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After creating entangled photons, how long does the entanglement last?

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  • $\begingroup$ Either you remember incorrectly or the article was wrong, entanglement notoriously cannot be used for faster than light communication. $\endgroup$ – fqq Oct 9 '19 at 17:57
  • $\begingroup$ The article was probably wrong(being about some chinese experiments) but the latest "delayed choice" experiments seem to suggest(at least to a non physicist like me) that it could be possible. $\endgroup$ – Cristi B Oct 9 '19 at 18:26
  • $\begingroup$ I have removed my comment about the article in the question wording since it leads to discussions beyond the scope of the original question. $\endgroup$ – Cristi B Oct 10 '19 at 7:15
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There isn't a simple answer to this because it depends on what the light interacts with.

If you start with two entangled photons then they form a single system described by a single wave function that isn't separable into the two photons. As long as this single system doesn't interact with anything else it remains unchanged. So if you manage to emit the photons along directions that head off into outer space without hitting anything then in principle the photons will remain a single entangled system forever.

If one (or both) of the photons hit something, let's suppose they hit a single molecule as an example, then now the photons and the molecule form an entangled system. That is we now have a single wave function that describes the photons and the molecule. And again if no further interaction occurs this single entangled system is in principle stable forever.

But in practice there are many interactions and the size of the entangled system grows rapidly. At some point the entangled system gets so complex that it decoheres and the entanglement is lost. Exactly how and why this happens depends on the system and its interactions. For simple systems like photons in a vacuum coherence can be sustained for a long time. For complex systems like qubits in a quantum computer decoherence occurs very rapidly, which is the current big problem with building quantum computers.

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Entanglement cannot be used to transfer information faster than light, unfortunately.

There is not a time limit to entanglement between two photons. Some pairs of photons emitted billions of years ago are still entangled because neither of the entangled members of the pair have ever interacted with anything.

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  • $\begingroup$ I find online that it's also destroyed after measuring the photon property. $\endgroup$ – Cristi B Oct 9 '19 at 18:32
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    $\begingroup$ That is true because measurement requires interaction. $\endgroup$ – S. McGrew Oct 9 '19 at 18:39
  • $\begingroup$ Also "delayed choice" experiments seem to show wave-colapse state can even be sent back in time. Wouldn't that qualify as information transfer?(I ask as a non-physicist, my background is engineer in Information Technology) $\endgroup$ – Cristi B Oct 9 '19 at 18:43
  • $\begingroup$ Actually, entanglement does not transfer information. It's more like a sort of synchronized random effect: measure one member of an entangled pair here, and you know what will happen if the other member is measured "over there", whether "over there" is in the past, present, or future. But there is no possibility of controlling the result of the measurement "here" (or "there") because the outcome is random, so no information is transferred. $\endgroup$ – S. McGrew Oct 9 '19 at 19:13
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    $\begingroup$ @S.McGrew Why "unfortunately"? $\endgroup$ – Norbert Schuch Oct 10 '19 at 13:36

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