  We investigate the quantum thermodynamics of two quantum systems, a two-level
system and a four-level quantum photocell, each driven by photon pulses as a
quantum heat engine. We set these systems to be in thermal contact only with a
cold reservoir while the heat (energy) source, conventionally given from a hot
thermal reservoir, is supplied by a sequence of photon pulses. The dynamics of
each system is governed by a coherent interaction due to photon pulses in terms
of the Jaynes-Cummings Hamiltonian together with the system-bath interaction
described by the Lindblad master equation. We calculate the thermodynamic
quantities for the two-level system and the quantum photocell including the
change in system energy, power delivered by photon pulses, power output to an
external load, heat dissipated to a cold bath, and entropy production. We
thereby demonstrate how a quantum photocell in the cold bath can operate as a
continuum quantum heat engine with the sequence of photon pulses continuously
applied. We specifically introduce the power efficiency of the quantum
photocell in terms of the ratio of output power delivered to an external load
with current and voltage to the input power delivered by the photon pulse. Our
study indicates a possibility that a quantum system driven by external fields
can act as an efficient quantum heat engine under non-equilibrium
thermodynamics.
