2,388 research outputs found
Bipartite entangled stabilizer mutually unbiased bases as maximum cliques of Cayley graphs
We examine the existence and structure of particular sets of mutually
unbiased bases (MUBs) in bipartite qudit systems. In contrast to well-known
power-of-prime MUB constructions, we restrict ourselves to using maximally
entangled stabilizer states as MUB vectors. Consequently, these bipartite
entangled stabilizer MUBs (BES MUBs) provide no local information, but are
sufficient and minimal for decomposing a wide variety of interesting operators
including (mixtures of) Jamiolkowski states, entanglement witnesses and more.
The problem of finding such BES MUBs can be mapped, in a natural way, to that
of finding maximum cliques in a family of Cayley graphs. Some relationships
with known power-of-prime MUB constructions are discussed, and observables for
BES MUBs are given explicitly in terms of Pauli operators.Comment: 8 pages, 1 figur
Large-scale horizontal flows in the solar photosphere II: Long-term behaviour and magnetic activity response
Recently, we have developed a method useful for mapping large-scale
horizontal velocity fields in the solar photosphere. The method was developed,
tuned and calibrated using the synthetic data. Now, we applied the method to
the series of Michelson Doppler Imager (MDI) dopplergrams covering almost one
solar cycle in order to get the information about the long-term behaviour of
surface flows. We have found that our method clearly reproduces the widely
accepted properties of mean flow field components, such as torsional
oscillations and a pattern of meridional circulation. We also performed a
periodic analysis, however due to the data series length and large gaps we did
not detect any significant periods. The relation between the magnetic activity
influencing the mean zonal motion is studied. We found an evidence that the
emergence of compact magnetic regions locally accelerates the rotation of
supergranular pattern in their vicinity and that the presence of magnetic
fields generally decelerates the rotation in the equatorial region. Our results
show that active regions in the equatorial region emerge exhibiting a constant
velocity (faster by 60 +/- 9 m/s than Carrington rate) suggesting that they
emerge from the base of the surface radial shear at 0.95 R_sun, disconnect from
their magnetic roots, and slow down during their evolution.Comment: 9 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
SpBase: the sea urchin genome database and web site
SpBase is a system of databases focused on the genomic information from sea urchins and related echinoderms. It is exposed to the public through a web site served with open source software (http://spbase.org/). The enterprise was undertaken to provide an easily used collection of information to directly support experimental work on these useful research models in cell and developmental biology. The information served from the databases emerges from the draft genomic sequence of the purple sea urchin, Strongylocentrotus purpuratus and includes sequence data and genomic resource descriptions for other members of the echinoderm clade which in total span 540 million years of evolutionary time. This version of the system contains two assemblies of the purple sea urchin genome, associated expressed sequences, gene annotations and accessory resources. Search mechanisms for the sequences and the gene annotations are provided. Because the system is maintained along with the Sea Urchin Genome resource, a database of sequenced clones is also provided
Stellar differential rotation from direct starspot tracking
On the Sun, the rotation periods of individual sunspots not only trace the
latitude dependence of the surface rotation rate, but also provide clues as to
the amount of subsurface fluid shear. In this paper we present the first
measurements of stellar differential rotation made by tracking the rotation of
individual starspots with sizes comparable to the largest sunspots. To achieve
this we re-analyse four sequences of densely-sampled, high signal-to-noise
echelle spectra of AB Doradus spanning several stellar rotations in 1996
December. Using spectral subtraction, least-squares deconvolution and
matched-filter analysis, we demonstrate that it is possible to measure directly
the velocity amplitudes and rotation periods of large numbers of individual
starspots at low to intermediate latitude. We derive values for the equatorial
rotation rate and the magnitude of the surface differential rotation, both of
which are in excellent agreement with those obtained by Donati & Collier
Cameron (1997) from cross-correlation of Doppler images derived a year earlier
in 1995 December, and with a re-analysis of the 1996 data by the method of
Donati et al (2000). The differences between the rotation rates of individual
spots and the fitted differential rotation law are substantially greater than
the observational errors. The smaller spots show a greater scatter about the
mean relation than the larger ones, which suggests that buffeting by turbulent
supergranular flows could be responsible.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Regulation of gene expression is associated with tolerance of the Arctic copepod Calanus glacialis to CO2-acidified sea water
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