Global optimization of universal unitary photonic integrated circuits

Universal unitary photonic integrated circuits (PICs) can have widespread applications in optical communication, deep learning, and quantum information processing, due to their ability to perform arbitrary unitary transformations in the optical domain. While large-scale universal unitary PICs have been demonstrated, their performances are fundamentally sensitive to fabrication errors. In this work, we propose a method to compensate for the errors by adding extra MZI stages and globally optimizing all the phase shifts using a simulated annealing algorithm. Furthermore, we experimentally demonstrate its feasibility using a 4×4 universal unitary PIC fabricated on a Si-on-insulator (SOI) platform.