  We describe a simple quantum mechanical method that can be used to obtain
accurate numerical results over long time scales for the spin correlation
tensor of an electron spin that is hyperfine coupled to a large number of
nuclear spins. This method does not suffer from the statistical errors that
accompany a Monte Carlo sampling of the exact eigenstates of the central spin
Hamiltonian obtained from the algebraic Bethe ansatz, or from the growth of the
truncation error with time in the time-dependent density matrix renormalization
group (t-DMRG) approach. As a result, it can be applied to larger central spin
problems than the algebraic Bethe ansatz, and for longer times than the t-DMRG
algorithm. It is therefore an ideal method to use to solve central spin
problems, and we expect that it will also prove useful for a variety of related
problems that arise in a number of different research fields.
