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Two-Slit Interference, One Photon at a Time

"The Essential Quantum Paradox"

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Conceptual Tour - Two-Slit Interference, One Photon at a Time (33 kb)
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Lab Courses:
Quantum Optics
Modern Physics
Video Demonstration (6:20)

To See Two-Slit Interference, One Photon at a Time in action go to the 6:20 mark in this video provided by Terry Klopcic of Kenyon College Physics Department. The Two Slit's Cricket is being used to make photon arrivals audible.

This video features Professor Ben Schumacher.


Two Slit Interference, One Photon at a Time

Two Slit Interference, One Photon at a Time

  • Perform Two-Slit Interference with Single Photon Source and Detector
  • Recreate Young's Two-Slit Measurement of the Wavelength of Light
  • Make Visual Observations of Two-Slit Interference
  • Explore Photon Counting Optimization
  • Compare and Contrast Two-Slit and Single-Slit Interference
  • Amplitude Modulation of Laser Source for Lock-In Detection
  • Wave-Particle Duality, in the Student's Hands

With Two-Slit Interference, One Photon at a Time, TeachSpin has built an apparatus that allows students to encounter wave-particle duality with photons, the quanta of light. With this instrument, students perform the seminal two-slit interference experiment with light, even at the limit of light intensities so low that they can record the arrival of individual photons at the detector. That raises the apparent paradox which has motivated the concept of duality: in the very interference experiment which makes possible the measurement of wavelength, one observes the arrival of light energy in particle-like quanta, individual photon events. How is it possible for light to propagate as if it were a wave and yet to be detected as if it were a particle? This paradox is the central theme in Richard Feynman's introduction to the fundamentals of quantum mechanics:

"We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by explaining how it works . . . In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics."

It is the purpose of this apparatus to make the phenomenon of light interference as concrete as possible, and to give students the hands-on familiarity which will allow them to confront duality in precise and operational terms. When they have finished, students might not fully understand the mechanism of duality - Feynman asserts that nobody really does - but they will certainly have had direct experience of the phenomenon itself. They will have confronted the essential quantum paradox.

The Collaboration:
You couldn’t take Dr. Jonathan Reichert’s Modern Physics course at SUNY Buffalo without reading and rereading his hero Richard Feynman’s description of “a phenomenon which . . contains the only mystery.”

David Van Baak It is no “mystery” then that when David Van Baak mentioned the “Two Slit Interference, One Photon at a Time” he had developed for his students, TeachSpin immediately asked him to collaborate on adding one to our catalog. As Feynman observed, this phenomenon “appears peculiar and mysterious to everyone – both to the novice and to the experienced physicist.” To Feynman, the idea of photon interference was so important that the discussion appears in two places in his Lectures On Physics; Volume 1, Chapter 37 and Volume III, Chapter I.

We have no doubt that every physics student and Ph.D. physicist has shared Feynman’s fascination with this “peculiar and mysterious” phenomenon. It is exciting to be able to make a hands-on version available to pique the curiosity of our current students.

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