In-Situ Lattice Polarization Measurement by Atomic Wave Scattering

Optical dipole traps and lattices have become indispensable tools in atomic physics and atom optics. Especially the accurate alignment of the beam polarization is crucial, because a deviation from purely linear polarization will result in state dependent AC-stark vector light shifts, which are proportional to the atoms' magnetic $m_F$ substates. Such shifts can be either utilized as a tool for state dependent atomic transport and the creation of artificial gauge fields, or, in contrast, could cause unwanted dephasing in quantum information processing and spectroscopic experiments. Here, we present an in-situ measurement method of an optical lattice's polarization purity by employing the Kapitza-Dirac effect - the scattering of atoms by a standing light wave: We create a Rubidium-87 (Rb) BEC and shine in an optical lattice at 790 nm that is tuned in between the $D_1$ and $D_2$ lines of Rb. At this wavelength, the scalar dipole potentials of both lines counteract and ideally cancel out, yielding a high sensitivity to vector light shifts for different $m_F$ states. By analysing the scattering of Rb atoms in the residual potential for different $m_F$ states, we can extract the lattice polarization with high accuracy below $10^{-3}$.