= Single-molecule experiment
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= Single-molecule measurement
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A <single-molecule experiment> resolves a response attributable to an individual <molecule>, rather than only an average over many <molecules>. Optical detection need not resolve its constituent <chemical atoms> in an image: repeated <photon absorption> by the same <molecule> can produce a measurable response. One detected <photon> is not automatically one different <molecule>.
The measured channel matters. Direct <photon absorption> detection measures a change in transmitted light. <Fluorescence> detection collects <photons> emitted after excitation; a <fluorescence excitation spectrum> varies the illuminating <frequency> while observing that emission. Both can probe excitation, but their backgrounds and readouts differ. A labeled particle cartoon explains the interpretation; it supplies no experimental evidence by itself.
Such measurements expose differences that a population average can conceal. Suppose two groups of <molecules> switch between different internal <quantum states>. Their average response could vary smoothly even though each <molecule> changes in discrete steps. Resolving an individual response tests that interpretation directly, subject to the measurement's time resolution, sensitivity, and particle assignment. The following experiment makes that assignment through spectral selection and controlled perturbations.
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