  A method is proposed to pin down an unambiguous proof for single molecule
surface enhanced Raman spectroscopy (SERS). The simultaneous use of two analyte
molecules enables a clear confirmation of the single (or few) molecule nature
of the signals. This method eliminates most of the uncertainties associated
with low dye concentrations in previous experiments. It further shows that
single-molecule signals are very common in SERS, both in liquids and on dry
substrates.
