  Errors in the control of quantum systems may be classified as unitary,
decoherent and incoherent. Unitary errors are systematic, and result in a
density matrix that differs from the desired one by a unitary operation.
Decoherent errors correspond to general completely positive superoperators, and
can only be corrected using methods such as quantum error correction.
Incoherent errors can also be described, on average, by completely positive
superoperators, but can nevertheless be corrected by the application of a
locally unitary operation that ``refocuses'' them. They are due to reproducible
spatial or temporal variations in the system's Hamiltonian, so that information
on the variations is encoded in the system's spatiotemporal state and can be
used to correct them. In this paper liquid-state nuclear magnetic resonance
(NMR) is used to demonstrate that such refocusing effects can be built directly
into the control fields, where the incoherence arises from spatial
inhomogeneities in the quantizing static magnetic field as well as the
radio-frequency control fields themselves. Using perturbation theory, it is
further shown that the eigenvalue spectrum of the completely positive
superoperator exhibits a characteristic spread that contains information on the
Hamiltonians' underlying distribution.
