I’m in a pop-sciency kind of mood this morning, so I’m going to dash off a quick post about “quantum teleportation”. It will have embarassingly few equations, but it might be entertaining. If you don’t follow me below the fold, I want you to remember this: quantum teleportation isn’t quite teleportation, not in the Star Trek sense of the word anyways.
So what is quantum so-called “teleportation”? It’s a laboratory technique, and so it’s best to begin by explaining what problem it solves.
Let’s suppose that you and I each have a baseball in hand, and that — for some reason — you’ve decided that you want your baseball to be in the exact same rotational state as mine. How are we going to accomplish this? Well, one approach would be to call me on the phone and ask “Hey, A.J., what state is your baseball in?” I would then look at the baseball, note that it was spinning around some axis with a certain angular velocity, and tell you what axis and what velocity. Then you’d align your baseball, spin it, and we’d be done.
This game is easy because baseballs are well described by classical mechanics. They’re very large objects, relative to atoms, and when I measure my baseball’s rotational state, I only have to disturb it very very slightly. I have to interact with it slightly (otherwise I won’t know anything about it), and this will change its state, but only in a very small way. Obviously, I could disturb it more, say by dropping it on the floor and seeing which way it rolled, but I don’t have to.
So what if you and I each have an electron instead of a baseball? Electrons do have a kind of rotational degree of freedom, called “spin”, which is like baseball rotation in that when you measure it, you should get an axis and a number. Things are a little weirder here in that the number can only take the values +1 and -1 (in appropriate units), but the basic idea is similar.
But our little game is vastly more difficult with electrons, for two closely related reasons. First, electrons don’t have to exist in definite spin states. My electron’s spin state could be +1, or it could be -1, or it could be 35% + 1 and 65% -1. This is the fundamental weirdness of quantum mechanics. Very large, very “classical” objects are (to a good approximation) always in definite rotational states, but very small, very “quantum” systems don’t have to be. Of course, if we couple a quantum object to a classical object, the combined system is basically a classical object.
And this is pretty much what is meant by “measurement” in quantum mechanics. We take a very quantum object and bludgeon it with a classical object. The delicate superpositions don’t actually go away, but for all practical purposes they are absorbed into the state of the classical measuring apparatus. (In particle physics, for example, one frequently measures a neutron’s momentum by allowing it to collide with a wall of steel.) The result: our quantum object is forced into a classical state, and we can tell what state that is by looking at our classical measuring device.
This is the second problem: You want to know what state my electron was in before I hammered it into a definite spin state. You want to know what the delicate superpositions were. So we’re going to need some better laboratory technique.
At this point, you can almost guess what correct technique is. I can’t hit the poor electron with a classical measuring device without misplacing a lot of information. But if I allow the electron to gently interact with another quantum object, I can perform a different kind of measurement. This time I won’t actually find out what state the electron is in; I won’t get a number from this experiment. Instead both the electron and the measurement apparatus will change state, but they’ll do so in a much more controlled way. The new state of the measurement apparatus will reflect the old state of the electron.
At this point, I could look at the measurement apparatus, get a number and ruin all of the superpositions. Or I can put the measurement apparatus in a box and mail it to you. Then, if we’ve chosen the right kind of measurement apparatus, you can allow the apparatus to interact with your system, effectively reversing the measurement I made with the apparatus. (The actual experimental technique is a bit more complicated, but not different in principle. We create an entangled pair of “quantum measuring devices” and each take one. Then I allow my half of the entangled pair to interact with my electron, make some classical measurements and pass them to you over the phone. Having this information, you can then let your electron and your half of the entangled pair interact in a way which puts your electron in my electron’s old state.)
The net effect, in any case, is that your electron and my electron have changed state. Your electron is now in the same state as my electron was, and my electron is in who knows what state.
This is “quantum teleportation”. It doesn’t let you transfer information faster than light; we still had to transfer the information by earthly means. (Frankly, talk of FTL is a dead giveaway that you’re dealing with lousy journalism.) It doesn’t let you “clone” electron states; we still had to disturb my electron.
But it is useful if you want to build a quantum computer, because it allows you to move information around between different logic gates without destroying any of the delicate superpositions that make quantum computing work. Some sort of quantum copying is almost certainly going to be necessary to combine lots of “quantum transistors” into a functioning quantum computer.