Radio-frequency quantum upconverters for spin metrology

Polarized nuclear spin ensembles are a robust and broadly applicable experimental tool. In spin ensembles in weak magnetic fields where Larmor precession frequencies are below 100 MHz, added noise contributions from the amplifier can limit experimental sensitivity. While dc SQUIDs have historically been used in spin-sensing experiments that require sensitive readout in this low frequency range, even the best available dc SQUIDs operate with noise several times above the Standard Quantum Limit. This work describes the Radio frequency Quantum Upconverter (RQU), a flux amplifier where the low frequency spin magnetization signal modulates the phase of a microwave readout tone. The imprecision noise of the RQU can be reduced by increasing the strength of the microwave tone, while the input inductance remains unchanged. This in-situ noise impedance tunability of an RQU will allow it to reach sensitivities unachievable with dc SQUIDs, enabling unprecedented sensitivity in experiments including spin-coupled dark matter detectors.