  We consider and compare candidate superconducting detector technologies that
might be applied to the readout of cavity-axion haloscopes and similar
fundamental physics experiments. We conclude that a transition edge sensor
(TES) configured with ballistic-phonon thermal isolation operated with a
superconducting transition temperature of order 30 mK would provide
quantum-limited detection performance at frequencies above 5 GHz. This would
permit the realisation of integrated homodyne detectors based on TESs that we
believe would make a unique contribution to a variety of fundamental physics
experiments, particularly those based on reading out microwave cavities in the
quantum ground state.
