Non-reciprocal components such as circulators, isolators and gyrators find utility in numerous microwave wireless applications, including high-power transmitters, simultaneous transmit-and-receive communication and radar systems, and emerging cryogenic quantum computing implementations. Today, such components are implemented using ferrite materials, which lose their reciprocity under the application of an external magnetic field.
However, ferrite materials are incompatible with semiconductor integrated-circuit fabrication processes, and therefore ferrite non-reciprocal components are difficult to miniaturize to chip scales, rendering them bulky and expensive. This has motivated significant research into non-magnetic non-reciprocal components over the past 50 years. In recent years, this research has been invigorated by breakthroughs in time-modulated non-reciprocal components, and their integration into silicon integrated circuits. This paper reviews the history of non-reciprocal electronics, surveys recent research results in the area, and describes outstanding directions for future research.