  Photons naturally solve the BosonSampling problem: sample the outputs of a
multi-photon experiment in a linear-optical interferometer. This is strongly
believed to be hard to do on a classical computer, and motivates the
development of technologies that enable precise control of multi-photon
interference in large interferometers. Here we report multi-photon experiments
in a 5-mode integrated interferometer. We use novel three-dimensional
manufacturing techniques to achieve simultaneous control of 25 independent
parameters that describe an arbitrary interferometer. We characterize the chip
using one- and two-photon experiments, and confirm the quantum mechanical
predictions for three-photon interference. Scaled up versions of this setup are
the most promising way to demonstrate the computational capability of quantum
systems, and may have applications in high-precision measurements and quantum
communication.
