- It is properly claimed that observing a particle means hitting it, no matter how gently, with light, a photon, which disturbs the particle you are attempting to detect. That is why the system is disturbed. However, there have been other experiments which claim non-demolition measurements. They don't measure using light or photons
I included the link to a 2007 article explaining the work of Serge Haroche and his collegues at Ecole Normale Supérieure. If you had followed Quantum Week, you would have not been surprised when Serge Haroche shared the Nobel Physics Prize this year with David J. Wineland for exactly this work—his measuring of photons without disturbing them.
Now Myk is always quoting Richard Feynman saying if he were able to quickly explain what he did, it wouldn't be worth a Nobel Prize. Einstein is also quoted as saying, "If you can't explain it simply, then you don't understand it well enough." Well, as we know in this crazy world, both may be true. Actually Feynman brings them together in this little episode.
Feynman was once asked by a Caltech faculty member to explain why spin 1/2 particles obey Fermi-Dirac statistics. He gauged his audience perfectly and said, "I'll prepare a freshman lecture on it." But a few days later he returned and said, "You know, I couldn't do it. I couldn't reduce it to the freshman level. That means we really don't understand it."Despite the complexity of Serge Haroche's prize winning work (as previewed by In Progress), here is a short video which does a decent job explaining it.
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