  A quantum thermal machine is an open quantum system coupled to hot and cold
thermal baths. Thus, its dynamics can be well understood using the concepts and
tools from non-Hermitian quantum systems. A hallmark of non-Hermiticity is the
existence of exceptional points where the eigenvalues of a non-Hermitian
Hamiltonian or an Liouvillian superoperator and their associated eigenvectors
coalesce. Here, we report the experimental realisation of a single-ion heat
engine and demonstrate the effect of the Liouvillian exceptional points on the
dynamics and the performance of a quantum heat engine. Our experiments have
revealed that operating the engine in the exact- and broken-phases, separated
by a Liouvillian exceptional point, respectively during the isochoric heating
and cooling strokes of an Otto cycle produces more work and output power and
achieves higher efficiency than executing the Otto cycle completely in the
exact phase where the system has an oscillatory dynamics and higher coherence.
This result opens interesting possibilities for the control of quantum heat
engines and will be of interest to other research areas that are concerned with
the role of coherence and exceptional points in quantum processes and in work
extraction by thermal machines.
