  Multi-photon propagation in connected structures - a quantum walk - offers
the potential for simulating complex physical systems and provides a route to
universal quantum computation. Increasing the complexity of quantum photonic
networks where the walk occurs is essential for many applications. Here, we
implement a quantum walk of indistinguishable photon pairs in a multimode fiber
supporting 380 modes. Using wavefront shaping, we control the propagation of
the two-photon state through the fiber in which all modes are coupled.
Excitation of arbitrary output modes of the system is realized by controlling
classical and quantum interferences. This experiment demonstrates a highly
multimode platform for multi-photon interference experiments and provides a
powerful method to program a general high-dimensional multiport optical
circuit. This work paves the way for the next generation of photonic devices
for quantum simulation, computing and communication.
