2,669 research outputs found
Mapping the Evolution of Optically-Generated Rotational Wavepackets in a Room Temperature Ensemble of D
A coherent superposition of rotational states in D has been excited by
nonresonant ultrafast (12 femtosecond) intense (2 10
Wcm) 800 nm laser pulses leading to impulsive dynamic alignment.
Field-free evolution of this rotational wavepacket has been mapped to high
temporal resolution by a time-delayed pulse, initiating rapid double
ionization, which is highly sensitive to the angle of orientation of the
molecular axis with respect to the polarization direction, . The
detailed fractional revivals of the neutral D wavepacket as a function of
and evolution time have been observed and modelled theoretically.Comment: 4 pages, 3 figures. Accepted for publication in Phys. Rev. A. Full
reference to follow.
Hopf algebras and characters of classical groups
Schur functions provide an integral basis of the ring of symmetric functions.
It is shown that this ring has a natural Hopf algebra structure by identifying
the appropriate product, coproduct, unit, counit and antipode, and their
properties. Characters of covariant tensor irreducible representations of the
classical groups GL(n), O(n) and Sp(n) are then expressed in terms of Schur
functions, and the Hopf algebra is exploited in the determination of
group-subgroup branching rules and the decomposition of tensor products. The
analysis is carried out in terms of n-independent universal characters. The
corresponding rings, CharGL, CharO and CharSp, of universal characters each
have their own natural Hopf algebra structure. The appropriate product,
coproduct, unit, counit and antipode are identified in each case.Comment: 9 pages. Uses jpconf.cls and jpconf11.clo. Presented by RCK at
SSPCM'07, Myczkowce, Poland, Sept 200
Zeros of Rydberg-Rydberg Foster Interactions
Rydberg states of atoms are of great current interest for quantum
manipulation of mesoscopic samples of atoms. Long-range Rydberg-Rydberg
interactions can inhibit multiple excitations of atoms under the appropriate
conditions. These interactions are strongest when resonant collisional
processes give rise to long-range C_3/R^3 interactions. We show in this paper
that even under resonant conditions C_3 often vanishes so that care is required
to realize full dipole blockade in micron-sized atom samples.Comment: 10 pages, 4 figures, submitted to J. Phys.
Stochastic Pulse Switching in a Degenerate Resonant Optical Medium
Using the idealized integrable Maxwell-Bloch model, we describe random
optical-pulse polarization switching along an active optical medium in the
Lambda-configuration with disordered occupation numbers of its lower energy
sub-level pair. The description combines complete integrability and stochastic
dynamics. For the single-soliton pulse, we derive the statistics of the
electric-field polarization ellipse at a given point along the medium in closed
form. If the average initial population difference of the two lower sub-levels
vanishes, we show that the pulse polarization will switch intermittently
between the two circular polarizations as it travels along the medium. If this
difference does not vanish, the pulse will eventually forever remain in the
circular polarization determined by which sub-level is more occupied on
average. We also derive the exact expressions for the statistics of the
polarization-switching dynamics, such as the probability distribution of the
distance between two consecutive switches and the percentage of the distance
along the medium the pulse spends in the elliptical polarization of a given
orientation in the case of vanishing average initial population difference. We
find that the latter distribution is given in terms of the well-known arcsine
law
Cognitive architectures as Lakatosian research programmes: two case studies
Cognitive architectures - task-general theories of the structure and function of the complete cognitive system - are sometimes argued to be more akin to frameworks or belief systems than scientific theories. The argument stems from the apparent non-falsifiability of existing cognitive architectures. Newell was aware of this criticism and argued that architectures should be viewed not as theories subject to Popperian falsification, but rather as Lakatosian research programs based on cumulative growth. Newell's argument is undermined because he failed to demonstrate that the development of Soar, his own candidate architecture, adhered to Lakatosian principles. This paper presents detailed case studies of the development of two cognitive architectures, Soar and ACT-R, from a Lakatosian perspective. It is demonstrated that both are broadly Lakatosian, but that in both cases there have been theoretical progressions that, according to Lakatosian criteria, are pseudo-scientific. Thus, Newell's defense of Soar as a scientific rather than pseudo-scientific theory is not supported in practice. The ACT series of architectures has fewer pseudo-scientific progressions than Soar, but it too is vulnerable to accusations of pseudo-science. From this analysis, it is argued that successive versions of theories of the human cognitive architecture must explicitly address five questions to maintain scientific credibility
An Absolute Flux Density Measurement of the Supernova Remnant Casseopia A at 32 GHz
We report 32 GHz absolute flux density measurements of the supernova remnant
Cas A, with an accuracy of 2.5%. The measurements were made with the 1.5-meter
telescope at the Owens Valley Radio Observatory. The antenna gain had been
measured by NIST in May 1990 to be .
Our observations of Cas A in May 1998 yield . We also report absolute flux density measurements of 3C48, 3C147, 3C286,
Jupiter, Saturn and Mars.Comment: 30 pages, 4 figures; accepted for publication by AJ. Revised
systematic error budget, corrected typos, and added reference
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