Measuring optical activity with unpolarized light: Ghost polarimetry

Quantifying the optical activity of a sample requires the precise measurement of the rotation of the plane of linear polarization of the transmitted light. Central to this notion is that the sample needs to be exposed to light of a defined polarization state. We show that by using a polarization-entangled photon source we can measure optical activity whilst illuminating a sample with unpolarized light. This not only allows for low light measurement of optical activity but also allows for the analysis of samples that would otherwise be perturbed if subjected to polarized light.