Absolute polarization measurement using a vector light shift

We have measured the vector light shift due to a cavity built-up optical lattice by using a variation of the Hanle effect with trapped Cs atoms, where the time-evolving population of all magnetic sublevels is measured in situ. The measurement is linearly sensitive to the electric field of the nonlinearly polarized light, which allows unprecedented sensitivity to absolute linear polarization quality, to the level of 10^-10 in fractional intensity. Our approach to measuring and improving linear polarization can be applied to electron electric dipole moment searches, optical lattice clocks, magnetometery, and quantum computing.