# Double slit experiment

I have an advanced amateur's(if there is such a thing) grasp of the concepts of quantum physics. However, the typical usage of the 'double slit experiment' to explain an observation of the differences between classical and quantum behaviours confuses me.

The specific case, of course, is when only a single partical is shot at the two slits and yet, over time, a compendium of such cases will retain the pattern of cancellation and reinforcement found with a large number of 'simultaneous' particles. My question is this: how can we, in fact, be sure that only a single particle is being emitted at a time in our experiment? Presumably there would be a high degree of uncertainty associated with such a precise declaration, wouldn't there?

For the record, I happen to think quantum theory is a pretty good approximation of our observed phenomena, I just don't completely understand this example.

sincerely,
thatwouldbeme

DrChinese
Gold Member
jeffceth said:
My question is this: how can we, in fact, be sure that only a single particle is being emitted at a time in our experiment? Presumably there would be a high degree of uncertainty associated with such a precise declaration, wouldn't there?

No, you would detect one particle at a time. The pattern that results from many particles then shows the interference.

ZapperZ
Staff Emeritus
This is an example of what DrChinese is describing.

http://www.optica.tn.tudelft.nl/education/photons.asp [Broken]

We can deduce that one photon passes through at a given time from the calculation of how many photons are there per second and, on avarage, the separation between them. Since the equipment is not several kilometers long by orders of magnitude, it is a safe bet that on average, one has only one photon at a time passing through the slits.

Zz.

Last edited by a moderator:
ZapperZ said:
This is an example of what DrChinese is describing.

http://www.optica.tn.tudelft.nl/education/photons.asp [Broken]

We can deduce that one photon passes through at a given time from the calculation of how many photons are there per second and, on avarage, the separation between them. Since the equipment is not several kilometers long by orders of magnitude, it is a safe bet that on average, one has only one photon at a time passing through the slits.

Zz.

I understand that we can calculate the average distance between photons and suppose it is the actual distance, but wouldn't the nature of quantum physics itself suggest that because in whatever medium the photons are being produced it requires a minimum charge to release a photon at all we essentially have the equivalent of a huge bank of capacitors charging up and releasing, which might tend to drastically increase simultaneous emissions if the energy source had any sort of wave period to it? Generally speaking, what sort of mechanisms might one employ to reduce the possibility of such an effect?

sincerely,
thatwouldbeme

p.s. DrChinese: do we really have devices that will reliably detect a single photon? In practice it seems we usually have to use a photon multiplier in order to measure what we suppose to be single-photon events.

Last edited by a moderator:
ZapperZ
Staff Emeritus
jeffceth said:
I understand that we can calculate the average distance between photons and suppose it is the actual distance, but wouldn't the nature of quantum physics itself suggest that because in whatever medium the photons are being produced it requires a minimum charge to release a photon at all we essentially have the equivalent of a huge bank of capacitors charging up and releasing, which might tend to drastically increase simultaneous emissions if the energy source had any sort of wave period to it? Generally speaking, what sort of mechanisms might one employ to reduce the possibility of such an effect?

I have no idea what you just said. If I use a laser (which is what is used in the experiment that I cited in the link), where is the "bank of capacitors" here?

Zz.

jeffceth said:
how can we, in fact, be sure that only a single particle is being emitted at a time in our experiment?
do we really have devices that will reliably detect a single photon?
Basically you’re questioning the ability of experimentalist’s hardware capabilities. To be sure they test them to build reliable methods. Low but uniform power output - into Filter densities and or crossed polar filters to bring the photon rate down. Lots of test exposures or Detector counts to confirm they can reliably produce one photon per some unit of time, whatever. Statistically you may have a chance of two coming close together. To avoid that issue, the problem becomes how fast can you send them to avoid having two come out together. After all it can be a long slow tedious work made worse if you insist on say one photon per minute. You want the highest rate that still works correctly by never sending two at once. Theoreticians need to trust the experimenters, and they do that by verifying the work. And even then getting it repeated by someone else.

Detecting a single photon?
As you say you have a grasp of the concepts of quantum physics, then you must know some about entanglement experiments. Those very much depend on testing correlations of detections of individual photons or particles don’t they?

DrChinese
Gold Member
jeffceth said:
p.s. DrChinese: do we really have devices that will reliably detect a single photon? In practice it seems we usually have to use a photon multiplier in order to measure what we suppose to be single-photon events.

Oh, absolutely. They can be calibrated to extremely high accuracy. In fact, entanglement is now routinely used to actually assist in detector calibration and analysis of detector efficiency and accuracy because of experiments like this one:

P. Grangier, G. Roger, and A. Aspect, "Experimental evidence for a photon anticorrelation effect on a beam splitter: A new light on single-photon interferences," Europhys. Lett. 1, 173-179 (1986).

To quote the authors: "A single photon can only be detected once." Using PDC, this is done routinely these days; check out this PDF of an undergraduate experiment that replicates the above: http://marcus.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf [Broken]. To many standard deviations of accuracy, individual photons are detected within time windows that support theoretical predictions. Check out the article to see how this is done. There are articles out there - I don't have a reference at my fingertip - that explain in greater detail how manufacturers rate their detectors and insure that they perform as described.

Last edited by a moderator:
Of course, entanglement. Yeah, I should have known that. I pretty much should have been able to answer that myself. Thanks for taking the time though, guys! I guess ultimately even with a high probability of multiple photon incidents the highest cancellation zones would still fail to get pretty much any hits, where as we would expect a large number of the single-photon cases to hit there otherwise, so it doesn't take a whole lot to produce a statistically significant result in that way, too.

jeffceth said:
even with a high probability of multiple photon incidents the ..........
I doubt a good experamenter would accept that, and would only use a very low probablity of multiple photons.