Quantum decay cannot be completely reversed. The 5% rule
  Using an exactly solvable model of the Wigner-Weisskopf atom it is shown that an unstable quantum state cannot be recovered completely by the procedure involving detection of the decay products followed by creation of the time reversed decay products state, as proposed in \cite{Son}. The universal lower bound on the recovery error is approximately equal to $5% $ of the \emph{error per cycle} - the dimensionless parameter characterizing decay process in the Markovian approximation. This result has consequences for the efficiency of quantum error correction procedures which are based on syndrome measurements and corrective operations. 
Robert Alicki