Two-qubit Quantum Logic Gate in Molecular Magnets

We proposed a scheme to realize a controlled-NOT quantum logic gate in a dimer of exchange coupled single-molecule magnets, $[\textrm{Mn}_4]_2$. We chosen the ground state and the three low-lying excited states of a dimer in a finite longitudinal magnetic field as the quantum computing bases and introduced a pulsed transverse magnetic field with a special frequency. The pulsed transverse magnetic field induces the transitions between the quantum computing bases so as to realize a controlled-NOT quantum logic gate. The transition rates between the quantum computing bases and between the quantum computing bases and other excited states are evaluated and analyzed.Comment: 7 pages, 2 figure

Lay epidemiology and the interpretation of low-risk drinking guidelines by adults in the United Kingdom

Aims To explore how the concept of lay epidemiology can enhance understandings of how drinkers make sense of current UK drinking guidelines. Methods Qualitative study using 12 focus groups in four sites in northern England and four sites in central Scotland. Participants were 66 male and female drinkers, aged between 19 and 65 years, of different socio-economic backgrounds. Data were analysed thematically using a conceptual framework of lay epidemiology. Results Current drinking guidelines were perceived as having little relevance to participants' drinking behaviours and were generally disregarded. Daily guidelines were seen as irrelevant by drinkers whose drinking patterns comprised heavy weekend drinking. The amounts given in the guidelines were seen as unrealistic for those motivated to drink for intoxication, and participants measured alcohol intake in numbers of drinks or containers rather than units. Participants reported moderating their drinking, but this was out of a desire to fulfil work and family responsibilities, rather than concerns for their own health. The current Australian and Canadian guidelines were preferred to UK guidelines, as they were seen to address many of the above problems. Conclusions Drinking guidelines derived from, and framed within, solely epidemiological paradigms lack relevance for adult drinkers who monitor and moderate their alcohol intake according to their own knowledge and risk perceptions derived primarily from experience. Insights from lay epidemiology into how drinkers regulate and monitor their drinking should be used in the construction of drinking guidelines to enhance their credibility and efficacy

The Theory and Simulation of Relativistic Jet Formation: Towards a Unified Model For Micro- and Macroquasars

I review recent progress in the theory of relativistic jet production, with special emphasis on unifying black hole sources of stellar and supermassive size. Observations of both classes of objects, as well as theoretical considerations, indicate that such jets may be launched with a spine/sheath flow structure, having a much higher Lorentz factor ($\sim 50$) near the axis and a lower speed ($\Gamma \sim 10$ or so) away from the axis. It has become clear that one can no longer consider models of accretion flows without also considering the production of a jet by that flow. Furthermore, the rotation rate of the black hole also must be taken into account. It provides a third parameter that should break the mass/accretion rate degeneracy and perhaps explain why some sources are radio loud and some radio quiet. Slow jet acceleration and collimation is expected theoretically, and can explain some of the observed features of AGN jet sources. Finally, relativistic jets launched by MHD/ED processes are Poynting flux dominated by nature, and are potentially unstable if there is significant entrainment of thermal material.Comment: 6 pages, 1 figure; in The physics of relativistic jets in the CHANDRA and XMM era, Bologna, 23-27 Sept 200

A Disc-Corona Model for a Rotating Black Hole

We propose a disc-corona model in which a geometrically thin, optically thick disc surrounds a Kerr black hole, and magnetic fields exert a time-steady torque on the inner edge of the accretion disc. The analytical expression of the total gravitational power is derived from the thin-disc dynamics equations by using this new boundary condition. It is shown that the magnetic torque can considerably enhance the amount of energy released in the disc-corona system. Furthermore, the global solutions of this disc-corona system are obtained numerically. We find that the fraction of the power dissipated into the corona in the total for such disc-corona system increases with the increasing dimensionless black hole spin parameter $a_\ast$, but is insensitive on the $\Delta \varepsilon$ which is the additional radiative efficiency parameter relevant to magnetic torque, for $\Delta\varepsilon > 1$. In addition, the emerged spectra from this disc-corona system are simulated by using Monte-Carlo method, and the effect of the different parameters on the output spectra is discussed

Communication and optimal hierarchical networks

We study a general and simple model for communication processes. In the model, agents in a network (in particular, an organization) interchange information packets following simple rules that take into account the limited capability of the agents to deal with packets and the cost associated to the existence of open communication channels. Due to the limitation in the capability, the network collapses under certain conditions. We focus on when the collapse occurs for hierarchical networks and also on the influence of the flatness or steepness of the structure. We find that the need for hierarchy is related to the existence of costly connections.Comment: 7 pages, 2 figures. NATO ARW on Econophysic