Recent atomic clock comparisons at NIST
The record of atomic clock frequency comparisons at NIST over the past half-decade provides one of the tightest constraints of any present-day temporal variations of the fundamental constants. Notably, the 6-year record of increasingly precise measurements of the absolute frequency of the Hg+ single-ion optical clock (using the cesium primary frequency standard NIST-F1) constrains the temporal variation of the fine structure constant α to less than 2 · 10−6yr−1 and offers a Local Position Invariance test in the framework of General Relativity. The most recent measurement of the frequency ratio of the Al+ and Hg+ optical clocks is reported with a fractional frequency uncertainty of ±5.2 · 10−17. The record of such measurements over the last year sensitively tests for a temporal variation of α and constrains α˙/α=(−1.6±2.3) . 10−17yr−1, consistent with zero.
Lorini, L., N. Ashby, A. Brusch, et al. 2008. "Recent atomic clock comparisons at NIST." The European Physical Journal Special Topics 163(1): 19-35.
The European Physical Journal Special Topics
Physics and Astronomy