  Within the lowest-order Born approximation, we present an exact calculation
of the time dynamics of the spin-boson model in the ohmic regime. We observe
non-Markovian effects at zero temperature that scale with the system-bath
coupling strength and cause qualitative changes in the evolution of coherence
at intermediate times of order of the oscillation period. These changes could
significantly affect the performance of these systems as qubits. In the biased
case, we find a prompt loss of coherence at these intermediate times, whose
decay rate is set by $\sqrt{\alpha}$, where $\alpha$ is the coupling strength
to the environment. We also explore the calculation of the next order Born
approximation: we show that, at the expense of very large computational
complexity, interesting physical quantities can be rigorously computed at
fourth order using computer algebra, presented completely in an accompanying
Mathematica file. We compute the $O(\alpha)$ corrections to the long time
behavior of the system density matrix; the result is identical to the reduced
density matrix of the equilibrium state to the same order in $\alpha$. All
these calculations indicate precision experimental tests that could confirm or
refute the validity of the spin-boson model in a variety of systems.
