  We study quantum impurity models as a platform for quantum thermometry. A
single quantum spin-1/2 impurity is coupled to an explicit, structured,
fermionic thermal environment which we refer to as the environment or bath. We
critically assess the thermometric capabilities of the impurity as a probe,
when its coupling to the environment is of Ising or Kondo exchange type. In the
Ising case, we find sensitivity equivalent to that of an idealized two-level
system, with peak thermometric performance obtained at a temperature that
scales linearly in the applied control field, independent of the coupling
strength and environment spectral features. By contrast, a richer thermometric
response can be realized for Kondo impurities, since strong probe-environment
entanglement can then develop. At low temperatures, we uncover a regime with a
universal thermometric response that is independent of microscopic details,
controlled only by the low-energy spectral features of the environment. The
many-body entanglement that develops in this regime means that low-temperature
thermometry with a weakly applied control field is inherently less sensitive,
while optimal sensitivity is recovered by suppressing the entanglement with
stronger fields.
