  The recent progress in nanotechnology [1,2] and single-molecule spectroscopy
[3-5] paves the way for cost-effective organic quantum optical technologies
emergent with a promise to real-life devices operating at ambient conditions.
In this letter, we harness $\pi$-conjugated segments of an organic ladder-type
polymer strongly coupled to a microcavity forming correlated collective dressed
states of light, so-called of exciton-polariton condensates. We explore an
efficient way for all-optical ultra-fast control over the macroscopic
condensate wavefunction via a single photon. Obeying Bose statistics,
exciton-polaritons exhibit an extreme nonlinearity undergoing bosonic
stimulation [6] which we have managed to trigger at the single-photon level.
Relying on the nature of organic matter to sustain stable excitons dressed with
high energy molecular vibrations we have developed a principle that allows for
single-photon nonlinearity operation at ambient conditions opening the door for
practical implementations like sub-picosecond switching, amplification and
all-optical logic at the fundamental limit of single light quanta.
