3,348 research outputs found
Voltage-controlled wavelength conversion by terahertz electro-optic modulation in double quantum wells
An undoped double quantum well (DQW) was driven with a terahertz (THz)
electric field of frequency \omega_{THz} polarized in the growth direction,
while simultaneously illuminated with a near-infrared (NIR) laser at frequency
\omega_{NIR}. The intensity of NIR upconverted sidebands
\omega_{sideband}=\omega_{NIR} + \omega_{THz} was maximized when a dc voltage
applied in the growth direction tuned the excitonic states into resonance with
both the THz and NIR fields. There was no detectable upconversion far from
resonance. The results demonstrate the possibility of using gated DQW devices
for all-optical wavelength shifting between optical communication channels
separated by up to a few THz.Comment: 3 pages, 6 figures. Figures 5 and 6 are JPEG files, figures/fig5.jpg
and fig6.jp
Demonstration of the spatial separation of the entangled quantum side-bands of an optical field
Quantum optics experiments on "bright" beams typically probe correlations
between side-band modes. However the extra degree of freedom represented by
this dual mode picture is generally ignored. We demonstrate the experimental
operation of a device which can be used to separate the quantum side-bands of
an optical field. We use this device to explicitly demonstrate the quantum
entanglement between the side-bands of a squeezed beam
Effect of Time of Access to Pasture and the Provision of a Total Mixed Ration on the Performance and Methane Production of High Yielding Dairy Cows
Allowing cows to spend time on pasture may improve their welfare, although high yielding cows are unable to consume sufficient amounts of grass to maintain milk yield and require supplementation (Charlton et al. 2011). The inclusion of grass in the diet of high yielding cows may have benefits as grass contains polyunsaturated fatty acids which can reduce methane production (Martin et al. 2008). Additionally, the soluble carbohydrate content in grass is higher in the afternoon which may increase intake (Trevaskis et al. 2004). The aim of the experiment was to determine the effects of timing of pasture access and the provision of access to total mixed ration (TMR) when at grass on the performance and methane production of high yielding dairy cows
A Bright Spatially-Coherent Compact X-ray Synchrotron Source
Each successive generation of x-ray machines has opened up new frontiers in
science, such as the first radiographs and the determination of the structure
of DNA. State-of-the-art x-ray sources can now produce coherent high brightness
keV x-rays and promise a new revolution in imaging complex systems on nanometre
and femtosecond scales. Despite the demand, only a few dedicated synchrotron
facilities exist worldwide, partially due the size and cost of conventional
(accelerator) technology. Here we demonstrate the use of a recently developed
compact laser-plasma accelerator to produce a well-collimated,
spatially-coherent, intrinsically ultrafast source of hard x-rays. This method
reduces the size of the synchrotron source from the tens of metres to
centimetre scale, accelerating and wiggling a high electron charge
simultaneously. This leads to a narrow-energy spread electron beam and x-ray
source that is >1000 times brighter than previously reported plasma wiggler and
thus has the potential to facilitate a myriad of uses across the whole spectrum
of light-source applications.Comment: 5 pages, 4 figure
A wider Europe? The view from Russia, Belarus and Ukraine
On the evidence of national surveys conducted between 2000 and 2006, there is a declining sense of European self-identity in the three Slavic post-Soviet republics of Russia, Belarus and Ukraine. Attitudes towards the European Union and the possibility of membership are broadly supportive, but with a substantial proportion who find it difficult to express a view, and substantial proportions are poorly informed in comparison with the general public in EU member or prospective member countries. Those who are better informed are more likely to favour EU membership and vice versa. Generally, socioeconomic characteristics (except for age and region) are relatively poor predictors of support for EU membership as compared with attitudinal variables. But âEuropeannessâ should not be seen as a given, and much will depend on whether EU member countries emphasize what is common to east and west or establish ânew dividing linesâ in place of those of the cold war
Experimental Demonstration of Frequency Autolocking an Optical Cavity Using a Time-Varying Kalman Filter
