Magnet-Free Nonreciprocity [Scanning the Section]

Electromagnetic waves, from radio signals to light, generally obey a strict symmetry in transmission: if they can travel from point A to point B, they can also travel backward from B to A with the same properties. This general symmetry, rooted in the fact that Maxwell’s equations are symmetric with respect to time and known as reciprocity, has profound implications on a variety of technologies and device functionalities. On the one hand, we count on reciprocity when we operate an antenna, in a variety of measurement schemes, and in analytical/numerical techniques to study the response of electromagnetic systems. On the other hand, reciprocity inherently limits the degree of control in manipulating electromagnetic waves and hence the overall functionality of many devices. For instance, breaking reciprocity enables the possibility of transmitting a signal without having to worry about backreflections, which may harm sensitive equipment or interfere with incoming data streams, and of transmitting and receiving signals through the same transducer at the same frequency and at the same time—the holy grail of full-duplex communication schemes. Fields as diverse as radar technologies, wireless communications, and optical and quantum computing all require nonreciprocal signal routing to enable reception of signal echoes while transmitting, data transmission without worrying about jammers, and to guarantee one-way streams of data that can be robustly processed.