Measurement of the Aharonov-Casher geometric phase with a separated-arm atom interferometer

In this letter, we report a measurement of the Aharonov-Casher (AC) geometric phase with our lithium atom interferometer. The AC phase appears when a particle carrying a magnetic dipole propagates in a transverse electric field. The first measurement of the AC phase was done with a neutron interferometer in 1989 by A. Cimmino \textit{et al.} (Phys. Rev. Lett. \textbf{63}, 380, 1989) and all the following experiments were done with Ramsey or Ramsey-Bord\'e interferometers with molecules or atoms. In our experiment, we use lithium atoms pumped in a single hyperfine-Zeeman sublevel and we measure the AC-phase by applying opposite electric fields on the two interferometer arms. Our measurements are in good agreement with the expected theoretical values and they prove that this phase is independent of the atom velocity.