text: Quantum devices can operate either continuously or by a reciprocating cycle. 
  Continuous devices include solar cells converting solar radiation to electrical power, thermoelectric  where the output is current and lasers where the output power is coherent light.
  The primary example of a continuous refrigerator is optical pumping and laser cooling. 
  Similarly to classical reciprocating engines, quantum heat engines also have a cycle that is divided into different strokes. A stroke is time segment in which a certain operation takes place (e.g. thermalization, or work extraction). Two adjacent strokes do not commute with each other. The most common reciprocating heat machines are the four-stroke machine, and the two-stroke machine. Reciprocating devices have been suggested operating either by the Carnot cycle or the  Otto cycle.
  In both types the quantum description allows to obtain equation of motion for the working medium
  and the heat flow from the reservoirs.