Ultracold atoms and precise time standards

2011 ◽  
Vol 369 (1953) ◽  
pp. 4078-4089 ◽  
Author(s):  
Gretchen K. Campbell ◽  
William D. Phillips
Keyword(s):  
Laser Cooling ◽  
International System ◽  
Microwave Frequency ◽  
Observation Time ◽  
Mass Motion ◽  
Frequency Standards ◽  
Systematic Uncertainties ◽  
System Of Units ◽  
Time Standards ◽  
Atomic Frequency

Experimental techniques of laser cooling and trapping, along with other cooling techniques, have produced gaseous samples of atoms so cold that they are, for many practical purposes, in the quantum ground state of their centre-of-mass motion. Such low velocities have virtually eliminated effects such as Doppler shifts, relativistic time dilation and observation-time broadening that previously limited the performance of atomic frequency standards. Today, the best laser-cooled, caesium atomic fountain, microwave frequency standards realize the International System of Units (SI) definition of the second to a relative accuracy of ≈3×10 −16 . Optical frequency standards, which do not realize the SI second, have even better performance: cold neutral atoms trapped in optical lattices now yield relative systematic uncertainties of ≈1×10 −16 , whereas cold-trapped ions have systematic uncertainties of 9×10 −18 . We will discuss the current limitations in the performance of neutral atom atomic frequency standards and prospects for the future.

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.

scholarly journals Spectral Bias Estimation of Historical HIRS Using IASI Observations for Improved Fundamental Climate Data Records

2009 ◽  
Vol 26 (7) ◽  
pp. 1378-1387 ◽  
Author(s):  
Changyong Cao ◽  
Mitch Goldberg ◽  
Likun Wang
Keyword(s):  
Measurement Errors ◽  
International System ◽  
Spectral Response ◽  
Observation Time ◽  
Lapse Rate ◽  
Polar Regions ◽  
Bias Estimation ◽  
Climate Data ◽  
Infrared Observation ◽  
System Of Units

Abstract A prerequisite for climate change detection from satellites is that the measurements from a series of historical satellites must be consistent and ideally made traceable to the International System of Units (SI). Unfortunately, this requirement is not met for the 14 High Resolution Infrared Radiation Sounders (HIRS) on the historical NOAA satellites, because the instrument was developed for weather forecasts and lacks accuracy and consistency across satellites. It is well known that for HIRS, differences in the spectral response functions (SRF) between instruments and their prelaunch measurement uncertainties often lead to observations of the atmosphere at different altitudes. As a result of the atmospheric lapse rate, they both can introduce significant intersatellite biases. The SRF-dependent biases are further mixed with other effects such as the diurnal cycle because of observation time differences and orbital drifts, on board calibration, and algorithm issues. In this study, the Infrared Atmospheric Sounding Interferometer (IASI) observations are used to calculate the radiances for the 14 Television Infrared Observation Satellite series N (TIROS-N; to MetOp-A) HIRS instruments in different climate regimes and seasons to separate the SRF-induced intersatellite biases from other factors. It is found that the calculated radiance ratio (a bias indicator) using IASI observations for the HIRS satellite pairs forms bell-shaped curves that vary with the HIRS model and channel as well as climate regimes. This suggests that a bias found in the polar regions at the Simultaneous Nadir Overpass (SNO) cannot be blindly used for bias correction globally; instead, the IASI/HIRS spectral bias bell curves should be used as a complement to more fully address the biases. These bell curves also serve as lookup charts for separating the bias due to true SRF differences from that caused by SRF prelaunch measurement errors to resolve the inconsistency, which sheds new light on reprocessing and reanalysis in generating fundamental climate data records from HIRS.

scholarly journals Time scales in the context of general relativity

2011 ◽  
Vol 369 (1953) ◽  
pp. 4131-4142 ◽  
Author(s):  
Bernard Guinot
Keyword(s):  
Relativistic Effect ◽  
International System ◽  
Centre Of Mass ◽  
Frequency Standards ◽  
The Earth ◽  
International Astronomical ◽  
System Of Units ◽  
Definition Of ◽  
Atomic Time

