Phase-sensitive detection with the Radio-Frequency Quantum Upconverter

The Radio-Frequency Quantum Upconverter (RQU) is a superconducting frequency upconverter designed to meet the need for better precision electromagnetic measurements at low frequencies. An RQU consists of a Josephson-junction interferometer embedded in a superconducting microwave resonator, and for small signals at MHz frequency, the interferometer presents a tunable inductance to the microwave circuit, creating a parametric interaction between the signal-frequency flux and the microwave mode. The upconverted signal appears as sidebands of the microwave carrier tone. RQUs with a three-junction interferometer design enable clean implementation of quantum upconversion protocols including sideband cooling, two-mode squeezing, and backaction-evading (BAE) detection. We discuss measurements with prototype RQUs, including phase-sensitive detection of a low frequency circuit which is a necessary prerequisite for evading quantum backaction noise. Additionally, we discuss BAE protocol applications in high-precision electromagnetic measurements including sub-μeV axion searches.