3,716 research outputs found
Dynamics of resource production and utilisation in two-component biosphere-human and terrestrial carbon systems
This paper analyses simple models for "production-utilisation" systems, reduced to two state variables for producers and utilisers, respectively. Two modes are distinguished: in "harvester" systems the resource utilisation involves active seeking on the part of the utilisers, while in "processor" systems, utilisers function as passive material processors. An idealised model of biosphere-human interactions provides an example of a harvester system, and a model of plant and soil carbon dynamics exemplifies a processor system. The biosphere-human interaction model exhibits a number of features in accord with experience, including a tendency towards oscillatory behaviour which in some circumstances results in limit cycles. The plant-soil carbon model is used to study the effect of random forcing of production (for example by weather and climate fluctuations), showing that with appropriate parameter choices the model can flip between active-biosphere and dormant-biosphere equilibria under the influence of random forcing. This externally-driven transition between locally stable states is fundamentally different from Lorenzian chaos. A behavioural difference between two-component processor and harvester systems is that harvester systems have a capacity for oscillatory behaviour while processor systems do not
Observation of Feshbach resonances between two different atomic species
We have observed three Feshbach resonances in collisions between lithium-6
and sodium-23 atoms. The resonances were identified as narrow loss features
when the magnetic field was varied. The molecular states causing these
resonances have been identified, and additional lithium-sodium resonances are
predicted. These resonances will allow the study of degenerate Bose-Fermi
mixtures with adjustable interactions, and could be used to generate ultracold
heteronuclear molecules
Formation Time of a Fermion Pair Condensate
The formation time of a condensate of fermionic atom pairs close to a
Feshbach resonance was studied. This was done using a phase-shift method in
which the delayed response of the many-body system to a modulation of the
interaction strength was recorded. The observable was the fraction of condensed
molecules in the cloud after a rapid magnetic field ramp across the Feshbach
resonance. The measured response time was slow compared to the rapid ramp,
which provides final proof that the molecular condensates reflect the presence
of fermion pair condensates before the ramp.Comment: 5 pages, 4 figure
Observation of Bose-Einstein Condensation of Molecules
We have observed Bose-Einstein condensation of molecules. When a spin mixture
of fermionic Li-6 atoms was evaporatively cooled in an optical dipole trap near
a Feshbach resonance, the atomic gas was converted into Li_2 molecules. Below
600 nK, a Bose-Einstein condensate of up to 900,000 molecules was identified by
the sudden onset of a bimodal density distribution. This condensate realizes
the limit of tightly bound fermion pairs in the crossover between BCS
superfluidity and Bose-Einstein condensation.Comment: 4 pages, 3 figure
Characterization of a 450-km Baseline GPS Carrier-Phase Link using an Optical Fiber Link
A GPS carrier-phase frequency transfer link along a baseline of 450 km has
been established and is characterized by comparing it to a phase-stabilized
optical fiber link of 920 km length, established between the two endpoints, the
Max-Planck-Institut f\"ur Quantenoptik in Garching and the
Physikalisch-Technische Bundesanstalt in Braunschweig. The characterization is
accomplished by comparing two active hydrogen masers operated at both
institutes. The masers serve as local oscillators and cancel out when the
double differences are calculated, such that they do not constitute a
limitation for the GPS link characterization. We achieve a frequency
instability of 3 x 10^(-13) in 30 s and 5 x 10^(-16) for long averaging times.
Frequency comparison results obtained via both links show no deviation larger
than the statistical uncertainty of 6 x 10^(-16). These results can be
interpreted as a successful cross-check of the measurement uncertainty of a
truly remote end fiber link.Comment: 14 pages, 6 figure
Photon-number dependent afterpulsing in superconducting nanostrip single-photon detectors
Superconducting nanostrip single-photon detectors (SNSPD) are wide-spread
tools in photonic quantum technologies. Here, we study afterpulsing in
commercial SNSPD made of amorphous superconducting material. We find that the
probability of an afterpulse is not a constant but depends on the mean number
of photons per light pulse including mean numbers much less than one. Our
observations exclude the electrical circuit as the primary cause of
afterpulsing. We propose a phenomenological model which qualitatively explains
our findings via the introduction of slowly relaxing "afterpulsing centers". We
argue that two-level systems in amorphous materials are the most plausible
physical candidates for the role of such afterpulsing centers
- …