We report on the design and implementation of a Field Programmable Josephson Amplifier (FPJA) -a compact and lossless superconducting circuit that can be programmed in situ by a set of microwave drives to perform reciprocal and nonreciprocal frequency conversion and amplification.In this work we demonstrate four modes of operation: frequency conversion (−0.5 dB transmission, −30 dB reflection), circulation (−0.5 dB transmission, −30 dB reflection, 30 dB isolation), phasepreserving amplification (gain > 20 dB, 1 photon of added noise) and directional phase-preserving amplification (−10 dB reflection, 18 dB forward gain, 8 dB reverse isolation, 1 photon of added noise). The system exhibits quantitative agreement with theoretical prediction. Based on a gradiometric Superconducting Quantum Interference Device (SQUID) with Nb/Al-AlOx/Nb Josephson junctions, the FPJA is first-order insensitive to flux noise and can be operated without magnetic shielding at low temperature. Due to its flexible design and compatibility with existing superconducting fabrication techniques, the FPJA offers a straightforward route toward on-chip integration with superconducting quantum circuits such as qubits or microwave optomechanical systems.

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,...

Quantum entanglement of mechanical systems emerges when distinct objects move with such a high degree of correlation that they can no longer be described separately. Although quantum mechanics presumably applies to objects of all sizes, directly observing entanglement becomes challenging as masses increase, requiring measurement and control with a vanishingly small error. Here, using pulsed electromechanics, we deterministically entangle two mechanical drumheads with masses of 70 picograms. Through nearly quantum-limited measurements of the position and momentum quadratures of both drums, we perform quantum state tomography and thereby directly observe entanglement. Such entangled macroscopic systems are poised to serve in fundamental tests of quantum mechanics, enable sensing beyond the standard quantum limit, and function as long-lived nodes of future quantum networks.

We present a new optomechanical device where the motion of a micromechanical membrane couples to a microwave resonance of a three-dimensional superconducting cavity. With this architecture, we realize ultrastrong parametric coupling, where the coupling rate not only exceeds the dissipation rates in the system but also rivals the mechanical frequency itself. In this regime, the optomechanical interaction induces a frequency splitting between the hybridized normal modes that reaches 88% of the bare mechanical frequency, limited by the fundamental parametric instability. The coupling also exceeds the mechanical thermal decoherence rate, enabling new applications in ultrafast quantum state transfer and entanglement generation.