Simulation of Radiofrequency Quantum Upconverters (RQUs) with WRspice

The Radiofrequency Quantum Upconverter (RQU) uses a Josephson-junction interferometer and a superconducting microwave resonator to upconvert low-frequency signals (<300 MHz) into microwave frequencies (4-6 GHz). By modulating the inductance of the interferometer, the RQU adjusts the GHz resonator's frequency via time-dependent flux coupled through the MHz resonator. This process parametrically upconverts signals, such as axion signals, into the high-frequency microwave band, supporting precision quantum measurements beyond the Standard Quantum Limit. To evaluate RQU performance, we simulate responsiveness and sensitivity using WRSpice, demonstrating tuning over several hundred MHz by applying a DC flux bias across multiple flux quanta. I will demonstrate how coupling strength of the MHz signal to the interferometer influences the responsivity and Q-factor of the resonators. Additionally, I will present simulations demonstrating the impact of inductively coupled MHz signal power on sideband heights for both reflection and transmission mode devices and validate these simulations against real-world chip data and analytical models, showing strong agreement. I further present progress on simulation of quantum protocols with an RQU including backaction evasion.