  There exist two formulations for quantum heat engine that models an energy
transfer between two microscopic systems. One is semi-classical scenario, and
the other is full quantum scenario. The former is formulated as a unitary
evolution for the internal system, and is adopted by the community of
statistical mechanics. In the latter, the whole process is formulated as
unitary, and is adopted by the community of quantum information. This paper
proposes a model for quantum heat engine that transfers energy from a
collection of microscopic systems to a macroscopic system like a fuel cell. In
such a situation, the amount of extracted work is visible for a human. For this
purpose, we formulate quantum heat engine as the measurement process whose
measurement outcome is the amount of extracted work. Under this model, we
derive a suitable energy conservation law and propose a more concrete submodel.
Then, we derive a novel trade-off relation between the measurability of the
amount of work extraction and the coherence of the internal system, which
examines the application of the semi-classical scenario to a heat engine
transferring an energy from a collection of microscopic systems to a
macroscopic system.
