1,048 research outputs found
Recommended from our members
A 2000-year annual record of snow accumulation rates for Law Dome, East Antarctica
Accurate high-resolution records of snow accumulation rates in Antarctica are crucial for estimating ice sheet mass balance and subsequent sea level change. Snowfall rates at Law Dome, East Antarctica, have been linked with regional atmospheric circulation to the mid-latitudes as well as regional Antarctic snowfall. Here, we extend the length of the Law Dome accumulation record from 750 years to 2035 years, using recent annual layer dating that extends to 22 BCE. Accumulation rates were calculated as the ratio of measured to modelled layer thicknesses, multiplied by the long-term mean accumulation rate. The modelled layer thicknesses were based on a power-law vertical strain rate profile fitted to observed annual layer thickness. The periods 380–442, 727–783 and 1970–2009 CE have above-average snow accumulation rates, while 663–704, 933–975 and 1429–1468 CE were below average, and decadal-scale snow accumulation anomalies were found to be relatively common (74 events in the 2035-year record). The calculated snow accumulation rates show good correlation with atmospheric reanalysis estimates, and significant spatial correlation over a wide expanse of East Antarctica, demonstrating that the Law Dome record captures larger-scale variability across a large region of East Antarctica well beyond the immediate vicinity of the Law Dome summit. Spectral analysis reveals periodicities in the snow accumulation record which may be related to El Niño–Southern Oscillation (ENSO) and Interdecadal Pacific Oscillation (IPO) frequencies
Escape from a metastable well under a time-ramped force
Thermally activated escape of an over-damped particle from a metastable well
under the action of a time-ramped force is studied. We express the mean first
passage time (MFPT) as the solution to a partial differential equation, which
we solve numerically for a model case. We discuss two approximations of the
MFPT, one of which works remarkably well over a wide range of loading rates,
while the second is easy to calculate and can provide a valuable first
estimate.Comment: 9 pages, including 2 figure
Resonance fluorescence in a band gap material: Direct numerical simulation of non-Markovian evolution
A numerical method of calculating the non-Markovian evolution of a driven
atom radiating into a structured continuum is developed. The formal solution
for the atomic reduced density matrix is written as a Markovian algorithm by
introducing a set of additional, virtual density matrices which follow, to the
level of approximation of the algorithm, all the possible trajectories of the
photons in the electromagnetic field. The technique is perturbative in the
sense that more virtual density matrices are required as the product of the
effective memory time and the effective coupling strength become larger. The
number of density matrices required is given by where is the number
of timesteps per memory time. The technique is applied to the problem of a
driven two-level atom radiating close to a photonic band gap and the
steady-state correlation function of the atom is calculated.Comment: 14 pages, 9 figure
Path Selection for Quantum Repeater Networks
Quantum networks will support long-distance quantum key distribution (QKD)
and distributed quantum computation, and are an active area of both
experimental and theoretical research. Here, we present an analysis of
topologically complex networks of quantum repeaters composed of heterogeneous
links. Quantum networks have fundamental behavioral differences from classical
networks; the delicacy of quantum states makes a practical path selection
algorithm imperative, but classical notions of resource utilization are not
directly applicable, rendering known path selection mechanisms inadequate. To
adapt Dijkstra's algorithm for quantum repeater networks that generate
entangled Bell pairs, we quantify the key differences and define a link cost
metric, seconds per Bell pair of a particular fidelity, where a single Bell
pair is the resource consumed to perform one quantum teleportation. Simulations
that include both the physical interactions and the extensive classical
messaging confirm that Dijkstra's algorithm works well in a quantum context.
