5,698 research outputs found
Optimal estimation of two-qubit pure-state entanglement
We present optimal measuring strategies for the estimation of the
entanglement of unknown two-qubit pure states and of the degree of mixing of
unknown single-qubit mixed states, of which N identical copies are available.
The most general measuring strategies are considered in both situations, to
conclude in the first case that a local, although collective, measurement
suffices to estimate entanglement, a non-local property, optimally.Comment: REVTEX, 9 pages, 1 figur
Formation of Episodic Magnetically Driven Radiatively Cooled Plasma Jets in the Laboratory
We report on experiments in which magnetically driven radiatively cooled
plasma jets were produced by a 1 MA, 250 ns current pulse on the MAGPIE pulsed
power facility. The jets were driven by the pressure of a toroidal magnetic
field in a ''magnetic tower'' jet configuration. This scenario is characterized
by the formation of a magnetically collimated plasma jet on the axis of a
magnetic ''bubble'', confined by the ambient medium. The use of a radial
metallic foil instead of the radial wire arrays employed in our previous work
allows for the generation of episodic magnetic tower outflows which emerge
periodically on timescales of ~30 ns. The subsequent magnetic bubbles propagate
with velocities reaching ~300 km/s and interact with previous eruptions leading
to the formation of shocks.Comment: 6 pages, 5 figures. Accepted for publication in Astrophysics & Space
Scienc
Searching for Far-Ultraviolet Auroral/Dayglow Emission from HD209458b
We present recent observations from the HST-Cosmic Origins Spectrograph aimed
at characterizing the auroral emission from the extrasolar planet HD209458b. We
obtained medium-resolution (R~18-20,000) far-ultraviolet (1150-1700A) spectra
at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar
baseline measurement at secondary eclipse. This analysis includes a catalog of
stellar emission lines and a star-subtracted spectrum of the planet. We present
an emission model for planetary H2 emission, and compare this model to the
planetary spectrum. No unambiguously identifiable atomic or molecular features
are detected, and upper limits are presented for auroral/dayglow line
strengths. An orbital velocity cross-correlation analysis finds a statistically
significant (3.8 sigma) feature at +15 (+/- 20) km/s in the rest frame of the
planet, at 1582 A. This feature is consistent with emission from H2 B-X (2-9)
P(4) (lambda_{rest} = 1581.11 A), however the physical mechanism required to
excite this transition is unclear. We compare limits on relative line strengths
seen in the exoplanet spectrum with models of ultraviolet fluorescence to
constrain the atmospheric column density of neutral hydrogen between the star
and the planetary surface. These results support models of short period
extrasolar giant planets with weak magnetic fields and extended atomic
atmospheres.Comment: Accepted to ApJ. 12 pages, 5 figures, 4 table
Numerical study of jets produced by conical wire arrays on the Magpie pulsed power generator
The aim of this work is to model the jets produced by conical wire arrays on
the MAGPIE generator, and to design and test new setups to strengthen the link
between laboratory and astrophysical jets. We performed the modelling with
direct three-dimensional magneto-hydro-dynamic numerical simulations using the
code GORGON. We applied our code to the typical MAGPIE setup and we
successfully reproduced the experiments. We found that a minimum resolution of
approximately 100 is required to retrieve the unstable character of the jet. We
investigated the effect of changing the number of wires and found that arrays
with less wires produce more unstable jets, and that this effect has magnetic
origin. Finally, we studied the behaviour of the conical array together with a
conical shield on top of it to reduce the presence of unwanted low density
plasma flows. The resulting jet is shorter and less dense.Comment: Accepted for publication in Astrophysics & Space Science. HEDLA 2010
conference procedings. Final pubblication will be available on Springe
`Classical' quantum states
We show that several classes of mixed quantum states in finite-dimensional
Hilbert spaces which can be characterized as being, in some respect, 'most
classical' can be described and analyzed in a unified way. Among the states we
consider are separable states of distinguishable particles, uncorrelated states
of indistinguishable fermions and bosons, as well as mixed spin states
decomposable into probabilistic mixtures of pure coherent states. The latter
were the subject of the recent paper by Giraud et. al., who showed that in the
lowest-dimensional, nontrivial case of spin 1, each such state can be
decomposed into a mixture of eight pure states. Using our method we prove that
in fact four pure states always suffice.Comment: revtex, 17 page
Experimental Studies of Magnetically Driven Plasma Jets
We present experimental results on the formation of supersonic, radiatively
cooled jets driven by pressure due to the toroidal magnetic field generated by
the 1.5 MA, 250 ns current from the MAGPIE generator. The morphology of the jet
produced in the experiments is relevant to astrophysical jet scenarios in which
a jet on the axis of a magnetic cavity is collimated by a toroidal magnetic
field as it expands into the ambient medium. The jets in the experiments have
similar Mach number, plasma beta and cooling parameter to those in protostellar
jets. Additionally the Reynolds, magnetic Reynolds and Peclet numbers are much
larger than unity, allowing the experiments to be scaled to astrophysical
flows. The experimental configuration allows for the generation of episodic
magnetic cavities, suggesting that periodic fluctuations near the source may be
responsible for some of the variability observed in astrophysical jets.
