New limit on the present temporal variation of the fine structure constant

A comparison of different atomic frequency standards over time can be used to perform a measurement of the present value of the temporal derivative of the fine structure constant a in a model-independent way without assumptions on constancy or variability of other parameters. We have measured an optical transition frequency at 688 THz in Yb+ with a cesium atomic clock at two times separated by 2.8 years and find that a variation of this frequency can be excluded within a 1 sigma relative uncertainty of 4.4 center dot 10(-15) yr(-1). Combined with recently published values for the constancy of other transition frequencies this measurement provides a limit on the present variability of a at the level of 2.0 center dot 10(-15) yr(-1). Constraints are also derived for the drift rates of other fundamental constants like the electron/proton mass ratio and the proton g-factor.