external degrees of freedom of the atoms. Due to the
permutational symmetry of the motional state, the cooperative spontaneous
emission, governed by a recently derived master equation [F. Damanet et al.,
Phys. Rev. A 93, 022124 (2016)], depends only on two decay rates and
and a single parameter describing the
dipole-dipole shifts. We solve the dynamics exactly for atoms,
numerically for up to 30 atoms, and obtain the large--limit by amean-field
approach. We find that there is a critical difference that
depends on beyond which superradiance is lost. We show that exact
non-trivial dark states (i.e. states other than the ground state with vanishing
spontaneous emission) only exist for , and that those states
(dark when ) are subradiant when .Comment: 14 pages, 8 figure
Muscle Cramping During a 161-km Ultramarathon: Comparison of Characteristics of Those With and Without Cramping
Background: This work sought to identify characteristics differing between those with and without muscle cramping during a 161-km ultramarathon.
Methods: In this observational study, race participants underwent body weight measurements before, during, and after the race; completed a post-race questionnaire about muscle cramping and “near” cramping (controllable, not reaching full-blown cramping), drinking strategies, and use of sodium supplementation during four race segments; and underwent a post-race blood draw for determination of serum sodium and blood creatine kinase (CK) concentrations.
Results: The post-race questionnaire was completed by 280 (74.5 %) of the 376 starters. A post-race blood sample was provided by 181 (61.1 %) of the 296 finishers, and 157 (53.0 %) of finishers completed the post-race survey and also provided a post-race blood sample. Among those who completed the survey, the prevalence of cramping and near cramping was 14.3 and 26.8 %, respectively, with greatest involvement being in the calf (54 %), quadriceps (44 %), and hamstring (33 %) muscles. Those with cramping or near cramping were more likely to have a prior history of muscle cramping during an ultramarathon (p \u3c 0.0001) and had higher blood CK concentrations (p = 0.001) than those without cramping. Weight change during the race, use of sodium supplements, intake rate of sodium in supplements, and post-race serum sodium concentration did not differ between those with and without cramping.
Conclusions: Muscle cramping is most common in those with a prior history of cramping and greater muscle damage during an ultramarathon, suggesting an association with relative muscular demand. Impaired fluid and sodium balance did not appear to be an etiology of muscle cramping during an ultramarathon
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