736 research outputs found
Inert states of spin-S systems
We present a simple but efficient geometrical method for determining the
inert states of spin-S systems. It can be used if the system is described by a
spin vector of a spin-S particle and its energy is invariant in spin rotations
and phase changes. Our method is applicable to an arbitrary S and it is based
on the representation of a pure spin state of a spin-S particle in terms of 2S
points on the surface of a sphere. We use this method to find candidates for
some of the ground states of spinor Bose-Einstein condensates.Comment: 4 pages, 2 figures, minor changes, references added, typos correcte
Induced superconductivity in noncuprate layers of the BiSrCaCuO high-temperature superconductor: Modeling of scanning tunneling spectra
We analyze how the coherence peaks observed in Scanning Tunneling
Spectroscopy (STS) of cuprate high temperature superconductors are transferred
from the cuprate layer to the oxide layers adjacent to the STS microscope tip.
For this purpose, we have carried out a realistic multiband calculation for the
superconducting state of BiSrCaCuO (Bi2212) assuming a
short range d-wave pairing interaction confined to the nearest-neighbor Cu
orbitals. The resulting anomalous matrix elements of the Green's
function allow us to monitor how pairing is then induced not only within the
cuprate bilayer but also within and across other layers and sites. The symmetry
properties of the various anomalous matrix elements and the related selection
rules are delineated.Comment: 9 pages, 2 figures. Accepted for publication in Phys. Rev.
Shortcut to a Fermi-Degenerate Gas of Molecules via Cooperative Association
We theoretically examine the creation of a Fermi-degenerate gas of molecules
via a photoassociation or Feshbach resonance applied to a degenerate Bose-Fermi
mixture of atoms. This problem raises an interest because, unlike bosons,
fermions in general do not behave cooperatively, so that the collective
conversion of, say, two million atoms into one million molecules is not to be
expected. Nevertheless, we find that the coupled Fermi system displays
collective Rabi-like oscillations and adiabatic passage between atoms and
molecules, thereby mimicking Bose-Einstein statistics. Cooperative association
of a degenerate mixture of Bose and Fermi gases could therefore serve as a
shortcut to a degenerate gas of Fermi molecules.Comment: 4 pages, 2 figures, submitted to PRL; v2: expanded intro, added
discussion on neglect of collisions and when mimicking should occu
Temporal Interferometry: A Mechanism for Controlling Qubit Transitions During Twisted Rapid Passage with Possible Application to Quantum Computing
In an adiabatic rapid passage experiment, the Bloch vector of a two-level
system (qubit) is inverted by slowly inverting an external field to which it is
coupled, and along which it is initially aligned. In twisted rapid passage, the
external field is allowed to twist around its initial direction with azimuthal
angle at the same time that it is inverted. For polynomial twist:
. We show that for , multiple avoided crossings
can occur during the inversion of the external field, and that these crossings
give rise to strong interference effects in the qubit transition probability.
The transition probability is found to be a function of the twist strength ,
which can be used to control the time-separation of the avoided crossings, and
hence the character of the interference. Constructive and destructive
interference are possible. The interference effects are a consequence of the
temporal phase coherence of the wavefunction. The ability to vary this
coherence by varying the temporal separation of the avoided crossings renders
twisted rapid passage with adjustable twist strength into a temporal
interferometer through which qubit transitions can be greatly enhanced or
suppressed. Possible application of this interference mechanism to construction
of fast fault-tolerant quantum CNOT and NOT gates is discussed.Comment: 29 pages, 16 figures, submitted to Phys. Rev.
Cold collisions between atoms in optical lattices
We have simulated binary collisions between atoms in optical lattices during
Sisyphus cooling. Our Monte Carlo Wave Function simulations show that the
collisions selectively accelerate mainly the hotter atoms in the thermal
ensemble, and thus affect the steady state which one would normally expect to
reach in Sisyphus cooling without collisions.Comment: 4 pages, 1 figur
Exact Soliton-like Solutions of the Radial Gross-Pitaevskii Equation
We construct exact ring soliton-like solutions of the cylindrically symmetric
(i.e., radial) Gross- Pitaevskii equation with a potential, using the
similarity transformation method. Depending on the choice of the allowed free
functions, the solutions can take the form of stationary dark or bright rings
whose time dependence is in the phase dynamics only, or oscillating and
bouncing solutions, related to the second Painlev\'e transcendent. In each case
the potential can be chosen to be time-independent.Comment: 8 pages, 7 figures. Version 2: stability analysis of the dark
solutio
Sudden death and sudden birth of entanglement in common structured reservoirs
We study the exact entanglement dynamics of two qubits in a common structured
reservoir. We demonstrate that, for certain classes of entangled states,
entanglement sudden death occurs, while for certain initially factorized
states, entanglement sudden birth takes place. The backaction of the
non-Markovian reservoir is responsible for revivals of entanglement after
sudden death has occurred, and also for periods of disentanglement following
entanglement sudden birth.Comment: 4 pages, 2 figure
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