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. An identical slide on the Laser Cart (Demonstration M1-11) can be used to display the double slit interference pattern.

The double slit used has slit width of 0.04mm and slit spacing of 0.250mm. With the single photon counter you can hear about nine interference maxima in the main diffraction maximum, pass through the diffraction minima on either side, and hear a few interference maxima in the second diffraction maximum, as seen in the photograph below.

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.

Click here for an interactive computerized model of the double slit experiment produced at Kansas State University.

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, Pasco fixed double slit slide, single photon photomultiplier tube in small student spectrometer, audio amplifier with loudspeaker, and oscilloscope.

SETUP TIME: 10 min.