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. ne structure constant a to less than 2 . 10(-16) 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 a and constrains. (alpha) over dot/alpha = (-1.6 +/- 2.3) . 10(-17) yr(-1), consistent with zero.