27,748 research outputs found
Effects of the sintering atmosphere on the superconductivity of SmFeAsO1-xFx compounds
A series of SmFeAsO1-xFx samples were sintered in quartz tubes filled with
air of different pressures. The effects of the sintering atmosphere on the
superconductivity were systematically investigated. The SmFeAsO1-xFx system
maintains a transition temperature (Tc) near 50 K until the concentration of
oxygen in quartz tubes increases to a certain threshold, after which Tc
decreases dramatically. Fluorine losses, whether due to vaporization, reactions
with starting materials, and reactions with oxygen, proved to be detrimental to
the superconductivity of this material. The deleterious effects of the oxygen
in the sintering atmosphere were also discussed in detail.Comment: 9 pages, 5 figure
Quantum Bit Regeneration
Decoherence and loss will limit the practicality of quantum cryptography and
computing unless successful error correction techniques are developed. To this
end, we have discovered a new scheme for perfectly detecting and rejecting the
error caused by loss (amplitude damping to a reservoir at T=0), based on using
a dual-rail representation of a quantum bit. This is possible because (1)
balanced loss does not perform a ``which-path'' measurement in an
interferometer, and (2) balanced quantum nondemolition measurement of the
``total'' photon number can be used to detect loss-induced quantum jumps
without disturbing the quantum coherence essential to the quantum bit. Our
results are immediately applicable to optical quantum computers using single
photonics devices.Comment: 4 pages, postscript only, figures available at
http://feynman.stanford.edu/qcom
Efficient spin control in high-quality-factor planar micro-cavities
A semiconductor microcavity embedding donor impurities and excited by a laser
field is modelled. By including general decay and dephasing processes, and in
particular cavity photon leakage, detailed simulations show that control over
the spin dynamics is significally enhanced in high-quality-factor cavities, in
which case picosecond laser pulses may produce spin-flip with high-fidelity
final states.Comment: 6 pages, 4 figure
Stability of superfluid Fermi gases in optical lattices
Critical velocities of superfluid Fermi gases in optical lattices are
theoretically investigated across the BCS-BEC crossover. We calculate the
excitation spectra in the presence of a superfluid flow in one- and
two-dimensional optical lattices. It is found that the spectrum of low-lying
Anderson-Bogoliubov (AB) mode exhibits a roton-like structure in the
short-wavelength region due to the strong charge density wave fluctuations, and
with increasing the superfluid velocity one of the roton-like minima reaches
zero before the single-particle spectrum does. This means that superfluid Fermi
gases in optical lattices are destabilized due to spontaneous emission of the
roton-like AB mode instead of due to Cooper pair breaking.Comment: 4 pages, 4 figures, conference proceeding for ISQM-TOKYO'0
Coating thermal noise of a finite-size cylindrical mirror
Thermal noise of a mirror is one of the limiting noise sources in the high
precision measurement such as gravitational-wave detection, and the modeling of
thermal noise has been developed and refined over a decade. In this paper, we
present a derivation of coating thermal noise of a finite-size cylindrical
mirror based on the fluctuation-dissipation theorem. The result agrees to a
previous result with an infinite-size mirror in the limit of large thickness,
and also agrees to an independent result based on the mode expansion with a
thin-mirror approximation. Our study will play an important role not only to
accurately estimate the thermal-noise level of gravitational-wave detectors but
also to help analyzing thermal noise in quantum-measurement experiments with
lighter mirrors.Comment: 13 pages, 4 figure
Nuclear spin-lattice relaxation in ferrimagnetic clusters and chains: A contrast between zero and one dimensions
Motivated by ferrimagnetic oligonuclear and chain compounds synthesized by
Caneschi et al., both of which consist of alternating manganese(II) ions and
nitronyl-nitroxide radicals, we calculate the nuclear spin-lattice relaxation
rate 1/T_1 employing a recently developed modified spin-wave theory. 1/T_1 as a
function of temperature drastically varies with the location of probe nuclei in
both clusters and chains, though the relaxation time scale is much larger in
zero dimension than in one dimension. 1/T_1 as a function of an applied field
in long chains forms a striking contrast to that in finite clusters, diverging
with decreasing field like inverse square root at low temperatures and
logarithmically at high temperatures.Comment: to be published in Phys. Rev. B 68 August 01 (2003
Dynamical tunneling in optical cavities
The lifetime of whispering gallery modes in a dielectric cavity with a
metallic inclusion is shown to fluctuate by orders of magnitude when size and
location of the inclusion are varied. We ascribe these fluctuations to
tunneling transitions between resonances quantized in different regions of
phase space. This interpretation is confirmed by a comparison of the classical
phase space structure with the Husimi distribution of the resonant modes. A
model Hamiltonian is introduced that describes the phenomenon and shows that it
can be expected in a more general class of systems.Comment: 8 pages LaTeX with 5 postscript figure
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