Trapped Ion Optical Frequency Standards for Laboratory Tests of Alpha-Variability

Abstract

Abstract. Optical transition frequencies of trapped laser-cooled ions have been measured so far with an uncertainty of 10 −14 with reference to primary cesium clocks. Systematic frequency shifts in these systems should be controllable to significantly higher accuracy. We have performed comparisons between frequency standards based on two independently stored 171 Yb + ions. The present experimental results indicate a relative instability (Allan standard deviation) of the optical frequency difference between the two systems of 1.0·10−15 at an averaging time of 1000 s and a mean frequency difference of 0.2 Hz for the reference frequency of 688 THz. Monitoring the absolute transition frequency over a period of a few years at this level of accuracy will lead to a stringent limit on the present value of the temporal derivative of the fine-structure constant. First results have been obtained with a frequency standard based on 199 Hg + at NIST and, more recently, also with Yb + . The combination of results obtained with different transitions can be used to separate the electronic and nuclear contributions to a drift of a transition frequency. We propose optical spectroscopy of a nuclear transition in 229 Th 3+ as a means for a precision monitoring of possible temporal variations in the strength of the nuclear forces.