Observation and mitigation of microwave echoes from dielectric defects in Josephson traveling-wave amplifiers

Abstract

Amplifying microwave signals with a noise level close to the minimum imposed by quantum mechanics is now routinely achieved with superconducting quantum devices. In particular, Josephson-based traveling-wave parametric amplifiers (JTWPAs) have shown record bandwidth with added noise close to the quantum limit [Macklin et al. Science 350, 307 (2015) and Planat et al. Phys. Rev. X 10, 021021 (2020)]. In this work, we report the appearance of echo signals emitted by JTWPAs driven by trains of high-power pulses near to or exceeding their dynamical range. These echoes have microsecond coherence, and we attribute their origin to microscopic defects in the amplifier’s dielectric layer. By analyzing the power and coherence of the echo signals as a function of temperature, we estimate the dielectric loss resulting from these defects and their impact on the quantum efficiency of the JTWPA. We introduce a mitigation technique (“BLAST”) to prevent the appearance of these echoes, which can alter measurements in experiments. This comprises an additional off-resonant high-power tone sent concurrently with each pulse. We demonstrate that this suppresses the spurious defect signals, and we recover the typical gain and noise figure within 95% of their low-power values in 300ns. These results can help to extend the use of JTWPAs in experiments where fast, high-power sequences are necessary to generate weak microwave responses from the system under study, and they also provide a path toward characterizing the dielectric losses of these devices in situ.

Source-reported materials — not catalogue approval

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