  Fluctuations strongly affect the dynamics and functionality of nanoscale
thermal machines. Recent developments in stochastic thermodynamics have shown
that fluctuations in many far-from-equilibrium systems are constrained by the
rate of entropy production via so-called thermodynamic uncertainty relations.
These relations imply that increasing the reliability or precision of an
engine's power output comes at a greater thermodynamic cost. Here we study the
thermodynamics of precision for small thermal machines in the quantum regime.
In particular, we derive exact relations between the power, power fluctuations,
and entropy production rate for several models of few-qubit engines (both
autonomous and cyclic) that perform work on a quantised load. Depending on the
context, we find that quantum coherence can either help or hinder where power
fluctuations are concerned. We discuss design principles for reducing such
fluctuations in quantum nano-machines, and propose an autonomous three-qubit
engine whose power output for a given entropy production is more reliable than
would be allowed by any classical Markovian model.
