6,661 research outputs found
Exploiting soliton decay and phase fluctuations in atom chip interferometry of Bose-Einstein condensates
We show that the decay of a soliton into vortices provides a mechanism for
measuring the initial phase difference between two merging Bose-Einstein
condensates. At very low temperatures, the mechanism is resonant, operating
only when the clouds start in anti-phase. But at higher temperatures, phase
fluctuations trigger vortex production over a wide range of initial relative
phase, as observed in recent experiments at MIT. Choosing the merge time to
maximize the number of vortices created makes the interferometer highly
sensitive to spatially varying phase patterns and hence atomic movement.Comment: 5 pages, 5 figure
M2 and the business cycle
Business cycles ; Money supply ; Monetary policy - United States
Quantifying Finite Temperature Effects in Atom Chip Interferometry of Bose-Einstein Condensates
We quantify the effect of phase fluctuations on atom chip interferometry of
Bose-Einstein condensates. At very low temperatures, we observe small phase
fluctuations, created by mean-field depletion, and a resonant production of
vortices when the two clouds are initially in anti-phase. At higher
temperatures, we show that the thermal occupation of Bogoliubov modes makes
vortex production vary smoothly with the initial relative phase difference
between the two atom clouds. We also propose a technique to observe vortex
formation directly by creating a weak link between the two clouds. The position
and direction of circulation of the vortices is subsequently revealed by kinks
in the interference fringes produced when the two clouds expand into one
another. This procedure may be exploited for precise force measurement or
motion detection.Comment: 7 pages, 5 figure
LATERAL-TORSIONAL VIBRATION OF A SIDE-LOADED ROTOR WITH ASYMMETRIC SHAFT STIFFNESS
Using energy equations a four degrees of freedom analytical model is developed for a two-disk rotor with shaft stiffness asymmetry. A radial constant force is applied to the outboard disk to emphasize the effects of gravity or aerodynamic side loading. Special emphasis is placed on characterizing the lateral and torsional vibration trends associated with shaft asymmetry which may be used to identify failing shafts in operational rotor systems. Simulation reveals distinct patterns in lateral and torsional response, with strong dependencies on the magnitude of the side load, magnitude of the asymmetry and proximity of the lateral and torsional natural frequencies. Notable interaction is also observed between the lateral and torsional response. Lateral response peaks are found to correlate to torsional response peaks under some conditions. An experiment is performed to compare the response of a real system with the simulated model
Scattering and absorption of ultracold atoms by nanotubes
We investigate theoretically how cold atoms, including Bose-Einstein
condensates, are scattered from, or absorbed by nanotubes with a view to
analysing recent experiments. In particular we consider the role of potential
strength, quantum reflection, atomic interactions and tube vibrations on atom
loss rates. Lifshitz theory calculations deliver a significantly stronger
scattering potential than that found in experiment and we discuss possible
reasons for this. We find that the scattering potential for dielectric tubes
can be calculated to a good approximation using a modified pairwise summation
approach, which is efficient and easily extendable to arbitrary geometries.
Quantum reflection of atoms from a nanotube may become a significant factor at
low temperatures, especially for non-metallic tubes. Interatomic interactions
are shown to increase the rate at which atoms are lost to the nanotube and lead
to non-trivial dynamics. Thermal nanotube vibrations do not significantly
increase loss rates or reduce condensate fractions, but lower frequency
oscillations can dramatically heat the cloud.Comment: 7 pages, 4 figure
Quantum reflection of ultracold atoms from thin films, graphene, and semiconductor heterostructures
We show that thin dielectric films can be used to enhance the performance of
passive atomic mirrors by enabling quantum reflection probabilities of over 90%
for atoms incident at velocities ~1 mm/s, achieved in recent experiments. This
enhancement is brought about by weakening the Casimir-Polder attraction between
the atom and the surface, which induces the quantum reflection. We show that
suspended graphene membranes also produce higher quantum reflection
probabilities than bulk matter. Temporal changes in the electrical resistance
of such membranes, produced as atoms stick to the surface, can be used to
monitor the reflection process, non-invasively and in real time. The resistance
change allows the reflection probability to be determined purely from
electrical measurements without needing to image the reflected atom cloud
optically. Finally, we show how perfect atom mirrors may be manufactured from
semiconductor heterostructures, which employ an embedded two-dimensional
electron gas to tailor the atom-surface interaction and so enhance the
reflection by classical means.Comment: 8 pages, 4 figure
More on coupling coefficients for the most degenerate representations of SO(n)
We present explicit closed-form expressions for the general group-theoretical
factor appearing in the alpha-topology of a high-temperature expansion of
SO(n)-symmetric lattice models. This object, which is closely related to
6j-symbols for the most degenerate representation of SO(n), is discussed in
detail.Comment: 9 pages including 1 table, uses IOP macros Update of Introduction and
Discussion, References adde
Transport of dipolar Bose-Einstein condensates in a one-dimensional optical lattice
We show that magnetic dipolar interactions can stabilize superfluidity in
atomic gases but the dipole alignment direction required to achieve this
varies, depending on whether the flow is oscillatory or continuous. If a
condensate is made to oscillate through a lattice, damping of the oscillations
can be reduced by aligning the dipoles perpendicular to the direction of
motion. However, if a lattice is driven continuously through the condensate,
superfluid behavior is best preserved when the dipoles are aligned parallel to
the direction of motion. We explain these results in terms of the formation of
topological excitations and tunnel barrier heights between lattice sites.Comment: 6 pages, 7 figure
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