scholarly journals Learning from the time and length redefinitions, and the metre demotion

2011 ◽  
Vol 369 (1953) ◽  
pp. 4090-4108 ◽  
Author(s):  
John L. Hall
Keyword(s):  
Laser Cooling ◽  
Optical Clock ◽  
Distance Measurements ◽  
Time Asymmetry ◽  
Spatial Anisotropy ◽  
Molecular Transition ◽  
Satellite Experiment ◽  
Satellite Spin ◽  
Mirror Motion ◽  
Better Than

After discussing several issues in a future redefinition of the kilogram, this paper considers the lessons that one might have learned from the analogous redefinitions of the metre and the second. The progress of length metrology was slow and steady, from seven digits reproducibility with the 1889 X-shaped metre prototype, to nine digits with Kr lamps, to 11 digits with the 1983 redefinition of the metre using the speed of light. With laser cooling, the Cs clock improved to 15, now 16, digits (and so also astronomical distance measurements could improve). Laser-cooled ions, and now atoms captured and cooled in an optical lattice, enable accuracy capability of three different optical frequency references to exceed 17 digits, i.e. better than time itself. The optical comb and related techniques vastly simplify frequency comparisons. Such progress stimulates a new satellite experiment, the STAR Mission (Space–Time Asymmetry Research). The goal is to test at the 1E–18 level frequency shifts owing to spatial anisotropy, position, gravitational potential and boost. The onboard optical clock will use stabilization to a molecular transition in I 2 or HCCH or CO 2 . The length etalons will be multiply redundant, with stability at the thermo-mechanical mirror motion limit. For a ULE glass etalon spacer (1987), I measure length creep approximately −1.5E–12/d, i.e. below 1E–14 over the 500 s satellite spin period.

scholarly journals When should we change the definition of the second?

2011 ◽  
Vol 369 (1953) ◽  
pp. 4109-4130 ◽  
Author(s):  
Patrick Gill
Keyword(s):  
Laser Cooling ◽  
Optical Lattice ◽  
Cold Atoms ◽  
International System ◽  
Atomic Clock ◽  
Optical Clock ◽  
Lattice Systems ◽  
System Of Units ◽  
Primary Standards ◽  
Definition Of

The microwave caesium (Cs) atomic clock has formed an enduring basis for the second in the International System of Units (SI) over the last few decades. The advent of laser cooling has underpinned the development of cold Cs fountain clocks, which now achieve frequency uncertainties of approximately 5×10 −16 . Since 2000, optical atomic clock research has quickened considerably, and now challenges Cs fountain clock performance. This has been suitably shown by recent results for the aluminium Al + quantum logic clock, where a fractional frequency inaccuracy below 10 −17 has been reported. A number of optical clock systems now achieve or exceed the performance of the Cs fountain primary standards used to realize the SI second, raising the issues of whether, how and when to redefine it. Optical clocks comprise frequency-stabilized lasers probing very weak absorptions either in a single cold ion confined in an electromagnetic trap or in an ensemble of cold atoms trapped within an optical lattice. In both cases, different species are under consideration as possible redefinition candidates. In this paper, I consider options for redefinition, contrast the performance of various trapped ion and optical lattice systems, and point to potential limiting environmental factors, such as magnetic, electric and light fields, collisions and gravity, together with the challenge of making remote comparisons of optical frequencies between standards laboratories worldwide.

