Two Slit Interference, One Photon at a Time by TeachSpin: The Instrument

Two Slit Interference, One Photon at a Time

Two Slit Interference, One Photon at a Time

Two Slit Interference, One Photon at a Time

The instrument consists of a black anodized aluminum U-Channel, a little over a meter in length, with a light tight removable cover. At one end, the student can select either of two light sources: a 670 nm, 1mW laser or a small flashlight bulb. The detection system at the opposite end is either a photodiode or a complete photon counting module.

Just in front of the light source is a single entrance slit. With either the laser or bulb illuminating this slit, the central maximum of the slit's diffraction pattern is aligned to cover a double slit assembly about 40 cm down the U-channel. Just past the double slit, a moveable "slit blocker" can be manipulated manually using a micrometer mounted on the outside of the U-channel. Using the slit blocker, students can compare the patterns created by the double slit to those created by either of the single slits. Three double slit assemblies, each of distinct slit spacing, are included.

At the far end of the U-channel is a moveable single slit, the detector slit. It, too, is attached to a translational stage actuated by a micrometer. Students move the detector slit across the interference pattern in front of either the photodiode or cathode of a photomultiplier to make quantitative measurements of either the light intensity or photon arrival rate as a function of position.

The black box with the brass front panel contains a complete photon counting module, as well as a photodiode detector connected a current-to-voltage converter. The box is supplied with a special flange and a light shutter. The photodiode is mounted on the outside of the shutter so that it is in the light path when the shutter is closed, and removed from the light path when the shutter is opened to let the light pass to the photomultiplier.

As in all TeachSpin apparatus, students are encouraged to explore the effect of changing a wide variety of parameters. In this instrument, the photomultiplier's high voltage supply and pulse-height discriminator level are both controlled and monitored from the front panel. It is also possible to inject a test pulse to calibrate the charge-sensitive preamplifier.

The entire photon-counting module can be detached from the U-channel and operated independently (requiring only a 15 volt regulated DC supply). This unit is thus available for other applications in the teaching or research laboratory, such as low intensity spectroscopy or photon correlation experiments.

The single-photon light source consists of a #47 flashlight bulb connected to a variable voltage-regulated power supply. The bulb is housed in a black plastic tube with a removable narrow-band green interference filter at the output end. A schematic is shown below.

Although flashlight bulbs such as this one generate about 10¹⁶ photons per second in ordinary use, operation at reduced power not only lowers the total production rate of photons, it also shifts the distribution toward longer wave length, thereby markedly reducing the production rate of "green" photons. The result is a thermal source of randomly produced photons, capable of giving a photon even rate at the detector in the range of 10¹ - 10⁵ per second. Thus, it is easy to take data in the "one photon at a time" regime in which the average waiting time for the next photon event vastly exceeds the time-of-flight for a photon through the apparatus.

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