7A55.20 SINGLE PHOTON INTERFERENCE

PURPOSE: To show the two slit experiment with single photons and to demonstrate wave-particle duality at the microscopic level

DESCRIPTION: With the acquisition of a photon detector that uses an image intensifier consisting of two successsive Micro Channel Plates this demonstration is now possible in the classroom.  Each single photon of light passes through the double slit arrangement and enters the detector.  Shown in the photos above are successive exposures. 

A sealed, lightproof  black box contains a small flash light bulb as a light source, a pinhole and a green filter,  a double slit and a photon detector.   Below is a movie of this demo:

Click on the image to see a movie of this demonstration.

The photon camera is a two stage micro-channel plate device. It is very sensitive to light and has an amplification factor of about a million.  The maximum sensitivity is in the green part of the light spectrum, where  the quantum efficiency is about 25% at 400 Angtsroms.  Currently we use a flashlight bulb as a source for the photons.  The images are acquired from the photon detector with an ARGUS image processor.  The ARGUS accumulates the photons continuously.  It consists of a photon counting computer and software that accumulates each photon and displays the integrated results on a TV screen.

THEORY: You may be thinking of photons as simply being particles. Of course, because in your physics textbook they’re shown as purple and blue globs of some radius. If this were the only truth then what interference pattern would you expect?

Based on this thinking, you would expect to see two large interference maxima, each being a single diffraction pattern resulting from each slit. This is because as the photons are considered to be particles, they would either go through one slit or another. However, based on the interference pattern forming on the screen from the experiment, this seems to not be true. In fact, there seems to be a multitude of maxima and minima.

Now you may be thinking that the photons are acting as waves? Well, since that’s the only answer you have left, what interference pattern would you now expect?

Based on this thinking, you would expect to see a multitude of waves in which the highest peak or maxima of greatest intensity would occur in the center of the interference pattern. On either side would be various maxima and minima in which the intensity decreases as you move farther away from the center of the pattern. Essentially, the result of Young’s Double Slit Experiment, which of course you remember.

And guess what? The interference pattern seems to fit Young’s results exactly. In the center of the diffraction pattern, the brightest fringes are found and as we move further away from the center, the intensity of the fringes dissipates.

However, can we really say that a photon acts only as a wave then?

Look at the interference pattern, right?

It is a wave pattern. Or, is It?

Actually, no. Suppose we put a detector near the slit, either in front or behind it. The detector would be able to pinpoint a single photon going through a single slit. In other words, we are able to clearly detect which slit a photon chooses to pass through. This behavior is characteristic of a particle. When the detector is present near the slits, the wave interference pattern is lost. Instead, now we see two single slit diffraction patterns, a result of each slit.

So now what? Photons are neither particles nor waves? Now you’ve run out of options. You might as well state that quantum mechanics is a bunch of baloney and tell your teacher that all that garbage he told you about waves and particles is false. Go ahead.

Apparently, the physics community is all about compromise. Imagine Einstein or Bohr becoming world leaders- plenty of peace would occur. Quantum mechanics states that since the photon exhibits both particle and wave behavior, then we simply can say that a photon exhibits wave-particle duality. Although we cannot deterministically detect where exactly the photon lands on the screen, the probability of its position can be calculated.

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REFERENCES: The Feynman Lectures on Physics. Vol. 3, R. P. Feynman, R. B. Leighton, & M. Sands, Addison-Wesley, 1965

Am. J. Phys. Vol. 64, No. 2, Feb 1996, Pages 184-188
A lecture demonstration of single photon interference
Wolfgang Rueckner and Paul Titcomb

EQUIPMENT: Single photon set up.

SETUP NOTES: Call at least 24 hours ahead. The low light imaging system consists of a Hamamatsu C2400 ICCD camera pictured below.  It is used without a lens and is connected to a power supply and an ARGUS image processor.