  We show how resonant laser spectroscopy of the trion optical transitions in a
self-assembled quantum dot can be used to determine the temperature of a nearby
electron reservoir. At finite magnetic field the spin-state occupation of the
Zeeman-split quantum dot electron ground states is governed by thermalization
with the electron reservoir via co-tunneling. With resonant spectroscopy of the
corresponding excited trion states we map out the spin occupation as a function
of magnetic field to establish optical thermometry for the electron reservoir.
We demonstrate the implementation of the technique in the sub-Kelvin
temperature range where it is most sensitive, and where the electron
temperature is not necessarily given by the cryostat base temperature.
