Circulators at the Quantum Limit: Recent Realizations of Quantum-Limited Superconducting Circulators and Related Approaches

A well-known component of RF systems is the circulator, a three-port device that allows signals to flow in one direction while inhibiting propagation in the reverse direction. Circulators play an important role in protecting sensitive RF devices from reflections and are used to isolate the transmitter from the receiver chains in monostatic radar architectures. Circulators are also fascinating on their own, providing a textbook example of how reciprocal symmetry is broken. This symmetry-the invariance of device transmission under the exchange of sources and detectors-is a very common property that applies to most passive microwave devices. For example, antenna gain does not change whether the antenna is used as a transmitter or a receiver. Likewise, attenuator loss does not depend on the attenuator's orientation.