  Isolating single molecules in the solid state has allowed fundamental
experiments in basic and applied sciences. When cooled down to liquid helium
temperature, certain molecules show transition lines, that are tens of
megahertz wide, limited only by the excited state lifetime. The extreme
flexibility in the synthesis of organic materials provides, at low costs, a
wide palette of emission wavelengths and supporting matrices for such single
chromophores. In the last decades, the controlled coupling to photonic
structures has led to an optimized interaction efficiency with light. Molecules
can hence be operated as single photon sources and as non-linear elements with
competitive performance in terms of coherence, scalability and compatibility
with diverse integrated platforms. Moreover, they can be used as transducers
for the optical read-out of fields and material properties, with the promise of
single-quanta resolution in the sensing of charges and motion. We show that
quantum emitters based on single molecules hold promise to play a key role in
the development of quantum science and technologies.
