3,534 research outputs found
Nonadiabatic Dynamics in Open Quantum-Classical Systems: Forward-Backward Trajectory Solution
A new approximate solution to the quantum-classical Liouville equation is
derived starting from the formal solution of this equation in forward-backward
form. The time evolution of a mixed quantum-classical system described by this
equation is obtained in a coherent state basis using the mapping
representation, which expresses quantum degrees of freedom in a
2N-dimensional phase space. The solution yields a simple non-Hamiltonian
dynamics in which a set of coherent state coordinates evolve in forward and
backward trajectories while the bath coordinates evolve under the influence of
the mean potential that depends on these forward and backward trajectories. It
is shown that the solution satisfies the differential form of the
quantum-classical Liouville equation exactly. Relations to other mixed
quantum-classical and semi-classical schemes are discussed.Comment: 28 pages, 1 figur
Multiple-time correlation functions for non-Markovian interaction: Beyond the Quantum Regression Theorem
Multiple time correlation functions are found in the dynamical description of
different phenomena. They encode and describe the fluctuations of the dynamical
variables of a system. In this paper we formulate a theory of non-Markovian
multiple-time correlation functions (MTCF) for a wide class of systems. We
derive the dynamical equation of the {\it reduced propagator}, an object that
evolve state vectors of the system conditioned to the dynamics of its
environment, which is not necessarily at the vacuum state at the initial time.
Such reduced propagator is the essential piece to obtain multiple-time
correlation functions. An average over the different environmental histories of
the reduced propagator permits us to obtain the evolution equations of the
multiple-time correlation functions. We also study the evolution of MTCF within
the weak coupling limit and it is shown that the multiple-time correlation
function of some observables satisfy the Quantum Regression Theorem (QRT),
whereas other correlations do not. We set the conditions under which the
correlations satisfy the QRT. We illustrate the theory in two different cases;
first, solving an exact model for which the MTCF are explicitly given, and
second, presenting the results of a numerical integration for a system coupled
with a dissipative environment through a non-diagonal interaction.Comment: Submitted (04 Jul 04
Natural DNA Uptake by Escherichia coli
Escherichia coli has homologues of the competence genes other species use for DNA uptake and processing, but natural competence and transformation have never been detected. Although we previously showed that these genes are induced by the competence regulator Sxy as in other gamma-proteobacteria, no conditions are known that naturally induce sxy expression. We have now tested whether the competence gene homologues encode a functional DNA uptake machinery and whether DNA uptake leads to recombination, by investigating the effects of plasmid-borne sxy expression on natural competence in a wide variety of E. coli strains. High- and low-level sxy expression alone did not induce transformation in any of the strains tested, despite varying the transforming DNA, its concentration, and the incubation conditions used. Direct measurements of uptake of radiolabelled DNA were below the limit of detection, however transformants were readily detected when recombination functions were provided by the lambda Red recombinase. This is the first demonstration that E. coli sxy expression can induce natural DNA uptake and that E. coli's competence genes do encode a functional uptake machinery. However, the amount of transformation cells undergo is limited both by low levels of DNA uptake and by inefficient DNA processing/recombination
Sodium Absorption From the Exoplanetary Atmosphere of HD189733b Detected in the Optical Transmission Spectrum
We present the first ground-based detection of sodium absorption in the
transmission spectrum of an extrasolar planet. Absorption due to the atmosphere
of the extrasolar planet HD189733b is detected in both lines of the NaI
doublet. High spectral resolution observations were taken of eleven transits
with the High Resolution Spectrograph (HRS) on the 9.2 meter Hobby-Eberly
Telescope (HET). The NaI absorption in the transmission spectrum due to
HD189733b is (-67.2 +/- 20.7) x 10^-5 deeper in the ``narrow'' spectral band
that encompasses both lines relative to adjacent bands. The 1-sigma error
includes both random and systematic errors, and the detection is >3-sigma. This
amount of relative absorption in NaI for HD189733b is ~3x larger than detected
for HD209458b by Charbonneau et al. (2002), and indicates these two
hot-Jupiters may have significantly different atmospheric properties.Comment: 12 pages, 2 figures; Accepted for publication in ApJ Letter
The hotspot conversion paradox
The contradiction between the long-term persistence of the chromosomal hotspots that initiate meiotic recombination and the self-destructive mechanism by which they act strongly suggests that our understanding of recombination is incomplete. This "hotspot paradox" has been reinforced by the finding that biased gene conversion also removes active hotspots from human sperm. To investigate the requirements for hotspot persistence, we developed a detailed computer simulation model of their activity and its evolutionary consequences. With this model, unopposed hotspot activity could drive strong hotspots from 50% representation to extinction within 70 generations. Although the crossing over that hotspots cause can increase population fitness, this benefit was always too small to slow the loss of hotspots. Hotspots could not be maintained by plausible rates of _de novo_ mutation, nor by crossover interference, which alters the frequency and/or spacing of crossovers. Competition among hotspots for activity-limiting factors also did not prevent their extinction, although the rate of hotspot loss was slowed. Key factors were the probability that the initiating hotspot allele is destroyed and the nonmeiotic contributions hotspots make to fitness. Experimental investigation of these deserves high priority, because until the paradox is resolved all components of the mechanism are open to doubt
Low- and high-frequency noise from coherent two-level systems
Recent experiments indicate a connection between the low- and high-frequency
noise affecting superconducting quantum systems. We explore the possibilities
that both noises can be produced by one ensemble of microscopic modes, made up,
e.g., by sufficiently coherent two-level systems (TLS). This implies a relation
between the noise power in different frequency domains, which depends on the
distribution of the parameters of the TLSs. We show that a distribution,
natural for tunneling TLSs, with a log-uniform distribution in the tunnel
splitting and linear distribution in the bias, accounts for experimental
observations.Comment: minor corrections, references adde
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