MG11 
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Limits on temporal variations of fundamental constants from comparisons of optical clocks

Optical frequency standards based on laser-cooled trapped ions have now reached relative uncertainties that are comparable to those achieved with the best caesium clocks. At the same time, they offer significantly improved stability because of the higher operating frequency. We will present recent results obtained with the Yb+ frequency standard at PTB. An interesting application is the search for a possible temporal variation of the fine structure constant based on comparisons between optical, electronic transition frequencies in different ions. The currently most stringent, model-independent upper limit for d ln alpha / dt is 2E-15 per year, based on measurements of the Yb+ frequency standard, the Hg+ frequency standard at NIST and the hydrogen 1S-2S frequency at MPQ against primary caesium clocks. More recent data allow to tighten this limit by nearly a factor of 4 and a significantly increased sensitivity to changes of alpha can be expected in the near future from direct optical frequency ratio measurements.