  The thermodynamic properties of quantum heat engines are stochastic owing to
the presence of thermal and quantum fluctuations. We here experimentally
investigate the efficiency and nonequilibrium entropy production statistics of
a spin-1/2 quantum Otto cycle. We first study the correlations between work and
heat within a cycle by extracting their joint distribution for different
driving times. We show that near perfect anticorrelation, corresponding to the
tight-coupling condition, can be achieved. In this limit, the reconstructed
efficiency distribution is peaked at the macroscopic efficiency and
fluctuations are strongly suppressed. We further test the second law in the
form of a joint fluctuation relation for work and heat. Our results
characterize the statistical features of a small-scale thermal machine in the
quantum domain and provide means to control them.

Quantum Physics;Statistical Mechanics