  Developments in the thermodynamics of small quantum systems envisage
non-classical thermal machines. In this scenario, energy fluctuations play a
relevant role in the description of irreversibility. We experimentally
implement a quantum heat engine based on a spin-1/2 system and nuclear magnetic
resonance techniques. Irreversibility at microscope scale is fully
characterized by the assessment of energy fluctuations associated with the work
and heat flows. We also investigate the efficiency lag related to the entropy
production at finite time. The implemented heat engine operates in a regime
where both thermal and quantum fluctuations (associated with transitions among
the instantaneous energy eigenstates) are relevant to its description.
Performing a quantum Otto cycle at maximum power, the proof-of-concept quantum
heat engine is able to reach an efficiency for work extraction
($\eta\approx42$%) very close to its thermodynamic limit ($\eta=44$%).
