Unified entropy production in finite quantum systems

In finite-dimensional quantum systems, temperature cannot be uniquely defined. This, in turn, implies that there are several ways to define entropy production in finite-dimensional quantum systems, because the classical entropy production depends on temperature. We propose a unified definition of entropy production based on the difference in quantum relative entropy with respect to reference states characterized by effective temperatures. We demonstrate that the proposed definition naturally decomposes into a Clausius-type entropy production and an additional contribution arising from the time dependence of the effective temperature. Furthermore, we show that requiring the entropy production rate to take the conventional form as the sum of the entropy change and the heat flow constrains the effective temperature to be either constant or equal to a specific energy-matching effective temperature. For general initial states, entropy production can become negative, in which case we derive lower bounds on entropy production and establish sufficient conditions for its non-negativity using the trace distance.

💡 Research Summary

The paper addresses a fundamental difficulty in quantum thermodynamics: in finite‑dimensional quantum systems the notion of temperature is not unique, and because classical entropy production depends on temperature, there is no consensus on how to define entropy production for such systems. The authors propose a unified definition that works for any choice of an “effective inverse temperature” β_eff(t). Their construction is based on the change of quantum relative entropy between the actual joint state of system and environment, ρ_SE(t), and a reference product state consisting of the reduced system state ρ_S(t) and a Gibbs state of the environment γ_E(β_eff(t)).

The central definition (Eq. 13) reads
ΔΣ(β₀,β_τ) = D