Searching for Axion Dark Matter: Quantum Sensors and the Dark Matter Radio

One of the most enduring mysteries in particle physics is the nature of the non-baryonic dark matter that makes up 85% of the matter in the universe. For several decades, most searches for this mysterious substance have focused on Weakly Interacting Massive Particles (WIMPs). Recently, there has been a surge in theoretical interest in ultra-light, wave-like dark matter candidates, including the strongly motivated QCD axion. I will describe a suite of experiments (including both NMR and electromagnetic coupling) that will be used to probe the QCD axion over more than six orders of magnitude of mass (~100 Hz to ~300 MHz), and the development of quantum sensors to enhance these searches. I will focus in particular on the Dark Matter Radio (DM Radio), an experiment searching for axions by their coupling to a lumped-element electromagnetic resonators. Quantum measurement techniques can be used to evade the standard quantum limit by the  exploitation of quantum correlations in the electromagnetic signals in a resonator. In fact, the use of quantum sensors is required to be able to fully probe the QCD axion over the mass range below 300 MHz. I will describe the RF Photon Upconverter (RPU), which can be used to implement techniques including backaction evasion to outperform the Standard Quantum Limit at the RF frequencies probed by DM Radio.