The Quantum Eraser - A 'Which Way' Demonstration (Last modified July 2009)

.....

Interactive Lecture using a Set of Demonstrations (30 minutes)
This lesson should be done after the interference lesson. This lesson explores the question of whether the object goes through one slit, both slits or niether. It does this through a series of demonstrations using a laser pointer, a double slit and a few polarizing filters. The students need a copy of the eraser worksheet. Details of the lesson and the answers to the worksheet can be found in the teacher's notes.

This lesson involves lasers which have safety concerns that you should be aware of.

This lesson is based on an article in Scientific American April 2007 and can be found here. I have made a few changes. First, I packaged the demonstration as a lesson in a PEOE(Predict Explain Observe Explain) format. Then Tahira Nasreen and Dan Blanchard of the York Region School Board, pointed out that is much easier and clearer if you use a prefabricated double slit instead of the pin suggested in the article. If you do not have a double slit you can make one using this pdf file, but it must be printed using a laser printer - not an inkjet. A colour laser printer is supposed to be best. Finally, I tried projecting the interference pattern onto tin foil. This makes the pattern brighter and more importantly, the light retains its polarization. This means that each student can rotate their polarizing filter and see what happens.

If you are interested in reading a bit more about quantum erasers there are three experiments summarized here.