Optical Implementation of 2 x 2 Universal Unitary Matrix Transformations

[EN] Unitary operations are a specific class of linear transformations that have become an essential ingredient for the realization of classical and quantum information processing. The ability of implementing any n-dimensional unitary signal transformation by using a reconfigurable optical hardware has recently led to the pioneering concept of programmable linear optical processor, whose basic building block (BB) must be correctly designed to guarantee that the whole system is able to perform nxn universal (i.e., arbitrary) unitary matrix transformations. Here, it is demonstrated that the present architectures of the BB do not fulfil the universal unitary property (at least) in 2x2 optical processors, limiting the number of unitary matrix transformations that may be generated. Aiming to solve this fundamental constraint, the theoretical tools required to analyze and design 2x2 universal unitary optical circuits and their corresponding BBs are presented. The consequences of this mathematical framework are explored, obtaining a simple route to implement different BB architectures, all of them guaranteeing a true universal unitary functionality in the resulting 2x2 optical processors. These findings may pave the way to revisit the design of high-dimensional unitary optical processors, unleashing the potential of programmable integrated photonics technology. This work was supported by the ERC-ADG-2016-741415 UMWP-Chip, ERC-POC-2019-859927 FPPAs, H2020-ICT-2019-021-871330 Neoteric, Generalitat Valenciana PROMETEO 2017/017 research excellency award, Infraestructuras FEDER IDIFEDER/2018/031, and the Spanish Juan de la Cierva Fellowship MICNN. Macho-Ortiz, A.; Pérez-López, D.; Capmany Francoy, J. (2021). Optical Implementation of 2 x 2 Universal Unitary Matrix Transformations. Laser & Photonics Review. 15(7):1-8. https://doi.org/10.1002/lpor.202000473