1,755 research outputs found
Strong Optomechanical Squeezing of Light
We create squeezed light by exploiting the quantum nature of the mechanical
interaction between laser light and a membrane mechanical resonator embedded in
an optical cavity. The radiation pressure shot noise (fluctuating optical force
from quantum laser amplitude noise) induces resonator motion well above that of
thermally driven motion. This motion imprints a phase shift on the laser light,
hence correlating the amplitude and phase noise, a consequence of which is
optical squeezing. We experimentally demonstrate strong and continuous
optomechanical squeezing of 1.7 +/- 0.2 dB below the shot noise level. The peak
level of squeezing measured near the mechanical resonance is well described by
a model whose parameters are independently calibrated and that includes thermal
motion of the membrane with no other classical noise sources.Comment: 12 pages, 8 figure
A physically-motivated quantisation of the electromagnetic field on curved spacetimes
Recently, Bennett et al. [Eur. J. Phys. 37:014001, 2016] presented a physically-motivated and explicitly gauge-independent scheme for the quantisation of the electromagnetic field in flat Minkowski space. In this paper we generalise this field quantisation scheme to curved spacetimes.Working within the standard assumptions of quantum field theory and only postulating the physicality of the photon, we derive the Hamiltonian, H, and the electric and magnetic field observables, E and B, without having to invoke a specific gauge. As an example, we quantise the electromagnetic field in the spacetime of an accelerated Minkowski observer, Rindler space, and demonstrate consistency with other field quantisation schemes by reproducing the Unruh effect
Cooperativity and Frustration in Protein-Mediated Parallel Actin Bundles
We examine the mechanism of bundling of cytoskeletal actin filaments by two
representative bundling proteins, fascin and espin. Small-angle X-ray studies
show that increased binding from linkers drives a systematic \textit{overtwist}
of actin filaments from their native state, which occurs in a linker-dependent
fashion. Fascin bundles actin into a continuous spectrum of intermediate twist
states, while espin only allows for untwisted actin filaments and
fully-overtwisted bundles. Based on a coarse-grained, statistical model of
protein binding, we show that the interplay between binding geometry and the
intrinsic \textit{flexibility} of linkers mediates cooperative binding in the
bundle. We attribute the respective continuous/discontinous bundling mechanisms
of fascin/espin to differences in the stiffness of linker bonds themselves.Comment: 5 pages, 3 figures, figure file has been corrected in v
Cavity optomechanics with Si3N4 membranes at cryogenic temperatures
We describe a cryogenic cavity-optomechanical system that combines Si3N4
membranes with a mechanically-rigid Fabry-Perot cavity. The extremely high
quality-factor frequency products of the membranes allow us to cool a MHz
mechanical mode to a phonon occupation of less than 10, starting at a bath
temperature of 5 kelvin. We show that even at cold temperatures
thermally-occupied mechanical modes of the cavity elements can be a limitation,
and we discuss methods to reduce these effects sufficiently to achieve ground
state cooling. This promising new platform should have versatile uses for
hybrid devices and searches for radiation pressure shot noise.Comment: 19 pages, 5 figures, submitted to New Journal of Physic
Metabolic flexibility as a major predictor of spatial distribution in microbial communities
A better understand the ecology of microbes and their role in the global ecosystem could be achieved if traditional ecological theories can be applied to microbes. In ecology organisms are defined as specialists or generalists according to the breadth of their niche. Spatial distribution is often used as a proxy measure of niche breadth; generalists have broad niches and a wide spatial distribution and specialists a narrow niche and spatial distribution. Previous studies suggest that microbial distribution patterns are contrary to this idea; a microbial generalist genus (Desulfobulbus) has a limited spatial distribution while a specialist genus (Methanosaeta) has a cosmopolitan distribution. Therefore, we hypothesise that this counter-intuitive distribution within generalist and specialist microbial genera is a common microbial characteristic. Using molecular fingerprinting the distribution of four microbial genera, two generalists, Desulfobulbus and the methanogenic archaea Methanosarcina, and two specialists, Methanosaeta and the sulfate-reducing bacteria Desulfobacter were analysed in sediment samples from along a UK estuary. Detected genotypes of both generalist genera showed a distinct spatial distribution, significantly correlated with geographic distance between sites. Genotypes of both specialist genera showed no significant differential spatial distribution. These data support the hypothesis that the spatial distribution of specialist and generalist microbes does not match that seen with specialist and generalist large organisms. It may be that generalist microbes, while having a wider potential niche, are constrained, possibly by intrageneric competition, to exploit only a small part of that potential niche while specialists, with far fewer constraints to their niche, are more capable of filling their potential niche more effectively, perhaps by avoiding intrageneric competition. We suggest that these counter-intuitive distribution patterns may be a common feature of microbes in general and represent a distinct microbial principle in ecology, which is a real challenge if we are to develop a truly inclusive ecology
Quantizing the electromagnetic field near two-sided semitransparent mirrors
This paper models light scattering through flat surfaces with finite transmission, reflection, and absorption rates, with wave packets approaching the mirror from both sides. While using the same notion of photons as in free space, our model also accounts for the presence of mirror images and the possible exchange of energy between the electromagnetic field and the mirror surface. To test our model, we derive the spontaneous decay rate and the level shift of an atom in front of a semitransparent mirror as a function of its transmission and reflection rates. When considering limiting cases and using standard approximations, our approach reproduces well-known results but it also paves the way for the modeling of more complex scenarios
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