Thermodynamics and fluctuation theorems for a strongly coupled open quantum system: an exactly solvable case

We illustrate recent results concerning the validity of the work fluctuation theorem in open quantum systems (Campisi et al 2009 Phys. Rev. Lett. 102 210401), by applying them to a solvable model of an open quantum system. The central role played by the thermodynamic partition function of the open quantum system, a two-level fluctuator with a strong quantum nondemolition coupling to a harmonic oscillator, is elucidated. The corresponding quantum Hamiltonian of mean force is evaluated explicitly. We study the thermodynamic entropy and the corresponding specific heat of this open system as a function of temperature and coupling strength and show that both may assume negative values at nonzero low temperatures.