Abstract The ability to engineer nonreciprocal interactions is an essential tool in modern communication technology as well as a powerful resource for building quantum networks. Aside from large reverse isolation, a nonreciprocal device suitable for applications must also show high efficiency (low insertion loss) and low output noise. Recent theoretical and experimental studies have shown that nonreciprocal behavior can be achieved in optomechanical systems, but performance in these last two attributes has been limited. Here we demonstrate an efficient, frequency-translating isolator based on the optomechanical interactions between microwave fields and a mechanically compliant vacuum gap capacitor. We use a four-mode coupling concept to achieve simultaneous reverse isolation of more than 20 dB and insertion loss less than 1.5 dB over a bandwidth of several kilohertz. One beneficial side effect of this coupling scheme is that optomechanical interactions naturally damp and cool the mechanical modes close to their quantum ground state. We quantitatively characterize the nonreciprocal noise performance of the device by verifying that the residual thermal mechanical noise is directionally routed solely to the input of the isolator. We detail a general analysis of four- mode nonreciprocity for both the driven response and the noise, applicable to microwave and optical frequencies alike. Unlike conventional isolators and circulators, these compact nonreciprocal parametric devices do not require a static magnetic field, and they allow for dynamic control of the direction of isolation. With these advantages, similar devices could enable programmable, high-efficiency connections between disparate nodes of quantum networks, even bridging the microwave and optical domains.
Physical Review X
nonreciprocity, optomechanics, isolator, superconducting circuits, frequency conversion