  Differences between the thermodynamic behavior of the three-level amplifier
(a quantum heat engine based on a thermally pumped laser) and the classical
Carnot cycle are usually attributed to the essentially quantum or discrete
nature of the former. Here we provide examples of a number of classical and
semiclassical heat engines, such as thermionic, thermoelectric and photovoltaic
devices, which all utilize the same thermodynamic mechanism for achieving
reversibility as the three-level amplifier, namely isentropic (but
non-isothermal) particle transfer between hot and cold reservoirs. This
mechanism is distinct from the isothermal heat transfer required to achieve
reversibility in cyclic engines such as the Carnot, Otto or Brayton cycles. We
point out that some of the qualitative differences previously uncovered between
the three-level amplifier and the Carnot cycle may be attributed to the fact
that they are not the same 'type' of heat engine, rather than to the quantum
nature of the three-level amplifier per se.
