When should we change the definition of the second?

Abstract

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.

Probing beyond the laser coherence time in optical clock comparisons

“…Still, laser frequency noise limits the stability of frequency comparisons well short of the limits imposed by atomic coherence [7], and has so far prevented the use of Heisenberg-limited measurements that realize a quantum enhancement in measurement stability [8,9]. High-stability optical clock comparisons are critical for the future redefinition of the SI second [10,11] and provide a key measurement tool for the parameters of fundamental physical theories [12][13][14], as well as relativistic geodesy with high spatial and temporal resolution [15][16][17]. While there has been a lot of recent progress both towards improving the frequency stability of clock lasers and developing measurement protocols aimed at circumventing clock laser noise using multiple atomic ensembles [18][19][20][21], it is likely that for the foreseeable future optical clock stability will continue to be limited by local oscillator noise.…”

Blackbody radiation shift of the B+clock transition

“…These improvements are expected to result in a new definition of the second [3]. Indeed, an optical clock using the Al + ion using quantum logic technology has been developed with a fractional frequency uncertainty of 8.6 × 10 −18 [4].…”

Dynamic polarizabilities for the low-lying states of Ca+

“…The identification of magic wavelengths and their use in making optical lattices has resulted in the development of optical lattice clocks which have the potential to exceed the performance characteristics of the existing standard for time, namely the cesium microwave clock [8][9][10][11][12].…”