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Science

Good timing for microwave technology

Ian S. Osborne

Published May 21, 2020• 0 citations
Publisher

Abstract

Metrology Timing standards around the world define the second using atomic clocks, specifically the microwave frequencies emitted from trapped atoms. Optical clocks, which are based on optical transitions of atoms, operate at much higher frequency and have been shown to exhibit better stability. Nakamura et al. demonstrate a framework that carries the improved stability of the optical domain over to microwaves (see the Perspective by Curtis). In addition to contributing to the eventual redefinition of the second based on optical clocks, this work will also lead to improvements in microwave-based technologies such as astronomical imaging and geodesy through very long baseline interferometry, radar, communications, and navigation systems. Science , this issue p. [889][1]; see also p. [825][2] [1]: /lookup/doi/10.1126/science.abb2473 [2]: /lookup/doi/10.1126/science.abb8629

DOI
10.1126/science.368.6493.840-n

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Metrology
Timing standards around the world define the second using atomic clocks, specifically the microwave frequencies emitted from trapped atoms. Optical clocks, which are based on optical transitions of atoms, operate at much higher frequency and have been shown to exhibit better stability. Nakamura et al. demonstrate a framework that carries the improved stability of the optical domain over to microwaves (see the Perspective by Curtis). In addition to contributing to the eventual redefinition of the second based on optical clocks, this work will also lead to improvements in microwave-based technologies such as astronomical imaging and geodesy through very long baseline interferometry, radar, communications, and navigation systems.
Science , this issue p. 889; see also p. 825
Good timing for microwave technology Timing standards around the world define the second using atomic clocks, specifically the microwave frequencies emitted from trapped atoms. Optical clocks, which are based on optical transitions of atoms, operate at much higher frequency and have been shown to exhibit better stability. Nakamura et al. demonstrate a framework that carries the improved stability of the optical domain over to microwaves (see the Perspective by Curtis). In addition to contributing to the eventual redefinition of the second based on optical clocks, this work will also lead to improvements in microwave-based technologies such as astronomical imaging and geodesy through very long baseline interferometry, radar, communications, and navigation systems. Science , this issue p. 889 ; see also p. 825
Optical atomic clocks based on laser-cooled trapped ions and atoms have advanced rapidly over the past decade. With fractional frequency uncertainties now surpassing 10 −18 , they are some of the most precise measurement tools ever built. Yet researchers are still pursuing new avenues of research to explore the fundamental limits to their stability, accuracy, and reproducibility. In this mini-review, we provide a survey of the current state of the art by describing the fundamental principles and techniques that underpin this progress, the architectures used to realize optical clocks, and the supporting laser technologies that are essential to their operation. We also examine the progress that has been made toward a redefinition of the second in the International System of Units and the inclusion of optical clocks into the global time and frequency metrology infrastructure. Finally, we discuss emerging applications of optical clocks and look at the prospects for making their precision more readily accessible to end users.
8half a century ago and have provided the definition of the second since 1967. The international atomic time (TAI) is currently kept by an ensemble of Cs clocks with a fractional uncertainty of Δν/ν0 ≈ 10 –15 , and some Cs fountain clocks 9,10 operate at even smaller uncertainties. Although Cs atomic clocks operate at the microwave transition of ν0 ≈ 9.2 GHz, using optical transition frequencies of several hundred terahertz should improve the fractional uncertainty Δν/ν0 by 4–5 orders of magnitude, as most measurement uncertainties Δν are independent of ν0 (except the Doppler shift Δ νD = (v/c)ν0, where v is the atomic velocity and c is the speed of light). The invention of the laser and the subsequent improvement of laser frequency stabilities 11 has allowed the realization of optical clocks to become a reality 2,3,12
Frequency and its inverse, time, are the most accurately measured quantities. Historically, improvements in the accuracy of clocks have enabled advances in navigation, communication, and science. Since 1967, the definition of the International System (SI) second has been based on the frequency of a microwave transition in cesium, and present day cesium atomic clocks have a fractional uncertainty near 10-16. Recently, a new type of atomic clock based on optical transitions has been developed, with a current fractional uncertainty near 10-18 (approximately one second divided by the age of the universe), and they are rapidly improving. This talk presents a brief summary of the development of optical atomic clocks, with a focus on the Al+ quantum-logic clock developed at NIST. We discuss the current state-of-the-art in optical clock performance, and describe new applications in sensing and fundamental physics. Future directions in optical atomic clock research are also considered.
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.