Fig. 2
From: Quantum engine efficiency bound beyond the second law of thermodynamics

Interaction of a cavity mode with thermal and non-thermal baths. a A cavity mode initialised in a coherent state decays into the surrounding electromagnetic-field bath to the vacuum state. b A cavity mode prepared in the vacuum state evolves to a squeezed-vacuum state due to its interaction with a squeezed bath. The circles and the ellipse represent the respective phase-space distributions60 of the field states