Reversibility of dynamics and multiple-quantum coherences
  In spin systems, the decay of the Loschmidt echo in the time-reversal experiment (evolution, perturbation, time-reversed evolution) is linked to the generation of multiple-quantum coherences. The approach is extended to other systems, and the general problem of reversibility of quantum dynamics is analyzed. 
A. K. Khitrin
quant-ph cond-mat.stat-mech math.DS