Qubit-based Single Photon Counters for Dark Matter Detection

The axion is a dark matter candidate particle, predicted by the Peccei-Quinn solution to the so-called "strong CP problem". If axions do indeed compose some part of the dark halos of galaxies (including the Milky Way) then these axions may be observed in the laboratory using a device appropriately referred to as a haloscope. Haloscopes induce an axion-to-photon conversion and resonantly accumulate the product photons in a microwave cavity. Assuming an axion mass of about 40 micro-eV (or equivalently, a photon frequency of 10 GHz), the expected signal power in a haloscope will be on the order of 10^-25 watts. Quantum mechanics imposes a limit on the performance of phase-preserving linear amplifiers, making the detection of this signal quite difficult. By coupling superconducting qubits to the haloscope cavity, we can count single microwave photons while improving our signal-to-noise ratio by several orders of magnitude relative to linear amplifiers.