Using Superconducting Qubits for Axion Dark Matter Detection

The axion is a potential solution to the strong CP problem in QCD and could account for the abundance of dark matter observed in the universe. In the presence of an applied magnetic field, the axion field will source a current used to drive a resonant cavity to single photon occupation. A transmon qubit operating as a microwave photon sensor is a viable readout system at frequencies where the added noise of quantum limited amplifiers overwhelms the signal rate. The use of a direct dispersive quantum non-demolition measurement of the photon number decouples the measurement back action from the experimental uncertainties. In this regime dark counts are the dominant sources of detector error. For a transmon qubit operating as a photon counter, dark counts occur due to qubit errors which occur with probability 1-10%. In order to mitigate the effect of individual qubit flip errors of the detector, repeated measurements of the same photon are performed. The error rate of the joint N measurements could be significantly suppressed. The detector errors are then no longer the dominant source of false positives when attempting to measure weak signals sourced by the dark matter.