# entangled photons and interference

**URL:** <https://boards.straightdope.com/t/entangled-photons-and-interference/584281>\
**Category:** Factual Questions\
**Created:** [June 4, 2011, 3:08am UTC](https://boards.straightdope.com/t/entangled-photons-and-interference/584281 "2011-06-04T03:08:29Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\
**Post date:** [June 4, 2011, 3:08am UTC](https://boards.straightdope.com/t/entangled-photons-and-interference/584281/1 "2011-06-04T03:08:29Z")

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I was reading an article which linked to a Wikipedia article on "delayed choice quantum eraser. [Delayed-choice quantum eraser - Wikipedia](http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#Introduction) It contains this excerpt, and I’ve underlined the part which really caught my eye:

> [@](#):
>
> In the process they learned that constraining the path taken by one of a pair of entangled photons inevitably controls the path taken by the partner photon. Further, if the partner photon is sent through a double-slit device and thus interferes with itself, then very surprisingly the first photon will also behave in a way consistent with its having interfered with itself, even though there is no double-slit device in its way.

My first guess at what this meant was that doing this a huge number of times will produce an interference pattern. But without the double slit that would be exceedingly amazing, and I’ve never heard of it.

But in any case, it actually doesn’t say that. So my question is: What exactly does the article mean by “a way consistent with its having interfered with itself”? Of course, being Wikipedia, that sentence be poorly phrased or just plain wrong.

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**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\
**Post date:** [June 4, 2011, 4:46am UTC](https://boards.straightdope.com/t/entangled-photons-and-interference/584281/2 "2011-06-04T04:46:30Z")

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In the dual-slit experiment the interference patterns appear even if you send photons through the device one at a time. It is as though each photon goes through BOTH slits and interferes with itself on the far side.

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**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\
**Post date:** [June 4, 2011, 9:36am UTC](https://boards.straightdope.com/t/entangled-photons-and-interference/584281/3 "2011-06-04T09:36:22Z")

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> [@rowrrbazzle](#):
>
> But in any case, it actually doesn’t say that. So my question is: What exactly does the article mean by “a way consistent with its having interfered with itself”? Of course, being Wikipedia, that sentence be poorly phrased or just plain wrong.

You’re right, it’s probably not that obvious what it means. It means you repeat the experiment mulitple times and you put all the results together and you get an interference pattern.

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**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\
**Post date:** [June 8, 2011, 7:26pm UTC](https://boards.straightdope.com/t/entangled-photons-and-interference/584281/4 "2011-06-08T19:26:32Z")

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So you’re saying what the article means is that the photon of the pair that does NOT go through double slits really does produce an interference pattern? Seriously?

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**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\
**Post date:** [June 8, 2011, 9:01pm UTC](https://boards.straightdope.com/t/entangled-photons-and-interference/584281/5 "2011-06-08T21:01:33Z")

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> [@The\_Hamster\_King](#):
>
> In the dual-slit experiment the interference patterns appear even if you send photons through the device one at a time. It is as though each photon goes through BOTH slits and interferes with itself on the far side.

To be clear, each photon impact with the target is a discrete point. The interference pattern comes about by looking at the spread of multiple photons after going through the slit; you get a distribution that is consistent with an interference pattern of a wave going through both slits, similar to the classic wave mechanics double slit experiment i.e. bands of greater and lesser intensity.

Stranger