Understanding the interface of quantum light and matter is critical for new high-precision measurements in imaging and spectroscopy, as well as in quantum photonics. We study this interaction in the context of (1) harnessing path- and energy-entangled photons in new optical spectroscopy to answer fundamental questions about how quantum light and matter interact, and (2) understanding condensed phase dynamics in the limit of single emitters, excitations, and photons. Our efforts rely on the development of new spectroscopic methods to study how structural parameters drive condensed phase processes, such as spin- and phonon-mediated dephasing, and how they can be overcome with materials design.

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