Quantum coherence of a single photon

Recent experiments with parametrically downconverted photons in a nonlinear optical material provide us with a new method to determine the path of a photon without disturbing it. In this paper, we will review experimental results illustrating that the mere possibility of identifying the path of a photon is enough to wipe out interference effects. We conclude that quantum coherence is merely the complementary manifestation of the photonʹs intrinsic path indistinguishability. An on-going experiment aimed at measuring the space-time relation in quantum coherence is also discussed. Potential applications are examined.