15,934 research outputs found
Active optical clock based on four-level quantum system
Active optical clock, a new conception of atomic clock, has been proposed
recently. In this report, we propose a scheme of active optical clock based on
four-level quantum system. The final accuracy and stability of two-level
quantum system are limited by second-order Doppler shift of thermal atomic
beam. To three-level quantum system, they are mainly limited by light shift of
pumping laser field. These limitations can be avoided effectively by applying
the scheme proposed here. Rubidium atom four-level quantum system, as a typical
example, is discussed in this paper. The population inversion between
and states can be built up at a time scale of s.
With the mechanism of active optical clock, in which the cavity mode linewidth
is much wider than that of the laser gain profile, it can output a laser with
quantum-limited linewidth narrower than 1 Hz in theory. An experimental
configuration is designed to realize this active optical clock.Comment: 5 page
Navigation in a small world with local information
It is commonly known that there exist short paths between vertices in a
network showing the small-world effect. Yet vertices, for example, the
individuals living in society, usually are not able to find the shortest paths,
due to the very serious limit of information. To theoretically study this
issue, here the navigation process of launching messages toward designated
targets is investigated on a variant of the one-dimensional small-world network
(SWN). In the network structure considered, the probability of a shortcut
falling between a pair of nodes is proportional to , where is
the lattice distance between the nodes. When , it reduces to the SWN
model with random shortcuts. The system shows the dynamic small-world (SW)
effect, which is different from the well-studied static SW effect. We study the
effective network diameter, the path length as a function of the lattice
distance, and the dynamics. They are controlled by multiple parameters, and we
use data collapse to show that the parameters are correlated. The central
finding is that, in the one-dimensional network studied, the dynamic SW effect
exists for . For each given value of in this
region, the point that the dynamic SW effect arises is ,
where is the number of useful shortcuts and is the average
reduced (effective) length of them.Comment: 10 pages, 5 figures, accepted for publication in Physical Review
Low-momentum Pion Enhancement Induced by Chiral Symmetry Restoration
The thermal and nonthermal pion production by sigma decay and its relation
with chiral symmetry restoration in a hot and dense matter are investigated.
The nonthermal decay into pions of sigma mesons which are popularly produced in
chiral symmetric phase leads to a low-momentum pion enhancement as a possible
signature of chiral phase transition at finite temperature and density.Comment: 3 pages, 2 figure
Connected Hopf algebras and iterated Ore extensions
We investigate when a skew polynomial extension T = R[x; {\sigma}, {\delta}]
of a Hopf algebra R admits a Hopf algebra structure, substantially generalising
a theorem of Panov. When this construction is applied iteratively in
characteristic 0 one obtains a large family of connected noetherian Hopf
algebras of finite Gelfand-Kirillov dimension, including for example all
enveloping algebras of finite dimensional solvable Lie algebras and all
coordinate rings of unipotent groups. The properties of these Hopf algebras are
investigated
Thermal and Nonthermal Pion Enhancements with Chiral Symmetry Restoration
The pion production by sigma decay and its relation with chiral symmetry
restoration in a hot and dense matter are investigated in the framework of the
Nambu-Jona-Lasinio model. The decay rate for the process sigma -> 2pion to the
lowest order in a 1/N_c expansion is calculated as a function of temperature T
and chemical potential mu. The thermal and nonthermal enhancements of pions
generated by the decay before and after the freeze-out present only in the
crossover region of the chiral symmetry transition. The strongest nonthermal
enhancement is located in the vicinity of the endpoint of the first-order
transition.Comment: Latex2e, 12 pages, 8 Postscript figures, submitted to Phys. Rev.
Visual grouping in menu interfaces
Menu interfaces often arrange options into semantic groups. This semantic structure is then usually conveyed to the user by supplementary visual grouping cues. We investigate whether these visual grouping cues actually help users locate items in menus faster, and whether there is potential for these powerful grouping cues to impede search when used inappropriately. Thirty-six participants performed known-item searches of word menus. These menus differed along three dimensions: (1) whether visual grouping cues were used, (2) whether items were semantically organized, and (3) the number of items belonging to each semantic group. Results show that the usefulness of visual grouping entirely depends on the underlying semantic structure of the menu. When menus were semantically organized, having visual grouping cues delineate the boundaries between large semantic groups resulted in the fastest search times. But when semantically unrelated items were visually grouped together, participants took far longer to locate targets. Menu designers should therefore take great care to avoid visually grouping semantically unrelated items as this has the potential to hinder menu interactions
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