  Room-temperature strong coupling of a single quantum emitter and a single
resonant plasmonic mode is a key resource for quantum information processing
and quantum sensing at ambient conditions. To beat dephasing, ultrafast energy
transfer is achieved by coupling single emitters to a plasmonic nanoresonator
with an extremely small mode volume and optimal spectral overlap. Typically,
normal mode splittings in luminescence spectra of single-emitter
strongly-coupled systems are provided as evidence for strong coupling and to
obtain rough estimates of the light-matter coupling strength g. However, a
complete anticrossing of a single emitter and a cavity mode as well as the
characterization of the uncoupled constituents is usually hard to achieve.
Here, we exploit the light-induced oxygen-dependent blue-shift of individual
CdSe/ZnS semiconductor quantum dots to tune their transition energy across the
resonance of a scanning plasmonic slit resonator after characterizing both
single emitter and nano resonator in their uncoupled states. Our results
provide clear proof of single-emitter strong light-matter coupling at ambient
condition as well as a value for the Rabi splitting at zero detuning 100 meV,
consistent with modeling, thereby opening the path towards plexitonic devices
that exploit single-photon nonlinearities at ambient conditions.
