  Nonreciprocal devices such as isolators and circulators are necessary to
protect sensitive apparatus from unwanted noise. Recently, a variety of
alternatives were proposed to replace ferrite-based commercial technologies,
with the motivation to be integrated with microwave superconducting quantum
circuits. Here, we review isolators realized with microwave optomechanical
circuits and present a gyrator-based picture to develop an intuition on the
origin of nonreciprocity in these systems. Such nonreciprocal optomechanical
schemes show promise as they can be extended to circulators and directional
amplifiers, with perspectives to reach the quantum limit in terms of added
noise.
