Measurement of dc SQUID backaction noise and correlations in the kHz - MHz range

DC SQUIDs are used to sensitively measure magnetic flux and current signals at kHz and MHz frequencies. Despite their widespread use, direct measurements of SQUID backaction noise and correlations between imprecision and backaction in this frequency range are not available. These measurements are needed to properly design axion searches such as DMRadio. I will present our effort to directly measure dc SQUID backaction noise and correlations at kHz and MHz frequencies. These measurements are done by coupling a high Q resonator in a dilution refrigerator to a dc SQUID through a tunable transformer. The tunable transformer allows in situ variation of the SQUID-resonator coupling strength, making it possible to probe the strength of the backaction noise sourced by the SQUID amplifier, and to determine correlations with imprecision noise. I will describe the design and construction of the resonator and superconducting gradiometric transformer and present data with tuned coupling. I will also discuss implications for experiments that use dc SQUIDs such as lumped element axion searches like DMRadio.