Two-level qubit heat engine — work, heat & efficiency
The quantum Otto cycle on a qubit: (1) isentropic compression — gap ω₁→ω₂ with no heat; (2) hot isochoric — qubit thermalises with hot bath T_h; (3) isentropic expansion — gap ω₂→ω₁; (4) cold isochoric — thermalises with cold bath T_c. Efficiency η = 1 − ω₁/ω₂ ≤ η_Carnot = 1 − T_c/T_h.