  We demonstrate a novel all-optical switch consisting of two molecules: a
primary fluorophore that can be switched between a fluorescent and a dark state
by light of different wavelengths, and a secondary chromophore that facilitates
switching. The interaction between the two molecules exhibits a distance
dependence much steeper than that of Forster resonance energy transfer. This
enables the switch to act as a ruler with the capability to probe distances
difficult to access by other spectroscopic methods, thus presenting a new tool
for the study of biomolecules at the single-molecule level.
