Quantum teleportation confuses people.
Maybe you’ve heard the buzzword, and you imagine science fiction become reality: teleporting people across the galaxy, or ansibles communicating faster than light. Maybe you’ve heard a bit more, and know that quantum teleportation can’t transfer information faster than light, that it hasn’t been used on something even as complicated as a molecule…and you’re still confused, because if so, why call it teleportation in the first place?
There’s a simple way to clear up this confusion. You just have to realize that classical teleportation is easy.
What do I mean by “classical teleportation”?
Let’s start with the simplest teleporter you could imagine. It scans you on one end, then vaporizes you, and sends your information to a teleportation pad on the other end. The other end uses that information to build a copy of your body from some appropriate raw materials, and there you are!
(If the machine doesn’t vaporize you, then you end up with an army of resurrected Derek Parfits.)
Doing this with a person is, of course, absurdly difficult, and well beyond the reach of current technology.
Do it with a document, though, and you’ve essentially invented the fax machine.
Yes, faxes don’t copy a piece of paper atom by atom, but they don’t need to: they just send what’s written on it. This sort of “classical teleportation” is commonplace. Trade Pokémon, and your Pikachu gets “classical teleported” from one device to another. Send an email, and your laptop teleports it to someone else. The ability to “classically teleport” is essential for computers to function, the idea that you can take the “important information” about something and copy it somewhere else.
Note that under this definition, “classical teleportation” is not faster than light. You still need to send a signal, between a “scanner” and a “printer”, and that’s only as fast as your signal normally is. Note also that the “printer” needs some “ink”, you still need the right materials to build or record whatever is being teleported over.
So suppose you’re building a quantum computer, one that uses the unique properties of quantum mechanics. Naturally, you want to be able to take a quantum state and copy it somewhere else. You need “quantum teleportation”. And the first thing you realize is that it’s harder than it looks.
The problem comes when you try to “scan” your quantum state. You might have heard quantum states described as “inherently uncertain” or “inherently indeterminate”. For this post, a better way to think about them is “inherently unknown”. For any quantum state, there is something you can’t know about its behavior. You can’t know which slit the next electron will go through, you can’t know whether Schrödinger’s cat is alive or dead. If you did, the state wouldn’t be quantum: no matter how you figure it out, there isn’t a way to discover which slit the electron will go through without getting rid of the quantum diffraction pattern.
This means that if you try to just “classically teleport” a quantum state, you lose the very properties you care about. To “scan” your state, you have to figure out everything important about it. The only way to do that, for an arbitrary state on your teleportation pad, is to observe its behavior. If you do that, though, you’ll end up knowing too much: a state whose behavior you know is not a quantum state, and it won’t do what you want it to on the other end. You’ve tried to “clone” it, and there’s a theorem proving you can’t.
(Note that this description should make sense even if you believe in a “hidden variable” interpretation of quantum mechanics. Those hidden variables have to be “non-local”, they aren’t close enough for your “scanner” to measure them.)
Since you can’t “classically teleport” your quantum state, you have to do something more subtle. That’s where “quantum teleportation” comes in. Quantum teleportation uses “entanglement”, long-distance correlations between quantum states. With a set of two entangled states, you can sneak around the “scanning” part, manipulating the states on one end to compute instructions that let someone use the other entangled particle to rebuild the “teleported” state.
Those instructions still have to be transferred normally, once again quantum teleportation isn’t faster than light. You still need the right kind of quantum state at your target, your “printer” still needs ink. What you get, though, is a way to transport the “inherently unknown” behavior of a quantum state, without scanning it and destroying the “mystery”. Quantum teleportation isn’t easier than classical teleportation, it’s harder. What’s exciting is that it’s possible at all.
On an unrelated topic, KKLT have fired back at their critics, with an impressive salvo of papers. (See also this one from the same day.) I don’t have the time or expertise to write a good post about this at the moment, currently hoping someone else does!