Test of Equivalence Principle at ${10}^{-8}$ Level by a Dual-species Double-diffraction Raman Atom Interferometer

We report an improved test of the weak equivalence principle by using a simultaneous ${}^{85}$Rb-${}^{87}$Rb dual-species atom interferometer. We propose and implement a four-wave double-diffraction Raman transition scheme for the interferometer, and demonstrate its ability in suppressing common-mode phase noise of Raman lasers after their frequencies and intensity ratios are optimized. The statistical uncertainty of the experimental data for E"{o}tv"{o}s parameter $\eta$ is $0.8\times {10}^{-8}$ at 3200 s. With various systematic errors corrected the final value is $\eta =(2.8\pm 3.0)\times {10}^{-8}$. The major uncertainty is attributed to the Coriolis effect.