Efficient Amplification and Non-Parametric Isolation of Flux Signals in a Radio-Frequency Quantum Upconverter

Radio-frequency Quantum Upconverters (RQUs) are sensors that enable quantum protocols like backaction evasion on input low-frequency RF signals (< 300 MHz), with applications to wave-like dark matter detection and spin sensing. RQUs use the inductance of an array of Josephson junctions to parametrically upconvert these RF signals to C-band microwave (4-8 GHz) tones, where mature quantum technologies are available. However, non-parametric coupling of the uncontrolled modes in the signal source degrades the coherence of the microwave resonators, impacting sensitivity. Lossy filters or high-inductance chokes are not used on these lines, because we want efficient parametric coupling to the RF signal in the Josephson junction array. I describe the design, operation, and performance of our 3-junction RQUs, which are biased in a way that prevents direct, non-parametric coupling between the microwave signal and the flux signal ports, optimizing resonator Q and device performance.