Two-state system driven by imperfect pi pulses: an estimate of the error   accumulation in bang-bang control methods
  The evolution of a two-state system driven by a sequence of imperfect pi pulses (with random phase or amplitude errors) is calculated. The resulting decreased fidelity is used to derive a plausible limit on the performance of "bang-bang" control methods for the suppression of decoherence. 
Julio Gea-Banacloche