15,803 research outputs found
Fermi Surface of Metallic VO from Angle-Resolved Photoemission: Mid-level Filling of Bands
Using angle resolved photoemission spectroscopy (ARPES) we report the first
band dispersions and distinct features of the bulk Fermi surface (FS) in the
paramagnetic metallic phase of the prototypical metal-insulator transition
material VO. Along the -axis we observe both an electron pocket and
a triangular hole-like FS topology, showing that both V 3 and
states contribute to the FS. These results challenge the existing
correlation-enhanced crystal field splitting theoretical explanation for the
transition mechanism and pave the way for the solution of this mystery.Comment: 5 pages, 4 figures plus supplement 12 pages, 3 figures, 1 tabl
Chaotic hysteresis in an adiabatically oscillating double well
We consider the motion of a damped particle in a potential oscillating slowly
between a simple and a double well. The system displays hysteresis effects
which can be of periodic or chaotic type. We explain this behaviour by
computing an analytic expression of a Poincar'e map.Comment: 4 pages RevTeX, 3 PS figs, uses psfig.sty. Submitted to Phys. Rev.
Letters. PS file also available at
http://dpwww.epfl.ch/instituts/ipt/berglund.htm
Catalysis in non--local quantum operations
We show how entanglement can be used, without being consumed, to accomplish
unitary operations that could not be performed with out it. When applied to
infinitesimal transformations our method makes equivalent, in the sense of
Hamiltonian simulation, a whole class of otherwise inequivalent two-qubit
interactions. The new catalysis effect also implies the asymptotic equivalence
of all such interactions.Comment: 4 pages, revte
High-energy scissors mode
All the orbital M1 excitations, at both low and high energies, obtained from
a rotationally invariant QRPA, represent the fragmented scissors mode. The
high-energy M1 strength is almost purely orbital and resides in the region of
the isovector giant quadrupole resonance. In heavy deformed nuclei the
high-energy scissors mode is strongly fragmented between 17 and 25 MeV (with
uncertainties arising from the poor knowledge of the isovector potential). The
coherent scissors motion is hindered by the fragmentation and for single transitions in this region. The cross
sections for excitations above 17 MeV are one order of magnitude larger for E2
than for M1 excitations even at backward angles.Comment: 20 pages in RevTEX, 5 figures (uuencoded,put with 'figures') accepted
for publication in Phys.Rev.
Using entanglement improves precision of quantum measurements
We show how entanglement can be used to improve the estimation of an unknown
transformation. Using entanglement is always of benefit, in improving either
the precision or the stability of the measurement. Examples relevant for
applications are illustrated, for either qubits and continuous variable
Relativistic quantum coin tossing
A relativistic quantum information exchange protocol is proposed allowing two
distant users to realize ``coin tossing'' procedure. The protocol is based on
the point that in relativistic quantum theory reliable distinguishing between
the two orthogonal states generally requires a finite time depending on the
structure of these states.Comment: 6 pages, no figure
Competing electric and magnetic excitations in backward electron scattering from heavy deformed nuclei
Important contributions to the cross sections of
low-lying orbital excitations are found in heavy deformed nuclei, arising
from the small energy separation between the two excitations with and 1, respectively. They are studied microscopically in QRPA using
DWBA. The accompanying response is negligible at small momentum transfer
but contributes substantially to the cross sections measured at for fm ( MeV)
and leads to a very good agreement with experiment. The electric response is of
longitudinal type for but becomes almost purely
transverse for larger backward angles. The transverse response
remains comparable with the response for fm
( MeV) and even dominant for MeV. This happens even at
large backward angles , where the dominance is
limited to the lower region.Comment: RevTeX, 19 pages, 8 figures included Accepted for publication in Phys
Rev
Constraining dark energy with Sunyaev-Zel'dovich cluster surveys
We discuss the prospects of constraining the properties of a dark energy
component, with particular reference to a time varying equation of state, using
future cluster surveys selected by their Sunyaev-Zel'dovich effect. We compute
the number of clusters expected for a given set of cosmological parameters and
propogate the errors expected from a variety of surveys. In the short term they
will constrain dark energy in conjunction with future observations of type Ia
supernovae, but may in time do so in their own right.Comment: 5 pages, 3 figures, 1 table, version accepted for publication in PR
The spike train statistics for consonant and dissonant musical accords
The simple system composed of three neural-like noisy elements is considered.
Two of them (sensory neurons or sensors) are stimulated by noise and periodic
signals with different ratio of frequencies, and the third one (interneuron)
receives the output of these two sensors and noise. We propose the analytical
approach to analysis of Interspike Intervals (ISI) statistics of the spike
train generated by the interneuron. The ISI distributions of the sensory
neurons are considered to be known. The frequencies of the input sinusoidal
signals are in ratios, which are usual for music. We show that in the case of
small integer ratios (musical consonance) the input pair of sinusoids results
in the ISI distribution appropriate for more regular output spike train than in
a case of large integer ratios (musical dissonance) of input frequencies. These
effects are explained from the viewpoint of the proposed theory.Comment: 22 pages, 6 figure
Trading quantum for classical resources in quantum data compression
We study the visible compression of a source E of pure quantum signal states,
or, more formally, the minimal resources per signal required to represent
arbitrarily long strings of signals with arbitrarily high fidelity, when the
compressor is given the identity of the input state sequence as classical
information. According to the quantum source coding theorem, the optimal
quantum rate is the von Neumann entropy S(E) qubits per signal.
We develop a refinement of this theorem in order to analyze the situation in
which the states are coded into classical and quantum bits that are quantified
separately. This leads to a trade--off curve Q(R), where Q(R) qubits per signal
is the optimal quantum rate for a given classical rate of R bits per signal.
Our main result is an explicit characterization of this trade--off function
by a simple formula in terms of only single signal, perfect fidelity encodings
of the source. We give a thorough discussion of many further mathematical
properties of our formula, including an analysis of its behavior for group
covariant sources and a generalization to sources with continuously
parameterized states. We also show that our result leads to a number of
corollaries characterizing the trade--off between information gain and state
disturbance for quantum sources. In addition, we indicate how our techniques
also provide a solution to the so--called remote state preparation problem.
Finally, we develop a probability--free version of our main result which may be
interpreted as an answer to the question: ``How many classical bits does a
qubit cost?'' This theorem provides a type of dual to Holevo's theorem, insofar
as the latter characterizes the cost of coding classical bits into qubits.Comment: 51 pages, 7 figure
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