Fundamental Limits of Electromagnetic Axion and Hidden-Photon Dark Matter Searches*

Ultralight, wavelike dark matter candidates can be detected via their coupling to electromagnetic modes. Recently, a number of experimental proposals have emerged to search for QCD axion dark matter over its entire allowed mass range (~1 peV - ~10meV). We describe the fundamental optimization of searches for for wavelike dark-matter candidates, including particularly the QCD axion, with electromagnetic sensors. The sensitivity of detection of the excitation of these modes is set by vacuum noise, thermal noise, and the noise added by amplification, and fundamental limits on optimized searches for QCD axion dark matter are imposed by the Standard Quantum Limit. Single-pole resonators can approach the fundamental limit of sensitivity for probes of axion dark matter, and the optimization of measurement backaction and sensitivity outside of the resonator bandwidth can increase scan rates by up to five orders of magnitude at low frequency. We also discuss the application of quantum sensing techniques to evade the Standard Quantum Limit on amplification to enable a complete probe of QCD axion dark matter below 1 micro eV.