**Definition: **

The *quantum Zeno effect* is a phenomenon in quantum physics where observing a particle prevents it from decaying as it would in the absence of the observation.

### Classical Zeno Paradox

The name comes from the classic logical (and scientific) paradox presented by ancient philosopher Zeno of Elea. In one of the more straightforward formulations of this paradox, in order to reach any distant point, you have to cross half of the distance to that point. But to reach that, you have to cross half that distance. But first, half of that distance. And so forth ... so that it turns out you actually have an infinite number of half-distances to cross and, therefore, you can't actually ever make it!

### Origins of the Quantum Zeno Effect

The quantum Zeno effect was originally presented in the 1977 paper "The Zeno's Paradox in Quantum Theory" (*Journal of Mathematical Physics*, PDF), written by Baidyanaith Misra and George Sudarshan.

In the article, the situation described is a radioactive particle (or, as described in the original article, an "unstable quantum system"). According to quantum theory, there is a given probability that this particle (or "system") will go through a decay in a certain period of time into a different state than the one in which it began.

However, Misra and Sudarshan proposed a scenario in which repeated observation of the particle actually prevents the transition into the decay state. This may certainly be reminiscent of the common idiom "a watched pot never boils," except instead of a mere observation about the difficulty of patience, this is an actual physical result that can be (and has been) experimentally confirmed.

### How the Quantum Zeno Effect Works

The physical explanation in quantum physics is complex, but fairly well understood. Let's begin by thinking of the situation as it just happens normally, without the quantum Zeno effect at work. The "unstable quantum system" described has two states, let's call them state A (the undecayed state) and state B (the decayed state).

If the system is not being observed, then over time it will evolve from the undecayed state into a superposition of state A and state B, with the probability of being in either state being based on time. When a new observation is made, the wavefunction that describes this superposition of states will collapse into either state A or B. The probability of which state it collapses into is based on the amount of time that has passed.

It's the last part which is key to the quantum Zeno effect. If you make a series of observations after short periods of time, the probability that the system will be in state A during each measurement is dramatically higher than the probability that the system will be in state B. In other words, the system keeps collapsing back into the undecayed state and never has time to evolve into the decayed state.

As counter-intuitive as this sounds, this has been experimentally confirmed (as has the following effect).

### Anti-Zeno Effect

There is evidence for an opposite effect, which is described in Jim Al-Khalili's *Paradox* as "the quantum equivalent of staring at a kettle and making it come to the boil more quickly. While still somewhat speculative, such research goes to the heart of some of the most profound and possibly important areas of science in the twenty-first century, such as working toward building what is called a quantum computer." This effect has been experimentally confirmed.