Short answer: The quantum Zeno effect is the inhibition of transitions between quantum states by frequent measurement of the state. A 1990 Physical Review A paper reports observing it in trapped beryllium ions, a 2008 Physical Review Letters paper reports freezing the growth of a field in a cavity by repeated photon-number measurements, and a 2014 Nature Communications paper reports Zeno dynamics in a rubidium Bose-Einstein condensate. These are laboratory experiments on small systems.
The effect
The 1990 abstract defines the effect as the inhibition of transitions between quantum states by frequent measurements of the state. It says the inhibition arises because measurement causes a collapse of the wave function, and if the time between measurements is short enough the wave function usually collapses back to the initial state.
1990: trapped ions
The authors say they observed the effect in a radio-frequency transition between two beryllium-9 ion ground-state hyperfine levels. The ions were confined in a Penning trap and laser cooled. Short pulses of light, applied at the same time as the radio-frequency field, made the measurements. If an ion was in one state it scattered a few photons, and if it was in the other it scattered none.
2008: freezing a field in a cavity
The 2008 authors report freezing the coherent evolution of a field in a cavity by repeated measurements of its photon number. They use circular Rydberg atoms dispersively coupled to the cavity mode for absorption-free photon counting. The measurements inhibit the growth of a field injected into the cavity by a classical source. They say this illustrates the backaction of photon-number determination onto the field phase, and that the residual growth of the field can be seen as a random walk of its amplitude in phase space. They say the experiment sheds light on the measurement process and opens perspectives for active quantum feedback. A summary on the journal page describes the result as light inside a cavity prevented from evolving by successive rapid measurements.
2014: Zeno dynamics
The 2014 Nature Communications paper starts from the point that it is generally impossible to probe a quantum system without disturbing it, but that the backaction of measurements and strong couplings can be used to tailor and protect coherent evolution, which it calls quantum Zeno dynamics. The authors demonstrate this with a rubidium Bose-Einstein condensate in a five-level Hilbert space, using measurements and strong couplings to dynamically disconnect groups of states and constrain the atoms to evolve coherently inside a two-level subregion. They describe it as realizing a dynamical superselection rule and as a step toward protecting and controlling quantum dynamics and quantum information processing.
Limits
The three papers are different systems and show different parts of the idea. The 2014 abstract's statement about quantum information processing is a stated direction, not a built device. The page text for the 1990 paper available here is the abstract, so details of the measured transition rates are not reported.
FAQ
What is the quantum Zeno effect?
Per the 1990 paper, inhibition of transitions between quantum states by frequent measurements of the state.
Why does measurement stop the transition?
The authors say measurement collapses the wave function, and with short enough intervals it collapses back to the initial state.
Has it been seen in different systems?
Yes: trapped ions in 1990, a cavity field in 2008 and a Bose-Einstein condensate in 2014, according to the three papers.
What is Zeno dynamics?
The 2014 authors describe using measurements and strong couplings to confine evolution to a subset of states.
Is it a practical error-protection tool?
The 2014 authors call it a step toward protecting and controlling quantum dynamics. No deployed system is described.
All field notes