Superconducting Parametric Amplifiers: The State of the Art in Josephson Parametric Amplifiers

In superconducting quantum computing, qubit state information is conveyed via low-power microwave fields. As such, ultralow-noise microwave amplification plays a central role in measuring these fields to quickly and accurately infer the qubit state. To this end, parametric amplification, once a well-known concept for low-noise amplifiers during the 1960s and 1970s, has become the leading technology for enabling highly efficient microwave measurements of these quantum circuits. While current research efforts have elements in common with much earlier work, there are some important differences that distinguish this new generation of amplifiers. Here, we try to give an overview of what this excitement is about, how these devices work, and where there are opportunities for improvement.