  The study shows that presence of the quantum coherent, unitary component of
the evolution of the system can improve constancy of heat engines, i.e.,
decrease fluctuations of the output power, in comparison with purely stochastic
setups. This enables to overcome the recently derived trade-off between
efficiency, power and constancy, which applies to classical Markovian
steady-state heat engines. The concept is demonstrated using a model system
consisting of two tunnel-coupled orbitals (i.e., electronic levels), each
attached to a separate electronic reservoir; such a setup can be realized, for
example, using quantum dots. Electronic transport is studied by means of the
exact Levitov-Lesovik formula in the case without the Coulomb interaction
between electrons, as well as applying a quantum master equation in the
interacting case. Constancy of the analyzed thermoelectric generator is
increased due to the fact that tunneling between the orbitals is associated
with a unitary evolution of the electron state instead of a stochastic Poisson
transition. This reduces stochasticity of the system, thus suppressing the
current and power fluctuations. Moreover, noise can be further reduced by the
Coulomb interaction between electrons which prevents the double occupancy of
the system.
