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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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My goal: Find ways to connect classical ideas to quantum ideas as much as possible and carefully show where they differ.

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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This is the best way to introduce quantum ideas earlier in the curriculum

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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It is powerful ideas, not advanced math that is important

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Part 1: The Photoelectric Effect

Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Quanta are too small to see by the eye, so how do we interrogate them?

Consider a weak electric field. If it drives on resonance, then after some time the oscillation should have a high enough amplitude, that the electron will become unbound and be emitted.

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Questions to ponder?

  • How do we know what the resonance frequencies of the electrons are?
  • Do we expect that it takes more time for electrons to be emitted for low amplitude than high amplitude fields?
  • How do we detect the emitted electrons?
  • What happens when we change the color of the light?

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Classical predictions for resonant excitation

  • Light must have a frequency whose energy is at least as large as the highest bound energy of the electron. (resonance matching)
  • When we emit electrons on resonance, the kinetic energy should only weakly depend on the intensity and frequency of light. (emission immediately on reaching threshold)
  • Low amplitude light takes longer to first emit electrons. (resonance takes time to build up)
  • We should see materials dependence on the minimal frequency for resonance matching.

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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What is observed in an experiment?

  • Low frequency light emits no electrons regardless of the intensity.
  • Electrons are emitted immediately with no delay when the frequency is high enough.
  • The threshold frequency varies with the metal used.
  • Higher intensity light emits more electrons; their maximal kinetic energy is independent of the intensity, but depends on the frequency of the light.

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Let’s explore with the Phet simulation

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Phet simulation website Worksheet

https://phet.colorado.edu/en/simulations/photoelectric

https://raw.githubusercontent.com/QMwC/Resources/master/photoelectric.pdf

Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Discussion on the photoelectric effect?

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Part 2: Single Photon Sources and Detection

Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Photon: Wave or particle?

  • Newton first proposed light was a particle because it cast sharp shadows

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  • Thomas Young showed it had wave properties from the two-slit experiment, but this was not widely accepted until Francois Arago illustrated Poincare’s spot

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  • Einstein later showed that the photoelectric effect suggests particles of light.

How to Introduce Quantum Ideas into an Introductory Physics Course

OPTYCs TYC Tandem Meeting 2025

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Different types of light sources

  • Classical light
    • Thermal sources (blackbody radiation)
      • Incandescent lightbulb
      • The sun
    • Stimulated emission sources
      • Light-emitting diode
      • Laser
  • Quantum light
    • Atomic
      • Spontaneous emission from atoms
    • Solid state
      • Quantum dots
    • Nonlinear crystal
      • Parametric down conversion

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Dim classical light is not a single photon source

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Even a dim laser!

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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How to detect single photons

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Use the photoelectric effect for single-photon sensitivity

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Amplify the single emitted electron many-fold to make a large enough current pulse it can be measured with classical equipment

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Reset to measure again after the event is recorded

Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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A single photon can be imaged with a PMT once and only once!!

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Because it is destroyed upon detection.

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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PMT’s can have dark counts

  • Any stray photon can set off the photomultiplier tube

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  • As can any stray electron (tickling the dragon’s tail)

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  • Dark counts are reduced when the detector is made colder

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  • But, they cannot be prevented

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Calcium cascade single photon source

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Let us now explore how we can verify we have a single photon source

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G2 worksheet

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https://raw.githubusercontent.com/QMwC/Resources/master/G2.pdf

Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Discussion about single photons?

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Please don’t just call them particles of light

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Curriculum for a more detailed treatment

  • Week one: The story of photons and electrons
  • Week two: The Stern-Gerlach experiment
  • Week three: Formalism for two-state quantum systems
  • Week four: Single and two-slit diffraction of light and advanced ideas (delayed choice, interaction-free measurements)
  • Week five: Using quantum computers to simulate single-quantum experiments: Stern-Gerlach, two-slit experiment (and delayed choice variant), Mach-Zehnder interferometer
  • Week six: Advanced quantum experiments (Neutron oscillations, atom interferometry, Franson interferometer, Hong-Ou-Mandel experiment, LIGO)
  • Week seven: The canonical commutation relation
  • Week eight: The simple harmonic oscillator
  • Week nine: Angular momentum and molecular rotations
  • Week ten: Hydrogen

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Curriculum for a more detailed treatment

  • Week one: The story of photons and electrons
  • Week two: The Stern-Gerlach experiment
  • Week three: Formalism for two-state quantum systems
  • Week four: Single and two-slit diffraction of light and advanced ideas (delayed choice, interaction-free measurements)
  • Week five: Using quantum computers to simulate single-quantum experiments: Stern-Gerlach, two-slit experiment (and delayed choice variant), Mach-Zehnder interferometer
  • Week six: Advanced quantum experiments (Neutron oscillations, atom interferometry, Franson interferometer, Hong-Ou-Mandel experiment, LIGO)
  • Week seven: The canonical commutation relation
  • Week eight: The simple harmonic oscillator
  • Week nine: Angular momentum and molecular rotations
  • Week ten: Hydrogen

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Additional resources

  • For the photoelectric effect, the UW tutorials have a nice one on the photoelectric effect.
  • My courses on edX, the github repository quantum mechanics for everyone, and the website quantum.georgetown.domains
  • My Youtube channel has many different levels of discussions
  • We have a number of recent Am. J. Phys. articles that discuss these ideas.
  • Ask me for advice at anytime.
  • We will be developing more materials this Spring.

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Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Thanks to

Leanne Doughty Jason Tran

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James.freericks@georgetown.edu

Georgetown quantum resources

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quantum.georgetown.domains

Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025

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Survey �https://georgetown.az1.qualtrics.com/jfe/form/SV_6W13rajWmCsGaWO

James.freericks@georgetown.edu

Georgetown quantum resources

https://quantum.georgetown.domains

Using classical mechanics reasoning to introduce quantum ideas

OPTYCs 2YC Workshop, November 22, 2025