Recent atomic clock comparisions 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 &#8226 10^-16yr^-1 and offers 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 &#8226 10^-17). The record of such measurements over the last year sensitively tests for a temporal variation of α and constraints (α/α = (–1.6 ± 2.3) &#8226 10^-17yr^-1), consistent with zero.