Control of decoherence via quantum zeno subspaces

We discuss three control strategies, whose objective is to counter the effects of decoherence: the first strategy hinges upon the quantum Zeno effect, the second makes use of frequent unitary interruptions ("bang-bang" pulses), and the third of a strong, continuous coupling. Decoherence can be suppressed only if the frequency tau(-1) of the measurements/pulses is large enough or if the coupling K is sufficiently strong. Otherwise, if tau(-1) or K are large, but not sufficiently large, all these control procedures accelerate decoherence