Noise and Sensitivity of Radio-Frequency Quantum Upconverters (RQUs)

Sensors that can measure better than the Standard Quantum Limit are needed for many scientific applications. We are developing sensors for continuous-variable quantum sensing at RF frequencies below 300 MHz, including axion detection and spin ensemble sensing. The Radio-Frequency Quantum Upconverter (RQU) is a resonant superconducting circuit that upconverts RF (<300 MHz) signals to higher frequency microwave signals (4-8 GHz). In this work, we describe the fundamental noise sources present in the RQU circuitry, expressed as imprecision and backaction noise, and describe the standard quantum limit in the context of our system. We describe the control parameters of the RQU (microwave drive power, and different flux bias parameters within the Josephson junction array) and how they can be tuned to increase measurement signal-to-noise ratio. We also describe the implementation of quantum protocols to operate beyond the standard quantum limit on single-quadrature measurements through quantum backaction evasion. These sensitivity capabilities will be crucial for successful implementation of axion detectors such as the DMRadio experiments.