20,407 research outputs found
Some Remarks on Quantum Coherence
There are many striking phenomena which are attributed to
``quantum coherence''. It is natural to wonder if there are new quantum
coherence effects waiting to be discovered which could lead to interesting
results and perhaps even practical applications. A useful starting point for
such discussions is a definition of ``quantum coherence''. In this article I
give a definition of quantum coherence and use a number of illustrations to
explore the implications of this definition. I point to topics of current
interest in the fields of cosmology and quantum computation where questions of
quantum coherence arise, and I emphasize the impact that interactions with the
environment can have on quantum coherence.Comment: 25 pages plain LaTeX, no figures. More references have been added and
typos have been corrected. Journal of Modern Optics, in press.
Imperial/TP/93-94/1
The variance-covariance method: Microdosimetry in time-varying low dose-rate radiation fields
The variance-covariance method is employed at low doses and in radiation fields of low dose rates from an241Am (4 nGy/s) and a90Sr (300 nGy/s) source. The preliminary applications and results illustrate some of the potential of the method, and show that the dose average of lineal energy or energy imparted can be determined over a wide range of doses and dose rates. The dose averages obtained with the variance-covariance method in time-varying fields, for which the conventional variance method is not suitable, agree well with results obtained under the condition of constant dose rate. The results are compared to data obtained in terms of the conventional single-event measurements. The method has evident advantages, such as facility and speed of measurement
Eigenvalues of even very nice Toeplitz matrices can be unexpectedly erratic
It was shown in a series of recent publications that the eigenvalues of
Toeplitz matrices generated by so-called simple-loop symbols admit
certain regular asymptotic expansions into negative powers of . On the
other hand, recently two of the authors considered the pentadiagonal Toeplitz
matrices generated by the symbol , which does not satisfy
the simple-loop conditions, and derived asymptotic expansions of a more
complicated form. We here use these results to show that the eigenvalues of the
pentadiagonal Toeplitz matrices do not admit the expected regular asymptotic
expansion. This also delivers a counter-example to a conjecture by Ekstr\"{o}m,
Garoni, and Serra-Capizzano and reveals that the simple-loop condition is
essential for the existence of the regular asymptotic expansion.Comment: 28 pages, 7 figure
Are the reactions a challenge for the factorized Pomeron at high energies?
We would like to point to the strong violation of the putative factorized
Pomeron exchange model in the reactions in the
high-energy region where this model works fairly well in all other cases.Comment: 4 pages, LaTex, 1 fig. in postscript, minor typos corrected, to be
published in Phys. Rev. D 60, 117503 (1999
Vector field mediated models of dynamical light velocity
A vector-tensor theory of gravity that was introduced in an earlier
publication is analyzed in detail and its consequences for early universe
cosmology are examined. The multiple light cone structure of the theory
generates different speeds of gravitational and matter wave fronts, and the
contraction of these light cones produces acausal, superluminary inflation that
can resolve the initial value problems of cosmology.Comment: 16 pages, uses amsar
On Quantum Mechanical Aspects of Microtubules
We discuss possible quantum mechanical aspects of MicroTubules (MT), based on
recent developments in quantum physics.We focus on potential mechanisms for
`energy-loss-free' transport along the microtubules, which could be considered
as realizations of Fr\"ohlich's ideas on the r\^ole of solitons for
superconductivity and/or biological matter. By representing the MT arrangements
as cavities,we present a novel scenario on the formation of macroscopic (or
mesoscopic) quantum-coherent states, as a result of the
(quantum-electromagnetic) interactions of the MT dimers with the surrounding
molecules of the ordered water in the interior of the MT cylinders. We suggest
specific experiments to test the above-conjectured quantum nature of the
microtubular arrangements inside the cell. These experiments are similar in
nature to those in atomic physics, used in the detection of the Rabi-Vacuum
coupling between coherent cavity modes and atoms. Our conjecture is that a
similar Rabi-Vacuum-splitting phenomenon occurs in the MT case.Comment: 26 pages LATEX (minor typos corrected no effect on conclusions
Towards realistic implementations of a Majorana surface code
Surface codes have emerged as promising candidates for quantum information
processing. Building on the previous idea to realize the physical qubits of
such systems in terms of Majorana bound states supported by topological
semiconductor nanowires, we show that the basic code operations, namely
projective stabilizer measurements and qubit manipulations, can be implemented
by conventional tunnel conductance probes and charge pumping via
single-electron transistors, respectively. The simplicity of the access scheme
suggests that a functional code might be in close experimental reach.Comment: 5 pages, 1 p. suppl.mat, PRL in pres
- …