Null gravitational redshift experiment with nonidentical atomic clocks

A test of the position invariance (LPI) principle embodied in the Einstein equivalence principle (EEP) has been performed via a ‘‘null’’ gravitational redshift experiment. The rate of a magnesium frequency standard has been compared with that of a cesium reference clock searching for a dependence on the solar gravitational potential during a period of 430 days. Because of the Earth’s orbital motion during the experiment, the solar potential in the laboratory had a peak-to-peak variation of 6.7×${10}^{\mathrm{-}10}$, allowing us to set an upper limit on the relative frequency variation of 7×${10}^{\mathrm{-}4}$ of the external potential. This result represents an improvement of more than one order of magnitude with respect to previous analogous tests of the LPI principle and leads also to a more rigorous limit on a possible spatial variation of the fine-structure constant..