, Volume 163, Issue 1, pp 19-35
Date: 14 Nov 2008

Recent atomic clock comparisons at NIST

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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 \(\dot{\alpha}/\alpha = (-1.6 \pm 2.3)\ .\ 10^{-17} {\rm yr}^{-1}\), consistent with zero.