Randomization of quantum relative phase in welcher Weg measurements

Welcher Weg (which-path) detectors where complementarity is enforced without altering the interfering beams are analysed in terms of the quantum relative phase. In such a case, the measurement disturbs the interference via random classical phase shifts. This applies when the interfering particles are atoms or photons. In the case of photons, the quantum relative phase coincides with the field phase difference.