4,654 research outputs found
First Experimental Evidence for Chaos-Assisted Tunneling in a Microwave Annular Billiard
We report on first experimental signatures for chaos-assisted tunneling in a
two-dimensional annular billiard. Measurements of microwave spectra from a
superconducting cavity with high frequency resolution are combined with
electromagnetic field distributions experimentally determined from a normal
conducting twin cavity with high spatial resolution to resolve eigenmodes with
properly identified quantum numbers. Distributions of so-called quasi-doublet
splittings serve as basic observables for the tunneling between whispering
gallery type modes localized to congruent, but distinct tori which are coupled
weakly to irregular eigenstates associated with the chaotic region in phase
space.Comment: 5 pages RevTex, 5 low-resolution figures (high-resolution figures:
http://linac.ikp.physik.tu-darmstadt.de/heiko/chaospub.html, to be published
in Phys. Rev. Let
Statistical mechanics of secondary structures formed by random RNA sequences
The formation of secondary structures by a random RNA sequence is studied as
a model system for the sequence-structure problem omnipresent in biopolymers.
Several toy energy models are introduced to allow detailed analytical and
numerical studies. First, a two-replica calculation is performed. By mapping
the two-replica problem to the denaturation of a single homogeneous RNA in
6-dimensional embedding space, we show that sequence disorder is perturbatively
irrelevant, i.e., an RNA molecule with weak sequence disorder is in a molten
phase where many secondary structures with comparable total energy coexist. A
numerical study of various models at high temperature reproduces behaviors
characteristic of the molten phase. On the other hand, a scaling argument based
on the extremal statistics of rare regions can be constructed to show that the
low temperature phase is unstable to sequence disorder. We performed a detailed
numerical study of the low temperature phase using the droplet theory as a
guide, and characterized the statistics of large-scale, low-energy excitations
of the secondary structures from the ground state structure. We find the
excitation energy to grow very slowly (i.e., logarithmically) with the length
scale of the excitation, suggesting the existence of a marginal glass phase.
The transition between the low temperature glass phase and the high temperature
molten phase is also characterized numerically. It is revealed by a change in
the coefficient of the logarithmic excitation energy, from being disorder
dominated to entropy dominated.Comment: 24 pages, 16 figure
Modern Electronic Techniques Applied to Physics and Engineering
Contains reports on three research projects
Modern Electronic Techniques Applied to Physics and Engineering
Contains reports on two research projects
Labyrinthine window rupture as a cause of acute sensorineural hearing loss
Labyrinthine window rupture (LWR) is one cause of acute sensorineural hearing loss and need for early exploration is clear for good improved hearing. Acute sensorineural hearing loss of 60Â dB or more treated from May 2006 to May 2010 were retrospectively analyzed. There were 21 ears of severe deafness, 18 ears of profound deafness, and 10 ears of total deafness. All patients were examined with temporal bone CT. Space-occupying lesions around the labyrinthine windows were suggestive images of LWR. Thirty-five ears were operated for LWR while 14 ears of SHL received conservative treatments. Fifty-seven percent of LWR improved 30Â dB or more after sealing of both labyrinthine windows. Of the 15 markedly recovered ears, 14 ears were operated within 2Â weeks from the onset. Of the five cured ears, four ears were operated within a week from the onset. As for the hearing prognosis of SHL, 88% of severe and profound deafness improved 30Â dB or more but total deafness did not improve more than 30Â dB. Exclusion of LWR from SHL and early surgical intervention in LWR will bring about good hearing prognosis to both LWR and SHL
Gas Accretion and Star Formation Rates
Cosmological numerical simulations of galaxy evolution show that accretion of
metal-poor gas from the cosmic web drives the star formation in galaxy disks.
Unfortunately, the observational support for this theoretical prediction is
still indirect, and modeling and analysis are required to identify hints as
actual signs of star-formation feeding from metal-poor gas accretion. Thus, a
meticulous interpretation of the observations is crucial, and this
observational review begins with a simple theoretical description of the
physical process and the key ingredients it involves, including the properties
of the accreted gas and of the star-formation that it induces. A number of
observations pointing out the connection between metal-poor gas accretion and
star-formation are analyzed, specifically, the short gas consumption time-scale
compared to the age of the stellar populations, the fundamental metallicity
relationship, the relationship between disk morphology and gas metallicity, the
existence of metallicity drops in starbursts of star-forming galaxies, the
so-called G dwarf problem, the existence of a minimum metallicity for the
star-forming gas in the local universe, the origin of the alpha-enhanced gas
forming stars in the local universe, the metallicity of the quiescent BCDs, and
the direct measurements of gas accretion onto galaxies. A final section
discusses intrinsic difficulties to obtain direct observational evidence, and
points out alternative observational pathways to further consolidate the
current ideas.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springe
Towards Quantum Repeaters with Solid-State Qubits: Spin-Photon Entanglement Generation using Self-Assembled Quantum Dots
In this chapter we review the use of spins in optically-active InAs quantum
dots as the key physical building block for constructing a quantum repeater,
with a particular focus on recent results demonstrating entanglement between a
quantum memory (electron spin qubit) and a flying qubit (polarization- or
frequency-encoded photonic qubit). This is a first step towards demonstrating
entanglement between distant quantum memories (realized with quantum dots),
which in turn is a milestone in the roadmap for building a functional quantum
repeater. We also place this experimental work in context by providing an
overview of quantum repeaters, their potential uses, and the challenges in
implementing them.Comment: 51 pages. Expanded version of a chapter to appear in "Engineering the
Atom-Photon Interaction" (Springer-Verlag, 2015; eds. A. Predojevic and M. W.
Mitchell
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Ribose 2âČ-O-methylation provides a molecular signature for the distinction of self and non-self mRNA dependent on the RNA sensor Mda5
The 5'-cap-structures of higher eukaryote mRNAs are ribose 2'-O-methylated. Likewise, a number of viruses replicating in the cytoplasm of eukayotes have evolved 2'-O-methyltransferases to modify autonomously their mRNAs. However, a defined biological role of mRNA 2'-O-methylation remains elusive. Here we show that viral mRNA 2'-O-methylation is critically involved in subversion of type-I-interferon (IFN-I) induction. We demonstrate that human and murine coronavirus 2'-O-methyltransferase mutants induce increased IFN-I expression, and are highly IFN-I sensitive. Importantly, IFN-I induction by 2'-O-methyltransferase-deficient viruses is dependent on the cytoplasmic RNA sensor melanoma differentiation-associated gene 5 (MDA5). This link between MDA5-mediated sensing of viral RNA and mRNA 2'-O-methylation suggests that RNA modifications, such as 2'-O-methylation, provide a molecular signature for the discrimination of self and non-self mRNA
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