PURPOSE: Demonstrate two-slit interference of single photons.

DESCRIPTION: Collimated light from a laser diode is incident on a double slit, creating interference. A photomultiplier tube sensitive to single photons is attached to the rotating telescope of the spectroscope opposite the light source. The signals from the tube are seen on the oscilloscope and heard using an audio amplifier and loudspeaker.

As the photon counter is slowly rotated the intensity of photons traces out the interference pattern for the double slit used. The slit can be removed from the spectrometer and placed in a laser beam to see what the intensity vs. angle curve looks like all at once. A slide is also included showing the double slit pattern.

SUGGESTIONS: Show this demonstration to indicate to the students the type of equipment and experiment involved, then show them the videodisc "Interference of Photons," which is old but contains a very good explanation of the key features of the experiment.

REFERENCES: (PIRA 7A55.20) See Demonstration Reference File for further information regarding this demonstration and its parts, including data on the single-photon counter and the slit.

EQUIPMENT: Laser diode with intensity slit, interference double slit, and single photon photomultiplier tube in small spectrometer, audio amplifier and loudspeaker, and oscilloscope on scope/TV cart.

SETUP TIME: 10 min.

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