Reflection and transmission in a neutron-spin test of the quantum Zeno effect

The dynamics of a quantum system undergoing frequent "measurements," leadin g to the so-called quantum Zeno effect, is examined on the basis of a neutr on-spin experiment recently proposed for its demonstration. Unlike in all p revious studies, the spatial degrees of freedom of the neutron are duly tak en into account. Their inclusion in the analysis is important for two reaso ns: first, neutron-reflection effects are shown to be very important; secon d, the evolution may rum out to be totally different from the ideal case. O ur results can be interpreted in terms of a rigorous theorem due to Misra a nd Sudarshan: indeed we clarify that, in contrast with a widespread belief, a quantum Zeno effect does not halt the evolution of a quantum system; it rather modifies it, by forcing the system to remain in a certain subspace, defined by the very measurement performed. [S1050-2947(99)00811-2].