Progress in fundamental physics--the search for new particles, forces, and laws of nature--relies on probing regimes of nature that have not yet been explored. This requires continuous development of ultra-sensitive detectors. In recent years, a number of experimental programs have begun to operate with sufficient precision that they have become limited not by technical sources of noise but by the very laws of quantum mechanics. While these limits have some history in the atomic, molecular, and optics community, they are relatively new to the high energy theory landscape. I'll give an overview of how quantum mechanics places limits on measurement sensitivity, and highlight a few key examples where these limits are playing an increasingly important role, including searches for dark matter, neutrinos, and quantum effects from gravity.