# Double Slit experiment with just one photon or electron

I just recently watched a video where they were explaining the double slit experiment.

I understand that the interference pattern would be observed if you send photons or electrons one after the other. The conclusion that the author seems to make is that it is as though the particle/packet of energy travels through both the slits after somehow splitting into two.

Has anyone done an experiment where they send just one particle? (And end the experiment) Do you observe two points on the screen in that case or just one?

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The conservation of energy still counts, so it would still show up at only one position. Only the probability of the 'particle' will be determined by the double-slit-interference-pattern. – fibonatic Jul 12 '13 at 12:20
BTW, The "one at a time" condition is not hard to achieve. You say "Well, the optical path is about 3 meters long so if we turn the source down until the rate is $10^{5}\,\mathrm{Bq}$ then the average occupancy of the device is $10^{-3}$." Or if that doesn't make you happy turn the rate down to 10,000 per second to get a mean occupancy of one in ten thousand and so on. – dmckee Jul 12 '13 at 12:47
The same Dr Quantum video in other Phys.SE posts: physics.stackexchange.com/search?q=url%3A%22DfPeprQ7oGc%22 – Qmechanic Jul 12 '13 at 13:29

Has anyone done an experiment where they send just one particle? (And end the experiment) Do you observe two points on the screen in that case or just one?

You don't observe two points - you get a probability. This is what the video was trying to indicate by showing dots on the screen. Those dots are the reason for postulating the role of probabilities in quantum mechanics.

The idea is that you can't predict where a single electron will hit the screen, the only thing you know is that some places are more probable than others. The probability is dictated by the probability density function which ostensibly can come from the quantum mechanical wave equation of the electron.

The probability distribution function for a single particle is the same function (aside from units/magnitude) as the intensity you see for a large number of particles. After all, that is always what we see when we observe the interference pattern. You just get enough particles that you can't distinguish single hits anymore.

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Thanks. I did not have enough rep to upvote this answer or fibonatic's comment, but yes, very useful. I was put off a bit by the idea of the particle splitting. I will post a separate question about that. – mehfoos Jul 12 '13 at 12:54
@user1218748 I would suggest doing just that. If a new curiosity stems off of a prior question then it makes sense to ask a new question. The wording of the video (or any double-slit intro) has lots of references to higher physics. For instance, saying that the electron travels every possible path references path integrals. I never understood those very well myself. – Alan Rominger Jul 12 '13 at 13:00

Yes, several people have done the experiment, and the photons/electrons are not split, you can get one hit at a time. Here is a second video .

The wikipedia two slit article has a build up of the electron interference pattern one at a time.

It is a probability pattern that shows the interference. The electrons and photons either go through one of the slits or the other, whole. Nothing mysterious is happening.

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

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Thanks for the images, but isn't an experiment where one can detect the slit the particle went through and observe the interference pattern a violation of complementary principle?! I have heard of the controversial Afshar experiment, but I was under the impression that experiments such as Wheeler's delayed choice experiment pretty much conclusively proved that observing the particle can influence the experiment. (Not merely because the observation leads to a causal effect) – mehfoos Jul 12 '13 at 14:23
Well, observing a particle changes the boundary conditions and a different solution and wave function would be necessary than the one of the undisturbed two slits. In the experiments linked in the wiki article they presume that the interference to the original is minimal. – anna v Jul 12 '13 at 16:48
This too is the better answer. Rephrasing: yes, the experiment has been done with a single particle, without observing which slit it went through. The result was always a single particle, never a split into two particles. The specific location of the single particle on the screen could not be predicted -- only the probability for each segment of the screen could be predicted. A single particle cannot prove the probability distribution -- it could hit anywhere. But a large number of particles, sent sequentially with gaps of time between them, conclusively prove the probability distribution. – randomstring Jul 15 '13 at 22:20
What experiments were recently made in which you know which way the particle went but you still get the inteference? – alex Nov 20 '13 at 19:00
@alex there is a link above to the wiki article, but the reference is behind a paywall. If you have access to a library you could see it. – anna v Nov 20 '13 at 19:35