  Estimating the temperature of a cold quantum system is difficult. Usually,
one measures a well-understood thermal state and uses that prior knowledge to
infer its temperature. In contrast, we introduce a method of thermometry that
assumes minimal knowledge of the state of a system and is potentially
non-destructive. Our method uses a universal temperature-dependence of the
quench dynamics of an initially thermal system coupled to a qubit probe that
follows from the Tasaki-Crooks theorem for non-equilibrium work distributions.
We provide examples for a cold-atom system, in which our thermometry protocol
may retain accuracy and precision at subnanokelvin temperatures.
