Editing Faster-than-light (section)

====Delayed choice quantum eraser====
{{Main|Delayed-choice quantum eraser}}

The [[delayed-choice quantum eraser]] is a version of the EPR paradox in which the observation (or not) of interference after the passage of a photon through a [[double slit experiment]] depends on the conditions of observation of a second photon entangled with the first. The characteristic of this experiment is that the observation of the second photon can take place at a later time than the observation of the first photon,<ref>
{{cite journal
|last1=Kim |first1=Yoon-Ho
|last2=Yu |first2=Rong
|last3=Kulik |first3=Sergei P.
|last4=Shih |first4=Yanhua
|last5=Scully |first5=Marlan O.
|year=2000
|title=Delayed "Choice" Quantum Eraser
|journal=[[Physical Review Letters]]
|volume=84 |issue=1 |pages=1–5
|arxiv=quant-ph/9903047
|bibcode=2000PhRvL..84....1K
|doi=10.1103/PhysRevLett.84.1
|pmid=11015820
|s2cid=5099293
}}</ref> which may give the impression that the measurement of the later photons "retroactively" determines whether the earlier photons show interference or not, although the interference pattern can only be seen by correlating the measurements of both members of every pair and so it cannot be observed until both photons have been measured, ensuring that an experimenter watching only the photons going through the slit does not obtain information about the other photons in an faster-than-light or backwards-in-time manner.<ref>
{{cite web
|last1=Hillmer |first1=R.
|last2=Kwiat |first2=P.|author2-link=Paul Kwiat
|date=16 April 2017
|title=Delayed-Choice Experiments
|url=https://www.scientificamerican.com/article/quantum-eraser-delayed-choice-experiments/
|website=[[Scientific American]]
}}</ref><ref>
{{cite web
|last=Motl |first=L.
|date=November 2010
|title=Delayed choice quantum eraser
|url=https://motls.blogspot.com/2010/11/delayed-choice-quantum-eraser.html
|website=[[The Reference Frame]]
}}</ref>