454 research outputs found
Stimulated Raman Adiabatic Passage (STIRAP) Among Degenerate-Level Manifolds
We examine the conditions needed to accomplish stimulated Raman adiabatic
passage (STIRAP) when the three levels (g, e and f) are degenerate, with
arbitrary couplings contributing to the pump-pulse interaction (g - e) and to
the Stokes-pulse interaction (e-f). We show that in general a sufficient
condition for complete population removal from the g set of degenerate states
for arbitrary, pure or mixed, initial state is that the degeneracies should not
decrease along the sequence g, e and f. We show that when this condition holds
it is possible to achieve the degenerate counterpart of conventional STIRAP,
whereby adiabatic passage produces complete population transfer. Indeed, the
system is equivalent to a set of independent three-state systems, in each of
which a STIRAP procedure can be implemented. We describe a scheme of unitary
transformations that produces this result. We also examine the cases when this
degeneracy constraint does not hold, and show what can be accomplished in those
cases. For example, for angular momentum states when the degeneracy of the g
and f levels is less than that of the e level we show how a special choice for
the pulse polarizations and phases can produce complete removal of population
from the g set. Our scheme can be a powerful tool for coherent control in
degenerate systems, because of its robustness when selective addressing of the
states is not required or impossible. We illustrate the analysis with several
analytically solvable examples, in which the degeneracies originate from
angular momentum orientation, as expressed by magnetic sublevels.Comment: 21 pages, 17 figure
Indirect Genetic Effects and the Spread of Infectious Disease: Are We Capturing the Full Heritable Variation Underlying Disease Prevalence?
Reducing disease prevalence through selection for host resistance offers a desirable alternative to chemical treatment. Selection for host resistance has proven difficult, however, due to low heritability estimates. These low estimates may be caused by a failure to capture all the relevant genetic variance in disease resistance, as genetic analysis currently is not taylored to estimate genetic variation in infectivity. Host infectivity is the propensity of transmitting infection upon contact with a susceptible individual, and can be regarded as an indirect effect to disease status. It may be caused by a combination of physiological and behavioural traits. Though genetic variation in infectivity is difficult to measure directly, Indirect Genetic Effect (IGE) models, also referred to as associative effects or social interaction models, allow the estimation of this variance from more readily available binary disease data (infected/non-infected). We therefore generated binary disease data from simulated populations with known amounts of variation in susceptibility and infectivity to test the adequacy of traditional and IGE models. Our results show that a conventional model fails to capture the genetic variation in infectivity inherent in populations with simulated infectivity. An IGE model, on the other hand, does capture some of the variation in infectivity. Comparison with expected genetic variance suggests that there is scope for further methodological improvement, and that potential responses to selection may be greater than values presented here. Nonetheless, selection using an index of estimated direct and indirect breeding values was shown to have a greater genetic selection differential and reduced future disease risk than traditional selection for resistance only. These findings suggest that if genetic variation in infectivity substantially contributes to disease transmission, then breeding designs which explicitly incorporate IGEs might help reduce disease prevalence
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