Fig. 5
From: Quantum engine efficiency bound beyond the second law of thermodynamics
A photonic Carnot cycle for a squeezed thermal bath. The cycle starts with a thermal state with frequency ωc and temperature Tc (lower left corner). In stroke 1, the mode undergoes an adiabatic compression to frequency ω2 = ωcTh/Tc and temperature Th > Tc. Thereafter, in the energising stroke 2, the frequency is slowly reduced to ωh ≤ ω2 while the mode is connected to the squeezed thermal bath, yielding a squeezed thermal steady state. Its ergotropy is extracted in stroke 3 by an ‘unsqueezing’ unitary operation, resulting in a thermal state with temperature Th. In stroke 4, the frequency is again adiabatically reduced to ω1 = ωhTc/Th such that the mode attains the temperature Tc. Finally, stroke 5 is an isothermal compression back to the initial state
