  Color centers in solids are the fundamental constituents of a plethora of
applications such as lasers, light emitting diodes and sensors, as well as the
foundation of advanced quantum information and communication technologies.
Their photoluminescence properties are usually studied under Stokes excitation,
in which the emitted photons are at a lower energy than the excitation ones. In
this work, we explore the opposite Anti-Stokes process, where excitation is
performed with lower energy photons. We report that the process is sufficiently
efficient to excite even a single quantum system, namely the germanium-vacancy
center in diamond. Consequently, we leverage the temperature-dependent,
phonon-assisted mechanism to realize an all-optical nanoscale thermometry
scheme that outperforms any homologous optical method employed to date. Our
results frame a promising approach for exploring fundamental light-matter
interactions in isolated quantum systems, and harness it towards the
realization of practical nanoscale thermometry and sensing.
