  Single molecule spectroscopy aims at unveiling often hidden but potentially
very important contributions of single entities to a system's ensemble
response. Albeit contributing tremendously to our ever growing understanding of
molecular processes the fundamental question of temporal evolution, or change,
has thus far been inaccessible, resulting in a static picture of a dynamic
world. Here, we finally resolve this dilemma by performing the first ultrafast
time-resolved transient spectroscopy on a single molecule. By tracing the
femtosecond evolution of excited electronic state spectra of single molecules
over hundreds of nanometres of bandwidth at room temperature we reveal their
non-linear ultrafast response in an effective 3-pulse scheme with fluorescence
detection. A first excitation pulse is followed by a phase-locked de-excitation
pulse-pair, providing spectral encoding with 25 fs temporal resolution. This
experimental realisation of true single molecule transient spectroscopy
demonstrates that two-dimensional electronic spectroscopy of single molecules
is experimentally in reach.
