4,227 research outputs found
Directed transport and Floquet analysis for a periodically kicked wavepacket at a quantum resonance
The dynamics of a kicked quantum mechanical wavepacket at a quantum resonance
is studied in the framework of Floquet analysis. It is seen how a directed
current can be created out of a homogeneous initial state at certain resonances
in an asymmetric potential. The almost periodic parameter dependence of the
current is found to be connected with level crossings in the Floquet spectrum.Comment: 8 pages, 4 figures, submitted to Phys. Rev.
Calculation of collective modes for the Bose-Hubbard model with confinement
The collective excitations in the Bose-Hubbard model in a trap are studied by
means of numerical diagonalization in one dimension. The strength function is
calculated for monopole and dipole perturbations, and moments of the strength
function are utilized in order to obtain information about the collective
behavior under external forces. In the superfluid regime, the spectrum is found
to be exhausted by one single frequency, while in systems that contain a Mott
insulating plateau several frequencies are excited. An explanation of recent
experimental findings in terms of a Mott plateau is suggested.Comment: 4 pages, 5 figures; submitted to PRA. One figure change
In search of an evolutionary coding style
In the near future, all the human genes will be identified. But understanding
the functions coded in the genes is a much harder problem. For example, by
using block entropy, one has that the DNA code is closer to a random code then
written text, which in turn is less ordered then an ordinary computer code; see
\cite{schmitt}.
Instead of saying that the DNA is badly written, using our programming
standards, we might say that it is written in a different style -- an
evolutionary style.
We will suggest a way to search for such a style in a quantified manner by
using an artificial life program, and by giving a definition of general codes
and a definition of style for such codes.Comment: 14 pages, 7 postscript figure
Ratchet effect for cold atoms in an optical lattice
The realization of a directed current for a quantum particle in a flashing
asymmetric potential is studied. It is found that a positive current, i.e. in
the direction expected for a conventional diffusive ratchet, can be attained at
short times in the limit where the potential is weak and quantum diffusion
dominates, while current reversal is obtained for stronger potentials. A single
parameter, the ratio between the kicking frequency and the optical lattice
potential strength, determines both the degree to which quantum effects
dominate, and the possibility of obtaining a ratchet current. The effect should
be readily observable in experiments.Comment: 4 pages, 3 figure
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