384 research outputs found
Fully permanent magnet atom chip for Bose-Einstein condensation
We describe a self-biased, fully permanent magnet atom chip used to study
ultracold atoms and to produce a Bose-Einstein condensate (BEC). The magnetic
trap is loaded efficiently by adiabatic transport of a magnetic trap via the
application of uniform external fields. Radio frequency spectroscopy is used
for in-trap analysis and to determine the temperature of the atomic cloud. The
formation of a Bose-Einstein condensate is observed in time of flight images
and as a narrow peak appearing in the radio frequency spectrum.Comment: changed title, substantial text modifications, journal reference
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Atom Chips: Fabrication and Thermal Properties
Neutral atoms can be trapped and manipulated with surface mounted microscopic
current carrying and charged structures. We present a lithographic fabrication
process for such atom chips based on evaporated metal films. The size limit of
this process is below 1m. At room temperature, thin wires can carry more
than 10A/cm current density and voltages of more than 500V. Extensive
test measurements for different substrates and metal thicknesses (up to 5
m) are compared to models for the heating characteristics of the
microscopic wires. Among the materials tested, we find that Si is the best
suited substrate for atom chips
A lattice of microtraps for ultracold atoms based on patterned magnetic films
We have realized a two dimensional permanent magnetic lattice of
Ioffe-Pritchard microtraps for ultracold atoms. The lattice is formed by a
single 300 nm magnetized layer of FePt, patterned using optical lithography.
Our magnetic lattice consists of more than 15000 tightly confining microtraps
with a density of 1250 traps/mm. Simple analytical approximations for the
magnetic fields produced by the lattice are used to derive relevant trap
parameters. We load ultracold atoms into at least 30 lattice sites at a
distance of approximately 10 m from the film surface. The present result
is an important first step towards quantum information processing with neutral
atoms in magnetic lattice potentials.Comment: 7 pages, 7 figure
Balance, growth and diversity of financial markets
A financial market comprising of a certain number of distinct companies is
considered, and the following statement is proved: either a specific agent will
surely beat the whole market unconditionally in the long run, or (and this "or"
is not exclusive) all the capital of the market will accumulate in one company.
Thus, absence of any "free unbounded lunches relative to the total capital"
opportunities lead to the most dramatic failure of diversity in the market: one
company takes over all other until the end of time. In order to prove this, we
introduce the notion of perfectly balanced markets, which is an equilibrium
state in which the relative capitalization of each company is a martingale
under the physical probability. Then, the weaker notion of balanced markets is
discussed where the martingale property of the relative capitalizations holds
only approximately, we show how these concepts relate to growth-optimality and
efficiency of the market, as well as how we can infer a shadow interest rate
that is implied in the economy in the absence of a bank.Comment: 25 page
Dynamically controlled toroidal and ring-shaped magnetic traps
We present traps with toroidal and ring-shaped topologies, based on
adiabatic potentials for radio-frequency dressed Zeeman states in a ring-shaped
magnetic quadrupole field. Simple adjustment of the radio-frequency fields
provides versatile possibilities for dynamical parameter tuning, topology
change, and controlled potential perturbation. We show how to induce toroidal
and poloidal rotations, and demonstrate the feasibility of preparing degenerate
quantum gases with reduced dimensionality and periodic boundary conditions. The
great level of dynamical and even state dependent control is useful for atom
interferometry.Comment: 6 pages, 4 figures. Paragraphs on gravity compensation and expected
trap lifetimes adde
Characterization of infectious and defective cloned avian hepadnavirus genomes
The infectivity in vivo, replication competence in vitro, and expression of viral genes of several molecularly cloned duck hepatitis B virus (DHBV) genomes were investigated. In addition, replication competence, core protein expression, and secretion of viral proteins were investigated for a grey heron hepatitis B virus genome. Except two, all DHBV isolates tested induced a systemic infection in Pekin ducks when injected as cloned viral DNA into the liver. After transfection of chicken hepatoma cells, both defective DHBV genomes expressed intracellular nucleocapsid and pre-S envelope proteins and secreted DHBs/pre-S particles into the medium. One of the defective DHBV genomes and HHBV produced within the cells replicative intermediates encapsidated in core particles and secreted virions, whereas the other defective DHBV genome did not and was unable to efficiently encapsidate the RNA pregenome. Comparative sequence analysis was performed to identify potential amino acid changes in viral proteins of both defective DHBV genomes. The data obtained demonstrate that most cloned avian hepadnaviruses are infectious or replication competent and suggest defects in envelope, polymerase or encapsidation function, respectively, in two cloned DHBV genomes
How many independent bets are there?
