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Suppose Winger's friend is placed into a black box, thoroughly isolated from the outside world.

He constantly observes an atom with a delay of some microseconds. According to Zeno effect, atom's lifetime will increase.

He writes his observations to a log.

After a day passed, the box is opened and Wigner's friend asked when the atom decayed.

Will the increased atom lifetime be seen by the observers who were outside of the box?

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A slight detail: spontaneous emission does not experience the quantum Zeno effect when observed, since the probability function is (for short times) linear. You need a different functional form to see the quantum Zeno effect. For instance, (Rabi) oscillations of a two level have a sinusoidal probability in time and are therefore quadratic at short times. The 'resetting' required in the quantum Zeno effect comes from observing the system by probing, and therefore influencing, its evolution.

So, to answer you question, if someone else comes along after you have been probing and measuring your atom, they would see that the atom's evolution has been altered. But this isn't surprising: unlike passively watching an atom decay, in this scenario you've been actively altering the state of the atom by your measurement apparatus.

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Wrong answer in the both parts. 1)Spontanous emission undergoes Quantum Zeno effect 2)Quantum Zeno effect in certain circumstances can be measured even for exponential probability function. 3)It is possible to watch the atom via a passive detector. It is not important for my question. – Anixx Dec 18 '12 at 16:55
"As a typical model, we consider a continuous measurement of the decay of an excited atom by a photodetector that detects a photon emitted from the atom upon decay." – Anixx Dec 18 '12 at 17:10
I'm a sucker for experimental papers. Bernu et al. ( has a very clear explanation (and data), take a look at the last paragraph on the first page. I suppose the point I want to make with this: any observation of a system requires coupling to that system, e.g. coupling an atom to a cavity and probing the cavity mode, so it's not surprising than an outside observer agrees that the evolution of the system is altered. – emarti Dec 19 '12 at 2:15

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