6,969 research outputs found
Kuramoto model with coupling through an external medium
Synchronization of coupled oscillators is often described using the Kuramoto
model. Here we study a generalization of the Kuramoto model where oscillators
communicate with each other through an external medium. This generalized model
exhibits interesting new phenomena such as bistability between synchronization
and incoherence and a qualitatively new form of synchronization where the
external medium exhibits small-amplitude oscillations. We conclude by
discussing the relationship of the model to other variations of the Kuramoto
model including the Kuramoto model with a bimodal frequency distribution and
the Millennium Bridge problem.Comment: 9 pages, 3 figure
Comment on "Evidence for Quantized Displacement in Macroscopic Nanomechanical Oscillators"
In a recent Letter, Gaidarzhy et al. [1] claim to have observed evidence for "quantized displacements" of a high-order mode of a nanomechanical oscillator. We contend that the methods employed by the authors are unsuitable in principle to observe such states for any harmonic mode
Ion trap transducers for quantum electromechanical oscillators
An enduring challenge for contemporary physics is to experimentally observe
and control quantum behavior in macroscopic systems. We show that a single
trapped atomic ion could be used to probe the quantum nature of a mesoscopic
mechanical oscillator precooled to 4K, and furthermore, to cool the oscillator
with high efficiency to its quantum ground state. The proposed experiment could
be performed using currently available technology.Comment: 4 pages, 2 figure
Exciton lifetime in InAs/GaAs quantum dot molecules
The exciton lifetimes in arrays of InAs/GaAs vertically coupled quantum
dot pairs have been measured by time-resolved photoluminescence. A considerable
reduction of by up to a factor of 2 has been observed as compared
to a quantum dots reference, reflecting the inter-dot coherence. Increase of
the molecular coupling strength leads to a systematic decrease of with
decreasing barrier width, as for wide barriers a fraction of structures shows
reduced coupling while for narrow barriers all molecules appear to be well
coupled. The coherent excitons in the molecules gain the oscillator strength of
the excitons in the two separate quantum dots halving the exciton lifetime.
This superradiance effect contributes to the previously observed increase of
the homogeneous exciton linewidth, but is weaker than the reduction of .
This shows that as compared to the quantum dots reference pure dephasing
becomes increasingly important for the molecules
A high-reflectivity high-Q micromechanical Bragg-mirror
We report on the fabrication and characterization of a micromechanical
oscillator consisting only of a free-standing dielectric Bragg mirror with high
optical reflectivity and high mechanical quality. The fabrication technique is
a hybrid approach involving laser ablation and dry etching. The mirror has a
reflectivity of 99.6%, a mass of 400ng, and a mechanical quality factor Q of
approximately 10^4. Using this micromirror in a Fabry Perot cavity, a finesse
of 500 has been achieved. This is an important step towards designing tunable
high-Q high-finesse cavities on chip.Comment: 3 pages, 2 figure
Enhancement of Cavity Cooling of a Micromechanical Mirror Using Parametric Interactions
It is shown that an optical parametric amplifier inside a cavity can
considerably improve the cooling of the micromechanical mirror by radiation
pressure. The micromechanical mirror can be cooled from room temperature 300 K
to sub-Kelvin temperatures, which is much lower than what is achievable in the
absence of the parametric amplifier. Further if in case of a precooled mirror
one can reach millikelvin temperatures starting with about 1 K. Our work
demonstrates the fundamental dependence of radiation pressure effects on photon
statistics.Comment: 14 pages, 7 figure
Self-cooling of a micro-mirror by radiation pressure
We demonstrate passive feedback cooling of a mechanical resonator based on
radiation pressure forces and assisted by photothermal forces in a high-finesse
optical cavity. The resonator is a free-standing high-reflectance micro-mirror
(of mass m=400ng and mechanical quality factor Q=10^4) that is used as
back-mirror in a detuned Fabry-Perot cavity of optical finesse F=500. We
observe an increased damping in the dynamics of the mechanical oscillator by a
factor of 30 and a corresponding cooling of the oscillator modes below 10 K
starting from room temperature. This effect is an important ingredient for
recently proposed schemes to prepare quantum entanglement of macroscopic
mechanical oscillators.Comment: 11 pages, 9 figures, minor correction
Statistical mechanics of transcription-factor binding site discovery using Hidden Markov Models
Hidden Markov Models (HMMs) are a commonly used tool for inference of
transcription factor (TF) binding sites from DNA sequence data. We exploit the
mathematical equivalence between HMMs for TF binding and the "inverse"
statistical mechanics of hard rods in a one-dimensional disordered potential to
investigate learning in HMMs. We derive analytic expressions for the Fisher
information, a commonly employed measure of confidence in learned parameters,
in the biologically relevant limit where the density of binding sites is low.
We then use techniques from statistical mechanics to derive a scaling principle
relating the specificity (binding energy) of a TF to the minimum amount of
training data necessary to learn it.Comment: 25 pages, 2 figures, 1 table V2 - typos fixed and new references
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Steady-state spin densities and currents
This article reviews steady-state spin densities and spin currents in
materials with strong spin-orbit interactions. These phenomena are intimately
related to spin precession due to spin-orbit coupling which has no equivalent
in the steady state of charge distributions. The focus will be initially on
effects originating from the band structure. In this case spin densities arise
in an electric field because a component of each spin is conserved during
precession. Spin currents arise because a component of each spin is continually
precessing. These two phenomena are due to independent contributions to the
steady-state density matrix, and scattering between the conserved and
precessing spin distributions has important consequences for spin dynamics and
spin-related effects in general. In the latter part of the article extrinsic
effects such as skew scattering and side jump will be discussed, and it will be
shown that these effects are also modified considerably by spin precession.
Theoretical and experimental progress in all areas will be reviewed
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