Quantum measurement of RF resonators to search for axion dark matter below 300 MHz

The QCD axion, which solves the strong CP problem in QCD, is one of the best motivated dark-matter candidates. In a strong dc magnetic field, QCD axion dark matter is detected electromagnetically as an extremely weak, narrowband ac electromagnetic signal at a frequency determined by the particle mass. At some frequencies (in particular, at MHz frequencies), it is impossible to rule out or detect candidate axion signals in particularly well motivated frequency ranges without achieving measurement sensitivity well below the Standard Quantum Limit (SQL).

At MHz frequencies, Fock state measurements and photon counting are not optimal sensing techniques because of contamination with thermal photons, but backaction evasion (BAE) can provide significant sensitivity improvement when detuned from the resonance. I will discuss the use of BAE with both single electromagnetic RF resonators and networked RF resonators. These techniques utilize the RF quantum upconverter (RQU), a multi-junction Josephson interferometer that couples MHz signals to microwave superconducting resonators, enabling both phase-insensitive measurement near the SQL and phase-sensitive techniques including BAE that can surpass the measurement sensitivity of an SQL amplifier.

I discuss the application of networked resonators to enhance axion detection, including coherently coupling two resonators (each with two quadratures), and applying BAE to delocalized measurement quadratures. This may be realized by coupling two networked RF resonators with frequencies near a MHz to a single microwave electromagnetic mode.