Test of the Universality of Free Fall with Atoms in Different Spin Orientations

We report a test of the universality of free fall by comparing the gravity acceleration of the ⁸⁷Rb atoms in mF=+1 versus those in mF=-1, of which the corresponding spin orientations are opposite. A Mach-Zehnder-type atom interferometer is exploited to alternately measure the free fall acceleration of the atoms in these two magnetic sublevels, and the resultant Eötvös ratio is ηS=(0.2±1.2)×10⁻⁷. This also gives an upper limit of 5.4×10⁻⁶ m⁻² for a possible gradient field of the spacetime torsion. The interferometer using atoms in mF=±1 is highly sensitive to the magnetic field inhomogeneity. A double differential measurement method is developed to alleviate the inhomogeneity influence, of which the effectiveness is validated by a magnetic field modulating experiment.