Quantum Zeno effect

Unstable quantum systems, such as radioactive  nuclei, are normally expected to decay into their stable state with a certain fixed probability – e.g. a constant radioactive decay rate.

However it has been predicted, and now experimentally confirmed, that if a passive measurement is made on the quantum system, then the wavefunction that describes the superimposition of the original and decayed states is forced to collapse into one or other state. (A measurement, in quantum terms, is actually just an interaction, perhaps with something as simple as a photon.)

Assuming the measurement is made sooner rather than later, then the likely collapsed state is still the undecayed state i.e. there has not been sufficient time elapsed to expect the decay to have taken place. After the measurement, the superposition effectively restarts again at the beginning. So if repeated frequent measurements are made, then the radioactive atom would be unlikely to decay – the decay rate would be inhibited. This is termed the Zeno effect. (This brings to mind the saying ‘a watched pot never boils’!)

However it seems that, depending on the rate and type of measurement, and environmental factors, it is possible to get the opposite happening – an accelerated radioactive decay rate. This is termed the anti-Zeno effect.

This is a difficult topic to understand, but here are a couple of references:

Science Direct

ThoughtCo – A good explanation