The paper "Programmable Photonic Circuits" by Bogaerts et al. (2020) discusses the emerging field of programmable photonic integrated circuits (PICs). These circuits, enabled by the maturity of integrated photonic technology, can be programmed in software to perform a wide range of functions on a single chip through waveguide meshes, tunable beam couplers, and optical phase shifters. The authors cover recent developments in building blocks, circuit architectures, electronic control, and programming strategies. They highlight applications in linear matrix operations, quantum information processing, and microwave photonics, emphasizing how these generic chips can accelerate future photonic circuit development. The paper also delves into the technology stack required for programmable PICs, including photonic chip fabrication, phase shifters, monitors, control loops, electronics, RF components, packaging, and programming algorithms. Finally, it explores potential applications in linear vector-matrix products, microwave photonics, optical beamforming, and sensing, and discusses the perspectives for programmable PICs in quantum information processing and artificial neural networks.The paper "Programmable Photonic Circuits" by Bogaerts et al. (2020) discusses the emerging field of programmable photonic integrated circuits (PICs). These circuits, enabled by the maturity of integrated photonic technology, can be programmed in software to perform a wide range of functions on a single chip through waveguide meshes, tunable beam couplers, and optical phase shifters. The authors cover recent developments in building blocks, circuit architectures, electronic control, and programming strategies. They highlight applications in linear matrix operations, quantum information processing, and microwave photonics, emphasizing how these generic chips can accelerate future photonic circuit development. The paper also delves into the technology stack required for programmable PICs, including photonic chip fabrication, phase shifters, monitors, control loops, electronics, RF components, packaging, and programming algorithms. Finally, it explores potential applications in linear vector-matrix products, microwave photonics, optical beamforming, and sensing, and discusses the perspectives for programmable PICs in quantum information processing and artificial neural networks.