966 research outputs found
Optimizing optical Bragg scattering for single-photon frequency conversion
We develop a systematic theory for optimising single-photon frequency
conversion using optical Bragg scattering. The efficiency and phase-matching
conditions for the desired Bragg scattering conversion as well as spurious
scattering and modulation instability are identified. We find that third-order
dispersion can suppress unwanted processes, while dispersion above the fourth
order limits the maximum conversion efficiency. We apply the optimisation
conditions to frequency conversion in highly nonlinear fiber, silicon nitride
waveguides and silicon nanowires. Efficient conversion is confirmed using full
numerical simulations. These design rules will assist the development of
efficient quantum frequency conversion between multicolour single photon
sources for integration in complex quantum networks.Comment: 9 pages, 14 figure
A stable, single-photon emitter in a thin organic crystal for application to quantum-photonic devices
Single organic molecules offer great promise as bright, reliable sources of
identical single photons on demand, capable of integration into solid-state
devices. It has been proposed that such molecules in a crystalline organic
matrix might be placed close to an optical waveguide for this purpose, but so
far there have been no demonstrations of sufficiently thin crystals, with a
controlled concentration of suitable dopant molecules. Here we present a method
for growing very thin anthracene crystals from super-saturated vapour, which
produces crystals of extreme flatness and controlled thickness. We show how
this crystal can be doped with a widely adjustable concentration of
dibenzoterrylene (DBT) molecules and we examine the optical properties of these
molecules to demonstrate their suitability as quantum emitters in nanophotonic
devices. Our measurements show that the molecules are available in the crystal
as single quantum emitters, with a well-defined polarisation relative to the
crystal axes, making them amenable to alignment with optical nanostructures. We
find that the radiative lifetime and saturation intensity vary little within
the crystal and are not in any way compromised by the unusual matrix
environment. We show that a large fraction of these emitters are able to
deliver more than photons without photo-bleaching, making them
suitable for real applications.Comment: 12 pages, 10 figures, comments welcom
An All Optical Fibre Quantum Controlled-NOT Gate
We report the first experimental demonstration of an optical controlled-NOT
gate constructed entirely in fibre. We operate the gate using two heralded
optical fibre single photon sources and find an average logical fidelity of 90%
and an average process fidelity of 0.83<F<0.91. On the basis of a simple model
we are able to conclude that imperfections are primarily due to the photon
sources, meaning that the gate itself works with very high fidelity.Comment: 4 pages, 4 figures, comments welcom
GPCRTree: online hierarchical classification of GPCR function
Background: G protein-coupled receptors (GPCRs) play important physiological roles transducing extracellular signals into intracellular responses. Approximately 50% of all marketed drugs target a GPCR. There remains considerable interest in effectively predicting the function of a GPCR from its primary sequence. Findings: Using techniques drawn from data mining and proteochemometrics, an alignment-free approach to GPCR classification has been devised. It uses a simple representation of a protein's physical properties. GPCRTree, a publicly-available internet server, implements an algorithm that classifies GPCRs at the class, sub-family and sub-subfamily level. Conclusion: A selective top-down classifier was developed which assigns sequences within a GPCR hierarchy. Compared to other publicly available GPCR prediction servers, GPCRTree is considerably more accurate at every level of classification. The server has been available online since March 2008 at URL: http://igrid-ext.cryst.bbk.ac.uk/gpcrtree
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An Extended Culture System that Supports Human Primordial Germ Cell-like Cell Survival and Initiation of DNA Methylation Erasure.
