Reports

Coherent superconducting circuits play a key role in explor-ing the interaction between light and matter at microwave frequencies, particularly at the level of single photons and artificial atoms where quantum effects dominate. Recent advances span quantum measurement (1–4), control (5–7), optics (8, 9), and information processing (10–12). Many of these developments have relied on ultra-low-noise Joseph-son parametric amplifiers (JPAs) (13–15) to detect micro-wave frequency signals with a sensitivity approaching the minimum allowed by quantum mechanics (16). Although several refinements have been incorporated into JPAs (17– 20), the basic architecture remains that of a cavity where anharmonicity is introduced via the nonlinear inductance of a Josephson junction. This architecture is well suited for detecting a few photons over a bandwidth of tens of mega-hertz; however, applications such as quantum computation and simulation with many bits motivate the development of a versatile, general purpose, quantum-limited microwave frequency amplifier with gigahertz-scale bandwidth and larger power handling capability. Semiconductor amplifiers based on high-electron-mobility transistors (HEMTs) fulfill some of these requirements, but generally achieve noise temperatures 10-20 times greater than the quantum limit. We introduce a style of Josephson amplifier which side-steps the primary limiting factor in JPAs- the use of a resonator to enhance the interaction be-tween incident waves and the Josephson nonlinearity. Instead, we use a long chain of Josephson junctions, forming a metamate-rial transmission line (21). We call this device a Josephso