Preliminary measurements of kinetic, magnetic and Poynting energy of the jets
in our experiments are presented and discussed, together with estimates of
their temperature and trapped toroidal magnetic field.Comment: 7 pages, 6 figures, accepted for publication in Astrophysics & Space
Scienc
The influence of organizational social responsibility on involvement behavior in community sport membership associations
Membershipâbased associations are critical to their local communities and the overall social impact of the nonprofit sector. This study examines how organizational social responsibility within nonprofit membership associations influences positive member involvement behaviors, including volunteering, speaking positively about the club, and member loyalty. Selfâadministered online questionnaires were completed by 735 members within seven grassroots membership associations in Ontario, Canada offering communityâbased sport programs. Results show that members are somewhat aware of and felt positively about their organizationâs socially responsible efforts. Awareness of these efforts had a positive direct effect on the involvement behaviors of members, including intention to stay involved with their club and speaking positively about their club to others (i.e., word of mouth). Membersâ level of social consciousness was found to have a positive direct effect on word of mouth. Furthermore, membersâ positive evaluation of sport clubsâ socially responsible initiatives was found to partially mediate the positive relationship between social consciousness and involvement behavior, as well as partially mediate the positive relationship between awareness of those efforts and involvement behavior. Results of this research provide grassroots membership associations with an inâdepth understanding of how their organizationâs efforts toward social responsibility influence member perceptions and behaviors, which may help them focus their efforts and more effectively manage their social change agenda moving forward.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155505/1/nml21406_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155505/2/nml21406.pd
A priori probability that a qubit-qutrit pair is separable
We extend to arbitrarily coupled pairs of qubits (two-state quantum systems)
and qutrits (three-state quantum systems) our earlier study (quant-ph/0207181),
which was concerned with the simplest instance of entangled quantum systems,
pairs of qubits. As in that analysis -- again on the basis of numerical
(quasi-Monte Carlo) integration results, but now in a still higher-dimensional
space (35-d vs. 15-d) -- we examine a conjecture that the Bures/SD (statistical
distinguishability) probability that arbitrarily paired qubits and qutrits are
separable (unentangled) has a simple exact value, u/(v Pi^3)= >.00124706, where
u = 2^20 3^3 5 7 and v = 19 23 29 31 37 41 43 (the product of consecutive
primes). This is considerably less than the conjectured value of the Bures/SD
probability, 8/(11 Pi^2) = 0736881, in the qubit-qubit case. Both of these
conjectures, in turn, rely upon ones to the effect that the SD volumes of
separable states assume certain remarkable forms, involving "primorial"
numbers. We also estimate the SD area of the boundary of separable qubit-qutrit
states, and provide preliminary calculations of the Bures/SD probability of
separability in the general qubit-qubit-qubit and qutrit-qutrit cases.Comment: 9 pages, 3 figures, 2 tables, LaTeX, we utilize recent exact
computations of Sommers and Zyczkowski (quant-ph/0304041) of "the Bures
volume of mixed quantum states" to refine our conjecture
On the volume of the set of mixed entangled states II
The problem of of how many entangled or, respectively, separable states there
are in the set of all quantum states is investigated. We study to what extent
the choice of a measure in the space of density matrices describing
N--dimensional quantum systems affects the results obtained. We demonstrate
that the link between the purity of the mixed states and the probability of
entanglement is not sensitive to the measure chosen. Since the criterion of
partial transposition is not sufficient to distinguish all separable states for
N > 6, we develop an efficient algorithm to calculate numerically the
entanglement of formation of a given mixed quantum state, which allows us to
compute the volume of separable states for N=8 and to estimate the volume of
the bound entangled states in this case.Comment: 14 pages in Latex, Revtex + epsf; 7 figures in .ps included (one new
figure in the revised version, several minor changes
The evolution of magnetic tower jets in the laboratory
The evolution of laboratory produced magnetic jets is followed numerically through three-dimensional, non-ideal magnetohydrodynamic simulations. The experiments are designed to study the interaction of a purely toroidal field with an extended plasma background medium. The system is observed to evolve into a structure consisting of an approximately cylindrical magnetic cavity with an embedded magnetically confined jet on its axis. The supersonic expansion produces a shell of swept-up shocked plasma which surrounds and partially confines the magnetic tower. Currents initially flow along the walls of the cavity and in the jet but the development of current-driven instabilities leads to the disruption of the jet and a re-arrangement of the field and currents. The top of the cavity breaks-up and a well collimated, radiatively cooled, 'clumpy' jet emerges from the system
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