We report on a conceptually new test of the equivalence principle performed by measuring the acceleration in Earth’s gravity field of two isotopes of strontium atoms, namely, the bosonic ${}^{88}\mathrm{Sr}$ isotope which has no spin versus the fermionic ${}^{87}\mathrm{Sr}$ isotope which has a half-integer spin. The effect of gravity on the two atomic species has been probed by means of a precision differential measurement of the Bloch frequency for the two atomic matter waves in a vertical optical lattice. We obtain the values $\eta =(0.2\pm 1.6)\times {10}^{-7}$ for the Eötvös parameter and $k=(0.5\pm 1.1)\times {10}^{-7}$ for the coupling between nuclear spin and gravity. This is the first reported experimental test of the equivalence principle for bosonic and fermionic particles and opens a new way to the search for the predicted spin-gravity coupling effects.

• Received 24 February 2014

DOI:https://doi.org/10.1103/PhysRevLett.113.023005