  We measure the quantum efficiency (QE) of individual dibenzoterrylene (DBT)
molecules embedded in para-dichlorobenzene at cryogenic temperatures. To
achieve this, we apply two distinct methods based on the maximal photon
emission and on the power required to saturate the zero-phonon line. We find
that the outcome of the two approaches are in good agreement, reporting a large
fraction of molecules with QE values above 50%, with some exceeding 70%.
Furthermore, we observe no correlation between the observed lower bound on the
QE and the lifetime of the molecule, suggesting that most of the molecules have
a QE exceeding the established lower bound. This confirms the suitability of
DBT for quantum optics experiments. In light of previous reports of low QE
values at ambient conditions, our results hint at the possibility of a strong
temperature dependence of the QE.

Quantum Physics;Mesoscale and Nanoscale Physics;Optics