Bias-Controlled Noise Suppression and Frequency Upconversion in a Superconducting Radiofrequency Quantum Upconverter (RQU)

The Radiofrequency Quantum Upconverter (RQU) integrates a three-Josephson-junction interferometer with a superconducting microwave resonator to translate low-frequency signals (<300 MHz) into the microwave domain (4–8 GHz). Through inductance modulation of the interferometer, the RQU dynamically tunes the GHz resonator's frequency via a time-varying flux coupled from the MHz resonator. This parametric process enables the upconversion of weak low-frequency signals into the microwave band, facilitating measurements that surpass the Standard Quantum Limit. We will present experimental data showing how the direct coupling between the input signal line and the GHz resonator is suppressed at specific DC bias points, thereby reducing high-frequency noise leakage into the input channel. Finally, we will discuss coupling strategies for integrating a MHz resonator with the low-frequency input to achieve efficient and low-noise upconversion.