5,602 research outputs found
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
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
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
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
Delay-Coordinates Embeddings as a Data Mining Tool for Denoising Speech Signals
In this paper we utilize techniques from the theory of non-linear dynamical
systems to define a notion of embedding threshold estimators. More specifically
we use delay-coordinates embeddings of sets of coefficients of the measured
signal (in some chosen frame) as a data mining tool to separate structures that
are likely to be generated by signals belonging to some predetermined data set.
We describe a particular variation of the embedding threshold estimator
implemented in a windowed Fourier frame, and we apply it to speech signals
heavily corrupted with the addition of several types of white noise. Our
experimental work seems to suggest that, after training on the data sets of
interest,these estimators perform well for a variety of white noise processes
and noise intensity levels. The method is compared, for the case of Gaussian
white noise, to a block thresholding estimator
Cold atoms near superconductors: Atomic spin coherence beyond the Johnson noise limit
We report on the measurement of atomic spin coherence near the surface of a
superconducting niobium wire. As compared to normal conducting metal surfaces,
the atomic spin coherence is maintained for time periods beyond the Johnson
noise limit. The result provides experimental evidence that magnetic near field
noise near the superconductor is strongly suppressed. Such long atomic spin
coherence times near superconductors open the way towards the development of
coherently coupled cold atom / solid state hybrid quantum systems with
potential applications in quantum information processing and precision force
sensing.Comment: Major revisions of the text for submission to New Journal of Physics
8 pages, 4 figure
Ad- and desorption of Rb atoms on a gold nanofilm measured by surface plasmon polaritons
Hybrid quantum systems made of cold atoms near nanostructured surfaces are
expected to open up new opportunities for the construction of quantum sensors
and for quantum information. For the design of such tailored quantum systems
the interaction of alkali atoms with dielectric and metallic surfaces is
crucial and required to be understood in detail. Here, we present real-time
measurements of the adsorption and desorption of Rubidium atoms on gold
nanofilms. Surface plasmon polaritons (SPP) are excited at the gold surface and
detected in a phase sensitive way. From the temporal change of the SPP phase
the Rubidium coverage of the gold film is deduced with a sensitivity of better
than 0.3 % of a monolayer. By comparing the experimental data with a Langmuir
type adsorption model we obtain the thermal desorption rate and the sticking
probability. In addition, also laser-induced desorption is observed and
quantified.Comment: 9 pages, 6 figure
Radio-frequency dressed lattices for ultracold alkali atoms
Ultracold atomic gases in periodic potentials are powerful platforms for exploring quantum physics in regimes dominated by many-body effects as well as for developing applications that benefit from quantum mechanical effects. Further advances face a range of challenges including the realization of potentials with lattice constants smaller than optical wavelengths as well as creating schemes for effective addressing and manipulation of single sites. In this paper we propose a dressed-based scheme for creating periodic potential landscapes for ultracold alkali atoms with the capability of overcoming such difficulties. The dressed approach has the advantage of operating in a low-frequency regime where decoherence and heating effects due to spontaneous emission do not take place. These results highlight the possibilities of atom-chip technology in the future development of quantum simulations and quantum technologies, and provide a realistic scheme for starting such an exploration
Brighter prospects? Assessing the franchise advantage using census data
This paper uses Census micro data to examine how starting a business as a franchise rather than an independent business affects its survival and growth prospects. We assess factors that influence the decision to become a franchisee and use various empirical approaches to correct for selection bias in our performance analyses. We find that franchised businesses on average exhibit higher survival rates than independent businesses, but importantly, the difference is small compared to claims in the trade press. The effect is also short lived: conditional on surviving a year or two, we no longer find survival (or growth) differences. We then explore two potential sources for this small survival advantage, namely franchisors’ screening process and the benefits arising from the brand and business know-how provided by franchisors. We find evidence that both of the sources contribute to the franchising advantage
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