  We study the correction of errors that have accumulated in an entangled state
of spins as a result of unknown local variations in the Zeeman energy (B) and
spin-spin interaction energy (J). A non-degenerate code with error rate kappa
can recover the original state with high fidelity within a time kappa^1/2 /
max(B,J) -- independent of the number of encoded qubits. Whether the
Hamiltonian is chaotic or not does not affect this time scale, but it does
affect the complexity of the error-correcting code.
