Although it is well known that a series of individual photons will build up an interference pattern in a double slit setup, there have been few (if any) experiments where such a pattern is built up by sending photons through one at a time. Here is a great experiment from one of the top quantum optics teams using entangled* photons: Time-resolved double-slit interference pattern measurement with entangled photons Piotr Kolenderski, Carmelo Scarcella, Kelsey D. Johnsen, Deny R. Hamel, Catherine Holloway, Lynden K. Shalm, Simone Tisa, Alberto Tosi, Kevin J. Resch, Thomas Jennewein Abstract: "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." http://arxiv.org/abs/1304.4943 The use of entangled photons allows the photons to be heralded (ie announced in advance). It is therefore clear that the pattern is built from independent photons. Some very interesting work, and a great demonstration of the theory! * Entangled photons normally do not produce interference patterns. But these have been specifically modified to prevent polarization attributes to allow which-slit information to be obtained. So interference is possible.