Part 1: Investigating Single Photon Interference

Timing: The full activity will take about 30 minutes. If you have very little time, you can just show the three short videos and discuss the last two questions in 10 minutes.

Key Concepts: This lesson focuses on the wave-particle dualityof light and other quantum objects.

Assumptions of Student Background Knowledge: This lesson assumes that the students are already familiar with double-slit inference with water waves, sound and light. This lesson can be used as an extension of the PI resource The Challenge of Quantum Reality.

Materials: Worksheets and student data sets to analyse the photon data. A data projector with internet connection to view the three videos.

The answers and extra information for teachers is in red and needs to be removed before photocopying and printing the student worksheet.

A double-slit experiment was done at the Institute of Quantum Computing using incredibly dim light and a row of 32detectorswhich were 0.1 mm apart. The detectorsrecorded photons arriving one at a time. They and measured where and when the photons arrived.

1)Graph the data that you have been given with time vertical.

This exercise isdesigned to demonstrate the random and individual arrival of the photons. This data is provided in thefile Single Photon Data.docx. Print one copy and cut into sets of ten data points for each student. The time measurements are precise to +/- 0.150 ns. Time is presented on the vertical axis – which is not usual except for spacetime diagrams and may concern some students. It is done this way so that it matches the way the data is presented in the video of the data that is viewed after question#3). I suspect that the K. Shalm - who made this video - chose this orientation because when you make the time vertical it looks identical to the pattern you get when the slits have a vertical dimension.

2)Does the information you have recorded show light behaving as a wave or as a particle?Explain.
It is behaving like a particle, because it is arriving at the detector at very specific locations and times. As well as demonstrating particle-like behaviour, this behaviour shows that it is not a classical wave. Classical waves are spread out and do not have a definite location.

Note: The particle nature of light was first hinted at by Planck in explaining black body radiation and further confirmed by Einstein and Millikan with the photoelectric effect. What really convinced the physicists was the explanation of the Compton Effect. Most textbooks take a historical approach and introduce the particle nature of light using these experiments. This makes no pedagogical sense. The particle nature of light is most clearly demonstrated using the double-slit experiment.

3)Combine your data with that of other students and plot it as a histogram where the height represents the number of times that light has arrived at that particular detector.

You can either have students plot this by hand using the grid below or you can have them enter the data electronically on one chart that is displayed by a data projector. A spreadsheet for this has been prepared as the file Single Photon Class Spreadsheet. The grid below shows the results for the first ten sets of data.

4)Does the histogram show light behaving like a wave or a particle? Explain.

Light’s particle-like nature is shown by the way it comes in at a particular time and place. However, with more data we see that the pattern is not random. It looks like there are several peaks. This becomes clearer as you add more data.

Particle behaviour would produce two piles, one behind each slit or one pile if the two piles overlapped. The resultsshow several peaks– an interference pattern. This is wave-like behaviour which means that light cannot be a classical particle.

Note: The data hasa minimum in the middle because the two sources were180o out of phase with each other. Students are much more familiar with in-phase patterns which have a peak in the middle. You need to choose whether to point this out or just deal with it if a student notices and asks. This extra detail about phase should be dealt with as simply as possible so that it doesn’t distract from the key concept of wave-particle duality.

In classical physics, objects either act like waves or like particles–they can’t be both. Waves are able to pass through each other and interfere but particles cannot. Particles are found in one place and time but waves are spread out. This experiment shows that light can behave like a wave and like a particle – but isn’t either of these classical objects. It is a new type of object, a quantum object, called a photon. The results for over sixteen thousand photons is shown in the video Double-slit interference buildup (

The video shows in less than two minutes what the class has been doing for the past 20 minutes. You could just screen the video – but would the particle nature of light really stick? The video and data used in this resource come from the following paper;

Time-resolved double-slit experiment with entangled photons
Piotr Kolenderski,Carmelo Scarcella,Kelsey D. Johnsen,Deny R. Hamel,Catherine Holloway,Lynden K. Shalm,Simone Tisa,Alberto Tosi,Kevin J. Resch,Thomas Jennewein 6 May 2013

More details about the photon experiments can be found in the paperwhich is very interesting and surprisingly readable. You can download it for free at

5)How are the results different for double-slit interference using objects likeelectrons (Controlled Double-slit electron buildup very large molecules (Real-time single-molecule imaging of quantum interference )

Both the electrons and large molecules show the same wave-particle duality as photons. The dimensions and the type of detectorsare different– but the fundamental result is the same.

The electron video is from the group in Nebraska shown in The Challenge of Quantum Reality. Details of the experiment can be found in the paper Controlled double-slit electron diffraction by Roger Bach, Damian Pope, Sy-Hwang Liou and Herman Batelaan.It discusses the history of the double-slit experiment with electrons and shows details of the effects of partially masking each slit. You can download it for free at

The video of large molecule interference is from the Vienna group shown in The Challenge of Quantum Reality. The molecules are phythalocyanine, C32 H18 N8! More details can be found in the paper Real-time single-molecule imaging of quantum interference byThomas Juffmann, Adriana Milic, Michael Müllneritsch, Peter Asenbaum, Alexander Tsukernik, Jens Tüxen, Marcel Mayor, Ori CheshnovskyMarkus Arndt. it costs $32.

6)A classmate of yours doesn’t understand why we need to have a new model for light and suggests that what we are seeing in the video is just like the interference of water waves. Water is made up of well-defined particles and it is easy to show water waves interfering. Explain how the interference of photons is fundamentally different from the interference of water molecules.

Water molecules interact with each other to form a fluid that passes through both slits and interferes with itself. In the experiment with photons, the light was so faint that only one photon was present at a time and yet an interference pattern was still formed over time. If the water were reduced to one molecule at a time, the interference pattern will disappear.

But wait! The third video showed the interference of molecules producing an interference pattern, one-by-one. This could also be done with water molecules – but it would be very different from the original water interference pattern. The deBroglie wavelength of a water molecule is around 100,000 times smaller and so the pattern will also be 100,000 times smaller. (= h/p ~ (10-33)/ (10-26 x 10) ~10-7 m)

Note: This question was inspired by Raymond Laflamme – the co-director of the Institute of Quantum Computing (IQC). In a casual discussion at lunch he made the mistake this mistake! He also very quickly, realized his error and corrected himself.