Driven two-level system: energy storage and extractable work
A quantum battery is a two-level system (qubit) charged by a coherent drive. The ergotropy ε is the maximum work extractable by a unitary — it equals E − E_passive where E_passive is the minimum energy over all unitaries. On resonance (δ=0) the Bloch vector undergoes Rabi oscillations, periodically saturating the ergotropy. Coherences boost extractable work beyond classical thermal bounds.