  Quantum random walks are the quantum counterpart of classical random walks,
and were recently studied in the context of quantum computation. A quantum
random walker is subject to self interference, leading to a remarkably
different behavior than that of classical random walks such as ballistic
propagation or localization due to disorder. Physical implementations of
quantum walks have only been made in very small scale systems severely limited
by decoherence. Here we show that the propagation of photons in waveguide
lattices, which have been studied extensively in recent years, are essentially
an implementation of quantum walks. Since waveguide lattices are easily
constructed at large scales and display negligible decoherence, they can serve
as an ideal and versatile experimental playground for the study of quantum
walks and quantum algorithms. We experimentally observe quantum walks in large
systems (~100 sites) and confirm quantum walks effects which were studied
theoretically, including ballistic propagation, disorder and boundary related
effects.

Quantum Physics;Other Condensed Matter;Optics