Quantum Walk Experiments Using a 32 × 32 Silicon Photonic Path-Independent Insertion Loss Switch as a Stable Multiport Interferometer

We study a 32 × 32 silicon photonic path-independent insertion loss switch used as a programmable multiport interferometer, with a robustness to optical losses appropriate for many classical and quantum photonic applications since any route through the interferometer has the same loss. Its operation stability was investigated by monitoring the fidelity (closeness to the ideal case) of single-photon quantum-walk experiments, where heralded single photons with a 10-nm bandwidth centered at 1547 nm were used as walkers in the interferometer. Fidelities of 0.98 and 0.97 were observed for 4- and 8-step quantum walks, respectively. The experiment took 21 hours to collect the data for the fidelity calculations. Similar fidelities were confirmed using narrowband classical light sources (200-kHz linewidth), for which a fidelity degradation as small as10⁻⁵/hour was achieved in the 45-hour free-running operation. The experiments demonstrate that our optical circuit has a wide range of potential applications to classical and quantum photonic processors based on a multiport input–output interference design.