Quantum efficiency of Josephson traveling wave parametric amplifiers with many-Mode processes

Abstract

Josephson traveling wave parametric amplifiers (JTWPAs) are widely used in superconducting qubit and microwave quantum optics experiments. Compared with cavity-based Josephson parametric amplifiers (JPAs), JTWPAs have ~10dB higher dynamic range and an order of magnitude higher bandwidth, exhibiting >20dB gain over several gigahertz of instantaneous bandwidth. The broad bandwidth and high dynamic range of JTWPAs allow for simultaneous readout of more than 20 frequency-multiplexed qubits. With these amplifiers, qubit readout fidelities above 99% have been achieved; however, current JTWPA have a full readout chain quantum efficiency of ~50% which is well below that of an ideal parametric amplifier. This thesis studies the effect of higher order modes on the quantum efficiency, identifies mitigation strategies, and proposes new designs of JTWPAs with improved quantum efficiency that can potentially increase qubit readout speed and fidelity.