8,932 research outputs found
Invariant and polynomial identities for higher rank matrices
We exhibit explicit expressions, in terms of components, of discriminants,
determinants, characteristic polynomials and polynomial identities for matrices
of higher rank. We define permutation tensors and in term of them we construct
discriminants and the determinant as the discriminant of order , where
is the dimension of the matrix. The characteristic polynomials and the
Cayley--Hamilton theorem for higher rank matrices are obtained there from
Universal field equations for metric-affine theories of gravity
We show that almost all metric--affine theories of gravity yield Einstein
equations with a non--null cosmological constant . Under certain
circumstances and for any dimension, it is also possible to incorporate a Weyl
vector field and therefore the presence of an anisotropy. The viability
of these field equations is discussed in view of recent astrophysical
observations.Comment: 13 pages. This is a copy of the published paper. We are posting it
here because of the increasing interest in f(R) theories of gravit
S-matrix poles and the second virial coefficient
For cutoff potentials, a condition which is not a limitation for the
calculation of physical systems, the S-matrix is meromorphic. We can express it
in terms of its poles, and then calculate the quantum mechanical second virial
coefficient of a neutral gas.
Here, we take another look at this approach, and discuss the feasibility,
attraction and problems of the method. Among concerns are the rate of
convergence of the 'pole' expansion and the physical significance of the
'higher' poles.Comment: 20 pages, 8 tables, submitted to J. Mol. Phy
Mapping auroral activity with Twitter
Twitter is a popular, publicly-accessible, social media service that has proven useful in mapping large-scale events in real-time. In this study, for the first time, the use of Twitter as a measure of auroral activity is investigated. Peaks in the number of aurora-related tweets are found to frequently coincide with geomagnetic disturbances (detection rate of 91%). Additionally, the number of daily aurora-related tweets is found to strongly correlate with several auroral strength proxies (ravg â 0.7). An examination is made of the bias for location and time of day within Twitter data, and a first order correction of these effects is presented. Overall, the results suggest that Twitter can provide both specific details about an individual aurora and accurate real-time indication of when, and even from where, an aurora is visible
Phase estimation for thermal Gaussian states
We give the optimal bounds on the phase-estimation precision for mixed
Gaussian states in the single-copy and many-copy regimes. Specifically, we
focus on displaced thermal and squeezed thermal states. We find that while for
displaced thermal states an increase in temperature reduces the estimation
fidelity, for squeezed thermal states a larger temperature can enhance the
estimation fidelity. The many-copy optimal bounds are compared with the minimum
variance achieved by three important single-shot measurement strategies. We
show that the single-copy canonical phase measurement does not always attain
the optimal bounds in the many-copy scenario. Adaptive homodyning schemes do
attain the bounds for displaced thermal states, but for squeezed states they
yield fidelities that are insensitive to temperature variations and are,
therefore, sub-optimal. Finally, we find that heterodyne measurements perform
very poorly for pure states but can attain the optimal bounds for sufficiently
mixed states. We apply our results to investigate the influence of losses in an
optical metrology experiment. In the presence of losses squeezed states cease
to provide Heisenberg limited precision and their performance is close to that
of coherent states with the same mean photon number.Comment: typos correcte
Secrecy content of two-qubit states
We analyze the set of two-qubit states from which a secret key can be
extracted by single-copy measurements plus classical processing of the
outcomes. We introduce a key distillation protocol and give the corresponding
necessary and sufficient condition for positive key extraction. Our results
imply that the critical error rate derived by Chau, Phys. Rev. A {\bf 66},
060302 (2002), for a secure key distribution using the six-state scheme is
tight. Remarkably, an optimal eavesdropping attack against this protocol does
not require any coherent quantum operation.Comment: 5 pages, RevTe
Ethnography of Language Planning and Policy
A decade ago, Hornberger & Johnson proposed that the ethnography of language planning and policy (ELPP) offers a useful way to understand how people create, interpret, and at times resist language policy and planning (LPP). They envisioned ethnographic investigation of layered LPP ideological and implementational spaces, taking up Hornberger\u27s plea five years earlier for language users, educators, and researchers to fill up and wedge open ideological and implementational spaces for multiple languages, literacies, identities, and practices to flourish and grow rather than dwindle and disappear. With roots going back to the 1980s and 1990s, ethnographic research in LPP had been gathering momentum since the turn of the millennium. This review encompasses selected ethnographic LPP research since 2000, exploring affordances and constraints of this research in yielding comparative and cumulative findings on how people interpret and engage with LPP initiatives. We highlight how common-sense wisdom about the perennial gap between policy and practice is given nuance through ethnographic research that identifies and explores intertwining dynamics of top-down and bottom-up LPP activities and processes, monoglossic and heteroglossic language ideologies and practices, potential equality and actual inequality of languages, and critical and transformative LPP research paradigms
On the geometry of four qubit invariants
The geometry of four-qubit entanglement is investigated. We replace some of
the polynomial invariants for four-qubits introduced recently by new ones of
direct geometrical meaning. It is shown that these invariants describe four
points, six lines and four planes in complex projective space . For
the generic entanglement class of stochastic local operations and classical
communication they take a very simple form related to the elementary symmetric
polynomials in four complex variables. Moreover, their magnitudes are
entanglement monotones that fit nicely into the geometric set of -qubit ones
related to Grassmannians of -planes found recently. We also show that in
terms of these invariants the hyperdeterminant of order 24 in the four-qubit
amplitudes takes a more instructive form than the previously published
expressions available in the literature. Finally in order to understand two,
three and four-qubit entanglement in geometric terms we propose a unified
setting based on furnished with a fixed quadric.Comment: 19 page
Aurorasaurus:a citizen science platform for viewing and reporting the aurora
A new, citizen science based, aurora observing and reporting platform has been developed with the primary aim of collecting auroral observations made by the general public to further improve the modeling of the aurora. In addition, the real-time ability of this platform facilitates the combination of citizen science observations with auroral oval models to improve auroral visibility nowcasting. Aurorasaurus provides easily understandable aurora information, basic gamification, and real-time location-based notification of verified aurora activity to engage citizen scientists. The Aurorasaurus project is one of only a handful of space weather citizen science projects and can provide useful results for the space weather and citizen science communities. Early results are promising with over 2,000 registered users submitting over 1,000 aurora observations and verifying over 1,700 aurora sightings posted on Twitter
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
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