719 research outputs found
Large atom number Bose-Einstein condensate of sodium
We describe the setup to create a large Bose-Einstein condensate containing
more than 120x10^6 atoms. In the experiment a thermal beam is slowed by a
Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical
dark-spot MOT in our experiments contains 2.0x10^10 atoms with a temperature of
320 microK and a density of about 1.0x10^11 atoms/cm^3. The sample is spin
polarized in a high magnetic field, before the atoms are loaded in the magnetic
trap. Spin polarizing in a high magnetic field results in an increase in the
transfer efficiency by a factor of 2 compared to experiments without spin
polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by
evaporative cooling. To suppress the 3-body losses at the end of the
evaporation the magnetic trap is decompressed in the axial direction.Comment: 11 pages, 12 figures, submitted to Review Of Scientific Instrument
Cavity Optomechanical Magnetometer
A cavity optomechanical magnetometer is demonstrated where the magnetic field
induced expansion of a magnetostrictive material is transduced onto the
physical structure of a highly compliant optical microresonator. The resulting
motion is read out optically with ultra-high sensitivity. Detecting the
magnetostrictive deformation of Terfenol-D with a toroidal whispering gallery
mode (TWGM) resonator a peak sensitivity of 400 nT/Hz^.5 was achieved with
theoretical modelling predicting that sensitivities of up to 500 fT/Hz^.5 may
be possible. This chip-based magnetometer combines high-sensitivity and large
dynamic range with small size and room temperature operation
Observation of shock waves in a large Bose-Einstein condensate
We observe the formation of shock waves in a Bose-Einstein condensate
containing a large number of sodium atoms. The shock wave is initiated with a
repulsive, blue-detuned light barrier, intersecting the BEC, after which two
shock fronts appear. We observe breaking of these waves when the size of these
waves approaches the healing length of the condensate. At this time, the wave
front splits into two parts and clear fringes appear. The experiment is modeled
using an effective 1D Gross-Pitaevskii-like equation and gives excellent
quantitative agreement with the experiment, even though matter waves with
wavelengths two orders of magnitude smaller than the healing length are
present. In these experiments, no significant heating or particle loss is
observed.Comment: 7 pages, 7 figure
Growth dynamics of a Bose-Einstein condensate in a dimple trap without cooling
We study the formation of a Bose-Einstein condensate in a cigar-shaped
three-dimensional harmonic trap, induced by the controlled addition of an
attractive "dimple" potential along the weak axis. In this manner we are able
to induce condensation without cooling due to a localized increase in the phase
space density. We perform a quantitative analysis of the thermodynamic
transformation in both the sudden and adiabatic regimes for a range of dimple
widths and depths. We find good agreement with equilibrium calculations based
on self-consistent semiclassical Hartree-Fock theory describing the condensate
and thermal cloud. We observe there is an optimal dimple depth that results in
a maximum in the condensate fraction. We also study the non-equilibrium
dynamics of condensate formation in the sudden turn-on regime, finding good
agreement for the observed time dependence of the condensate fraction with
calculations based on quantum kinetic theory.Comment: v1: 9 pages, 7 figures, submitted to Phys. Rev. A; v2: 10 pages, 8
figures, fixed typos, added references, additional details on experimental
procedure, values of phase-space density, new figure and discussion on
effects of three-body loss in Appendix B (replaced with published version
Versatile two-dimensional potentials for ultra-cold atoms
We propose and investigate a technique for generating smooth two-dimensional
potentials for ultra-cold atoms based on the rapid scanning of a far-detuned
laser beam using a two-dimensional acousto-optical modulator (AOM). We
demonstrate the implementation of a feed-forward mechanism for fast and
accurate control of the spatial intensity of the laser beam, resulting in
improved homogeneity for the atom trap. This technique could be used to
generate a smooth toroidal trap that would be useful for static and dynamic
experiments on superfluidity and persistent currents with ultra-cold atoms.Comment: 8 pages, 5 figure
Recommendations for the Determination of Nutrients in Seawater to High Levels of Precision and Inter-Comparability using Continuous Flow Analysers
The Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) brings together scientists with interests in physical oceanography, the carbon cycle, marine biogeochemistry and ecosystems, and other users and collectors of ocean interior data to develop a sustained global network of hydrographic sections as part of the Global Ocean Climate Observing System. A series of manuals and guidelines are being produced by GO-SHIP which update those developed by the World Ocean Circulation Experiment (WOCE) in the early 1990s. Analysis of the data collected in WOCE suggests that improvements are needed in the collection of nutrient data if they are to be used for determining change within the ocean interior. Production of this manual is timely as it coincides with the development of reference materials for nutrients in seawater (RMNS). These RMNS solutions will be produced in sufficient quantities and be of sufficient quality that they will provide a basis for improving the consistency of nutrient measurements both within and between cruises.