Selection and amplification of a single optical frequency comb mode for laser cooling of the strontium atoms in an optical clock

2015 ◽  
Vol 107 (15) ◽  
pp. 151104 ◽  
Author(s):  
Hui Liu ◽  
Mojuan Yin ◽  
Dehuan Kong ◽  
Qinfang Xu ◽  
Shougang Zhang ◽  
...  
Keyword(s):  
Laser Cooling ◽  
Frequency Comb ◽  
Optical Frequency Comb ◽  
Optical Clock ◽  
Optical Frequency

scholarly journals AN INSTRUMENTAL BASIS FOR MULTISPECTRAL LIDAR WITH SPECTRALLY-RESOLVED DISTANCE MEASUREMENTS

2019 ◽  
Vol XLII-2/W13 ◽  
pp. 1121-1126
Author(s):  
D. Salido-Monzú ◽  
A. Wieser
Keyword(s):  
Femtosecond Laser ◽  
Laser Scanning ◽  
Distance Measurement ◽  
Distance Measurements ◽  
Spectral Bands ◽  
Laser Scanners ◽  
Propagation Medium ◽  
Set Up ◽  
Spectrally Resolved ◽  
Better Than

<p><strong>Abstract.</strong> Hyperspectral solutions augment laser scanning technology with material probing capabilities by measuring target reflectance along with topography. We propose a novel instrumental basis that enables also spectrally-resolved distance measurement with sufficient sensitivity as to access dispersive phenomena on the reflecting target and along the propagation medium, further enhancing the material analysis capabilities of hyperspectral approaches. To this end we have extended distance metrology using intermode beat notes of a mode-locked femtosecond laser to cover flexibly selected spectral regions. The approach is based on an ultra-broadband source derived from a femtosecond laser via coherent supercontinuum generation. Herein we provide a first demonstration of the successful application of this technique to reflectorless measurements and thus its feasibility for multispectral LiDAR. We use a table-top experimental set-up to assess the approach by measuring distance, spectrally-resolved relative distance and reflectance to 5 different material samples on 5 multiplexed contiguous spectral bands of 50 nm in the range of 600&amp;thinsp;nm to 850&amp;thinsp;nm. We have achieved a distance measurement precision and accuracy better than 100&amp;thinsp;&amp;mu;m using integration times of about 30&amp;thinsp;ms, with promising perspectives regarding scalability to practical distances. The spectrally-resolved distance measurements also show repeatable material-dependent profiles with differences between materials up to several tenths of mm in some spectral bands. Combined with simultaneously acquired reflectance estimations, these profiles enable collecting additional target information, indicating the potential of the approach to enhance the material probing capabilities of prospective multispectral laser scanners.</p>

scholarly journals An Accurate Measurement Method for Azimuth Pointing of Spaceborne Synthetic Aperture Radar Antenna Beams Based on Ground Receiver

Sensors ◽  
2018 ◽  
Vol 18 (8) ◽  
pp. 2626 ◽  
Author(s):  
Weibin Liang ◽  
Zengzeng Jia ◽  
Lihong Kang ◽  
Jun Hong ◽  
Bin Lei ◽  
...  
Keyword(s):  
Synthetic Aperture Radar ◽  
Low Earth Orbit ◽  
Synthetic Aperture ◽  
Complex Sampling ◽  
Satellite Experiment ◽  
Range Migration ◽  
Radar Antenna ◽  
Over The Top ◽  
Better Than ◽  
Aperture Radar

The paper proposes a new method for measuring the azimuth pointing of spaceborne synthetic aperture radar (SAR) antenna beams based on the ground receiver, which can receive and record complex sampling data of the pulse signals transmitted from the spaceborne SAR. The center of the antenna pattern is extracted from the complex sampling data amplitude envelope to obtain the time when the beam main lobe center irradiates the ground receiver, and the range migration information is extracted from the complex sampling data to obtain the time when the satellite is over the top of the ground receiver. The results of Chinese civilian remote sensing GaoFen-3 SAR satellite experiment data processing show that the measurement accuracy of this method is better than 0.002°, which can be applied to the accurate measurement of azimuth pointing of various low Earth orbit (LEO) SAR antenna beams.

scholarly journals PhotoWeb redshift: boosting photometric redshift accuracy with large spectroscopic surveys