Simulating about three hundred heterogeneous paths, comparing our path cost and
the total work along the path gives a coefficient of determination of 0.88 or
better.Comment: 12 pages, 8 figure
Optical Confinement of a Bose-Einstein Condensate
Bose-Einstein condensates of sodium atoms have been confined in an optical
dipole trap using a single focused infrared laser beam. This eliminates the
restrictions of magnetic traps for further studies of atom lasers and
Bose-Einstein condensates. More than five million condensed atoms were
transferred into the optical trap. Densities of up to of Bose condensed atoms were obtained, allowing for a measurement of
the three-body decay rate constant for sodium condensates as . At lower densities, the observed 1/e
lifetime was more than 10 sec. Simultaneous confinement of Bose-Einstein
condensates in several hyperfine states was demonstrated.Comment: 5 pages, 4 figure
Boojums and the Shapes of Domains in Monolayer Films
Domains in Langmuir monolayers support a texture that is the two-dimensional
version of the feature known as a boojum. Such a texture has a quantifiable
effect on the shape of the domain with which it is associated. The most
noticeable consequence is a cusp-like feature on the domain boundary. We report
the results of an experimental and theoretical investigation of the shape of a
domain in a Langmuir monolayer. A further aspect of the investigation is the
study of the shape of a ``bubble'' of gas-like phase in such a monolayer. This
structure supports a texture having the form of an inverse boojum. The
distortion of a bubble resulting from this texture is also studied. The
correspondence between theory and experiment, while not perfect, indicates that
a qualitative understanding of the relationship between textures and domain
shapes has been achieved.Comment: replaced with published version, 10 pages, 13 figures include
Sea Water Radiocarbon Evolution in the Gulf of Alaska: 2002 Observations
Oceanic uptake and transport of bomb radiocarbon as {sup 14}CO{sub 2} created by atmospheric nuclear weapons testing in the 1950s and 1960s has been a useful diagnostic to determine the carbon transfer between the ocean and atmosphere. In addition, the distribution of radiocarbon in the ocean can be used as a tracer of oceanic circulation. Results obtained from samples collected in the Gulf of Alaska in the summer of 2002 provide a direct comparison with results in the 1970s during GEOSECS and in the early 1990s during WOCE. The open gyre values are 20-40{per_thousand} more negative than those documented in 1991 and 1993 (WOCE) although the general trends as a function of latitude are reproduced. Surface values are still significantly higher than pre-bomb levels ({approx}-105{per_thousand} or lower). In the central gyre, we observe {Delta}{sup 14}C-values that are lower in comparison to GEOSECS (stn 218) and WOCE P16/P17 to a density of {approx}26.8{sigma}t. This observation is consistent with the overall decrease in surface {Delta}{sup 14}C values, and reflects the erosion of the bomb-{sup 14}C transient. We propose that erosion of the bomb-{sup 14}C transient is accomplished by entrainment of low {sup 14}C water via vertical exchange within the Gulf of Alaska and replenishment of surface and sub-thermocline waters with waters derived from the far northwest Pacific
Networking - A Statistical Physics Perspective
Efficient networking has a substantial economic and societal impact in a
broad range of areas including transportation systems, wired and wireless
communications and a range of Internet applications. As transportation and
communication networks become increasingly more complex, the ever increasing
demand for congestion control, higher traffic capacity, quality of service,
robustness and reduced energy consumption require new tools and methods to meet
these conflicting requirements. The new methodology should serve for gaining
better understanding of the properties of networking systems at the macroscopic
level, as well as for the development of new principled optimization and
management algorithms at the microscopic level. Methods of statistical physics
seem best placed to provide new approaches as they have been developed
specifically to deal with non-linear large scale systems. This paper aims at
presenting an overview of tools and methods that have been developed within the
statistical physics community and that can be readily applied to address the
emerging problems in networking. These include diffusion processes, methods
from disordered systems and polymer physics, probabilistic inference, which
have direct relevance to network routing, file and frequency distribution, the
exploration of network structures and vulnerability, and various other
practical networking applications.Comment: (Review article) 71 pages, 14 figure
Effects of interatomic collisions on atom laser outcoupling
We present a computational approach to the outcoupling in a simple
one-dimensional atom laser model, the objective being to circumvent
mathematical difficulties arising from the breakdown of the Born and Markov
approximations. The approach relies on the discretization of the continuum
representing the reservoir of output modes, which allows the treatment of
arbitrary forms of outcoupling as well as the incorporation of non-linear terms
in the Hamiltonian, associated with interatomic collisions. By considering a
single-mode trapped condensate, we study the influence of elastic collisions
between trapped and free atoms on the quasi steady-state population of the
trap, as well as the energy distribution and the coherence of the outcoupled
atoms.Comment: 25 pages, 11 figures, to appear in J. Phys.
Generic model of an atom laser
We present a generic model of an atom laser by including a pump and loss term
in the Gross-Pitaevskii equation. We show that there exists a threshold for the
pump above which the mean matter field assumes a non-vanishing value in
steady-state. We study the transient regime of this atom laser and find
oscillations around the stationary solution even in the presence of a loss
term. These oscillations are damped away when we introduce a position dependent
loss term. For this case we present a modified Thomas-Fermi solution that takes
into account the pump and loss. Our generic model of an atom laser is analogous
to the semi-classical theory of the laser.Comment: 15 pages, including 5 figures, submitted to Phys. Rev. A, revised
manuscript, file also available at
http://www.physik.uni-ulm.de/quan/users/kne
- …