777 research outputs found
Fragmented condensation in Bose-Hubbard trimers with tunable tunnelling
We consider a Bose-Hubbard trimer, i.e. an ultracold Bose gas populating
three quantum states. The latter can be either different sites of a triple-well
potential or three internal states of the atoms. The bosons can tunnel between
different states with variable tunnelling strength between two of them. This
will allow us to study; i) different geometrical configurations, i.e. from a
closed triangle to three aligned wells and ii) a triangular configuration with
a -phase, i.e. by setting one of the tunnellings negative. By solving the
corresponding three-site Bose-Hubbard Hamiltonian we obtain the ground state of
the system as a function of the trap topology. We characterise the different
ground states by means of the coherence and entanglement properties. For small
repulsive interactions, fragmented condensates are found for the -phase
case. These are found to be robust against small variations of the tunnelling
in the small interaction regime. A low-energy effective many-body Hamiltonian
restricted to the degenerate manifold provides a compelling description of the
-phase degeneration and explains the low-energy spectrum as excitations of
discrete semifluxon states
Application of multiple-wireless to a visual localisation system for emergency services
Abstract—In this paper we discuss the application of multiplewireless technology to a practical context-enhanced service system called ViewNet. ViewNet develops technologies to support enhanced coordination and cooperation between operation teams in the emergency services and the police. Distributed localisation of users and mapping of environments implemented over a secure wireless network enables teams of operatives to search and map an incident area rapidly and in full coordination with each other and with a control centre. Sensing is based on fusing absolute positioning systems (UWB and GPS) with relative localisation and mapping from on-body or handheld vision and inertial sensors. This paper focuses on the case for multiple-wireless capabilities in such a system and the benefits it can provide. We describe our work of developing a software API to support both WLAN and TETRA in ViewNet. It also provides a basis for incorporating future wireless technologies into ViewNet. I
Reputation based selfishness prevention techniques for mobile ad-hoc networks
Mobile ad-hoc networks require nodes to cooperate in the relaying of data from source to destination. However, due to their limited resources, selfish nodes may be unwilling to forward packets, which can deteriorate the multi-hop connectivity. Different reputation-based protocols have been proposed to cope with selfishness in mobile ad-hoc networks. These protocols utilize the watchdog detection mechanism to observe the correct relaying of packets, and to compile information about potential selfish nodes. This information is used to prevent the participation of selfish nodes in the establishment of multi-hop routes. Despite its wide use, watchdog tends to overestimate the selfish behavior of nodes due to the effects of radio transmission errors or packet collisions that can be mistaken for intentional packet drops. As a result, the availability of valid multi-hop routes is reduced, and the overall performance deteriorates. This paper proposes and evaluates three detection techniques that improve the ability of selfishness prevention protocols to detect selfish nodes and to increase the number of valid routes.Ingeniería, Industria y Construcció
Freezing of He-4 and its liquid-solid interface from Density Functional Theory
We show that, at high densities, fully variational solutions of solid-like
type can be obtained from a density functional formalism originally designed
for liquid 4He. Motivated by this finding, we propose an extension of the
method that accurately describes the solid phase and the freezing transition of
liquid 4He at zero temperature. The density profile of the interface between
liquid and the (0001) surface of the 4He crystal is also investigated, and its
surface energy evaluated. The interfacial tension is found to be in
semiquantitative agreement with experiments and with other microscopic
calculations. This opens the possibility to use unbiased DF methods to study
highly non-homogeneous systems, like 4He interacting with strongly attractive
impurities/substrates, or the nucleation of the solid phase in the metastable
liquid.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
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