Catch the eavesdropper

This activity teaches how participants in QKD protocols detect eavesdroppers.

One round per team. Each round, one or two teams are assigned the “eavesdropper” role. The rest get a shared basis, e.g. X, Y, or Z. The goal is to find the eavesdropper(s), the teams that don’t know the shared basis. If the eavesdropper survives, they get a point. If they get caught, the rest of the teams win a point.

A Qubi gets passed around and measured by each team. Results are recorded, then announced afterwards (perhaps in reverse order). From the results, teams have to figure out who the eavesdropper(s) are.

For example, if all the non-eavesdroppers measure in the right basis, and the results are 0,0,0,1,0,0, then we know an eavesdropper is team 4. Some measurement results aren’t so obvious, and might let the eavesdroppers convince the others that someone else is the eavesdropper… especially when there are two, and the eavesdroppers can lie about their measurement results.

This game illuminates the way that BB84 participants can catch eavesdroppers, by noticing that they received different results when they should have been the same.