  Room temperature detection of single quantum emitters has had a broad impact
in fields ranging from biophysics to material science, photophysics, or even
quantum optics. These experiments have exclusively relied on the efficient
detection of fluorescence. An attractive alternative would be to employ direct
absorption, or more correctly expressed "extinction" measurements. Indeed,
small nanoparticles have been successfully detected using this scheme in
reflection and transmission. Coherent extinction detection of single emitters
has also been reported at cryogenic temperatures, but their room temperature
implementation has remained a great laboratory challenge owing to the expected
weak signal-to-noise ratio. Here we report the first extinction study of a
single quantum emitter at ambient condition. We obtain a direct measure for the
extinction cross section of a single semiconductor nanocrystal both during and
in the absence of fluorescence, for example in the photobleached state or
during blinking off-times. Our measurements pave the way for the detection and
absorption spectroscopy of single molecules or clusters of atoms even in the
quenched state.