2020 ◽  
Vol 636 ◽  
pp. A90 ◽  
Author(s):  
M. Shuntov ◽  
J. Pasquet ◽  
S. Arnouts ◽  
O. Ilbert ◽  
M. Treyer ◽  
...  
Keyword(s):  
Large Scale ◽  
Distribution Functions ◽  
Distance Measurements ◽  
Photometric Redshifts ◽  
Star Forming ◽  
Photometric Redshift ◽  
Probability Distribution Functions ◽  
Galaxy Sample ◽  
New Generation ◽  
Better Than

Improving distance measurements in large imaging surveys is a major challenge to better reveal the distribution of galaxies on a large scale and to link galaxy properties with their environments. As recently shown, photometric redshifts can be efficiently combined with the cosmic web extracted from overlapping spectroscopic surveys to improve their accuracy. In this paper we apply a similar method using a new generation of photometric redshifts based on a convolution neural network (CNN). The CNN is trained on the SDSS images with the main galaxy sample (SDSS-MGS, r ≤ 17.8) and the GAMA spectroscopic redshifts up to r ∼ 19.8. The mapping of the cosmic web is obtained with 680 000 spectroscopic redshifts from the MGS and BOSS surveys. The redshift probability distribution functions (PDF), which are well calibrated (unbiased and narrow, ≤120 Mpc), intercept a few cosmic web structures along the line of sight. Combining these PDFs with the density field distribution provides new photometric redshifts, zweb, whose accuracy is improved by a factor of two (i.e., σ ∼ 0.004(1 + z)) for galaxies with r ≤ 17.8. For half of them, the distance accuracy is better than 10 cMpc. The narrower the original PDF, the larger the boost in accuracy. No gain is observed for original PDFs wider than 0.03. The final zweb PDFs also appear well calibrated. The method performs slightly better for passive galaxies than star-forming ones, and for galaxies in massive groups since these populations better trace the underlying large-scale structure. Reducing the spectroscopic sampling by a factor of 8 still improves the photometric redshift accuracy by 25%. Finally, extending the method to galaxies fainter than the MGS limit still improves the redshift estimates for 70% of the galaxies, with a gain in accuracy of 20% at low z where the resolution of the cosmic web is the highest. As two competing factors contribute to the performance of the method, the photometric redshift accuracy and the resolution of the cosmic web, the benefit of combining cosmological imaging surveys with spectroscopic surveys at higher redshift remains to be evaluated.

Determining the Validity and Accuracy of Multiple Activity-Tracking Devices in Controlled and Free-Walking Conditions

2018 ◽  
Vol 32 (8) ◽  
pp. 1671-1678 ◽  
Author(s):  
Daniel V. Gaz ◽  
Thomas M. Rieck ◽  
Nolan W. Peterson ◽  
Jennifer A. Ferguson ◽  
Darrell R. Schroeder ◽  
...  
Keyword(s):  
Actual Number ◽  
Activity Tracking ◽  
Distance Measurements ◽  
Cross Sectional ◽  
Actual Distance ◽  
Activity Trackers ◽  
Measured Distance ◽  
The Difference ◽  
Tracking Devices ◽  
Better Than

Purpose: Clinicians and fitness professionals are increasingly recommending the use of activity trackers. This study compares commercially available activity tracking devices for step and distance accuracy in common exercise settings. Design: Cross sectional. Setting: Rochester, Minnesota. Participants: Thirty-two men (n = 10) and women (n = 22) participated in the study. Measures: Researchers manually counted steps and measured distance for all trials, while participants wore 6 activity tracking devices that measured steps and distance. Analysis: We computed the difference between the number of steps measured by the device and the actual number of steps recorded by the observers, as well as the distance displayed by the device and the actual distance measured. Results: The analyses showed that both the device and walking trials affected the accuracy of the results (steps or distance, P < .001). Hip-based devices were more accurate and consistent for measuring step count. No significant differences were found among devices or locations for the distance measured. Conclusions: Hip-based activity tracking devices varied in accuracy but performed better than their wrist-based counterparts for step accuracy. Distance measurements for both types of devices were more consistent but lacked accuracy.

