Frequency metrology on the 4s 2S1/2 - 4p 2P1/2 transition in the calcium   ion for a comparison with quasar data
  High accuracy frequency metrology on the 4s 2S1/2 - 4p 2P1/2 transition in calcium ions is performed using laser cooled and crystallized ions in a linear Paul trap. Calibration is performed with a frequency comb laser, resulting in a transition frequency of f=755222766.2(1.7) MHz. The accuracy presents an improvement of more than one order of magnitude, and will facilitate a comparison with quasar data in a search for a possible change of the fine structure constant on a cosmological time scale. 
A.L. Wolf, S.A. van den Berg, C. Gohle, E.J. Salumbides, W. Ubachs,   K.S.E. Eikema