  In the regime of weak bath coupling and low temperature we demonstrate
numerically for the spin-boson dynamics the equivalence between two widely used
but seemingly different roads of approximation, namely the path integral
approach and the Bloch-Redfield theory. The excellent agreement between these
two methods is corroborated by a novel efficient analytical high-frequency
approach: it well approximates the decay of quantum coherence via a series of
damped coherent oscillations. Moreover, a suitably tuned control field can
selectively enhance or suppress quantum coherence.
