Work and energy gain of heat-pumped quantized amplifiers

We investigate heat-pumped single-mode amplifiers of quantized fields in high-Q cavities based on noninverted two-level systems. Their power generation is shown to crucially depend on the capacity of the quantum state of the field to accumulate useful work. By contrast, the energy gain of the field is shown to be insensitive to its quantum state. Analogies and differences with masers are explored.