  Abstract We perform an experiment in which a quantum heat engine works under
two reservoirs, one at a positive spin temperature and the other at an
effective negative spin temperature i.e., when the spin system presents
population inversion. We show that the efficiency of this engine can be greater
than that when both reservoirs are at positive temperatures. We also
demonstrate the counter-intuitive result that the Otto efficiency can be beaten
only when the quantum engine is operating in the finite-time mode.
