Redfieldtheorie

Redfieldtheorie is a quantum master equation approach that describes the dynamics of an open quantum system interacting weakly with an environment or bath. Developed by Albert G. Redfield in the 1950s, the theory is derived from second‑order perturbation theory applied to the von Neumann equation for the total system plus bath. After tracing out the bath degrees of freedom, a time‑convolutionless equation for the reduced density matrix of the system is obtained. The resulting Redfield equation contains dissipative terms expressed in terms of bath correlation functions and the system’s energy eigenstates.

Key assumptions of Redfieldtheorie are the Born approximation, which treats the system‑bath coupling as weak, and the Markov approximation, which neglects memory effects by extending the upper time limit to infinity. In practice, a secular or rotating‑wave approximation is often applied to ensure complete positivity of the density matrix, although the original Redfield equation does not guarantee this property.

The theory is widely used in magnetic resonance, energy‑transfer spectroscopy, and the study of quantum heat engines, providing insight into coherence, dephasing, and relaxation processes. Its perturbative nature limits accuracy when the coupling is strong or when non‑Markovian effects are significant. Alternative approaches, such as the Gorini–Kossakowski–Sudarshan–Lindblad master equation or path‑integral techniques, are employed in these regimes to obtain more reliable results.