Test of Einstein Equivalence Principle for 0-spin and half-integer-spin atoms: Search for spin-gravity coupling effects

We report on a conceptually new test of the equivalence principle performed by measuring the acceleration in Earths gravity field of two isotopes of strontium atoms, namely, the bosonic $^{88}$Sr isotope which has no spin vs the fermionic $^{87}$Sr isotope which has a half-integer spin. The effect of gravity upon 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 $eta = (0.2pm 1.6)times10^{-7}$ for the Eotvos parameter and $k=(0.5pm1.1)times10^{-7}$ 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.