We experimentally realize a nanoscale stochastic Stirling heat engine operating in the underdamped regime. The setup involves an optically levitated silica particle that is subjected to a power-varying optical trap and periodically coupled to a cold or hot reservoir via switching on or off of the active feedback cooling. We conduct a systematic investigation of the engine's performance and find that both the output work and efficiency approach their theoretical limits under quasistatic conditions. Furthermore, we examine the dependence of the output work fluctuation on the cycle time and the temperature difference between the hot and cold reservoirs. We observe that the distribution has a Gaussian profile in the quasistatic regime, whereas it becomes asymmetric and non-Gaussian as the cycle duration time decreases. This non-Gaussianity is qualitatively attributed to the strong autocorrelation of the particle's position within a cycle in the nonequilibrium regime. Our experiments provide valuable insights into stochastic thermodynamics in the underdamped regime and open up possibilities for the design of future nanomachines.

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• Optomechanics
• Stochastic thermodynamics
• Heat engines
• Nanoparticles