The quantum Otto cycle uses a two-level system (energy gap ℏω) as working substance. Four strokes: isentropic compression (ωc→ωh), hot isochoric (thermalize with Th), isentropic expansion (ωh→ωc), cold isochoric (thermalize with Tc). Efficiency η = 1 − ωc/ωh, bounded by Carnot ηC = 1 − Tc/Th.