  The double-slit experiment strikingly demonstrates the wave-particle duality
of quantum objects. In this famous experiment, particles pass one-by-one
through a pair of slits and are detected on a distant screen. A distinct
wave-like pattern emerges after many discrete particle impacts as if each
particle is passing through both slits and interfering with itself. While the
direct event-by-event buildup of this interference pattern has been observed
for massive particles such as electrons, neutrons, atoms and molecules, it has
not yet been measured for massless particles like photons. Here we present a
temporally- and spatially-resolved measurement of the double-slit interference
pattern using single photons. We send single photons through a birefringent
double-slit apparatus and use a linear array of single-photon detectors to
observe the developing interference pattern. The analysis of the buildup allows
us to compare quantum mechanics and the corpuscular model, which aims to
explain the mystery of single-particle interference. Finally, we send one
photon from an entangled pair through our double-slit setup and show the
dependence of the resulting interference pattern on the twin photon's measured
state. Our results provide new insight into the dynamics of the buildup process
in the double-slit experiment, and can be used as a valuable resource in
quantum information applications.