The benefits of portfolio diversification is a central tenet implicit to
modern financial theory and practice. Linked to diversification is the notion
of breadth. Breadth is correctly thought of as the number of in- dependent bets
available to an investor. Conventionally applications us- ing breadth
frequently assume only the number of separate bets. There may be a large
discrepancy between these two interpretations. We uti- lize a simple
singular-value decomposition (SVD) and the Keiser-Gutman stopping criterion to
select the integer-valued effective dimensionality of the correlation matrix of
returns. In an emerging market such as South African we document an estimated
breadth that is considerably lower than anticipated. This lack of
diversification may be because of market concentration, exposure to the global
commodity cycle and local currency volatility. We discuss some practical
extensions to a more statistically correct interpretation of market breadth,
and its theoretical implications for both global and domestic investors.Comment: Less technical rewrite. 12 Pages, 6 Figures (.eps
Sagnac interferometry with a single atomic clock
The Sagnac effect enables interferometric measurements of rotation with high precision. Using matter waves instead of light promises resolution enhancement by orders of magnitude that scales with particle mass. So far, the paradigm for matter wave Sagnac interferometry relies on DeBroglie waves and thus on free propagation of atoms either in free fall or within waveguides. However, the Sagnac effect can be expressed as a propertime difference experienced by two observers moving in opposite directions along closed paths and has indeed been measured with atomic clocks flown around Earth. Inspired by this, we investigate an interferometer comprised of a single atomic clock. The Sagnac effect manifests as a phase shift between trapped atoms in different internal states after transportation along closed paths in opposite directions, without any free propagation. With analytic models, we quantify limitations of the scheme arising from atomic dynamics and finite temperature. Furthermore, we suggest an implementation with previously demonstrated technology
Three-dimensional character of atom-chip-based rf-dressed potentials
We experimentally investigate the properties of radio-frequency-dressed
potentials for Bose-Einstein condensates on atom chips. The three-dimensional
potential forms a connected pair of parallel waveguides. We show that
rf-dressed potentials are robust against the effect of small magnetic-field
variations on the trap potential. Long-lived dipole oscillations of condensates
induced in the rf-dressed potentials can be tuned to a remarkably low damping
rate. We study a beam-splitter for Bose-Einstein condensates and show that a
propagating condensate can be dynamically split in two vertically separated
parts and guided along two paths. The effect of gravity on the potential can be
tuned and compensated for using a rf-field gradient.Comment: 9 pages, 7 figure
Two-dimensional array of microtraps with atomic shift register on a chip
Arrays of trapped atoms are the ideal starting point for developing registers
comprising large numbers of physical qubits for storing and processing quantum
information. One very promising approach involves neutral atom traps produced
on microfabricated devices known as atom chips, as almost arbitrary trap
configurations can be realised in a robust and compact package. Until now,
however, atom chip experiments have focused on small systems incorporating
single or only a few individual traps. Here we report experiments on a
two-dimensional array of trapped ultracold atom clouds prepared using a simple
magnetic-film atom chip. We are able to load atoms into hundreds of tightly
confining and optically resolved array sites. We then cool the individual atom
clouds in parallel to the critical temperature required for quantum degeneracy.
Atoms are shuttled across the chip surface utilising the atom chip as an atomic
shift register and local manipulation of atoms is implemented using a focused
laser to rapidly empty individual traps.Comment: 6 pages, 4 figure
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