  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 have 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-converting microwave isolator based on the
optomechanical interactions between electromagnetic fields and a mechanically
compliant vacuum gap capacitor. We achieve simultaneous reverse isolation of
more than 20 dB and insertion loss less than 1.5 dB over a bandwidth of 5 kHz.
We characterize the nonreciprocal noise performance of the device, observing
that the residual thermal noise from the mechanical environments is routed
solely to the input of the isolator. Our measurements show quantitative
agreement with a general coupled-mode theory. Unlike conventional isolators and
circulators, these compact nonreciprocal 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
efficiently bridging the microwave and optical domains.
