# Is the double slit experiment performed measuring single photons?

As a hobby physicist I don't understand how the double slit experiment is performed in terms of single photons. Is the experiment really conducted by sending and measuring single photons? Is there not always interference of some kind (except for some device like the LHC)?

If yes: what would happen if the experiment is conducted over a prolonged period of time, for instance: 1 photon every n hours. Would they theoretically interfere with "themselves"? (don't know if i'm still making sense here).

If no: why is the interference considered to be originating from the sent photons and not from an external influence of some sort?

Thanks!

• – probably_someone Feb 22 '18 at 18:00
• More on single photons in YDSE: physics.stackexchange.com/q/76162/2451 , physics.stackexchange.com/q/70855/2451 and links therein. – Qmechanic Feb 22 '18 at 22:00
• @probably_someone and @Qmechanic - both your links provide very useful explanations. I understand that with CdS Quantum Dots it is possible to release a controlled frequency of photons. I still have trouble however converting these constructs to the configuration where two single bands result on the measurement device. What needs to happen to the configuration in order for the released photons to exert particle like behavior? (my current understanding is that you need to 'measure' which slit it goes through thereby 'collapsing the wavefunction',but how can you measure a single photon twice?) – Ropstah Feb 22 '18 at 22:25

Yes, there are single photon through a double slit experiments , as also single electron ones.

Here is one single photon at a time:

. Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

You can see the single photons hitting on the left frame, the hits seem random. By the time the accumulation has reached 1000 frames(about 50.000 photons counting the ones in the single frame) the interference pattern starts appearing, and it is the classical interference of light in the far right (~25.000.000 photons).

The external influence is the boundary condition: two slits d1 width each, d2 distance apart. Photon hits the boundary conditions. One sees the probability distribution for a single photon to scatter off these specific double slits. Each photon follows that probability distribution, except individually it looks random. In the accumulation the wave nature of the photon wavefunction is demonstrated.

The timing would have no influence on the pattern if the experimental geometry remains fixed.

• Appreciate the answer. I will comment 'upstairs' for those links also provide very useful information with which I can make my question more clear. – Ropstah Feb 22 '18 at 22:19
• Reading my comment it seems as if I haven't digested your answer which I have. I can't seem to wrap my head around the "single photon" concept properly, if i try to visualize this then it seem to make sense that a single photo "interferes" with itself just by the angle at which it is sent through the slit. I understand that if a slit has the same width as a single photon and if the entire sheet is made a few inches thick that there is only a single trajectory the photon can follow. But as soon as these parameters change it is logical for a photon to interfere with the boundary of the slit? – Ropstah Feb 22 '18 at 23:09
• "only a single trajectory the photon can follow" - in order to 'go through the sheet' onto the measuring device at the back. Widening the slit and thinning the sheet creates more space for the photons to make it through the sheet onto the measuring device. However a photon passing the edge of the slit would behave a little differently than one going exactly through the middle right? – Ropstah Feb 22 '18 at 23:12
• The photon is an elementary point particle. The dimensions have to do with with the frequency/wavelength since E=h*nu . Wide slits , widely separated, will not give strong interference ( some diffraction always happens at the edges). – anna v Feb 23 '18 at 6:54
• Here is for classical light philschatz.com/physics-book/contents/m42519.html . The surprise of quantum mechanics is that finally Newton was vindicated, and it is particles (photons) that build up light, but not in a simple summation, but in a quantum mechanical superposition of wavefunction. The wave nature of the photon lies in the probability distribution assigned by its wavefunction in a particular boundary conditions system. – anna v Feb 23 '18 at 6:54