Reducing the linewidth of an atom laser by feedback

A continuous atom laser will almost certainly have a linewidth dominated by the effect of the atomic interaction energy, which turns fluctuations in the condensate atom number into fluctuations in the condensate frequency. These correlated fluctuations mean that information about the atom number could be used to reduce the frequency fluctuations, by controlling a spatially uniform potential. We show that feedback based on a physically reasonable quantum non-demolition measurement of the atom number of the condensate in situ can reduce the linewidth enormously.Comment: 5 pages, 1 figur

Effects of twin-beam squashed light on a three-level atom

An electro-optical feedback loop can make in-loop light (squashed light) which produces a photocurrent with noise below the standard quantum limit (such as squeezed light). We investigate the effect of squashed light interacting with a three-level atom in the cascade configuration and compare it to the effects produced by squeezed light and classical noise. It turns out that one master equation can be formulated for all three types of light and that this unified formalism can also be applied to the evolution of a two-level atom. We show that squashed light does not mimic all aspects of squeezed light, and in particular, it does not produce the characteristic linear intensity dependence of the population of the upper-most level of the cascade three-level atom. Nevertheless, it has nonclassical transient effects in the de-excitation.Comment: 12 pages, 6 figure

Control of perennial weeds based on weed biology and environmental considerations

Timing, type and duration of tillage operations and cropping practices are of importance for reducing the regeneration of perennial weeds and also for minimizing the environmental impacts of such operations. In the present work and with reference to previous publications we find that, based on the knowledge gained on growth patterns, sensitivity to competition, growth potential of roots and mechanical and management measures it is possible to coordinate control of perennial weeds with considerations of yield and reduced environmental impact

Pure-state quantum trajectories for general non-Markovian systems do not exist

Since the first derivation of non-Markovian stochastic Schr\"odinger equations, their interpretation has been contentious. In a recent Letter [Phys. Rev. Lett. 100, 080401 (2008)], Di\'osi claimed to prove that they generate "true single system trajectories [conditioned on] continuous measurement". In this Letter we show that his proof is fundamentally flawed: the solution to his non-Markovian stochastic Schr\"odinger equation at any particular time can be interpreted as a conditioned state, but joining up these solutions as a trajectory creates a fiction.Comment: 4 page

The strength of the radial-breathing mode in single-walled carbon nanotubes

We show by ab initio calculations that the electron-phonon coupling matrix element M of the radial breathing mode in single-walled carbon nanotubes depends strongly on tube chirality. For nanotubes of the same diameter the coupling strength |M|^2 is up to one order of magnitude stronger for zig-zag than for armchair tubes. For (n,m) tubes M depends on the value of (n-m) mod 3, which allows to discriminate semiconducting nano tubes with similar diameter by their Raman scattering intensity. We show measured resonance Raman profiles of the radial breathing mode which support our theoretical predictions

Mining cosmic dust from the blue ice lakes of Greenland

Extraterrestrial material, most of which invisible settles to Earth's surface as dust particles smaller than a millimeter in size were investigated. Particles of 1/10 millimeter size fall at a rate of one/sq m/yr collection of extraterrestrial dust is important because the recovered cosmic dust particles can provide important information about comets. Comets are the most important source of dust in the solar system and they are probably the major source of extraterrestrial dust that is collectable at the Earth's surface. A new collection site for cosmic dust, in an environment where degradation by weathering is minimal is reported. It is found that the blue ice lakes on the Greenland ice cap provide an ideal location for collection of extraterrestrial dust particles larger than 0.1 mm in size. It is found that the lakes contain large amounts of cosmic dust which is much better preserved than similar particles recovered from the ocean floor

First-principles calculations of the vibrational properties of bulk CdSe and CdSe nanowires

We present first-principles calculations on bulk CdSe and CdSe nanowires with diameters of up to 22 \AA. Density functional linear combination of atomic orbitals and plane wave calculations of the electronic and structural properties are presented and discussed. We use an iterative, symmetry-based method to relax the structures into the ground state. We find that the band gap depends on surface termination. Vibrational properties in the whole Brillouin zone of bulk CdSe and the zone-center vibrations of nanowires are calculated and analyzed. We find strongly size-dependent and nearly constant modes, depending on the displacement directions. A comparison with available experimental Raman data is be given