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Photon emitters placed in an optical cavity experience an environment that changes how they are coupled to the surrounding light field. In the weak-coupling regime, the extraction of light from the emitter is enhanced. But more profound effects emerge when single-emitter strong coupling occurs: mixed states are produced that are part light, part matter1,2, forming building blocks for quantum information systems and for ultralow-power switches and lasers3-6. Such cavity quantum electrodynamics has until now been the preserve of low temperatures and complicated fabrication methods, compromising its use5,7,8. Here, by scaling the cavity volume to less than 40 cubic nanometres and using host-guest chemistry to align one to ten protectively isolated methylene-blue molecules, we reach the strong-coupling regime at room temperature and in ambient conditions. Dispersion curves from more than 50 such plasmonic nanocavities display characteristic light-matter mixing, with Rabi frequencies of 300 millielectronvolts for ten methyleneblue molecules, decreasing to 90 millielectronvolts for single molecules-matching quantitative models. Statistical analysis of vibrational spectroscopy time series and dark-field scattering spectra provides evidence of single-molecule strong coupling. This dressing of molecules with light can modify photochemistry, opening up the exploration of complex natural processes such as photosynthesis9 and the possibility of manipulating chemical bonds10.
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Creating strongly coupled mixed states from visible light and individual emitters is severely compromised by the hundred-fold difference in their spatial localization. To overcome this, high-quality cavities are used to boost interaction times and enhance coupling strengths. However, in larger cavities the longer round trip for photons to return to the same emitter decreases the coupling, which scales as g1/ V, where V is the effective cavity volume and g is the coupling energy. This coupling has to exceed both the cavity loss rate, κ, and the emitter scattering rate, γ, in order for energy to cycle back and forth between matter and light components, requiring 2g > γ, κ (ref. 11). For cryogenic emitters5,6 (laser-cooled atoms, vacancies in diamond, or semiconductor quantum dots), the suppressed emitter scattering allows large cavities (with a high quality factor, Q, which is proportional to κ-1) to reach strong coupling. Severe technical challenges, however, restrict the energy, bandwidth, size and complexity of devices. Progress towards room-temperature devices has been limited by the unavoidable...