Preferential Trade Agreements and the Optimal Liberalisation of Agricultural Trade

Recent years have seen a rapid growth in the number of preferential trade agreements (PTAs) between developed and developing economies. Typically however many of these PTAs only incorporate a partial liberalisation of food and agricultural trade by developed economies. This paper reports the results from simulations conducted using a global comparative static model CGE model that has been calibrated with data from the GTAP database (version 5). Using the EU RSA FTA as an example the results indicate that the optimal degree of food trade liberalisation by the EU is less than 100 percent, and declines appreciably after the optimum. Qualitatively similar results emerge for South Africa. However, the welfare gains for South Africa increase rapidly with the increasing liberalisation of EU food and agricultural trade, while the welfare gains for the EU increase slowly with the increasing liberalisation of South African food trade. These results indicate that bilateral trade negotiations between developing and developed countries may involve a complex bargaining process, wherein the payoffs from different strategies are neither necessarily intuitively obvious nor are they necessarily consistent with the full liberalisation of food trade by developed economies

A kinetic model for the action of a resistance efflux pump

ArsA is the catalytic subunit of the arsenical pump, coupling ATP hydrolysis to the efflux of arsenicals through the ArsB membrane protein. It is a paradigm for understanding the structure-function of the nucleotide binding domains (NBD) of medically important efflux pumps, such as P-glycoprotein, because it has two sequence-related, interacting NBD, for which the structure is known. On the basis of a rigorous analysis of the pre-steady-state kinetics of nucleotide binding and hydrolysis, we propose a model in which ArsA alternates between two mutually exclusive conformations as follows: the ArsA1 conformation in which the A1 site is closed but the A2 site open; and the ArsA2 conformation, in which the A1 and A2 sites are open and closed, respectively. Antimonite elicits its effects by sequestering ArsA in the ArsA1 conformation, which catalyzes rapid ATP hydrolysis at the A2 site to drive ArsA between conformations that have high (nucleotide-bound ArsA) and low affinity (nucleotide-free ArsA) for Sb(III). ArsA potentially utilizes this process to sequester Sb(III) from the medium and eject it into the channel of ArsB

Interactions between unidirectional quantized vortex rings

We have used the vortex filament method to numerically investigate the interactions between pairs of quantized vortex rings that are initially traveling in the same direction but with their axes offset by a variable impact parameter. The interaction of two circular rings of comparable radii produce outcomes that can be categorized into four regimes, dependent only on the impact parameter; the two rings can either miss each other on the inside or outside, or they can reconnect leading to final states consisting of either one or two deformed rings. The fraction of of energy went into ring deformations and the transverse component of velocity of the rings are analyzed for each regime. We find that rings of very similar radius only reconnect for a very narrow range of the impact parameter, much smaller than would be expected from geometrical cross-section alone. In contrast, when the radii of the rings are very different, the range of impact parameters producing a reconnection is close to the geometrical value. A second type of interaction considered is the collision of circular rings with a highly deformed ring. This type of interaction appears to be a productive mechanism for creating small vortex rings. The simulations are discussed in the context of experiments on colliding vortex rings and quantum turbulence in superfluid helium in the zero temperature limit

No Effect of Steady Rotation on Solid 4^4He in a Torsional Oscillator

We have measured the response of a torsional oscillator containing polycrystalline hcp solid $^{4}$He to applied steady rotation in an attempt to verify the observations of several other groups that were initially interpreted as evidence for macroscopic quantum effects. The geometry of the cell was that of a simple annulus, with a fill line of relatively narrow diameter in the centre of the torsion rod. Varying the angular velocity of rotation up to 2\,rad\,s$^{-1}$ showed that there were no step-like features in the resonant frequency or dissipation of the oscillator and no history dependence, even though we achieved the sensitivity required to detect the various effects seen in earlier experiments on other rotating cryostats. All small changes during rotation were consistent with those occurring with an empty cell. We thus observed no effects on the samples of solid $^4$He attributable to steady rotation.Comment: 8 pages, 3 figures, accepted in J. Low Temp. Phy