We propose and demonstrate a new autolocking scheme using a three-mirror ring cavity consisting of a linear quadratic regulator and a time-varying Kalman filter. Our technique does not require a frequency scan to acquire resonance. We utilize the singular perturbation method to simplify our system dynamics and to permit the application of linear control techniques. The error signal combined with the transmitted power is used to estimate the cavity detuning. This estimate is used by a linear time-varying Kalman filter which enables the implementation of an optimal controller. The experimental results validate the controller design, and we demonstrate improved robustness to disturbances and a faster locking time than a traditional proportional-integral controller. More important, the time-varying Kalman filtering approach automatically reacquires lock for large detunings, where the error signal leaves its linear capture range, a feat which linear time-invariant controllers cannot achieve. © 2016 American Physical Society
Getting elections right? Measuring electoral integrity
Holding elections has become a global norm. Unfortunately, the integrity of elections varies strongly, ranging from âfree and fairâ elections with genuine contestation to âfaçadeâ elections marred by manipulation and fraud. Clearly, electoral integrity is a topic of increasing concern. Yet electoral integrity is notoriously difficult to measure, and hence taking stock of the available data is important. This article compares cross-national data sets measuring electoral integrity. The first part evaluates how the different data sets (a) conceptualize electoral integrity, (b) move from concepts to indicators, and (c) move from indicators to data. The second part analyses how different data sets code the same elections, seeking to explain the sources of disagreement about electoral integrity. The sample analysed comprises 746 elections in 95 third and fourth wave regimes from 1974 until 2009. I find that conceptual and measurement choices affect disagreement about election integrity, and also find that elections of lower integrity and post-conflict elections generate higher disagreement about election integrity. The article concludes with a discussion of results and suggestions for future research
Strong-field terahertz-optical mixing in excitons
Driving a double-quantum-well excitonic intersubband resonance with a
terahertz (THz) electric field of frequency \omega_{THz} generated terahertz
optical sidebands \omega=\omega_{THz}+\omega_{NIR} on a weak NIR probe. At high
THz intensities, the intersubband dipole energy which coupled two excitons was
comparable to the THz photon energy. In this strong-field regime the sideband
intensity displayed a non-monotonic dependence on the THz field strength. The
oscillating refractive index which gives rise to the sidebands may be
understood by the formation of Floquet states, which oscillate with the same
periodicity as the driving THz field.Comment: 4 pages, 6 figure
Unconditional Continuous Variable Dense Coding
We investigate the conditions under which unconditional dense coding can be
achieved using continuous variable entanglement. We consider the effect of
entanglement impurity and detector efficiency and discuss experimental
verification. We conclude that the requirements for a strong demonstration are
not as stringent as previously thought and are within the reach of present
technology
Development of an Interpretive Simulation Tool for the Proton Radiography Technique
Proton radiography is a useful diagnostic of high energy density (HED)
plasmas under active theoretical and experimental development. In this paper we
describe a new simulation tool that interacts realistic laser-driven point-like
proton sources with three dimensional electromagnetic fields of arbitrary
strength and structure and synthesizes the associated high resolution proton
radiograph. The present tool's numerical approach captures all relevant physics
effects, including effects related to the formation of caustics.
Electromagnetic fields can be imported from PIC or hydrodynamic codes in a
streamlined fashion, and a library of electromagnetic field `primitives' is
also provided. This latter capability allows users to add a primitive, modify
the field strength, rotate a primitive, and so on, while quickly generating a
high resolution radiograph at each step. In this way, our tool enables the user
to deconstruct features in a radiograph and interpret them in connection to
specific underlying electromagnetic field elements. We show an example
application of the tool in connection to experimental observations of the
Weibel instability in counterstreaming plasmas, using particles
generated from a realistic laser-driven point-like proton source, imaging
fields which cover volumes of mm. Insights derived from this
application show that the tool can support understanding of HED plasmas.Comment: Figures and tables related to the Appendix are included in the
published journal articl
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