Test of the universality of free fall with atoms in different spin   orientations
  We report a test of the universality of free fall (UFF) by comparing the gravity acceleration of the $^{87}$Rb atoms in $m_F=+1$ versus that in $m_F=-1$, where the corresponding spin orientations are opposite. A Mach-Zehnder-type atom interferometer is exploited to sequentially measure the free fall acceleration of the atoms in these two magnetic sublevels, and the resultant E$\rm{\ddot{o}}$tv$\rm{\ddot{o}}$s ratio is ${\eta _S} =(0.2\pm1.2)\times 10^{-7}$. This also gives an upper limit of $1.1\times 10^{-21}$ GeV/m for possible gradient field of the spacetime torsion. The interferometer using atoms in $m_F=\pm 1$ is highly sensitive to the magnetic field inhomogeneity, and a double differential measurement method is developed to alleviate the inhomogeneity influence. Moreover, a proof experiment by modulating the magnetic field is performed, which validates the alleviation of the inhomogeneity influence in our test. 
Xiao-Chun Duan, Xiao-Bing Deng, Min-Kang Zhou, Ke Zhang, Wen-Jie Xu,   Feng Xiong, Yao-Yao Xu, Cheng-Gang Shao, Jun Luo, and...
physics.atom-ph physics.atm-clus