https://doi.org/10.1140/epjst/e2008-00807-7
Recent atomic clock comparisons at NIST
1
Time and Frequency Division, National Institute of Standards and Technology, Boulder, CO, 80305, USA
2
Istituto Nazionale di Ricerca Metrologica INRiM, Strada delle Cacce 91, 10135 Torino, Italy
3
Department of Physics and Astronomy, Oberlin College, Oberlin, OH, 44074, USA
Corresponding author: l.lorini@inrim.it
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 
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 
. The record of such measurements over the last year sensitively tests for a temporal variation of α and constrains 
, consistent with zero.
© EDP Sciences, Springer-Verlag, 2008
