Experimental test of the collapse time of a delocalized photon state.

We investigate whether the collapse of the quantum state of a single photon split between two space-like separated places takes a nonvanishing time. We realize this by using a source of heralded single photons, then splitting the resulting single photon state and letting it propagate over distances much larger than the experimental time resolution times the speed of light c. We find no additional delay within our accuracy and set a lower limit for the speed of collapse of the quantum state to 1550c.