Microwave Nonreciprocity

In this article, we provide a comprehensive review of past and present efforts in the field of microwave nonreciprocity with an emphasis on the most commonly used nonreciprocal devices, namely gyrators, isolators, and circulators. After discussing the origin and history of such efforts, we elaborate on the pros and cons of four distinct approaches to break reciprocity which include magnetic biasing of ferrite materials, intrinsic nonreciprocity of solid-state devices, nonlinearities with geometric asymmetries, and finally, periodic spatiotemporal variation. We subsequently pay due attention to the spatiotemporal approach and show that the recently emerging proposals to achieve magnet-free nonreciprocity with linear, periodically time-varying circuits can compete with traditional ferrite devices and even outperform them in several metrics, thus opening the door to exciting venues for miniaturized low-cost and high-performance nonreciprocal devices with numerous applications ranging from full-duplex communications and quantum computing to biomedical imaging and radar systems.