15,186 research outputs found
Temporal Ordering in Quantum Mechanics
We examine the measurability of the temporal ordering of two events, as well
as event coincidences. In classical mechanics, a measurement of the
order-of-arrival of two particles is shown to be equivalent to a measurement
involving only one particle (in higher dimensions). In quantum mechanics, we
find that diffraction effects introduce a minimum inaccuracy to which the
temporal order-of-arrival can be determined unambiguously. The minimum
inaccuracy of the measurement is given by dt=1/E where E is the total kinetic
energy of the two particles. Similar restrictions apply to the case of
coincidence measurements. We show that these limitations are much weaker than
limitations on measuring the time-of-arrival of a particle to a fixed location.Comment: New section added, arguing that order-of-arrival can be measured more
accurately than time-of-arrival. To appear in Journal of Physics
Quantum Monte Carlo scheme for frustrated Heisenberg antiferromagnets
When one tries to simulate quantum spin systems by the Monte Carlo method,
often the 'minus-sign problem' is encountered. In such a case, an application
of probabilistic methods is not possible. In this paper the method has been
proposed how to avoid the minus sign problem for certain class of frustrated
Heisenberg models. The systems where this method is applicable are, for
instance, the pyrochlore lattice and the Heisenberg model. The method
works in singlet sector. It relies on expression of wave functions in dimer
(pseudo)basis and writing down the Hamiltonian as a sum over plaquettes. In
such a formulation, matrix elements of the exponent of Hamiltonian are
positive.Comment: 19 LaTeX pages, 6 figures, 1 tabl
Dipoles in Graphene Have Infinitely Many Bound States
We show that in graphene charge distributions with non-vanishing dipole
moment have infinitely many bound states. The corresponding eigenvalues
accumulate at the edges of the gap faster than any power
Flux networks in metabolic graphs
A metabolic model can be represented as bipartite graph comprising linked
reaction and metabolite nodes. Here it is shown how a network of conserved
fluxes can be assigned to the edges of such a graph by combining the reaction
fluxes with a conserved metabolite property such as molecular weight. A similar
flux network can be constructed by combining the primal and dual solutions to
the linear programming problem that typically arises in constraint-based
modelling. Such constructions may help with the visualisation of flux
distributions in complex metabolic networks. The analysis also explains the
strong correlation observed between metabolite shadow prices (the dual linear
programming variables) and conserved metabolite properties. The methods were
applied to recent metabolic models for Escherichia coli, Saccharomyces
cerevisiae, and Methanosarcina barkeri. Detailed results are reported for E.
coli; similar results were found for the other organisms.Comment: 9 pages, 4 figures, RevTeX 4.0, supplementary data available (excel
On a certain class of semigroups of operators
We define an interesting class of semigroups of operators in Banach spaces,
namely, the randomly generated semigroups. This class contains as a remarkable
subclass a special type of quantum dynamical semigroups introduced by
Kossakowski in the early 1970s. Each randomly generated semigroup is
associated, in a natural way, with a pair formed by a representation or an
antirepresentation of a locally compact group in a Banach space and by a
convolution semigroup of probability measures on this group. Examples of
randomly generated semigroups having important applications in physics are
briefly illustrated.Comment: 11 page
The Hartree limit of Born's ensemble for the ground state of a bosonic atom or ion
The non-relativistic bosonic ground state is studied for quantum N-body
systems with Coulomb interactions, modeling atoms or ions made of N "bosonic
point electrons" bound to an atomic point nucleus of Z "electron" charges,
treated in Born--Oppenheimer approximation. It is shown that the (negative)
ground state energy E(Z,N) yields the monotonically growing function (E(l N,N)
over N cubed). By adapting an argument of Hogreve, it is shown that its limit
as N to infinity for l > l* is governed by Hartree theory, with the rescaled
bosonic ground state wave function factoring into an infinite product of
identical one-body wave functions determined by the Hartree equation. The proof
resembles the construction of the thermodynamic mean-field limit of the
classical ensembles with thermodynamically unstable interactions, except that
here the ensemble is Born's, with the absolute square of the ground state wave
function as ensemble probability density function, with the Fisher information
functional in the variational principle for Born's ensemble playing the role of
the negative of the Gibbs entropy functional in the free-energy variational
principle for the classical petit-canonical configurational ensemble.Comment: Corrected version. Accepted for publication in Journal of
Mathematical Physic
Solving simultaneously Dirac and Ricatti equations
We analyse the behaviour of the Dirac equation in with Lorentz scalar
potential. As the system is known to provide a physical realization of
supersymmetric quantum mechanics, we take advantage of the factorization method
in order to enlarge the restricted class of solvable problems. To be precise,
it suffices to integrate a Ricatti equation to construct one-parameter families
of solvable potentials. To illustrate the procedure in a simple but relevant
context, we resort to a model which has proved useful in showing the phenomenon
of fermion number fractionalization
O-6 Optical Property Degradation of the Hubble Space Telescope's Wide Field Camera-2 Pick Off Mirror
Degradation in the performance of optical components can be greatly affected by exposure to the space environment. Many factors can contribute to such degradation including surface contaminants; outgassing; vacuum, UV, and atomic oxygen exposure; temperature cycling; or combinations of parameters. In-situ observations give important clues to degradation processes, but there are relatively few opportunities to correlate those observations with post-flight ground analyses. The return of instruments from the Hubble Space Telescope (HST) after its final servicing mission in May 2009 provided such an opportunity. Among the instruments returned from HST was the Wide-Field Planetary Camera-2 (WFPC-2), which had been exposed to the space environment for 16 years. This work focuses on the identifying the sources of degradation in the performance of the Pick-off mirror (POM) from WFPC-2. Techniques including surface reflectivity measurements, spectroscopic ellipsometry, FTIR (and ATR-FTIR) analyses, SEM/EDS, X-ray photoelectron spectroscopy (XPS) with and without ion milling, and wet and dry physical surface sampling were performed. Destructive and contact analyses took place only after completion of the non-destructive measurements. Spectroscopic ellipsometry was then repeated to determine the extent of contaminant removal by the destructive techniques, providing insight into the nature and extent of polymerization of the contaminant layer
Casimir forces in a T operator approach
We explore the scattering approach to Casimir forces. Our main tool is the
description of Casimir energy in terms of transition operators, as presented in
Kenneth and Klich, Phys. Rev. Lett. 97, 160401 (2006). We study the convergence
properties of the formula and how to utilize it, together with scattering data
to compute the force. We illustrate the approach by describing the force
between scatterers in 1d and 3d,, and in particular show how it may be applied
in order to study the interaction between two spherical bodies at all
distances
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