8,196 research outputs found
Searching for Stars in Compact High-Velocity Clouds. II
We address the hypothesis that High Velocity Clouds correspond to the
"missing" dwarf galaxies of the Local Group predicted by cosmological
simulations. To this end, we present optical and near-infrared photometry of
five additional High Velocity Clouds, one of which produces Lyman series
absorption on the sight line towards the Quasar Ton S210, with sufficient
resolution and sensitivity to enable the detection of an associated stellar
content. We do not detect significant stellar populations intrinsic to any of
the five clouds. In combination with the results from our paper I, which had
yielded non detections of stellar content in another five cases, we find that
there is a 50% chance of getting a null result in ten trials if fewer than 7%
of all High Velocity Clouds contain stars. We conclude that the population of
High Velocity Clouds is an unlikely repository for the "missing" dwarfs of the
Local Group.Comment: 6 pages, 3 figures. submitted to MNRA
Searching for Stars in Compact High-Velocity Clouds. I First Results from VLT and 2MASS
We investigate the hypothesis that compact high-velocity clouds (CHVC) are
the "missing" dwarf galaxies of the Local Group, by searching them for
populations of resolved stars. To this end we conducted two distinct tests
based on optical and near-infrared single-star photometry. The optical and the
near-infrared experiments complement one another; the optical data help us to
rule out distant populations but they are restricted to the central regions of
the gas distributions, whereas the near-infrared photometry allows us to set
limits on nearby populations spread over the typical cloud size. First, we
discuss deep optical single-star photometry of five CHVCs in the V and I
filters, obtained with the FORS instrument at the Very Large Telecope (VLT). We
find that their optical colour-magnitude diagrams are indistinguishable from
that of a population of Galactic stars, and attribute all of the resolved stars
to Galactic foreground. We present simulations which address the question of
how much of a "normal" dwarf-galaxy type population we might hide in the data.
A Kolmogorov-Smirnov test allows us to set very stringent limits on the absence
of a resolved stellar population in CHVCs. Second, we also culled near infrared
single-star photometry in the J, H, and K_S bands for four of the CHVCs from
the Two Micron All Sky Survey (2MASS). The infrared data do not reveal any
stellar contents in the CHVCs which resembles that of nearby dwarf galaxies
either, and are explained with Galactic foreground as well. We interpret our
null detections to indicate that the five CHVCs investigated by us do not host
an associated stellar content which is similar to that of the known dwarf
galaxies of the Local Group. These CHVCs are very likely pure hydrogen clouds
in which no star formation has taken place over cosmic time.Comment: 13 pages, 10 figures, accepted for publication by MNRA
Relative "-Numerical Ranges for Applications in Quantum Control and Quantum Information
Motivated by applications in quantum information and quantum control, a new
type of "-numerical range, the relative "-numerical range denoted
, is introduced. It arises upon replacing the unitary group U(N) in
the definition of the classical "-numerical range by any of its compact and
connected subgroups .
The geometric properties of the relative "-numerical range are analysed in
detail. Counterexamples prove its geometry is more intricate than in the
classical case: e.g. is neither star-shaped nor simply-connected.
Yet, a well-known result on the rotational symmetry of the classical
"-numerical range extends to , as shown by a new approach based on
Lie theory. Furthermore, we concentrate on the subgroup , i.e. the -fold tensor product of SU(2),
which is of particular interest in applications. In this case, sufficient
conditions are derived for being a circular disc centered at
origin of the complex plane. Finally, the previous results are illustrated in
detail for .Comment: accompanying paper to math-ph/070103
Optimal Control for Generating Quantum Gates in Open Dissipative Systems
Optimal control methods for implementing quantum modules with least amount of
relaxative loss are devised to give best approximations to unitary gates under
relaxation. The potential gain by optimal control using relaxation parameters
against time-optimal control is explored and exemplified in numerical and in
algebraic terms: it is the method of choice to govern quantum systems within
subspaces of weak relaxation whenever the drift Hamiltonian would otherwise
drive the system through fast decaying modes. In a standard model system
generalising decoherence-free subspaces to more realistic scenarios,
openGRAPE-derived controls realise a CNOT with fidelities beyond 95% instead of
at most 15% for a standard Trotter expansion. As additional benefit it requires
control fields orders of magnitude lower than the bang-bang decouplings in the
latter.Comment: largely expanded version, superseedes v1: 10 pages, 5 figure
The landscape of quantum transitions driven by single-qubit unitary transformations with implications for entanglement
This paper considers the control landscape of quantum transitions in
multi-qubit systems driven by unitary transformations with single-qubit
interaction terms. The two-qubit case is fully analyzed to reveal the features
of the landscape including the nature of the absolute maximum and minimum, the
saddle points and the absence of traps. The results permit calculating the
Schmidt state starting from an arbitrary two-qubit state following the local
gradient flow. The analysis of multi-qubit systems is more challenging, but the
generalized Schmidt states may also be located by following the local gradient
flow. Finally, we show the relation between the generalized Schmidt states and
the entanglement measure based on the Bures distance
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