  Quantum walks have a host of applications, ranging from quantum computing to
the simulation of biological systems. We present an intrinsically stable,
deterministic implementation of discrete quantum walks with single photons in
space. The number of optical elements required scales linearly with the number
of steps. We measure walks with up to 6 steps and explore the
quantum-to-classical transition by introducing tunable decoherence. Finally, we
also investigate the effect of absorbing boundaries and show that decoherence
significantly affects the probability of absorption.
