  Quantum information processing (QIP) requires thorough assessment of
decoherence. Atoms or ions prepared for QIP often become addressed by radiation
within schemes of alternating microwave-optical double resonance. A
well-defined amount of decoherence may be applied to the system when spurious
resonance light is admitted simultaneously with the driving radiation. This
decoherence is quantified in terms of longitudinal and transversal relaxation.
It may serve for calibrating observed decoherence as well as for testing
error-correcting quantum codes.
