Preserving Quantum States : A Super-Zeno Effect

We construct an algorithm for suppressing the transitions of a quantum mechanical system, initially prepared in a subspace P of the full Hilbert space of the system, to outside this subspace by subjecting it to a sequence of unequally spaced short-duration pulses. Each pulse multiplies the amplitude of the vectors in the subspace by -1. The number of pulses required by the algorithm to limit the leakage probability to $\epsilon$ in time $T$ increases as $T \exp[ \sqrt{\log(T^2/\epsilon)}]$, compared to $T^2 \epsilon^{-1}$ in the standard quantum Zeno effect.