Boltzmann optical thermometry for cryogenics.

We propose and implement an optical technique to access the local temperature of an erbium doped crystal by probing the electron spin population under magnetic field. We reliably extract the sample temperature in the range 2-7 K. We additionally discuss the suitability of our method as a primary standard for cryogenic thermometry. By adding an auxiliary heating laser, we are able to measure the interface conductance between the dielectric crystal and the cold plate of the cryostat by exploring different cooling configurations.