  We report an axion dark matter search with a haloscope equipped with a
microwave photon counter. The haloscope is a tunable high quality factor
3-dimensional microwave cavity placed in a magnetic field. The photon counter,
operated cyclically, maps an incoming microwave photon onto the state of a
superconducting transmon qubit. The measurement protocol continuously monitors
the power emitted by the haloscope cavity as well as the dark count background,
and enables tuning of the cavity frequency to probe different axion masses.
With this apparatus we enhance by a factor 20 the search speed that can be
reached with quantum-limited linear amplifiers, and set a new standard for
probing the existence of axions with resonant detectors.