The development of an in vitro system in which human primordial germ cell-like cells (hPGCLCs) are generated from human pluripotent stem cells (hPSCs) has been invaluable to further our understanding of human primordial germ cell (hPGC) specification. However, the means to evaluate the next fundamental steps in germ cell development have not been well established. In this study we describe a two dimensional extended culture system that promotes proliferation of specified hPGCLCs, without reversion to a pluripotent state. We demonstrate that hPGCLCs in extended culture undergo partial epigenetic reprogramming, mirroring events described in hPGCs in vivo, including a genome-wide reduction in DNA methylation and maintenance of depleted H3K9me2. This extended culture system provides a new approach for expanding the number of hPGCLCs for downstream technologies, including transplantation, molecular screening, or possibly the differentiation of hPGCLCs into gametes by in vitro gametogenesis
Measurements of shoaling internal waves and turbulence in an estuary
Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 118 (2013): 273–286, doi:10.1029/2012JC008154.The shoaling of horizontally propagating internal waves may represent an important source of mixing and transport in estuaries and coastal seas. Including such effects in numerical models demands improvements in the understanding of several aspects of the energetics, especially those relating to turbulence generation, and observations are needed to build this understanding. To address some of these issues in the estuarine context, we undertook an intensive field program for 10 days in the summer of 2008 in the St. Lawrence Estuary. The sampling involved shore-based photogrammetry, ship-based surveys, and an array of moorings in the shoaling region that held both conventional and turbulence-resolving sensors. The measurements shed light on many aspects of the wave shoaling process. Wave arrivals were generally phase-locked with the M2 tide, providing hints about far-field forcing. In the deeper part of the study domain, the waves propagated according to the predictions of linear theory. In intermediate-depth waters, the waves traversed the field site perpendicularly to isobaths, a pattern that continued as the waves transformed nonlinearly. Acoustic Doppler velocimeters permitted inference of the turbulent energetics, and two main features were studied. First, during a period of shoaling internal waves, turbulence dissipation rates exceeded values associated with tidal shear by an order of magnitude. Second, the evolving spectral signatures associated with a particular wave-shoaling event suggest that the turbulence is at least partly locally generated. Overall, the results of this study suggest that parameterizations of wave-induced mixing could employ relatively simple dynamics in deep water, but may have to handle a wide suite of turbulence generation and transport mechanisms in inshore regions.The work was supported
by the Killam Foundation, the Natural Sciences and Engineering Research
Council of Canada, the Canadian Foundation for Innovation, the Canadian
Foundation for Climate and Atmospheric Sciences, and the Canadian Department
of Fisheries and Oceans.2013-07-3
Hybrid photonic circuit for multiplexed heralded single photons
A key resource for quantum optics experiments is an on-demand source of
single and multiple photon states at telecommunication wavelengths. This letter
presents a heralded single photon source based on a hybrid technology approach,
combining high efficiency periodically poled lithium niobate waveguides,
low-loss laser inscribed circuits, and fast (>1 MHz) fibre coupled
electro-optic switches. Hybrid interfacing different platforms is a promising
route to exploiting the advantages of existing technology and has permitted the
demonstration of the multiplexing of four identical sources of single photons
to one output. Since this is an integrated technology, it provides scalability
and can immediately leverage any improvements in transmission, detection and
photon production efficiencies.Comment: 5 pages, double column, 3 figure
Integrated spatial multiplexing of heralded single photon sources
The non-deterministic nature of photon sources is a key limitation for single
photon quantum processors. Spatial multiplexing overcomes this by enhancing the
heralded single photon yield without enhancing the output noise. Here the
intrinsic statistical limit of an individual source is surpassed by spatially
multiplexing two monolithic silicon correlated photon pair sources,
demonstrating a 62.4% increase in the heralded single photon output without an
increase in unwanted multi-pair generation. We further demonstrate the
scalability of this scheme by multiplexing photons generated in two waveguides
pumped via an integrated coupler with a 63.1% increase in the heralded photon
rate. This demonstration paves the way for a scalable architecture for
multiplexing many photon sources in a compact integrated platform and achieving
efficient two photon interference, required at the core of optical quantum
computing and quantum communication protocols.Comment: 10 pages, 3 figures, comments welcom
Single-photon-level sub-Doppler pump-probe spectroscopy of rubidium
We propose and demonstrate pump-probe spectroscopy of rubidium absorption
which reveals the sub-Doppler hyperfine structure of the S P (D2) transitions. The counter propagating pump
and probe lasers are independently tunable in frequency, with the probe
operating at the single-photon-level. The two-dimensional spectrum measured as
the laser frequencies are scanned shows fluorescence, Doppler-broadened
absorption dips and sub-Doppler features. The detuning between the pump and
probe lasers allows compensation of the Doppler shift for all atomic velocities
in the room temperature vapor, meaning we observe sub-Doppler features for all
atoms in the beam. We detail a theoretical model of the system which
incorporates fluorescence, saturation effects and optical pumping and compare
this with the measured spectrum, finding a mean absolute percentage error of
4.17\%. In the future this technique could assist in frequency stabilization of
lasers, and the single-photon-level probe could be replaced by a single photon
source.Comment: 5 page paper, 4 page supplemental material. Comments welcom
Examining adherence to activity monitoring devices to improve physical activity in adults with cardiovascular disease: A systematic review
Background
Activity monitoring devices are currently being used to facilitate and monitor physical activity. No prior review has examined adherence to the use of activity monitoring devices amongst adults with cardiovascular disease.
Methods
Literature from June 2012 to October 2017 was evaluated to examine the extent of adherence to any activity monitoring device used to collect objective physical activity data. Randomized control trials comparing usual care against the use of an activity monitoring device, in a community intervention for adults from any cardiovascular diagnostic group, were included. A systematic search of databases and clinical trials registers was conducted using Joanna Briggs Institute methodology.
Results
Of 10 eligible studies, two studies reported pedometer use and eight accelerometer use. Six studies addressed the primary outcome. Mean adherence was 59.1% (range 39.6% to 85.7%) at last follow-up. Studies lacked equal representation by gender (28.6% female) and age (range 42 to 82 years).
Conclusion
This review indicates that current research on activity monitoring devices may be overstated due to the variability in adherence. Results showed that physical activity tracking in women and in young adults have been understudied
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