Quantum master equation

A quantum master equation is a generalization of the idea of a master equation. Rather than just a system of differential equations for a set of probabilities, quantum master equations are differential equations for the entire density matrix, including off-diagonal elements. A density matrix with only diagonal elements can be modeled as a classical random process, therefore such an "ordinary" master equation is considered classical. Off-diagonal elements represent quantum coherence which is a physical characteristic that is intrinsically quantum mechanical.

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