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Quantum cooling by unitary transformations

We study the unitary time evolution of a simple quantum Hamiltonian describing a heat engine coupled to two heat baths. The engine is modeled as a three-level system. Each heat bath consists of a single harmonic oscillator. The engine is operated via time-dependent external fields. The S-matrix of the thermodynamic cycle is obtained in analytic form. We conjecture that the spectrum of this S-matrix contains a continuous part, and that this is a requirement for the operation as a heat engine. Energy currents flow in both directions through the engine. The balance of these currents determines whether the engine performs work or whether its operation requires the application of external forces.