Measuring clicks as photons arrive one by one

We now try an even more sophisticated experiment, in which we replace the screen by an array of photomultipliers. Each time a photon arrives we note the time and position. The hairline slits have reduced the beam intensity even further. In general only one photomultiplier goes off at a time, and we hear a series of discrete clicks. The light definitely behaves as a stream of particles.

Each photon passes through the apparatus 'on its own', the next photon is at a distance of about 10 km. It is difficult to construct a theory of interference between photons. There is no other photon or wave with which it could interfere!

The photons appear to be arriving in a random manner, rather like a shower of hailstones.

Photons arriving one by one

Screen Array of photomultipliers

Figure 14.4 Young's experiment using very dim light and photomultiplier detectors. (As before, the diagram is not drawn to scale. The slits are much narrower and closer together.)

Figure 14.5 'The pattern grows'. Results of an experiment reported by Delft University. (The 'dark' fringes are actually the areas where photons landed.)

A research project on discrete photon effects which was carried out by two third year students at Delft University of Technology, Niels Vegter and Thijs Wendrich, working under the supervision of Dr S.F. Pereira, obtained the interference patterns shown in Figure 14.5. (The experimental arrangement involved an image intensifier coupled to a highly sensitive photographic film, using gradually increasing exposure times — a slight variation on the method described above.)

At first just a few photons land on the screen. They seem to be distributed in a random manner. Gradually the pattern of 'hits' builds up, and we note a surprising result. There are areas where photons never land. We have 'dark fringes', as predicted by the wave theory of light. Eventually we finish up with bright and dark fringes, exactly as in the 'regular' Young's slit experiment.

The particles somehow follow the interference pattern of waves. There is a law of nature which prohibits the photons from landing in certain areas. Even though each photon arrives on its own, it appears to be

subject to the laws of interference of waves, which have passed through two neighbouring slits!

We know that a photon arrives at the slits and is detected by the photomultiplier, but we do not know the path it followed. All we can tell is that both slits were open. As soon as we close one slit, the pattern of regular dark fringes disappears, and we get back to the single slit diffraction pattern. By 'closing one door' we have made it possible for the photon to go to an area were it could not go before, where there had been a dark fringe when both doors were open.

According to wave theory, the waves emerging from the two slits interfere and give rise to the fringes. But how does the photon manage to be in two places at the one time and to interfere with itself?

Suppose that we try to resolve the matter by making a small adjustment to our apparatus. We insert a small detector beside each slit to tell us which way each photon goes. When a signal from one of the two detectors coincides with a hit on the screen we will have tracked the photon. We will know which way the photon went. Apart from inserting the detectors, we have not interfered with the experiment in any other way. The result is remarkable. The interference pattern on the screen changes. Instead of interference fringes typical of Young's double slit experiment, we get back to characteristic single slit diffraction.

The act of making the measurement has interfered with the result of the experiment. Many experiments of this type have been done with electrons and every single one has confirmed this result. The answer is that we cannot determine where any particular electron or photon goes, because the very act of making a measurement changes the situation.

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