Characterization of dc SQUID damping on superconducting resonant circuits*

The high bandwidth, high dynamic range, low noise characteristics, and maturity of dc SQUIDs make them a versatile tool for a variety of precision measurements, including readout of resonant circuits in applications such as quantum information, gravitational wave detection, and dark-matter detection. The dc SQUID is a lossy active circuit with a dynamic input impedance that varies based on its detailed design, temperature, magnetic flux, current, voltage operating points, and applied feedback. Coupling a dc SQUID to a resonant circuit can modify the circuit's resonance frequency, quality factor, noise, and impedance. We present experimental measurements of dc SQUID-induced damping effects on highly coupled, lumped-element resonators in the 500kHz-1MHz regime.