3,311,262 research outputs found
Quantum Computing with Globally Controlled Exchange-type Interactions
If the interaction between qubits in a quantum computer has a non-diagonal
form (e.g. the Heisenberg interaction), then one must be able to "switch it
off" in order to prevent uncontrolled propagation of states. Therefore, such QC
schemes typically demand local control of the interaction strength between each
pair of neighboring qubits. Here we demonstrate that this degree of control is
not necessary: it suffices to switch the interaction collectively - something
that can in principle be achieved by global fields rather than with local
manipulations. This observation may offer a significant simplification for
various solid state, optical lattice and NMR implementations.Comment: 3 pages inc. 3 figure
Quantum Computing in Arrays Coupled by 'Always On' Interactions
It has recently been shown that one can perform quantum computation in a
Heisenberg chain in which the interactions are 'always on', provided that one
can abruptly tune the Zeeman energies of the individual (pseudo-)spins. Here we
provide a more complete analysis of this scheme, including several
generalizations. We generalize the interaction to an anisotropic form
(incorporating the XY, or Forster, interaction as a limit), providing a proof
that a chain coupled in this fashion tends to an effective Ising chain in the
limit of far off-resonant spins. We derive the primitive two-qubit gate that
results from exploiting abrupt Zeeman tuning with such an interaction. We also
demonstrate, via numerical simulation, that the same basic scheme functions in
the case of smoothly shifted Zeeman energies. We conclude with some remarks
regarding generalisations to two- and three-dimensional arrays.Comment: 16 pages (preprint format) inc. 3 figure
Perceptions of time in relation to climate change
Time is at the heart of understanding climate change, from the perspective of both natural and social scientists. This article selectively reviews research on time perception and temporal aspects of decision making in sociology and psychology. First we briefly describe the temporal dimensions that characterize the issue of climate change. Second, we review relevant theoretical approaches and empirical findings. Then we propose an integration of these insights for the problem of climate change and discuss mismatches between the human mind, surrounding social dynamics, and climate change. Finally, we discuss the implications of this article for understanding and responding to climate change, and make suggestions on how we can use the strengths of the human mind and social dynamics to communicate climate change in its temporal context.This article is categorized under:
Climate, History, Society, Culture > Ideas and Knowledge
Perceptions, Behavior, and Communication of Climate Change > Perceptions of Climate Change
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An RXTE study of M87 and the core of the Virgo cluster
We present hard X-ray observations of the nearby radio galaxy M87 and the
core of the Virgo cluster using the Rossi X-ray Timing Explorer. These are the
first hard X-ray observations of M87 not affected by contamination from the
nearby Seyfert 2 galaxy NGC4388. Thermal emission from Virgo's intracluster
medium is clearly detected and has a spectrum indicative of kT=2.5keV plasma
with approximately 25% cosmic abundances. No non-thermal (power-law) emission
from M87 is detected in the hard X-ray band, with fluctuations in the Cosmic
X-ray Background being the limiting factor. Combining with ROSAT data, we infer
that the X-ray spectrum of the M87 core and jet must be steep (Gamma_core>1.90$
and Gamma_jet>1.75), and we discuss the implications of this result. In
particular, these results are consistent with M87 being a mis-aligned BL-Lac
object.Comment: 8 pages, 2 postscript figures. Accepted for publication in Ap
Universality in the Gravitational Stretching of Clocks, Waves and Quantum States
There are discernible and fundamental differences between clocks, waves and
physical states in classical physics. These fundamental concepts find a common
expression in the context of quantum physics in gravitational fields; matter
and light waves, quantum states and oscillator clocks become quantum synonymous
through the Planck-Einstein-de Broglie relations and the equivalence principle.
With this insight, gravitational effects on quantum systems can be simply and
accurately analyzed. Apart from providing a transparent framework for
conceptual and quantitative thinking on matter waves and quantum states in a
gravitational field, we address and resolve with clarity the recent
controversial discussions on the important issue of the relation and the
crucial difference between gravimetery using atom interferometers and the
measurement of gravitational time dilation.Comment: Gravity Research Foundation honorable mention, 201
Faint counts as a function of morphological type in a hierarchical merger model
The unprecedented resolution of the refurbished Wide Field and Planetary
Camera 2 (WFPC2) on the Hubble Space Telescope (HST) has led to major advances
in our understanding of galaxy formation. The high image quality in the Medium
Deep Survey and Hubble Deep Field has made it possible, for the first time, to
classify faint distant galaxies according to morphological type. These
observations have revealed a large population of galaxies classed as irregulars
or which show signs of recent merger activity. Their abundance rises steeply
with apparent magnitude, providing a likely explanation for the large number of
blue galaxies seen at faint magnitudes. We demonstrate that such a population
arises naturally in a model in which structure forms hierarchically and which
is dynamically dominated by cold dark matter. The number counts of irregular,
spiral and elliptical galaxies as a function of magnitude seen in the HST data
are well reproduced in this model.We present detailed predictions for the
outcome of spectroscopic follow-up observations of the HST surveys. By
measuring the redshift distributions of faint galaxies of different
morphological types, these programmes will provide a test of the hierarchical
galaxy formation paradigm and might distinguish between models with different
cosmological parameters.Comment: 5 pages, 3 postscript figures included. To be published as a Letter
in Monthly Notices of the RAS. Postscript version available at
http://star-www.dur.ac.uk/~cmb/counts.htm
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