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From frozen Coulomb decoherence to thermalization: Classical and quantum density-matrix routes to Balescu–Lenard and Landau kinetics
We study the density-matrix kinetics of a charged test particle in two types of Coulomb environment. For a prescribed Gaussian potential, averaging the random-unitary dynamics leads to a Born–Novikov memory equation; Novikov’s identity is exact, but its closure requires a second-order approximation. An equilibrium quantum bath instead yields a Born–Redfield equation with ordered correlators and KMS spectral asymmetry. The resulting transition rates determine momentum-population kinetics, while a local Wigner–Boltzmann equation follows at leading spatial-gradient order. In the small-transfer limit, recoil and KMS asymmetry generate friction and diffusion from the same spectrum, consistently with the Einstein relation. For a Vlasov/RPA Coulomb bath, we recover the test-particle Balescu–Lenard diffusion tensor and its leading-logarithmic Landau limit. Static disorder has a different weak-scattering kinetic limit and produces elastic redistribution without thermalization. We also show that the population rates admit completely positive, but nonunique, extensions to quantum coherences. The connection between the two branches therefore lies in their Coulomb momentum-transfer kernels, not in an identical microscopic master equation.