Nonreciprocal Electronic Devices: A Hypothesis Turned Into Reality

One of the earliest references to reciprocity as an underlying symmetry within a system dates back to the 1828 work of Green in electrostatics, relating the interchanging electric potential and charge density [1]. Through the next several decades, some of the brightest mathematicians and physicists in history, including Helmholtz, Rayleigh, and Lorentz, made various observations related to reciprocity in different branches of physics such as optics, acoustics, elastodynamics, and electromagnetics and formulated a number of theorems to explain these observations [2], [3]. Even today, more than 100 years after Lorentz's proof of reciprocity in electromagnetics, his theory is still widely in use.