314 research outputs found
Coherent Graphene Devices: Movable Mirrors, Buffers and Memories
We theoretically report that, at a sharp electrostatic step potential in
graphene, massless Dirac fermions can obtain Goos-H\"{a}nchen-like shifts under
total internal reflection. Based on these results, we study the coherent
propagation of the quasiparticles along a sharp graphene \emph{p-n-p} waveguide
and derive novel dispersion relations for the guided modes. Consequently,
coherent graphene devices (e.g. movable mirrors, buffers and memories) induced
only by the electric field effect can be proposed.Comment: 12 pages, 5 figure
Superradiance in spin- particles: Effects of multiple levels
We study the superradiance dynamics in a dense system of atoms each of which
can be generally a spin- particle with an arbitrary half-integer. We
generalize Dicke's superradiance point of view to multiple-level systems, and
compare the results based on a novel approach we have developed in {[}Yelin
\textit{et al.}, arXiv:quant-ph/0509184{]}. Using this formalism we derive an
effective two-body description that shows cooperative and collective effects
for spin- particles, taking into account the coherence of transitions
between different atomic levels. We find that the superradiance, which is
well-known as a many-body phenomenon, can also be modified by multiple level
effects. We also discuss the feasibility and propose that our approach can be
applied to polar molecules, for their vibrational states have multi-level
structure which is partially harmonic.Comment: 11 pages, 7 figure
Fast mode of rotating atoms in one-dimensional lattice rings
We study the rotation of atoms in one-dimensional lattice rings. In
particular, the "fast mode", where the ground state atoms rotate faster than
the stirring rotating the atoms, is studied both analytically and numerically.
The conditions for the transition to the fast mode are found to be very
different from that in continuum rings. We argue that these transition
frequencies remain unchanged for bosonic condensates described in a mean field.
We show that Fermionic interaction and filling factor have a significant effect
on the transition to the fast mode, and Pauli principle may suppress it
altogether.Comment: 4 pages, 5 figure
Diffusion induced decoherence of stored optical vortices
We study the coherence properties of optical vortices stored in atomic
ensembles. In the presence of thermal diffusion, the topological nature of
stored optical vortices is found not to guarantee slow decoherence. Instead the
stored vortex state has decoherence surprisingly larger than the stored
Gaussian mode. Generally, the less phase gradient, the more robust for stored
coherence against diffusion. Furthermore, calculation of coherence factor shows
that the center of stored vortex becomes completely incoherent once diffusion
begins and, when reading laser is applied, the optical intensity at the center
of the vortex becomes nonzero. Its implication for quantum information is
discussed. Comparison of classical diffusion and quantum diffusion is also
presented.Comment: 5 pages, 2 figure
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