Recurrent multiport interferometers for fault-tolerant programmable photonics

Universal linear programmable photonic circuits, composed of an array of Mach-Zehnder interferometers (MZIs), are fidelity-limited devices and are prohibitively difficult to scale. To address these limitations, we propose a compact and non-feedforward recurrent MZI topology wherein light can propagate in both directions. Our recurrent mesh achieves O(N) scaling by building imperfect beamsplitters in series to build perfect transformations. We theoretically examine the error tolerance of our proposed recurrent interferometer under imperfect mesh parameters. Our numerical results indicate that our proposed recurrent mesh permits more than an order of magnitude reduction in implementing unitary transformations.