Experiments with Downconverted Photons: the Quantum Eraser

This experiment investigates properties of light at the single photon level. We generate pairs of 915.8-nm photons through parametric downconversion by sending a beam of 457.9-nm light through a BBO crystal. These downconverted photons are detected using two avalanche photodiodes. We then create a distinguishable characteristic in one arm of a Mach-Zehnder interferometer by adding a half waveplate. This will destroy the single photon interference pattern that is observed when single photons are sent through an interferometer with indistinguishable arms. We will regain the interference pattern by adding a 45° polarizer in front of our avalanche photodiode. This polarizer effectively erases the distinguishing information added by the half waveplate.