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. 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. DOI: 10.1103/PhysRevX.7.031001 Subject Areas: Acoustics, Condensed Matter Physics, Quantum PhysicsMany branches of physics and engineering employ nonreciprocal devices to route signals along desired paths of measurement networks. Conceptually, the simplest nonreciprocal element is the isolator, a two-port device that transmits signals from the first to the second port but strongly attenuates in the reverse direction [1]. Placing an ideal isolator (or its close relative, the circulator) between two systems allows the first system to influence the second but not vice versa. This nonreciprocal functionality enables, for example, telecommunication antennas to transmit and receive signals at the same time. Another example relevant for future applications is quantum signal processing, where the strict demands of quantum measurement require isolators with high performance in several metrics, including not only large isolation, but also high efficiency and low noise [2].Well-established technology uses magnetic materials to achieve nonreciprocity for both microwave and optical frequencies [3][4][5]. While these conventional devices have enabled much of the progress in classical and quantum signal processing, overcoming their limitations could lead to exciting new developments in both areas. For example, these components are typically bulky, not chip compatible, and incompatible with superconducting technology because they require strong magnetic fields. Signal losses due to these conventional nonreciprocal devices have now become the bottleneck for the overall efficiency of,...