Key Stage 5 – Quantum quirks

Notes for teachers

At a glance

Scientists at The University of Oxford are utilising the quantum quirks of light to study how single photons could be used to provide un-hackable data protection, quantum computers and powerful microscopes. This activity is suitable as a follow-on lesson after students have studied Young's double slit experiment. It asks them to interpret data from a single photon double slit experiment as an introduction to quantum theory.

Learning Outcomes

  • Students write predictions and conclusions using data
  • Students describe what the single photon double slit experiment shows about the nature of light

Each student will need

  • Copy of page 1-4 of the pupil worksheet. Pages 2 -4 should be cut in half to produce two A5 sheets.

Possible Lesson Activities

  1. Starter activity
  2. Question the students: What is light? Ask them to write down a'tweet' in 140 characters. Invite a few students to read out their explanation and record on the board some of the keywords they use.
  3. Depending on previous knowledge, students may mention waves, electromagnetic radiation and photons.
  1. Main activity: The single photon double slit experiment
  2. Give each student a copy of page 1 of the pupil worksheet and ask them to read through the information.Take time to recap the double slit experiment and discuss an explanation of the results as light behaving as a wave and interference taking place.
  3. Introduce the class to an updated version of the double-slit experiment where single photons are fired through the slits, one at a time. Discuss that we can assume that each photon will go through one slit or the other before being detected by the photodiode. The start of the Veritasium video in the weblinks below can be shown to the class to explain this.
  4. Give each student a copy of page 2 of the pupil worksheet. Ask them to make a prediction on the graph axis.
  5. Then, give each student a copy of page 3. This shows the data produced after running the experiment for one second. They should see that the spread of the location of photons on the photodiode looks random. It looks as though the photons have travelled through each slit in equal numbers. Students may have predicted that the photons would land in two defined areas behind the slits. The spread can be explained if we assume that some photons are deflected by the edges of the slits.
  6. Finally, give students page 4 of the student worksheet which shows the data after 5 minutes. A pattern has emerged. They should notice that this is similar to the interference pattern produced when a constant stream of photons is used. Discuss how this is evidence that the photons are behaving like waves. How can we explain how single photons can interfere with themselves, as only one went through one slit at a time? Somehow, each photon must be contributing to the wave-like behaviour to produce the interference pattern. Photons can act as both particles and waves. This is known as particle-wave duality.
  1. Plenary
  2. Discuss the simple fact that we can't explain these results using anything we can relate to - photons do not behave in the same way as macroscopic objects - welcome to the quantum world!
  3. To add to this counter-intuitive process ask the students to consider these quantum quirks:

- If you measure or observe the path of each photon then their wave function will collapse and they will revert to acting as particles (so you will not get an interference pattern).

- A pair of entangled photons can be produced. If one photon is measured, then it will collapse into its original state along with its pair (even though they could be separated by a long distance).

  • Show the class the animation ‘Run for your Light’ which shows some applications of single photon technology which utilise these properties of photons.

Weblinks

Run for your Light animation

Laser light show which can be used for the starter.

Entertaining article about how we have come to understand the nature of light - suitable for student background reading.

Video from Veritasium about single photon interference. Show up to 2:52 for a recap of Young's Double slit experiment plus the equipment used for the single photon experiment.