361 research outputs found
Analysis and calibration of absorptive images of Bose-Einstein condensate at non-zero temperatures
We describe the method allowing quantitative interpretation of absorptive
images of mixtures of BEC and thermal atoms which reduces possible systematic
errors associated with evaluation of the contribution of each fraction. By
using known temperature dependence of the BEC fraction, the analysis allows
precise calibration of the fitting results. The developed method is verified in
two different measurements and compares well with theoretical calculations and
with measurements performed by another group.Comment: 17 pages, 8 figure
Optically-induced lensing effect on a Bose-Einstein condensate expanding in a moving lattice
We report the experimental observation of a lensing effect on a Bose-Einstein
condensate expanding in a moving 1D optical lattice. The effect of the periodic
potential can be described by an effective mass dependent on the condensate
quasi-momentum. By changing the velocity of the atoms in the frame of the
optical lattice we induce a focusing of the condensate along the lattice
direction. The experimental results are compared with the numerical predictions
of an effective 1D theoretical model. Besides, a precise band spectroscopy of
the system is carried out by looking at the real-space propagation of the
atomic wavepacket in the optical lattice.Comment: 5 pages, 4 figures; minor changes applied and typos corrected; a new
paragraph added; some references updated; journal reference adde
A Comparison of Hydrographically and Optically Derived Mixed Layer Depths
Efforts to understand and model the dynamics of the upper ocean would be significantly advanced given the ability to rapidly determine mixed layer depths (MLDs) over large regions. Remote sensing technologies are an ideal choice for achieving this goal. This study addresses the feasibility of estimating MLDs from optical properties. These properties are strongly influenced by suspended particle concentrations, which generally reach a maximum at pycnoclines. The premise therefore is to use a gradient in beam attenuation at 660 nm (c660) as a proxy for the depth of a particle-scattering layer. Using a global data set collected during World Ocean Circulation Experiment cruises from 1988-1997, six algorithms were employed to compute MLDs from either density or temperature profiles. Given the absence of published optically based MLD algorithms, two new methods were developed that use c660 profiles to estimate the MLD. Intercomparison of the six hydrographically based algorithms revealed some significant disparities among the resulting MLD values. Comparisons between the hydrographical and optical approaches indicated a first-order agreement between the MLDs based on the depths of gradient maxima for density and c660. When comparing various hydrographically based algorithms, other investigators reported that inherent fluctuations of the mixed layer depth limit the accuracy of its determination to 20 m. Using this benchmark, we found a similar to 70% agreement between the best hydrographical-optical algorithm pairings
Strontium optical lattice clocks for practical realization of the metre and secondary representation of the second
We present a system of two independent strontium optical lattice standards
probed with a single shared ultra-narrow laser. The absolute frequency of the
clocks can be verified by the use of Er:fiber optical frequency comb with the
GPS-disciplined Rb frequency standard. We report hertz-level spectroscopy of
the clock line and measurements of frequency stability of the two strontium
optical lattice clocks.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in Meas. Sci. Technol. The publisher is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at
doi:10.1088/0957-0233/26/7/07520
Stratospheric-trace-gas-profile retrievals from balloon-borne limb imaging of mid-infrared emission spectra
The Limb Imaging Fourier Transform Spectrometer Experiment (LIFE) instrument is a balloon-borne prototype of a satellite instrument designed to take vertical images of atmospheric limb emission spectra in the 700–1400 cm−1 wavenumber range from the upper-troposphere–lower-stratosphere (UTLS) altitude region of the atmosphere. The prototype builds on the success of past and existing instruments while reducing the complexity of the imaging design. This paper details the results of a demonstration flight on a stabilized stratospheric balloon gondola from Timmins, Canada, in August 2019. Retrievals of vertical trace gas profiles for the important greenhouse gases H2O, O3, CH4, and N2O, as well as HNO3, are performed using an optimal estimation approach and the SASKTRAN radiative transfer model. The retrieved profiles are compared to approximately coincident observations made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) solar occultation and Microwave Limb Sounder (MLS) instruments. An evaluation of the LIFE measurements is performed, and areas of improvement are identified. This work increases the overall technical readiness of the approach for
future balloon, aircraft, and space applications.</p
Fibre-optic delivery of time and frequency to VLBI station
The quality of Very Long Baseline Interferometry (VLBI) radio observations
predominantly relies on precise and ultra-stable time and frequency (T&F)
standards, usually hydrogen masers (HM), maintained locally at each VLBI
station. Here, we present an operational solution in which the VLBI
observations are routinely carried out without use of a local HM, but using
remote synchronization via a stabilized, long-distance fibre-optic link. The
T&F reference signals, traceable to international atomic timescale (TAI), are
delivered to the VLBI station from a dedicated timekeeping laboratory.
