  The time evolution of some quantum states can be slowed down or even stopped
under frequent measurements. This is the usual quantum Zeno effect. Here, we
report an operator quantum Zeno effect, in which the evolution of some physical
observables is slowed down through measurements even though the quantum state
changes randomly with time. Based on the operator quantum Zeno effect, we show
how we can protect quantum information from decoherence with two-qubit
measurements, realizable with noisy two-qubit interactions.
