# Using photons to detect electrons and vice versa

• AnssiH
Is it possible to do this kind of experiment with photons and electrons existing in the same space, or is the double slit an ideal setup for this experiment?f

#### AnssiH

Something I've been wondering for a looong time...

In Feynman Lectures on Physics V1 Ch37, Feynman describes an electron double-slit experiment where the route of the electron is revealed by the electron scattering light on its way:

We shall now try the following experiment. To our electron apparatus we add a very strong light source, placed behind the wall and between the two holes. We know that electric charges scatter light. So when an electron passes, however it does pass, on its way to the detector, it will scatter some light to our eye, and we can see where the electron goes. If, for instance, an electron were to take the path via hole 2 that is sketched in Fig. 37-4, we should see a flash of light coming from the vicinity of the place marked A in the figure. If an electron passes through hole 1 we would expect to see a flash from the vicinity of the upper hole. If it should happen that we get light form both places at the same time...

...every time we hear a click from our electron detector (at the backstop) we also see a flash of light either near hole 1 or near hole 2, but never both at once. And we observe the same result no matter where we put the detector. From this observation we conclude that when we look at the electrons we find that electron go either through one hole or the other.

And then he goes on to describe that the interference pattern of the electrons disappears if the electrons are "watched" this way with light.

There are few additional details I would like to know about such experiments:

1. Suppose we have a high-flux experiment with constantly visible electron interference pattern on a screen. To make the interference pattern disappear, is it enough to just point a light across the supposed paths of the electrons? Just flick a switch and the interference pattern of electrons disappears?

2. Is the effect the same if you point a laser beam so that it crosses by either hole?

3. What happens if you point a laser so that it crosses the entrances of the holes? We will know the path of each electron, but they have yet to actually pass through the holes.

4. Does it matter whether or not you "watch" the photons that reveal the electron routes. I.e. is the "interaction between photons and electrons" enough to collapse the wave function or do the photons themselves also need to be captured in some sense?

5. What happens if you perform an experiment where it is the light passing the double slit and a beam of electrons being shot across the supposed paths of light?

And last but definitely not least, I can't help but think of a variation to Afshar's experiment:
http://en.wikipedia.org/wiki/Afshar_experiment

He placed wires in locations where the interference pattern would have dark fringes, and captured the photons after having passed the wires, and found that the "photons avoid the wires" (so to speak) even though the lense after the wires is such that we can tell which hole the photon came through (assuming the light behaves the way we imagine it to at the lense).

What if we have an interference pattern of electrons and instead of wires we use laser beams in locations where the electrons should not be found. Will we find that the electrons pass the laser beams unaffected by them?

Now what if we shift the location of laser beams so that they should detect the electrons passing by? In this case, depending on how it would behave (I cannot even begin to guess anymore :) we might detect an interference pattern by the lasers and still find out which way each electron came through after having passed the laserbeams?

I hope someone has some data about these sorts of experiments...

-Anssi

Anyone?

Anyone have bunch of photons and electrons lying around so we can test? :)

1. yes (uh.. actually, probably no, if the flux is too high, because the information gets erased again) 2. yes 3. same 4. no 5. same if that worked 6. interesting experiment.. if you trust Afshar's result then seems you should get the same with electron beams and laser wires.

Thank you very much for the reply. Just couple of follow-up questions.

1. yes (uh.. actually, probably no, if the flux is too high, because the information gets erased again)

Can this always be countered by making the "photon flux" higher also?

3. same

#3 (using a laser to detect the electron before it enters a hole), is your answer based on a prediction of quantum models, or has this kind of experiment been done? How far from the holes can we place the detection device to make the interference pattern disappear? I suppose placing it right on the electron gun doesn't do much, or does it?

5. same if that worked

I was wondering just now; wouldn't it happen that the electrons would scatter the photons all over the place?

-Anssi

1. In the original experiment, you can basically turn the interference on and off just by switching one of the lasers on and off (choosing whether to measure each electron's which-path information). But if the electron and photon flux are both very high then you will sometimes measure an electron coming through both slits at the same time, in which case you can no longer distinguish them (your which-path information is lost) and so there should be interference again.

3. What matters is whether you can, in principle, determine which path the electron took. As you suggest, a measurement right at the electron gun likely won't give you enough information to determine which path they take, so interference should still appear. As you move the laser toward the slits, your measurement will tell you (with gradually greater confidence) which path each electron takes, and so you would expect the fringes to gradually dissappear.

For the record though, my answers here are based only on my own understanding/intuition, although that is something I've developed through detailed study of various published experiments and so forth.

5. Yeah, the electron's motion isn't very significantly perturbed by the photon, so you can't really use the electron to measure which path a photon takes. Even if you could.. you would need an extremely high electron intensity (to guarantee every photon through that slit interacts with an electron) and also this would scatter those photons in every direction (so only really light from the other slit would reach the screen). So, yes, you'd see fringes when the electrons are turned off, and they'd disappear when the electrons are turned up, but the total intensity would also decrease, and it would not illustrate the quantum effect you're interested in.

But in principle, the quantum effect isn't limited to electron interference patterns, and you could indeed produce the effect with photon interference.. you'd just need to choose a which-path measurement mechanism that is appropriate for photons.

1. In the original experiment, you can basically turn the interference on and off just by switching one of the lasers on and off (choosing whether to measure each electron's which-path information). But if the electron and photon flux are both very high then you will sometimes measure an electron coming through both slits at the same time, in which case you can no longer distinguish them (your which-path information is lost) and so there should be interference again.

3. What matters is whether you can, in principle, determine which path the electron took. As you suggest, a measurement right at the electron gun likely won't give you enough information to determine which path they take, so interference should still appear. As you move the laser toward the slits, your measurement will tell you (with gradually greater confidence) which path each electron takes, and so you would expect the fringes to gradually dissappear.

For the record though, my answers here are based only on my own understanding/intuition, although that is something I've developed through detailed study of various published experiments and so forth.

Ok, that is good to put on record. What you are saying is what I think should happen based on what I know about quantum behaviour, yet it would be interesting to see this effect right in front of my eyes with high-flux experiments... And it would be interesting to see the variation to Afshar's experiment where we should see individual electrons being detected at the lasers with evident interference, and still gather them to individual detectors after the lense... :I

-Anssi

Personally, I'm interpreting the Afshar experiment as having the wire grid collapse the wave function so (in a holographic kind of way) that what is detected behind the lens is not truly measuring which slit the photon actually went through. I don't think it will behave differently at low intensity. I basically think the experiment is equivalent to first measuring position (perfectly), then measuring momentum (perfectly), then leaving others to guess (against HUP) that momentum hadn't changed since well prior the position measurement.

At the moment I think it is just impossible for many photons to be measured in the would-be place of the dark fringes, unless there is some way in-principle to know afterward which path those photons took (so as to prevent the interference manouvering them to elsewhere). On the other hand, would replacing the wires with a slitted mirror suffice?

Last edited: