  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.
