  Projective measurements are an essential element of quantum mechanics. In
most cases, they cause an irreversible change of the quantum system on which
they act. However, measurements can also be used to stabilize quantum states
from decay processes, which is known as the quantum Zeno effect (QZE). Here, we
demonstrate this effect for the case of a superposition state of a nuclear spin
qubit, using an ancilla to perform the measurement. As a result, the quantum
state of the qubit is protected against dephasing without relying on an
ensemble nature of NMR experiments. We also propose a scheme to protect an
arbitrary state by using QZE.
