302 research outputs found
Seam tracking performance of a Coaxial Weld Vision System and pulsed welding
This report describes a continuation of a series of tests on the Coaxial Weld Vision System at MSFC. The ability of the system to compensate for transients associated with pulsed current welding is analyzed. Using the standard image processing approach for root pass seam tracking, the system is also tested for the ability to track the toe of a previous weld bead, for tracking multiple pass weld joints. This Coaxial Weld Vision System was developed by the Ohio State University (OSU) Center for Welding Research and is a part of the Space Shuttle Main Engine Robotic Welding Development System at MSFC
Micro-mechanical oscillator ground state cooling via intracavity optical atomic excitations
We predict ground state cooling of a micro-mechanical oscillator, i.e. a
vibrating end-mirror of an optical cavity, by resonant coupling of mirror
vibrations to a narrow internal optical transition of an ensemble of two level
systems. The particles represented by a collective mesoscopic spin model
implement, together with the cavity, an efficient, frequency tailorable zero
temperature loss channel which can be turned to a gain channel of pump. As
opposed to the case of resolved-sideband cavity cooling requiring a small
cavity linewidth, one can work here with low finesses and very small cavity
volumes to enhance the light mirror and light atom coupling. The tailored loss
and gain channels provide for efficient removal of vibrational quanta and
suppress reheating. In a simple physical picture of sideband cooling, the atoms
shape the cavity profile to enhance/inhibit scattering into higher/lower energy
sidebands. The method should be applicable to other cavity based cooling
schemes for atomic and molecular gases as for molecular ensembles coupled to
stripline cavities
Scaling properties of cavity-enhanced atom cooling
We extend an earlier semiclassical model to describe the dissipative motion
of N atoms coupled to M modes inside a coherently driven high-finesse cavity.
The description includes momentum diffusion via spontaneous emission and cavity
decay. Simple analytical formulas for the steady-state temperature and the
cooling time for a single atom are derived and show surprisingly good agreement
with direct stochastic simulations of the semiclassical equations for N atoms
with properly scaled parameters. A thorough comparison with standard free-space
Doppler cooling is performed and yields a lower temperature and a cooling time
enhancement by a factor of M times the square of the ratio of the atom-field
coupling constant to the cavity decay rate. Finally it is shown that laser
cooling with negligible spontaneous emission should indeed be possible,
especially for relatively light particles in a strongly coupled field
configuration.Comment: 7 pages, 5 figure
Ultra-cold atoms in an optical cavity: two-mode laser locking to the cavity avoiding radiation pressure
The combination of ultra-cold atomic clouds with the light fields of optical
cavities provides a powerful model system for the development of new types of
laser cooling and for studying cooperative phenomena. These experiments
critically depend on the precise tuning of an incident pump laser with respect
to a cavity resonance. Here, we present a simple and reliable experimental
tuning scheme based on a two-mode laser spectrometer. The scheme uses a first
laser for probing higher-order transversal modes of the cavity having an
intensity minimum near the cavity's optical axis, where the atoms are confined
by a magnetic trap. In this way the cavity resonance is observed without
exposing the atoms to unwanted radiation pressure. A second laser, which is
phase-locked to the first one and tuned close to a fundamental cavity mode
drives the coherent atom-field dynamics.Comment: 7 pages, 7 figure
Cavity Assisted Nondestructive Laser Cooling of Atomic Qubits
We analyze two configurations for laser cooling of neutral atoms whose
internal states store qubits. The atoms are trapped in an optical lattice which
is placed inside a cavity. We show that the coupling of the atoms to the damped
cavity mode can provide a mechanism which leads to cooling of the motion
without destroying the quantum information.Comment: 12 page
Collective Sideband Cooling in an Optical Ring Cavity
We propose a cavity based laser cooling and trapping scheme, providing tight
confinement and cooling to very low temperatures, without degradation at high
particle densities. A bidirectionally pumped ring cavity builds up a resonantly
enhanced optical standing wave which acts to confine polarizable particles in
deep potential wells. The particle localization yields a coupling of the
degenerate travelling wave modes via coherent photon redistribution. This
induces a splitting of the cavity resonances with a high frequency component,
that is tuned to the anti-Stokes Raman sideband of the particles oscillating in
the potential wells, yielding cooling due to excess anti-Stokes scattering.
Tight confinement in the optical lattice together with the prediction, that
more than 50% of the trapped particles can be cooled into the motional ground
state, promise high phase space densities.Comment: 4 pages, 1 figur
Cold atoms in a high-Q ring-cavity
We report the confinement of large clouds of ultra-cold 85-Rb atoms in a
standing-wave dipole trap formed by the two counter-propagating modes of a
high-Q ring-cavity. Studying the properties of this trap we demonstrate loading
of higher-order transverse cavity modes and excite recoil-induced resonances.Comment: 4 pages, 4 figure
Manipulation of Cold Atomic Collisions by Cavity QED Effects
We show how the dynamics of collisions between cold atoms can be manipulated
by a modification of spontaneous emission times. This is achieved by placing
the atomic sample in a resonant optical cavity. Spontaneous emission is
enhanced by a combination of multiparticle entanglement together with a higher
density of modes of the modified vacuum field, in a situation akin to
superradiance. A specific situation is considered and we show that this effect
can be experimentally observed as a large suppression in trap-loss rates.Comment: RevTex, 2 EPS figures; scheduled for Phys. Rev. Lett. 19 Feb 01, with
minor change
Low energy expansion of the four-particle genus-one amplitude in type II superstring theory
A diagrammatic expansion of coefficients in the low-momentum expansion of the
genus-one four-particle amplitude in type II superstring theory is developed.
This is applied to determine coefficients up to order s^6R^4 (where s is a
Mandelstam invariant and R^4 the linearized super-curvature), and partial
results are obtained beyond that order. This involves integrating powers of the
scalar propagator on a toroidal world-sheet, as well as integrating over the
modulus of the torus. At any given order in s the coefficients of these terms
are given by rational numbers multiplying multiple zeta values (or
Euler--Zagier sums) that, up to the order studied here, reduce to products of
Riemann zeta values. We are careful to disentangle the analytic pieces from
logarithmic threshold terms, which involves a discussion of the conditions
imposed by unitarity. We further consider the compactification of the amplitude
on a circle of radius r, which results in a plethora of terms that are
power-behaved in r. These coefficients provide boundary `data' that must be
matched by any non-perturbative expression for the low-energy expansion of the
four-graviton amplitude.
The paper includes an appendix by Don Zagier.Comment: JHEP style. 6 eps figures. 50 page
- …