Towards 1967, the accuracy of caesium frequency standards reached such a level that the relativistic effect could not be ignored anymore. Corrections began to be applied for the gravitational frequency shift and for distant time comparisons. However, these corrections were not applied to an explicit theoretical framework. Only in 1991 did the International Astronomical Union provide metrics (then improved in 2000) for a definition of space–time coordinates in reference systems centred at the barycentre of the Solar System and at the centre of mass of the Earth. In these systems, the temporal coordinates (coordinate times) can be realized on the basis of one of them, the International Atomic Time (TAI), which is itself a realized time scale. The definition and the role of TAI in this context will be recalled. There remain controversies regarding the name to be given to the unit of coordinate times and to other quantities appearing in the theory. However, the idea that astrometry and celestial mechanics should adopt the usual metrological rules is progressing, together with the use of the International System of Units, among astronomers.

scholarly journals New SI and precision measurements: an interview with Tianchu Li

2020 ◽  
Vol 7 (12) ◽  
pp. 1837-1840
Author(s):  
Jin Wang
Keyword(s):  
International System ◽  
Precision Measurements ◽  
Weights And Measures ◽  
Frequency Standards ◽  
System Of Units ◽  
Constant E ◽  
Most Significant Change ◽  
History Of ◽  
Metre Convention ◽  
New Si

Abstract On 13–16 November 2018, the 26th General Conference of Weights and Measures (CGPM) was held in Paris. The conference adopted Resolution A on ‘Revision of the International System of Units (SI).’ According to Resolution A: four of the SI basic units, namely kilograms, amps, kelvin and mole, are defined by the Planck constant h, the basic charge constant e, the Boltzmann constant k and the Avogadro constant NA, respectively. This establishes the basic quantities and units in SI on a series of constants. The new SI was officially launched on 20 May 2019. This is the most significant change and a milestone in the history of metrology since the Metre Convention was signed in 20 May 1875. Professor Tianchu Li, an academician of the Chinese Academy of Engineering, has been working on time and frequency standards for 37 years. In this interview, Prof. Li reviews the quantization and constant evolutions of the second and meter, and introduces the redefinitions of ampere, kelvin, kilogram and mole, and their significance for precision measurements.

Dimensions of plane and solid angles and their units in the International System of Units (SI)

2020 ◽  
pp. 26-32
Author(s):  
M. I. Kalinin ◽  
L. K. Isaev ◽  
F. V. Bulygin
Keyword(s):  
Solid Angle ◽  
International System ◽  
International Committee ◽  
Plane Angle ◽  
Arc Length ◽  
Weights And Measures ◽  
International System Of Units ◽  
System Of Units ◽  
In Series ◽  
Solid Angles

The situation that has developed in the International System of Units (SI) as a result of adopting the recommendation of the International Committee of Weights and Measures (CIPM) in 1980, which proposed to consider plane and solid angles as dimensionless derived quantities, is analyzed. It is shown that the basis for such a solution was a misunderstanding of the mathematical formula relating the arc length of a circle with its radius and corresponding central angle, as well as of the expansions of trigonometric functions in series. From the analysis presented in the article, it follows that a plane angle does not depend on any of the SI quantities and should be assigned to the base quantities, and its unit, the radian, should be added to the base SI units. A solid angle, in this case, turns out to be a derived quantity of a plane angle. Its unit, the steradian, is a coherent derived unit equal to the square radian.

Possible investigation of relativistic effects with the aid of molecular and atomic frequency standards

1961 ◽  
Vol 75 (9) ◽  
pp. 3-59 ◽  
Author(s):  
N.G. Basov ◽  
Oleg N. Krokhin ◽  
A.N. Oraevskii ◽  
G.M. Strakhovskii ◽  
B.M. Chikhachev
Keyword(s):  
Relativistic Effects ◽  
Frequency Standards ◽  
Atomic Frequency
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