Irwin invented the transition-edge sensor (TES) and pioneered SQUID-multiplexed readout now used throughout CMB and dark-matter detector arrays; his group builds quantum-limited electromagnetic sensors for axion dark matter searches (DMRadio) and cryogenic calorimeters, pushing sensitivity to the standard quantum limit and beyond -- a field of quantum sensing that, like ensemble NV-diamond magnetometry reaching pT/√Hz sensitivities, trades off bandwidth and volume for extreme field sensitivity.