Joseph F. Keithley Award For Advances in Measurement Science (2021): Quantum Metrology Using Superconducting Parametric Amplifiers*

The superconducting Josephson tunnel junction is a unique circuit element, which behaves as a nonlinear inductance with vanishingly small dissipation when cooled to well below its critical temperature. When incorporated into a circuit topology where quantum bound states can be readily resolved and addressed, one obtains a quantum bit suitable for sensing, communication, and computation. In the opposite quasi-continuum limit, nonlinear oscillatory circuits can be constructed for three- and four-wave mixing with near-quantum-limited noise performance. Josephson junctions can be embedded within standing-wave cavities or traveling-wave architectures for enhanced interaction with an input microwave frequency signal, resulting in high-gain amplification. We describe the basic design principles of contemporary Josephson parametric amplifiers, discuss performance limits, and highlight quantum applications in state readout, real-time feedback, and weak measurement.