The present disclosure generally relates to techniques for measuring a phase difference between a first set of photons and a second set of photons. The techniques can include directing the first set of photons to a first parametric downconverter, directing the second set of photons to a second parametric downconverter, directing photons output from an exotic photon source to the first downconverter and directing photons output from the first parametric downconverter to a first beam splitter, directing photons output from an exotic photon source to the first beam splitter, directing photons output from the second parametric downconverter to a second beam splitter, directing photons output from the first beam splitter to the second beam splitter, detecting photons output from the second beam splitter, deriving, from the detecting, a phase difference between the first set of photons and the second set of photons, and outputting the phase difference.