  The comparison of different atomic transition frequencies over time can be
used to determine the present value of the temporal derivative of the fine
structure constant alpha 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 ^{171}Yb+ with a cesium atomic clock at two
times separated by 2.8 years and find a value for the fractional variation of
the frequency ratio $f_{\rm Yb}/f_{\rm Cs}$ of $(-1.2\pm 4.4)\cdot 10^{-15}$
yr$^{-1}$, consistent with zero. Combined with recently published values for
the constancy of other transition frequencies this measurement sets an upper
limit on the present variability of alpha at the level of $2.0\cdot 10^{-15}$
yr$^{-1}$, corresponding so far to the most stringent limit from laboratory
experiments.
