How is it even possible that a single photon can be isolated and "shot" through a slit? What tool or mechanism or setup allows such an incredible feat, especially since the photon has no mass?


Have a look at this double slit single photon experiment:

dblslit single phot

In 2003, A. Weis and R. Wynands at the University of Bonn (Germany) designed a lecture demonstration experiment of single photon interference from a double slit. Light from a laser pointer was so strongly attenuated that at each instant there was only a single photon between the double slit and the detector. The diffracted light was recorded by a single photon imaging camera consisting of an image intensifier (multichannel plate, MCP) followed by phosphor screen and a CCD camera. When adding consecutive camera frames one sees the gradual appearance of the smooth classical interference pattern

Each dot on a camera frame appearing at (x,y) is the footprint of a single photon. The accumulation which gives the interference pattern is the probability density distribution for the quantum mechanical problem "photon impinging on two slits". The photon is "shot" at the two slits.

Photons show their particle nature by the (x,y) footprint at the camera frame, and their wave nature in the probability distribution . The classical electromagnetic wave which gives the same pattern as the single accumulation, is a confluence of an enormous number of photons.

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    $\begingroup$ It is important to emphasize that this should not be envisioned as photons traveling in particular trajectories and hitting the detectors at these specific locations. The EM field was interacting with the detectors everywhere and with Fermi's Golden Rule establish these "hits" with a certain probability. It is a didactic point since the OP asked about getting photons through a slit like a localized bullet which can't be done. $\endgroup$ – Jan Bos Oct 9 '16 at 8:34
  • $\begingroup$ @JanBos one hit is the footprint of one photon. The electric and magnetic field information is carried in the complex wave function of the photon whose Psi*Psi gives the probability of finding the footprint at (x,y) $\endgroup$ – anna v Oct 9 '16 at 8:41

There is one very simple and very decisive experiment which proves the existence of photons beyond any doubt, and it involves exactly what the OP is asking about: shooting one photon at a time. You shoot photons at a half-silvered mirror and have two detectors, one for each path of the photon. Each time you shoot a photon, one of the detectors clicks. Either the photon got through or it was reflected. This proves light is made of photons: becuase if it were a wave, sometimes it would split in half and you would get clicks in both detectors.

This experiment has never been done. Oh, there are "similar" experiments involving something called parametric down-conversion which people claim amounts to exactly the same thing, but the actual experiment as described above has never been done. The reason is that there are no pea-shooters for photons. You can't shoot individual photons at will at a target.

I explain more about this in my blogpost, "There Are No Pea-Shooters For Photons".


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