Active Optical Clock Based on Laser Cooling of Alkali-metal Atoms

2021 ◽  
Author(s):  
Tiantian Shi ◽  
Duo Pan ◽  
Wei Zhuang ◽  
Xiaolei Guan ◽  
Jianxiang Miao ◽  
...  
Keyword(s):  
Alkali Metal ◽  
Laser Cooling ◽  
Optical Clock ◽  
Metal Atoms ◽  
Active Optical Clock

Time and Latitude in South Africa

1972 ◽  
Vol 1 ◽  
pp. 27-38
Author(s):  
J. Hers
Keyword(s):  
South Africa ◽  
Cape Town ◽  
Astronomical Observations ◽  
Royal Observatory ◽  
The Republic ◽  
Astronomical Determination ◽  
Limited Accuracy ◽  
Better Than

In South Africa the modern outlook towards time may be said to have started in 1948. Both the two major observatories, The Royal Observatory in Cape Town and the Union Observatory (now known as the Republic Observatory) in Johannesburg had, of course, been involved in the astronomical determination of time almost from their inception, and the Johannesburg Observatory has been responsible for the official time of South Africa since 1908. However the pendulum clocks then in use could not be relied on to provide an accuracy better than about 1/10 second, which was of the same order as that of the astronomical observations. It is doubtful if much use was made of even this limited accuracy outside the two observatories, and although there may – occasionally have been a demand for more accurate time, it was certainly not voiced.

Localization of immunolabels by electron microscopy and image averaging

1983 ◽  
Vol 41 ◽  
pp. 282-283
Author(s):  
J. Frank ◽  
P.-Y. Sizaret ◽  
A. Verschoor ◽  
J. Lamy
Keyword(s):  
Electron Microscopy ◽  
Single Particle ◽  
Attachment Site ◽  
Uranyl Acetate ◽  
Limulus Polyphemus ◽  
Resolution Limit ◽  
Electron Microscopic ◽  
Image Averaging ◽  
Top View ◽  
Better Than

The accuracy with which the attachment site of immunolabels bound to macromolecules may be localized in electron microscopic images can be considerably improved by using single particle averaging. The example studied in this work showed that the accuracy may be better than the resolution limit imposed by negative staining (∽2nm).The structure used for this demonstration was a halfmolecule of Limulus polyphemus (LP) hemocyanin, consisting of 24 subunits grouped into four hexamers. The top view of this structure was previously studied by image averaging and correspondence analysis. It was found to vary according to the flip or flop position of the molecule, and to the stain imbalance between diagonally opposed hexamers (“rocking effect”). These findings have recently been incorporated into a model of the full 8 × 6 molecule.LP hemocyanin contains eight different polypeptides, and antibodies specific for one, LP II, were used. Uranyl acetate was used as stain. A total of 58 molecule images (29 unlabelled, 29 labelled with antl-LPII Fab) showing the top view were digitized in the microdensitometer with a sampling distance of 50μ corresponding to 6.25nm.

Closing the GAP—A 5Å Scanning Microscope

1969 ◽  
Vol 27 ◽  
pp. 6-7
Author(s):  
A. V. Crewe
Keyword(s):  
Normal Form ◽  
The Other ◽  
Resolving Power ◽  
Scanning Microscope ◽  
Conventional Microscope ◽  
Other Hand ◽  
Closing The Gap ◽  
Thermal Sources ◽  
Better Than ◽  
Transmission Microscope

We have become accustomed to differentiating between the scanning microscope and the conventional transmission microscope according to the resolving power which the two instruments offer. The conventional microscope is capable of a point resolution of a few angstroms and line resolutions of periodic objects of about 1Å. On the other hand, the scanning microscope, in its normal form, is not ordinarily capable of a point resolution better than 100Å. Upon examining reasons for the 100Å limitation, it becomes clear that this is based more on tradition than reason, and in particular, it is a condition imposed upon the microscope by adherence to thermal sources of electrons.

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