0
$\begingroup$

I am a non physicist so please bear with me here. I am just trying to see if my understanding of the implications of the delayed quantum eraser experiment is even remotely accurate.

If the apparatus was set up to record the the pattern from the double slit experiment on a photographic plate which required later development.

Then without detecting which path (slit) single photons are fired you would expect a interference pattern when the plate was developed (yes?)

Or if a detector recorded information (about which slit single photons pass) you would expect the wave function to collapse and see just two lines (corresponding to the slits) when the plate was developed (yes?)

Finally if the information about the path is destroyed before the plate is developed we would see the interference pattern again (yes?)

So what would happen if the information about the path of travel was hidden (perhaps on an encrypted hard drive or even in an unseen sealed envelope)

If before I develop the plate I look at the information on the hard drive or in the envelope the paths of the photons are established the wave function collapses and I would see two lines (yes?)

If I destroy the hard drive (magnetically or physically) or burn the envelope the information about the path of travel is destroyed forever and when I develop my photographic plate I would see the interference pattern (yes?)

Would this be a bit like the Schrodinger's cat scenario where the undeveloped photographic plate would have both the two lines and interference pattern until I decide to peak at the information or destroy it

Thanks for reading and I would really appreciate any insights you can provide me with :)

$\endgroup$
2
$\begingroup$

The most significant thing to understand, in my opinion, is that there isn't an interference pattern — the very device that splits the information about the photon spoils any possibility of such a thing appearing.

Instead, what happens in the delayer choice eraser experiment is that the eraser outputs a signal that categorizes each individual detection event into one of two categories. If your apparatus lets you plot just the events corresponding to just one of the categories, only then will you see an interference pattern.

And, of course, if you lose the information that lets you separate the events into categories, then you will never be able to see the individual patterns.

If you like, it's probably reasonable to think of the result as superimposing the two different interference patterns atop one another; and since the two patterns are perfectly out of phase, the result looks identical to no interference at all.

Since your thought experiment records all of the events together on the same photographic plate, you never had the ability to identify which events fall into which category, so you couldn't recover the two individual interference patterns even if you wanted to.

$\endgroup$
  • $\begingroup$ Thank you for your reply , so when you say : The most significant thing to understand, in my opinion, is that there isn't an interference pattern — the very device that splits the information about the photon spoils any possibility of such a thing appearing, do you mean the detector will collapse the wave function resulting in two lines? Is this true even when the information collected by the detector is destroyed? $\endgroup$ – Nutmeg Jun 15 '17 at 19:37
1
$\begingroup$

Then without detecting which path (slit) single photons are fired you would expect a interference pattern when the plate was developed (yes?)

Yes. But only if the detector was not even turned on. As soon as it could in principle record the path, even if it does not output it, the interference is lost.

Or if a detector recorded information (about which slit single photons pass) you would expect the wave function to collapse and see just two lines (corresponding to the slits) when the plate was developed (yes?)

Yes.

Finally if the information about the path is destroyed before the plate is developed we would see the interference pattern again (yes?)

No. You have made the classical mistake of confusing "observation" in the physical sense, i.e. any kind of interaction with the outside world (equivalent to a measurement), and a human "observing" something. It is like saying "as long as nobody is looking at the detector screen, there will be interference". This is the wrong way of looking at it.

The information is stored as soon as the photons hit the plate. They make chemical changes within the photosensitive layer, which is just not visible to us humans without developing it. But it is still there. All fancy things like encrypting stuff so that it is not available to us for observation has nothing to do with the quantum nature of things. The information is there, regardless if anyone is reading it. Erasing it after it has left the system will not change the outcome. Only when you erase the information in a way that in can not - even in principle - leave the system, then the interference pattern can be restored (as is the case in the classical quantum eraser using polarizers).

$\endgroup$
0
$\begingroup$

I would describe it this way. A photon or an electron is a "quant" which means it is a particle as well as a wave. Suppose the particle travels at some speed, but the wave is immediately there over the full path, but you can't "see" it until the particle arrived as well. As in the double slit experiment with delayed choice eraser (https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser), the two entangled quants from the SPDC from the quant who came through one of the splits immediately create the wave to D0 as well as one of D1-D4. The wave, when it connects to D1 or D4 says "hey, I've been detected", and forces the particle of the other quant to not interfere, thus making a dot in D0 that will result in a smudge. Would that be a simplification of what might happen?

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.