The relative phase between spatially separated component waves of a single photon can be measured by joint interference with a second photon emitted by a known source. In the case of a single such phase (i.e. two component waves), the probability for a successful measurement is one half. This method can be implemented with current experimental techniques.
@article{arxiv.quant-ph/0511129,
title = {How to Measure the Nonlocal Phase of a Single Photon},
author = {Noam Erez},
journal= {arXiv preprint arXiv:quant-ph/0511129},
year = {2007}
}