Today is the official release date for the paperback edition of How to Teach Physics to Your Dog, so I wanted to write up something cool about quantum physics to mark the occasion. I looked around the house for inspiration, and most of what we have lying around the house is SteelyKid’s toys. Thus, I will now explain the physics of quantum teleportation using SteelyKid’s toys:
“I could hardly expect to get away with that, could I. No, I’m happy to have your contributions– the book is about talking physics with you, after all. Just break in if you have something to add, and I’ll put it into the blog post.”
“When I have something to add, you mean. Because I’m going to have stuff to add, you know.”
“Oh, I know it…”
The phenomenon known as “quantum teleportation” (a slightly unfortunate name) involves using quantum entanglement– the strange phenomenon Einstein called “spukhafte fernwirkung” or “spooky action at a distance”– to transmit the state of a quantum system from one place to another without disturbing it. Explaining teleportation thus involves both a sender and a receiver of the quantum state, who are traditionally referred to as “Alice” and “Bob.” Since SteelyKid doesn’t have many anthropomorphic toys, though, we’ll use these two:
Appa and Bertha the Big Bear (It’s a little disconcerting, by the way, to watch SteelyKid playing with these two, and remember that not all that long ago, she was significantly smaller than either of them…).
So, the scenario is this: Appa has a quantum object, whose exact state he doesn’t know, so it is in some superposition of all the available states at the same time. This could be anything– an atom with two possible states, an electron spin, a polarized photon. We’ll represent Appa’s unknown state by this little brown-and-white dog:
And he wants to transmit this state to Bertha, who is a long distance away:
So, how can he send this state to Bertha and make sure she gets exactly the state that he starts with?
“That does seem like the obvious course of action, but in fact, it’s not possible. There’s a mathematical rule about quantum physics, helpfully called the ‘no-cloning theorem,’ that says it’s impossible to make a perfect copy of a single quantum state unless you already know something about the state.”