Cavity Microwave Searches for Cosmological Axions

This chapter will cover the search for dark matter axions based on microwave cavity experiments proposed by Pierre Sikivie. The topic begins with a brief overview of halo dark matter and the axion as a candidate. The principle of resonant conversion of axions in an external magnetic field will be described as well as practical considerations in optimizing the experiment as a signal-to-noise problem. A major focus of the lecture will be the two complementary strategies for ultra-low noise detection of the microwave photons - the ``photon-as-wave'' approach (i.e. conventional heterojunction amplifiers and soon to be quantum-limited SQUID devices), and ``photon-as-particle'' (i.e. Rydberg-atom single-quantum detection). Experimental results will be presented; these experiments have already reached well into the range of sensitivity to exclude plausible axion models, for limited ranges of mass. The section will conclude with a discussion of future plans and challenges for the microwave cavity experiment.