Programmable photonic unitary circuits for light computing

TL;DR

This paper reviews the design principles and recent advancements in programmable photonic unitary circuits, emphasizing integrated platforms, different design strategies, and approaches to scalability for light-based computing and quantum applications.

Contribution

It provides a comprehensive classification and analysis of various design strategies for programmable photonic unitary circuits, highlighting recent progress and scalability solutions.

Findings

Classification of design strategies based on unitary operation dimensionality

Discussion of physical axes like temporal and frequency domains for scalability

Analysis of trade-offs in different photonic circuit implementations

Abstract

Unitarity serves as a fundamental concept for characterizing linear and conservative wave phenomena in both classical and quantum systems. Developing platforms that perform unitary operations on light waves in a uni-versal and programmable manner enables the emulation of complex light-matter interactions and the execution of general-purpose functionalities for wave manipulations, photonic computing, and quantum circuits. Recent-ly, numerous approaches to implementing programmable photonic unitary circuits have been proposed and demonstrated, each employing different design strategies that distinctly impact overall device performance. Here, we review foundational design principles and recent achievements in the implementation of programma-ble photonic unitary circuits, with a particular focus on integrated photonic platforms. We classify the design strategies based on the dimensionality of nontrivial unit operations in their building blocks: lower-dimensional unitary units, such as SU(2) operations, and higher-dimensional ones, such as Fourier transforms. In each cate-gory, recent efforts to leverage alternative physical axes, such as the temporal and frequency domains, to ad-dress scalability challenges are also reviewed. We discuss the underlying concepts, design procedures, and trade-offs of each design strategy, especially in relation to light-based computing.