Moreover, we describe a proof-of-concept experiment where the VLBI station is
synchronized to a remote strontium optical lattice clock during the
observation.Comment: 8 pages, 8 figures, matches the version published in A&A, section
Astronomical instrumentatio
Experimenting an optical second with strontium lattice clocks
Progress in realizing the SI second had multiple technological impacts and
enabled to further constraint theoretical models in fundamental physics.
Caesium microwave fountains, realizing best the second according to its current
definition with a relative uncertainty of 2-4x10^(-16), have already been
superseded by atomic clocks referenced to an optical transition, both more
stable and more accurate. Are we ready for a new definition of the second? Here
we present an important step in this direction: our system of five clocks
connects with an unprecedented consistency the optical and the microwave
worlds. For the first time, two state-of-the-art strontium optical lattice
clocks are proven to agree within their accuracy budget, with a total
uncertainty of 1.6x10^(-16). Their comparison with three independent caesium
fountains shows a degree of reproducibility henceforth solely limited at the
level of 3.1x10^(-16) by the best realizations of the microwave-defined second.Comment: 9 pages, 4 figures, 2 table
Free-fall expansion of finite-temperature Bose-Einstein condensed gas in the non Thomas-Fermi regime
We report on our study of the free-fall expansion of a finite-temperature
Bose-Einstein condensed cloud of 87Rb. The experiments are performed with a
variable total number of atoms while keeping constant the number of atoms in
the condensate. The results provide evidence that the BEC dynamics depends on
the interaction with thermal fraction. In particular, they provide experimental
evidence that thermal cloud compresses the condensate.Comment: 8 pages, 4 figure
Thermomechanical Fatigue Behavior of a Silicon Carbide Fiber-Reinforced Calcium Aluminosilicate Composite
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65874/1/j.1151-2916.1993.tb04022.x.pd
Spatial heterodyne observations of water (SHOW) from a high-altitude airplane: characterization, performance, and first results
The Spatial Heterodyne Observations of Water instrument (SHOW) is a
limb-sounding satellite prototype that utilizes the Spatial Heterodyne
Spectroscopy (SHS) technique, operating in a limb-viewing configuration, to
observe limb-scattered sunlight in a vibrational band of water vapour within
a spectral window from 1363 to 1366 nm. The goal is to retrieve high
vertical and horizontal resolution measurements of water vapour in the upper
troposphere and lower stratosphere. The prototype instrument has been
configured for observations from NASA's ER-2 high-altitude airborne remote
science airplane. Flying at a maximum altitude of ∼21.34 km with a
maximum speed of ∼760 km h−1, the
ER-2 provides a stable platform to simulate observations from a low-earth
orbit satellite. Demonstration flights were performed from the ER-2 during an
observation campaign from 15 to 22 July 2017. In this paper, we present the
laboratory characterization work and the level 0 to level 1 processing of
flight data that were obtained during an engineering flight performed on
18 July 2017. Water vapour profile retrievals are presented and compared to
in situ radiosonde measurements made of the same approximate column of air.
These measurements are used to validate the SHOW measurement concept and
examine the sensitivity of the technique.</p
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