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 −6 yr −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 , consistent with zero.