Dual Slit: how are single electrons/photons detected?

[Buckethead]

Can anyone describe in detail the exact device used when detecting single photon/electron presence through a slit in a dual slit experiment. For an electron I suspect a coil is used to detect the electron passing through without destroying it, but for a photon, it's not possible to detect a single photon going through a slit without interacting with it (ccd or whatever) in which case the photon would not actually complete it's journey. So how it is done?

 

[Strilanc]

Can anyone describe in detail the exact device used when detecting single photon/electron presence through a slit in a dual slit experiment. For an electron I suspect a coil is used to detect the electron passing through without destroying it, but for a photon, it's not possible to detect a single photon going through a slit without interacting with it (ccd or whatever) in which case the photon would not actually complete it's journey. So how it is done?

Disclaimer: I am not a physicist and *definitely* not an experimental physicist. My ideas will be subtly or blatantly wrong.

One thing you could do is polarize the photons vertically, and place a waveplate (... I think that's the right thing ...) over one of the slits to adjust photons going through that slit to have horizontal polarization. As long as you don't then cancel the polarization differences somehow, the interference would disappear. I think.

 

[zonde]

but for a photon, it's not possible to detect a single photon going through a slit without interacting with it (ccd or whatever) in which case the photon would not actually complete it's journey. So how it is done?

It is as Strilanc said. You place differently oriented wave plates in front of the slits. Only you don't see interference even if you do not "look" at polarization of photons. Only when you place polarizer before detector so that both polarizations get through with equal chance but are the same after polarizer (obviously) you restore interference pattern.

I think that this setup was described as "A Do-It-Yourself Quantum Eraser" but I can't find nice link right now.

 

[StevieTNZ]

I think that this setup was described as "A Do-It-Yourself Quantum Eraser" but I can't find nice link right now.

Maybe it be this? -- http://www.scientificamerican.com/article/slide-show-do-it-yourself-diy-quantum-eraser/

 

[Buckethead]

Thank you for your replies. The eraser link does not show polaIrizers, but is interesting nontheless. I didn't know that a photon could scatter another photon. I thought photons just pass through each other. And for this type of detection doesn't the act of scattering disturb the photon, nulling the experiment?

But back to the detector...I imagine that the polarizers for each slit are 90 degrees from each other. And I can also imagine that without the additional polarizers by the detectors there would be no interference (how could there be when the waves are not oriented the same way?) and that by restoring the parallel polarization by putting in additional polarizers, interference would be restored, but where is the photon detector in all this. I seem to be missing something that is probably obvious.

Thanks

 

[zonde]

I didn't know that a photon could scatter another photon. I thought photons just pass through each other.

Nothing in this setup suggests that photons scatter photons. Photons are supposed to be "marked" by polarizing them.

But back to the detector...I imagine that the polarizers for each slit are 90 degrees from each other. And I can also imagine that without the additional polarizers by the detectors there would be no interference (how could there be when the waves are not oriented the same way?) and that by restoring the parallel polarization by putting in additional polarizers, interference would be restored, but where is the photon detector in all this. I seem to be missing something that is probably obvious.

You mean - photon detector at the slits? If so there is none. The same photon can't be detected twice. Probably you misunderstood me if it seemed like I suggest something like that.

 

[Buckethead]

Nothing in this setup suggests that photons scatter photons. Photons are supposed to be "marked" by polarizing them.You mean - photon detector at the slits? If so there is none. The same photon can't be detected twice. Probably you misunderstood me if it seemed like I suggest something like that.

The "scattered photon" was from the link that StevieTNZ offered up where a beam of light is used to interfere (scatter) the photon going through the slit. That was a tangent to the setup using the wave plates that you were suggesting.

"Only when you place polarizer before detector..." is what you said and I understand that to mean the detector is the final destination (the phosphor or whatever). I don't see in this setup how you can detect which slit the photon is going through (and thus destroying the interference pattern).

Thanks,

 

[Swamp Thing]

I remember reading this article soon after it was published. At that time I was a bit unsure whether this is really a case of quantum erasure.

Is it not true that orthogonally polarized waves can never interfere mutually, in any case? So the fact that the interference is removed by the first (composite) polarizer could be attributed to this fact alone, without invoking the "which way" concept. The 45 degree polarizer - the so-called eraser - is now "measuring" the polarization of the mixture of [H + V] photons, and the result of the measurement is a new state into which both the H and V states have to "collapse". All the photons are now in this new state, so that interference is restored.

IIUC, a truly "which-way" experiment with erasure would involve an opportunity for a particle to interact with a quantum entity which could later, in principle, be used as a record of the particle's path. Can we say that the polarizer plays this role? To quote from another parallel thread about erasure, from this forum :

To me 'it seems' that in some simple cases decoherence can be undone...

In the Sci. Am. experiment there is no decoherence that is being undone!

 

[f95toli]

Maybe I misunderstood what the OP what the OP was asking, but I interpreted the question as being "technical", i.e. which piece of kit is used to detect single photons/electrons. Adding polarizers won't make it any easier to actually detect anything.

Anyway,. the answer to the question is: It depends
Detecting single photons and electrons is not actually that difficult if you use the "right" parameters. There are commercially single photon detectors for many wavelengths and detecting electrons is also fairly easy if their energies are high enough.
The technical details for how this is done really depends on what you are trying to detect, there are many different types of detectors and they all have pros and cons; most of them also only cover parts of the spectrum (I am only really familiar with the high-performance superconducting detectors that are used for e.g. telecom wavelength). It is important to realize that there is nothing "fundamentally" hard about detecting single photons/electrons, it basically just comes down to clever engineering.

 

[zonde]

The "scattered photon" was from the link that StevieTNZ offered up where a beam of light is used to interfere (scatter) the photon going through the slit. That was a tangent to the setup using the wave plates that you were suggesting.

As I understand you are speaking about this page:
http://www.scientificamerican.com/media/inline/DD39218F-E7F2-99DF-39D45DA3DD2602A1_p95.gif
it describes hypothetical experiment with massive particles. While first two points are ok, I would not take third point as a fact without references.

"Only when you place polarizer before detector..." is what you said and I understand that to mean the detector is the final destination (the phosphor or whatever). I don't see in this setup how you can detect which slit the photon is going through (and thus destroying the interference pattern).

This is shown in points 3. and 4. from slide show. If you have horizontally polarized photons from one slit and vertically polarized photons from the other slit then by placing polarizer horizontally or vertically you will detect photons only from first slit or from second slit.

Please do not ask me to defend this idea about complementarity between which-way information/interference pattern. I don't think it's very rigorous and sometimes it's misleading. But if you are interested in experiments I will gladly tell you what I know.

 

[Nugatory]

IIUC, a truly "which-way" experiment with erasure would involve an opportunity for a particle to interact with a quantum entity which could later, in principle, be used as a record of the particle's path.

It's been done. Check out the rather clever method that Kim et al. used in their delayed choice quantum eraser experiment to generate which-way information for photons without interfering with their ability to reach the screen.

 

[atyy]

Can anyone describe in detail the exact device used when detecting single photon/electron presence through a slit in a dual slit experiment. For an electron I suspect a coil is used to detect the electron passing through without destroying it, but for a photon, it's not possible to detect a single photon going through a slit without interacting with it (ccd or whatever) in which case the photon would not actually complete it's journey. So how it is done?

Maybe http://arxiv.org/abs/1311.3625 Nondestructive Detection of an Optical Photon by Reiserer, Ritter and Rempe?