90,926 research outputs found
Entanglement and entropy engineering of atomic two-qubit states
We propose a scheme employing quantum-reservoir engineering to controllably
entangle the internal states of two atoms trapped in a high finesse optical
cavity. Using laser and cavity fields to drive two separate Raman transitions
between metastable atomic ground states, a system is realized corresponding to
a pair of two-state atoms coupled collectively to a squeezed reservoir.
Phase-sensitive reservoir correlations lead to entanglement between the atoms,
and, via local unitary transformations and adjustment of the degree and purity
of squeezing, one can prepare entangled mixed states with any allowed
combination of linear entropy and entanglement of formation.Comment: 4 pages, 5 figures, REVTe
ScotGrid: A Prototype Tier 2 Centre
ScotGrid is a prototype regional computing centre formed as a collaboration
between the universities of Durham, Edinburgh and Glasgow as part of the UK's
national particle physics grid, GridPP. We outline the resources available at
the three core sites and our optimisation efforts for our user communities. We
discuss the work which has been conducted in extending the centre to embrace
new projects both from particle physics and new user communities and explain
our methodology for doing this.Comment: 4 pages, 4 diagrams. Presented at Computing for High Energy and
Nuclear Physics 2004 (CHEP '04). Interlaken, Switzerland, September 200
Thermal design study of an air-cooled plug-nozzle system for a supersonic cruise aircraft
A heat-transfer design analysis has been made of an air-cooled plug-nozzle system for a supersonic-cruise aircraft engine. The proposed 10deg half-angle conical plug is sting supported from the turbine frame. Plug cooling is accomplished by convection and film cooling. The flight profile studied includes maximum afterburning from takeoff to Mach 2.7 and supersonic cruise at Mach 2.7 with a low afterburner setting. The calculations indicate that, for maximum afterburning, about 2 percent of the engine primary flow, removed after the second stage of the nine-stage compressor, will adequately cool the plug and sting support. Ram air may be used for cooling during supersonic-cruise operations, however. Therefore, the cycle efficiency penalty paid for air cooling the plug and sting support should be low
Entanglement consumption of instantaneous nonlocal quantum measurements
Relativistic causality has dramatic consequences on the measurability of
nonlocal variables and poses the fundamental question of whether it is
physically meaningful to speak about the value of nonlocal variables at a
particular time. Recent work has shown that by weakening the role of the
measurement in preparing eigenstates of the variable it is in fact possible to
measure all nonlocal observables instantaneously by exploiting entanglement.
However, for these measurement schemes to succeed with certainty an infinite
amount of entanglement must be distributed initially and all this entanglement
is necessarily consumed. In this work we sharpen the characterisation of
instantaneous nonlocal measurements by explicitly devising schemes in which
only a finite amount of the initially distributed entanglement is ever
utilised. This enables us to determine an upper bound to the average
consumption for the most general cases of nonlocal measurements. This includes
the tasks of state verification, where the measurement verifies if the system
is in a given state, and verification measurements of a general set of
eigenstates of an observable. Despite its finiteness the growth of entanglement
consumption is found to display an extremely unfavourable exponential of an
exponential scaling with either the number of qubits needed to contain the
Schmidt rank of the target state or total number of qubits in the system for an
operator measurement. This scaling is seen to be a consequence of the
combination of the generic exponential scaling of unitary decompositions
combined with the highly recursive structure of our scheme required to overcome
the no-signalling constraint of relativistic causality.Comment: 32 pages and 14 figures. Updated to published versio
A representative sample of Be Stars I: Sample Selection, Spectral Classification and Rotational Velocities
We present a sample of 58 Be stars containing objects of spectral types O9 to
B8.5 and luminosity classes III to V. We have obtained 3670 - 5070 Angstrom
spectra of the sample which are used to derive spectral types and rotational
velocities. We discuss the distribution of spectral types and rotational
velocities obtained and conclude that there are no significant selection
effects in our sample.Comment: 10 Pages, 9 Figures, Accepted for publication in A&A
The star formation efficiency and its relation to variations in the initial mass function
We investigate how the dynamical state of a turbulently supported, 1000 solar
mass, molecular cloud affects the properties of the cluster it forms, focusing
our discussion on the star formation efficiency (SFE) and the initial mass
function (IMF). A variety of initial energy states are examined in this paper,
ranging from clouds with PE = 0.1 KE to clouds with PE = 10 KE, and for both
isothermal and piece-wise polytropic equations of state (similar to that
suggested by Larson). It is found that arbitrary star formation efficiencies
are possible, with strongly unbound clouds yielding very low star formation
efficiencies. We suggest that the low star formation efficiency in the
Maddelena cloud may be a consequence of the relatively unbound state of its
internal structure. It is also found that competitive accretion results in the
observed IMF when the clouds have initial energy states of PE >= KE. We show
that under such conditions the shape of the IMF is independent of time in the
calculations. This demonstrates that the global accretion process can be
terminated at any stage in the cluster's evolution, while still yielding a
distribution of stellar masses that is consistent with the observed IMF. As the
clouds become progressively more unbound, competitive accretion is less
important and the protostellar mass function flattens. These results predict
that molecular clouds should be permeated with a distributed population of
stars that follow a flatter than Salpeter IMF.Comment: 8 pages, 6 figures, accepted by MNRAS for publictaion. Now available
through the 'Online Early' schem
The Akulov-Volkov Lagrangian, Symmetry Currents and Spontaneously Broken Extended Supersymmetry
A generalization of the Akulov-Volkov effective Lagrangian governing the self
interactions of the Nambu-Goldstone fermions associated with spontaneously
broken extended supersymmetry as well as their coupling to matter is presented
and scrutinized. The resulting currents associated with R-symmetry,
supersymmetry and space-time translations are constructed and seen to form a
supermultiplet structure.Comment: 17 pages, LaTeX; Title, abstract and introduction changes, references
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