Exploring Entanglement in the Double Slit Experiment

[Trollfaz]

In the double slit experiment , if one place only one detector around one slit, while leaving the other slit unchecked, the interference pattern vanishes. Does this show entanglement?

 

[Drakkith]

No, why would it?

 

[Trollfaz]

If the detector did not click, one will know that the particle passed through the other slit.

 

[Drakkith]

If the detector did not click, one will know that the particle passed through the other slit.

What other particle? The photon source emits single photons, not entangled pairs.

 

[UsableThought]

If the detector did not click, one will know that the particle passed through the other slit.

But that is only observing that a single particle goes through one slit rather than another - nothing spooky about that, is there?

I am a complete newbie when it comes to reading up on QT, but If I have the history correct, the first proposal for what later was to be called entanglement was the EPR thought experiment in 1935, which stipulated a radioactive particle which decays into two equal-mass particles; this led Schrödinger to propose entanglement; which in general involves pairs or groups of particles with a common origin & with quantum states that can't be described separately, even if they are far apart; or so I read, though I don't claim to understand yet.

Whereas with the double-slit experiment, for most of its history the point hasn't been to explore entanglement but rather wave vs. particle behaviors; i.e. that light behaved like a wave in showing interference (Young, 1801), and did the same even down to single photons as they accumulated (Taylor, 1909); and later that not only did closing a slit remove the interference effect, but so did putting detectors at the slits.

A later variation of the double-slit apparently does involve entanglement; this is the "delayed choice quantum eraser" first presented in 1999. By coincidence, earlier today I was reading about a DIY home version you can do of this experiment, described in May 2007 in Scientific American; although I haven't read enough yet to know how similar or different this DIY version is from the original.

 

[Nugatory]

A later variation of the double-slit apparently does involve entanglement; this is the "delayed choice quantum eraser" first presented in 1999.

That experiment is using entanglement as a clever way of measuring the photon's position without absorbing it. Just because we use this technique in a double-slit experiment doesn't mean that the experiment is demonstrating entanglement.

 

[UsableThought]

That experiment is using entanglement as a clever way of measuring the photon's position without absorbing it. Just because we use this technique in a double-slit experiment doesn't mean that the experiment is demonstrating entanglement.

Having thought about it a bit more -

I'm not at all clear that the distinction applies in this case. It's true that I'm not clear as to anything involving QT; but to me, "use" and "demonstrate" are ordinary words here; so we could as well be talking about hammering a nail as about QT when it comes to understanding the intent.

Obviously there can be a distinction drawn between what an experiment's main point is - what is intended to show - versus the technical means used to demonstrate this point; however in this case, if I look at both the original study & Wikipedia's reporting on that study, entanglement seems to have been very much one of the aspects the authors wanted to demonstrate; and in fact they say as much in their conclusion.

Some excerpts:

Link to the original study is here; and this is the abstract – quite short; bold is mine:

This paper reports a “delayed choice quantum eraser” experiment proposed by Scully and Dru ̈hl in 1982. The experimental results demonstrated the possibility of simultaneously observing both particle-like and wave-like behavior of a quantum via quantum entanglement. The which-path or both-path information of a quantum can be erased or marked by its entangled twin even after the registration of the quantum.

Similarly the link to the Wikipedia article is here; and this is the lead paragraph - bold is mine:

A delayed choice quantum eraser experiment, first performed by Yoon-Ho Kim, R. Yu, S. P. Kulik, Y. H. Shih and Marlan O. Scully,[1] and reported in early 1999, is an elaboration on the quantum eraser experiment that incorporates concepts considered in Wheeler's delayed choice experiment. The experiment was designed to investigate peculiar consequences of the well-known double-slit experiment in quantum mechanics, as well as the consequences of quantum entanglement.

And here is the beginning of the subsection in that article titled "Does delayed choice violate causality?":

Experiments that involve entanglement exhibit phenomena that may make some people doubt their ordinary ideas about causal sequence. In the delayed choice quantum eraser, an interference pattern will form on D0 even if which-path data pertinent to photons that form it are only erased later in time than the signal photons that hit the primary detector. Not only that feature of the experiment is puzzling; D0 can, in principle at least, be on one side of the universe, and the other four detectors can be "on the other side of the universe" to each other.[21]:197f

However, the interference pattern can only be seen retroactively once the idler photons have been detected and the experimenter has had information about them available, with the interference pattern being seen when the experimenter looks at particular subsets of signal photons that were matched with idlers that went to particular detectors.[21]:197

And back to the study itself again, here is the concluding paragraph; I have bolded the last sentence:

In conclusion, we have realized a quantum eraser experiment of the type proposed in ref. [3]. The experimental results demonstrate the possibility of observing both particle-like and wave-like behavior of a light quantum via quantum mechanical entanglement. The which-path or both-path information of a quantum can be erased or marked by its entangled twin even after the registration of the quantum.

That sentence seems to show that they wanted not merely to use entanglement, but to demonstrate something interesting about having used it. Which makes it part of the study's focus.

 

[DrChinese]

... Link to the original study is here; and this is the abstract ...

This is a great paper, and there are a number of PF members that are familiar with this and related variations. But I guess I didn't see a particular question.

I suspect everyone here would agree with you that this experiment uses entanglement to demonstrate some very interesting elements of QM. One of those elements being issues around time ordering: in many cases, ordering of events has no impact on the outcomes. That is true even when it defies classical causal ordering.

 

[UsableThought]

But I guess I didn't see a particular question.

In response to the OP's original question, I said it was my understanding that most of the double-slit experiments historically have not been concerned with entanglement; I then mentioned the quantum eraser experiment as a possible exception - I had come across a mention of it but hadn't looked at it yet.

@Nugatory then said, in response to my comment, that "Just because we use this technique in a double-slit experiment doesn't mean that the experiment is demonstrating entanglement." I interpreted this, perhaps mistakenly, as going against what I read in the study itself. Or not; I'm just trying to clarify so I understand as much as I can, bearing in mind that the technical details are over my head.