Tomography of the optical polarization rotation induced by a single quantum dot in a cavity

A major challenge in the development of atom-photon and photon-photon quantum gates is to provide output photons in a pure quantum state, as opposed to incoherent superpositions. Here we introduce a tomography approach to describe the optical response of a cavity quantum electrodynamics device, by analyzing the polarization density matrix of the reflected photons in the Poincaré sphere. Applying this approach to an electrically controlled quantum dot (QD)-cavity device, we show that the superposition of emitted single photons with directly reflected photons leads to a large rotation of the output polarization, by 20° both in latitude and longitude in the Poincaré sphere, with a polarization purity remaining above 84%. The QD resonance fluorescence is shown to contribute to the polarization rotation via its coherent part, while its incoherent part contributes to degrading the polarization purity. This polarization tomography technique allows discriminating between various decoherence processes, a powerful tool for solid-state quantum technologies.