A novel method for the production of aluminium nitride

A method has been developed to produce thick (> 400 mu m) AlN surface layers oil aluminium plates at 540 degrees C, under nitrogen at atmospheric pressure. A critical element of the process is the use of Mg powder placed in close proximity to the Al plate surface. The Mg reduces/disrupts the natural, protective oxide film on the Al surface. The nitride layers form through two distinct modes, one growing outward from the Al plate surface and the other growing into the Al. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Beach face and berm morphodynamics fronting a coastal lagoon

This study documents two different modes of berm development: (I) vertical growth at spring tides or following significant beach cut due to substantial swash overtopping, and (2) horizontal progradation at neap tides through the formation of a proto-berm located lower and further seaward of the principal berm. Concurrent high-frequency measurements of bed elevation and the associated wave runup distribution reveal the details of each of these berm growth modes. In mode I sediment is eroded from the inner surf and lower swash zone where swash interactions are prevalent. The net transport of this sediment is landward only, resulting in accretion onto the upper beach face and over the berm crest. The final outcome is a steepening of the beach face gradient, a change in the profile shape towards concave and rapid vertical and horizontal growth of the berm. In mode 2 sediment is eroded from the lower two-thirds of the active swash zone during the rising tide and is transported both landward and seaward. On the falling tide sediment is eroded from the inner surf and transported landward to backfill the zone eroded on the rising tide. The net result is relatively slow steepening of the beach face, a change of the profile shape towards convex, and horizontal progradation through the formation of a neap berm. The primary factor determining which mode of berm growth occurs is the presence or absence of swash overtopping at the time of sediment accumulation on the beach face. This depends on the current phase of the spring-neap tide cycle, the wave runup height (and indirectly offshore wave conditions) and the height of the pre-existing berm. A conceptual model for berm morphodynamics is presented, based on sediment transport shape functions measured during the two modes of berm growth. (c) 2006 Elsevier B.V. All rights reserved.

Entanglement of two-mode Bose-Einstein condensates

We investigate the entanglement characteristics of two general bimodal Bose-Einstein condensates-a pair of tunnel-coupled Bose-Einstein condensates and the atom-molecule Bose-Einstein condensate. We argue that the entanglement is only physically meaningful if the system is viewed as a bipartite system, where the subsystems are the two modes. The indistinguishibility of the particles in the condensate means that the atomic constituents are physically inaccessible and, thus, the degree of entanglement between individual particles, unlike the entanglement between the modes, is not experimentally relevant so long as the particles remain in the condensed state. We calculate the entanglement between the two modes for the exact ground state of the two bimodal condensates and consider the dynamics of the entanglement in the tunnel-coupled case.

Entangled two-photon source using biexciton emission of an asymmetric quantum dot in a cavity

A semiconductor based scheme has been proposed for generating entangled photon pairs from the radiative decay of an electrically pumped biexciton in a quantum dot. Symmetric dots produce polarization entanglement, but experimentally realized asymmetric dots produce photons entangled in both polarization and frequency. In this work, we investigate the possibility of erasing the “which-path” information contained in the frequencies of the photons produced by asymmetric quantum dots to recover polarization-entangled photons. We consider a biexciton with nondegenerate intermediate excitonic states in a leaky optical cavity with pairs of degenerate cavity modes close to the nondegenerate exciton transition frequencies. An open quantum system approach is used to compute the polarization entanglement of the two-photon state after it escapes from the cavity, measured by the visibility of two-photon interference fringes. We explicitly relate the two-photon visibility to the degree of the Bell-inequality violation, deriving a threshold at which Bell-inequality violations will be observed. Our results show that an ideal cavity will produce maximally polarization-entangled photon pairs, and even a nonideal cavity will produce partially entangled photon pairs capable of violating a Bell-inequality.

Ena/VASP proteins can regulate distinct modes of actin organization at cadherin-adhesive contacts

Functional interactions between classical cadherins and the actin cytoskeleton involve diverse actin activities, including filament nucleation, cross-linking, and bundling. In this report, we explored the capacity of Ena/VASP proteins to regulate the actin cytoskeleton at cadherin-adhesive contacts. We extended the observation that Ena/vasodilator-stimulated phosphoprotein (VASP) proteins localize at cell-cell contacts to demonstrate that E-cadherin homophilic ligation is sufficient to recruit Mena to adhesion sites. Ena/VASP activity was necessary both for F-actin accumulation and assembly at cell-cell contacts. Moreover, we identified two distinct pools of Mena within individual homophilic adhesions that cells made when they adhered to cadherin-coated substrata. These Mena pools localized with Arp2/3-driven cellular protrusions as well as at the tips of cadherin-based actin bundles. Importantly, Ena/VASP activity was necessary for both modes of actin activity to be expressed. Moreover, selective depletion of Ena/VASP proteins from the tips of cadherin-based bundles perturbed the bundles without affecting the protrusive F-actin pool. We propose that Ena/VASP proteins may serve as higher order regulators of the cytoskeleton at cadherin contacts through their ability to modulate distinct modes of actin organization at those contacts.

Quantum gate based on Stark tunable nanocrystal interactions with ultrahigh-Q/V field modes in fused silica microcavities

We investigate the use of nanocrystal quantum dots as a quantum bus element for preparing various quantum resources for use in photonic quantum technologies. Using the Stark-tuning property of nanocrystal quantum dots as well as the biexciton transition, we demonstrate a photonic controlled-NOT (CNOT) interaction between two logical photonic qubits comprising two cavity field modes each. We find the CNOT interaction to be a robust generator of photonic Bell states, even with relatively large biexciton losses. These results are discussed in light of the current state of the art of both microcavity fabrication and recent advances in nanocrystal quantum dot technology. Overall, we find that such a scheme should be feasible in the near future with appropriate refinements to both nanocrystal fabrication technology and microcavity design. Such a gate could serve as an active element in photonic-based quantum technologies.

Two men with placards during Aldermaston Peace march in 1964

Two men with placards during Aldermaston Peace march, Sunday, April 5th 1964. The Aldermaston march covered the distance between Ipswich and Brisbane, Australia, walked in relays covering approximately two miles each. Most relay sections were sponsored by one or more individual organisations.