If you’re a regular reader of ReducedMass then you’ll remember an article from a week ago on quantum cryptography, and hopefully it wasn’t mind numbingly boring and you actually read it as well. Before that, i had written an article trying to explain some of the quantum weirdness using Bells inequality. Now, this time (as promised in the last article) I’ll tie these two together into one ultra-safe and delicious quantum cryptography protocol that involves Bells inequality!

I know it’s a mean thing to do, but I’m afraid i’ll have to assume you’ve read the two previous articles here. I’ll try and make my explanations clear and sum up the main points from the old articles before using them, so hopefully you can gain something from this without having read them, but truthfully, it’ll be hard. But enough with the sobering forewords, lets dive right in!

Now if you’ll remember, two entangled photon will break the so-called Bell inequality, while un-entangled (completely unconnected) photons would not. A way to measure the bell inequality would be to have a source of entangled photons, have it send one photon towards Alice, one towards Bob and have them measure their polarization in different basis (for example Bob in +-45° and Alice in the horizontal/vertical). Now imagine that someone (lets call her Eve) intercepts the photon coming over to Bob, measures it, and resends it. It is impossible for Eve to measure the entangled photon, and send another one on-to Bob that is also entangled with Alice. So if Eve intercepts the photons, Alice and Bob will see that the Bell inequality is NOT broken, and therefore they will know that someone was listening in!

So in essence what they do, without going too deeply into it, is combine the encryption protocol we discussed in the previous article with this property of entangled photons. So you have a source sending out entangled photons to Alice and Bob, they will measure it in a random basis, and get either 0 or 1 as an outcome. Alice calls Bob and tells him what basis she measured in, and if he measured in the same one, they will know they measured the same thing, and they’ll keep that bit for their one-time-pad. BUT! and here’s the kicker, if they did NOT have the same basis, that is the equivilant of testing the Bell inequality! So they will use those results to see if it (the inequality) is broken or not. If it IS broken, then they know the photons were entangled and no-one was listening, and if it’s NOT broken they know the code has been compromised and they throw it away.