  We propose employing a quantum heat engine as a sensitive probe for thermal
baths. In particular, we study a single-atom Otto engine operating in an open
thermodynamic cycle. Owing to its cyclic nature, the engine is capable of
translating small temperature differences between two baths into a macroscopic
oscillation in a flywheel. We present analytical and numerical modeling of the
quantum dynamics of the engine and estimate it to be capable of detecting
temperature differences as small as 2 $\mu$K. This sensitivity can be further
improved by utilizing quantum resources such as squeezing of the ion motion.
The proposed scheme does not require quantum state initialization and is able
to detect small temperature differences even at high base temperatures.
