Interference has been observed in the mixing of signal photons produced by spontaneous down-conversion in two nonlinear crystals, when the optical path difference through the interferometer greatly exceeds the coherence length of the light. The interference shows up as a periodic modulation of the spectrum. We demonstrate that the interference is nonclassical and connected with the entangled state of the signal and idler photons, by transferring the long optical delay from the path of the signal photons to the path of the idler photons. Even though the idlers are not interfering directly, the effect is found to be unchanged, but the interference is lost when the idler photons are blocked. We discuss the extent to which one photon carries the modulated spectrum.

• Received 24 August 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.3041