Improving the Quantum Efficiency of Josephson Traveling Wave Parametric Amplifiers*

Josephson traveling wave parametric amplifiers (JTWPAs) with gigahertz of simultaneous bandwidth and near-quantum-limited noise performance have become workhorse amplifiers for a wide variety of microwave superconducting quantum experiments. However, the best reported intrinsic quantum efficiency of such amplifiers remains at least 20% below that of an ideal phase-preserving amplifier. Here, we systematically analyze the quantum efficiency of a standard resonantly phase-matched JTWPA using a multi-mode input-output theory framework. We identify the higher order sidebands as a significant noise source that prevents the JTWPA from attaining quantum limited performance. Finally, we report our progress towards designing JTWPAs which mitigate these noise mechanisms. Such devices may advance applications such as continuous quantum error correction, measurement-based quantum feedback protocols, and dark matter detection.