How do scientists/researchers isolate a single photon (for single photon sources)?

How do they know they have isolated it? Is it really totally "isolated"? What is the photon isolated in?

Sorry if this is a basic or general question, just really interested to know what this means, considering that a "vacuum" isn't really empty space.

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    $\begingroup$ One can never "isolate" single photons. One can design experiments, in which mostly single photons are being detected. That doesn't mean that there are no other photons in that experiment at the same time, or that all photons are being detected or that there is never a case when two or more photons are being detected at the same time. Indeed, in most experiments one has to do some very careful statistical analysis to take care of all possible cases (loss of photons and simultaneous detection of two or more). Do you have a specific experiment in mind for further discussion? $\endgroup$
    – CuriousOne
    Sep 7, 2014 at 2:37

2 Answers 2


This is an instructive video on the double slit experiment with photons which is experimenting with single photons. At about 2.5 minutes it explains how the experiment is done with single photons. In a nutshell, by lowering the intensity of a light source to the point of zero emitted by the source and then slowly increasing it.

The detection hinges on the detector of single photons, in this case a photo multiplier. Photo multipliers work by multiplying the input from a single hit into an avalanche that is detectable electronically. For this detector since it has not been isolated from cosmic rays a noise background exists.

If one googles "single photon detectors" a number of commercial detectors come up, using different techniques. One can choose a detector that will fulfill the accuracies necessary in his/her experimental needs.

  • $\begingroup$ I don't understand how you would set up such a slit to pass a photon through. How would you do that? physics.stackexchange.com/questions/38440/… $\endgroup$
    – Lance
    Sep 12, 2014 at 4:00
  • $\begingroup$ There are good answers to the link you provided for why it did not work for him. Usually to ensure collimation of the beam one first passes it through a single slit, which sorts out the phases because it creates a point source type of optics. (Then I wonder whether the glass acted as detector at the slits with refelections, for example). here are simple instructions instructables.com/id/How-To-Make-a-Simple-Double-Slit/step2/…. The single photon is attained as I said by lowering the intensity to zero signal and then slowly slowly raising it. $\endgroup$
    – anna v
    Sep 12, 2014 at 4:09
  • $\begingroup$ Thanks for that link. But still, a slit cut out of paper is far from a quantum straight line which could split a single photon. For example, paper as seen through a Scanning Electron Microscope is far from straight. paperproject.org/semgallery.html There is a lot else that can be happening there, what am I missing? $\endgroup$
    – Lance
    Sep 12, 2014 at 4:43
  • $\begingroup$ @LancePollard A single photon IS NOT SPLIT. It has a probability of either going through one slit or another. This probability comes from the quantum mechanical solution, which nature does with great facility, of the boundary value problem "two slits at x distance of y width , one photon of frequency nu with perpendicular momentum". The solution of this gives a state function whose square gives the probability of the single photon to go through one or the other slit and how much it will diverge on the screen. The wave is a PROBABILITY WAVE IN THE XY OF THE SCREEN. Not an energy wave $\endgroup$
    – anna v
    Sep 12, 2014 at 5:06

The double slit experiment supposedly proves a single photon also acts in both wave and particle fashion. A photon is a quanta of photon particles. Sure they will act as a wave and also particles it is made of. The experiment is true only if the slits are small enough to allow a single photon particle or a single photon particle is used to experiment. You cannot make water wave with one water molecule.

  • 1
    $\begingroup$ This answer is incorrect. The double-slit experiment gives the same interference pattern whether the light intensity is large (that is, the continuum limit) or small (the particle limit). An interference patter still appears if the slits’ widths and separations are many times larger than the photon wavelength; for instance you can make a visible-light double-slit experiment using a razor blade to remove paint from a microscope slide. $\endgroup$
    – rob
    Apr 15, 2022 at 13:36

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