Quantum Money

I’ve been trying to read Introduction to Quantum Computation and Information, a collection of papers on the subject used as an introduction to the field. I can’t claim to understand it, but there are some interesting parts nonetheless.

One is an example of Quantum Cryptology in the form of Quantum Money, a theoretical idea which could prevent counterfeiting by storing sequences of photons within banknotes, along with a serial number. The photons would be randomly polarized during printing, and the bank would keep a secret record of photon polarizations for each banknote. It would be infeasible to copy the banknotes, as measuring unknown quantum states disturbs the states irreversibly. As the bank knows the quantum states within each banknote, it can determine whether the banknote is a copy. This is a little simplified, but you get the idea.

Anyway, such an idea is not amazing today, but the theory for this was developed in 1970 or so by Stephen Weisner, who mysteriously did not publish it until 1983. Apparently his ideas were way ahead of their time and only taken up later, for example, by scientists who published the first known Quantum Key Distribution (QKD) system in 1984. The first working QKD was first demonstrated in 1989, and the first network protected by quantum cryptography was deployed only this year.