text: Continuous quantum engines are the quantum analogues of turbines. The work output mechanism is coupling to an external periodic field, typically the electromagnetic field. Thus the heat engine is a model for a laser. 
  The models differ by the choice of their working substance
  and heat source and sink. Externally driven two-level, three level four-level and coupled harmonic oscillators have been studied.
  The periodic driving splits the energy level structure of the working medium. This splitting allows the two level engine to couple
  selectively to the hot and cold baths and produce power. On the other hand, ignoring this splitting in the derivation of the equation of motion  will violate the second law of thermodynamics.
  Non thermal fuels have been considered for quantum heat engines. The idea is to increase the energy content of the hot bath without
  increasing its entropy. This can be achieved by employing coherence or a squeezed thermal bath. These devices do not violate the second law of thermodynamics.