Triggered/sequential star formation? A multi-phase ISM study around the prominent IRDC G18.93-0.03

G18.93-0.03 is a prominent dust complex within an 0.8deg long filament, with the molecular clump G18.93/m being IR dark from near IR wavelength up to 160mu. Spitzer composite images show an IR bubble spatially associated with G18.93. We use GRS 13CO and IRAM 30m H13CO+ data to disentangle the spatial structure of the region. From ATLASGAL submm data we calculate the gas mass, while we use the H13CO+ line width to estimate its virial mass. Using HERSCHEL data we produce temperature maps from fitting the SED. With the MAGPIS 20cm and SuperCOSMOS Halpha data we trace the ionized gas, and the VGPS HI survey provides information on the atomic hydrogen gas. We show that the bubble is spatially associated with G18.93, located at a kinematic near distance of 3.6kpc. With 280Msun, the most massive clump within G18.93 is G18.93/m. The virial analysis shows that it may be gravitationally bound and has neither Spitzer young stellar objects nor mid-IR point sources within. Fitting the SED reveals a temperature distribution that decreases towards its center, but heating from the ionizing source puts it above the general ISM temperature. We find that the bubble is filled by HII gas, ionized by an O8.5 star. Between the ionizing source and the IR dark clump G18.93/m we find a layered structure, from ionized to atomic to molecular hydrogen, revealing a PDR. Furthermore, we identify an additional velocity component within the bubble's 8mu emission rim at the edge of the infrared dark cloud and speculate that it might be shock induced by the expanding HII region. While the elevated temperature allows for the build-up of larger fragments, and the shock induced velocity component may lead to additional turbulent support, we do not find conclusive evidence that the massive clump G18.93/m is prone to collapse because of the expanding HII region.Comment: Accepted for publication in A&

Attosecond sampling of arbitrary optical waveforms

Advances in the generation of ultrashort laser pulses, and the emergence of new research areas such as attosecond science, nanoplasmonics, coherent control, and multidimensional spectroscopy, have led to the need for a new class of ultrafast metrology that can measure the electric field of complex optical waveforms spanning the ultraviolet to the infrared. Important examples of such waveforms are those produced by spectral control of ultrabroad bandwidth pulses, or by Fourier synthesis. These are typically tailored for specific purposes, such as to increase the photon energy and flux of high-harmonic radiation, or to control dynamical processes by steering electron dynamics on subcycle time scales. These applications demand a knowledge of the full temporal evolution of the field. Conventional pulse measurement techniques that provide estimates of the relative temporal or spectral phase are unsuited to measure such waveforms. Here we experimentally demonstrate a new, all-optical method for directly measuring the electric field of arbitrary ultrafast optical waveforms. Our method is based on high-harmonic generation (HHG) driven by a field that is the collinear superposition of the waveform to be measured with a stronger probe laser pulse. As the delay between the pulses is varied, we show that the field of the unknown waveform is mapped to energy shifts in the high-harmonic spectrum, allowing a direct, accurate, and rapid retrieval of the electric field with subcycle temporal resolution at the location of the HHG

Simplified Quantum Process Tomography

We propose and evaluate experimentally an approach to quantum process tomography that completely removes the scaling problem plaguing the standard approach. The key to this simplification is the incorporation of prior knowledge of the class of physical interactions involved in generating the dynamics, which reduces the problem to one of parameter estimation. This allows part of the problem to be tackled using efficient convex methods, which, when coupled with a constraint on some parameters allows globally optimal estimates for the Kraus operators to be determined from experimental data. Parameterising the maps provides further advantages: it allows the incorporation of mixed states of the environment as well as some initial correlation between the system and environment, both of which are common physical situations following excitation of the system away from thermal equilibrium. Although the approach is not universal, in cases where it is valid it returns a complete set of positive maps for the dynamical evolution of a quantum system at all times.Comment: Added references to interesting related work by Bendersky et a