Optimal design of error-tolerant reprogrammable multiport interferometers

Photonic information processing demands programmable multiport interferometers capable of implementing arbitrary transfer matrices, for which planar meshes of error-sensitive Mach-Zehnder interferometers are usually exploited. We propose an alternative design that uses a single static beam-splitter and a variable phase shift as the building block. The design possesses superior resilience to manufacturing errors and losses without extra elements added into the scheme. Namely, the power transmissivities of the static BSs can take arbitrary values in the range from $\approx{}1/2$ to $\approx{}4/5$. We show that the fraction of transfer matrices non-implementable by the interferometers of our design diminishes rapidly with their size.