  Cryogenic temperatures are the prerequisite for many advanced scientific
applications and technologies. The accurate determination of temperature in
this range and at the submicrometer scale is, however, nontrivial. This is due
to the fact that temperature reading in cryogenic conditions can be inaccurate
due to optically induced heating. Here, we present an ultralow power, optical
thermometry technique that operates at cryogenic temperatures. The technique
exploits the temperature dependent linewidth broadening measured by resonant
photoluminescence of a two level system, a germanium vacancy color center in a
nanodiamond host. The proposed technique achieves a relative sensitivity of 20%
1/K, at 5 K. This is higher than any other all optical nanothermometry method.
Additionally, it achieves such sensitivities while employing excitation powers
of just a few tens of nanowatts, several orders of magnitude lower than other
traditional optical thermometry protocols. To showcase the performance of the
method, we demonstrate its ability to accurately read out local differences in
temperatures at various target locations of a custom-made microcircuit. Our
work is a definite step towards the advancement of nanoscale optical
thermometry at cryogenic temperatures.
