  A heat engine operating in the one-shot finite-size regime, where systems
composed of a small number of quantum particles interact with hot and cold
baths and are restricted to one-shot measurements, delivers fluctuating work.
Further, engines with lesser fluctuation produce a lesser amount of
deterministic work. Hence, the heat-to-work conversion efficiency stays well
below the Carnot efficiency. Here we overcome this limitation and attain Carnot
efficiency in the one-shot finite-size regime, where the engines allow the
working systems to simultaneously interact with two baths via the semi-local
thermal operations and reversibly operate in a one-step cycle. These engines
are superior to the ones considered earlier in work extraction efficiency, and,
even, are capable of converting heat into work by exclusively utilizing
inter-system correlations. We formulate a resource theory for quantum heat
engines to prove the results.
