9,865 research outputs found
Photon statistics and dynamics of Fluorescence Resonance Energy Transfer
We report high time-resolution measurements of photon statistics from pairs
of dye molecules coupled by fluorescence resonance energy transfer (FRET). In
addition to quantum-optical photon antibunching, we observe photon bunching on
a timescale of several nanoseconds. We show by numerical simulation that
configuration fluctuations in the coupled fluorophore system could account for
minor deviations of our data from predictions of basic Forster theory. With
further characterization we believe that FRET photon statistics could provide a
unique tool for studying DNA mechanics on timescales from 10^-9 to 10^-3 s.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Ribonucleolytic resection is required for repair of strand displaced nonhomologous end-joining intermediates
Nonhomologous end-joining (NHEJ) pathways repair DNA double-strand breaks (DSBs) in eukaryotes and many prokaryotes, although it is not reported to operate in the third domain of life, archaea. Here, we describe a complete NHEJ complex, consisting of DNA ligase (Lig), polymerase (Pol), phosphoesterase (PE), and Ku from a mesophillic archaeon, Methanocella paludicola (Mpa). Mpa Lig has limited DNA nick-sealing activity but is efficient in ligating nicks containing a 3′ ribonucleotide. Mpa Pol preferentially incorporates nucleoside triphosphates onto a DNA primer strand, filling DNA gaps in annealed breaks. Mpa PE sequentially removes 3′ phosphates and ribonucleotides from primer strands, leaving a ligatable terminal 3′ monoribonucleotide. These proteins, together with the DNA end-binding protein Ku, form a functional NHEJ break-repair apparatus that is highly homologous to the bacterial complex. Although the major roles of Pol and Lig in break repair have been reported, PE’s function in NHEJ has remained obscure. We establish that PE is required for ribonucleolytic resection of RNA intermediates at annealed DSBs. Polymerase-catalyzed strand-displacement synthesis on DNA gaps can result in the formation of nonligatable NHEJ intermediates. The function of PE in NHEJ repair is to detect and remove inappropriately incorporated ribonucleotides or phosphates from 3′ ends of annealed DSBs to configure the termini for ligation. Thus, PE prevents the accumulation of abortive genotoxic DNA intermediates arising from strand displacement synthesis that otherwise would be refractory to repair
Magnetic Trapping of Cold Bromine Atoms
Magnetic trapping of bromine atoms at temperatures in the milliKelvin regime
is demonstrated for the first time. The atoms are produced by photodissociation
of Br molecules in a molecular beam. The lab-frame velocity of Br atoms is
controlled by the wavelength and polarization of the photodissociation laser.
Careful selection of the wavelength results in one of the pair of atoms having
sufficient velocity to exactly cancel that of the parent molecule, and it
remains stationary in the lab frame. A trap is formed at the null point between
two opposing neodymium permanent magnets. Dissociation of molecules at the
field minimum results in the slowest fraction of photofragments remaining
trapped. After the ballistic escape of the fastest atoms, the trapped slow
atoms are only lost by elastic collisions with the chamber background gas. The
measured loss rate is consistent with estimates of the total cross section for
only those collisions transferring sufficient kinetic energy to overcome the
trapping potential
A new bound of the ℒ2[0, T]-induced norm and applications to model reduction
We present a simple bound on the finite horizon ℒ2/[0, T]-induced norm of a linear time-invariant (LTI), not necessarily stable system which can be efficiently computed by calculating the ℋ∞ norm of a shifted version of the original operator. As an application, we show how to use this bound to perform model reduction of unstable systems over a finite horizon. The technique is illustrated with a non-trivial physical example relevant to the appearance of time-irreversible phenomena in statistical physics
On the Distributed Compression of Quantum Information
The problem of distributed compression for correlated quantum sources is considered. The classical version of this problem was solved by Slepian and Wolf, who showed that distributed compression could take full advantage of redundancy in the local sources created by the presence of correlations. Here it is shown that, in general, this is not the case for quantum sources, by proving a lower bound on the rate sum for irreducible sources of product states which is stronger than the one given by a naive application of Slepian–Wolf. Nonetheless, strategies taking advantage of correlation do exist for some special classes of quantum sources. For example, Devetak and Winter demonstrated the existence of such a strategy when one of the sources is classical. Optimal nontrivial strategies for a different extreme, sources of Bell states, are presented here. In addition, it is explained how distributed compression is connected to other problems in quantum information theory, including information-disturbance questions, entanglement distillation and quantum error correction
Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements
The production of the elements heavier than iron via slow neutron captures
(the s process) is a main feature of the contribution of asymptotic giant
branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos.
However, our understanding of the main neutron source, the 13C(alpha,n)16O
reaction, is still incomplete. It is commonly assumed that in AGB stars mixing
beyond convective borders drives the formation of 13C pockets. However, there
is no agreement on the nature of such mixing and free parameters are present.
By means of a parametric model we investigate the impact of different mixing
functions on the final s-process abundances in low-mass AGB models. Typically,
changing the shape of the mixing function or the mass extent of the region
affected by the mixing produce the same results. Variations in the relative
abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are
generally within +/-0.2 dex, similar to the observational error bars. We
conclude that other stellar uncertainties - the effect of rotation and of
overshoot into the C-O core - play a more important role than the details of
the mixing function. The exception is at low metallicity, where the Pb
abundance is significantly affected. In relation to the composition observed in
stardust SiC grains from AGB stars, the models are relatively close to the data
only when assuming the most extreme variation in the mixing profile.Comment: 17 pages, 8 figures, 6 tables, accepted for publications on Monthly
Notices of the Royal Astronomical Societ
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