Practical Implementations of Photonic Quantum Walks on Graphs

Discrete-time quantum walks have become a subject of great interest because of their potential applications for quantum information processing tasks. Quantum walk-based algorithms provide significant speedup over classical algorithms in a range of tasks including search algorithms, solving the element distinctness problem, and evaluating NAND trees. Up to this point, work on quantum walk-based algorithms has been largely theoretical, as discrete-time quantum walk systems are difficult to implement beyond a few time steps. Here we demonstrate a step forward in producing practical experimental realization of optical quantum walks by means of directionally-unbiased linear optical multiports, which have recently been experimentally demonstrated in the form of tabletop setups. These multiports or other related devices, such as reversible optical tritters, can be considered as additional fundamental blocks for quantum walk applications, and can serve as scattering vertices for practical implementations of optical graph systems.