This manual is a guide to suggested best practice in performing nutrient measurements at sea. It provides a detailed set of advice on laboratory practice for all the procedures surrounding the use of 1
gas-segmented continuous flow analysers (CFA) for the determination of dissolved nutrients (usually ammonium, nitrate, nitrite, phosphate and silicate) at sea. It does not proscribe the use of a particular instrument or related chemical method as these are well described in other publications.
The manual provides a brief introduction to the CFA method, the collection and storage of samples, considerations in the preparation of reagents and the calibrations of the system. It discusses how RMNS solutions can be used to “track” the performance of a system during a cruise and between cruises. It provides a format for the meta-data that need to be reported along side the sample data at the end of a cruise so that the quality of the reported data can be evaluated and set in context relative to other data sets.
Most importantly the central manual is accompanied by a set of nutrient standard operating procedures (NSOPs) that provide detailed information on key procedures that are necessary if best quality data are to be achieved consistently. These cover sample collection and storage, an example NSOP for the use of a CFA system at sea, high precision preparation of calibration solutions, assessment of the true calibration blank, checking the linearity of a calibration and the use of internal and externally prepared reference solutions for controlling the precision of data during a cruise and between cruises. An example meta-data report and advice on the assembly of the quality control and statistical data that should form part of the meta-data report are also given
Vortex Motion Noise in Micrometre-Sized Thin Films of the Amorphous Nb0.7Ge0.3 Weak-Pinning Superconductor
We report high-resolution measurements of voltage (V) noise in the mixed
state of micrometre-sized thin films of amorphous Nb0.7Ge0.3, which is a good
representative of weak-pinning superconductors. There is a remarkable
difference between the noise below and above the irreversibility field Birr.
Below Birr, in the presence of measurable pinning, the noise at small applied
currents resembles shot noise, and in the regime of flux flow at larger
currents decreases with increasing voltage due to a progressive ordering of the
vortex motion. At magnetic fields B between Birr and the upper critical field
Bc2 flux flow is present already at vanishingly small currents. In this regime
the noise scales with (1-B/Bc2)^2 V^2 and has a frequency (f) spectrum of 1/f
type. We interpret this noise in terms of the properties of strongly driven
depinned vortex systems at high vortex density.Comment: 8 pages, 5 figures, version accepted for publication in PR
Diversity Arrays Technology (DArT) and next-generation sequencing combined: genome-wide, high throughput, highly informative genotyping for molecular breeding of Eucalyptus
Measurement of the 3s3p 3P1 lifetime in magnesium using a magneto-optical trap
We demonstrate an accurate method for measuring the lifetime of
long-lived metastable magnetic states using a magneto-optical trap
(MOT). Through optical pumping, the metastable (3s3p) (3)P(1) level is
populated in a standard MOT. During the optical pumping process, a
fraction of the population is captured in the magnetic quadrupole field
of the MOT. When the metastable atoms decay to the (3s(2)) (1)S(0)
ground state they are recaptured into the MOT. In this system no
alternative cascading transition is possible. The lifetime of the
metastable level is measured directly as an exponential load time of the
MOT. We have experimentally tested our method by measuring the lifetime
of the (3s3p) (3)P(1) of (24)Mg. This lifetime has been measured
numerous times previously, but with quite different results. Using our
method we find the (3s3p) (3)P(1) lifetime to be (4.4 +/- 0.2) ms.
Theoretical values point toward a lower value for the lifetime
Qualitative Evaluation of Common Quantitative Metrics for Clinical Acceptance of Automatic Segmentation:a Case Study on Heart Contouring from CT Images by Deep Learning Algorithms
Organs-at-risk contouring is time consuming and labour intensive. Automation by deep learning algorithms would decrease the workload of radiotherapists and technicians considerably. However, the variety of metrics used for the evaluation of deep learning algorithms make the results of many papers difficult to interpret and compare. In this paper, a qualitative evaluation is done on five established metrics to assess whether their values correlate with clinical usability. A total of 377 CT volumes with heart delineations were randomly selected for training and evaluation. A deep learning algorithm was used to predict the contours of the heart. A total of 101 CT slices from the validation set with the predicted contours were shown to three experienced radiologists. They examined each slice independently whether they would accept or adjust the prediction and if there were (small) mistakes. For each slice, the scores of this qualitative evaluation were then compared with the Sørensen-Dice coefficient (DC), the Hausdorff distance (HD), pixel-wise accuracy, sensitivity and precision. The statistical analysis of the qualitative evaluation and metrics showed a significant correlation. Of the slices with a DC over 0.96 (N = 20) or a 95% HD under 5 voxels (N = 25), no slices were rejected by the readers. Contours with lower DC or higher HD were seen in both rejected and accepted contours. Qualitative evaluation shows that it is difficult to use common quantification metrics as indicator for use in clinic. We might need to change the reporting of quantitative metrics to better reflect clinical acceptance
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