In the question Double Slit experiment with just one photon or electron, one of the answers says

There have been experiments recently where one can detect which the slit the particle went through and still the interference pattern appears.

This had 6 votes. Is it correct? If you put a detector to find out which way the particle went through won't that destroy the interference?

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    $\begingroup$ link to article? $\endgroup$ Nov 20, 2013 at 19:19
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    $\begingroup$ I remember reading that (and sadly forget where). The slit determination was of course an "indirect" observation. $\endgroup$ Nov 20, 2013 at 19:26
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    $\begingroup$ Presumably talking about anna v's anwer here. You should have linked the answer you meant. $\endgroup$ Nov 20, 2013 at 19:30
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    $\begingroup$ The most recent link in the wiki paragraph I linked to in my answer is behind a paywall newscientist.com/article/… . Not even the names of the authors are given. I remember that they used a difuse source method to reach the result of knowing which slit it came through, but cannot find an article $\endgroup$
    – anna v
    Nov 20, 2013 at 19:43
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    $\begingroup$ What about this? arstechnica.com/science/2012/05/… I found it linked as reference 30 in this Wiki article: en.wikipedia.org/wiki/… $\endgroup$
    – BMS
    Nov 20, 2013 at 22:08

2 Answers 2


If you put a detector to find out which way the particle went through won't that destroy the interference?


You even don't need a detector for destroying interference pattern. Take a look at this experiment. They just put two perpendicular polarizing filters after the slits. So that photon coming out of one slit will be polarized in one way, and photon coming out from second slit - will be polarized differently. So they encoded "which-slit" information into photons passing slits, so called "marker". This marker will then destroyed interference pattern, with or without photon polarization detector at the end. So conclusion is that not a photon detector destroys interference pattern, but rather a "which-slit" information encoding into traveling photons.


In the aforementioned experiment researchers has entangled a pair of photons. Later one photon from an entangled pair was directed to a slits and another was directed to some measurement device which detected which slit "sibling-photon" has taken. They exploited a quantum entanglement - strange thing in QM when measuring one particle you can say what the other sibling-particle "is doing". So in this experiment you see wave-particle duality at once, but from different photons - the one who passes slits behaves like a wave, and the one who passes into detector behaves like a particle.


Sometimes yes, observation of very small particles influence those particles because of the extremely small size of the particle that is attempted to be observed. I guess it depends, this can be both right and wrong. Sometimes you can observe it and influence the photon to become like a particle or wave as you observe it. Interference can also happen in other ways even in a well controlled experiment because of the extremely small size of the particle. So basically in conclusion you can still detect interference in some experiments even without a photon detector.


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