Tuesday, November 14, 2006

Spooky Entanglement

We become what we behold. We shape our tools, and then our tools shape us. ~ Marshall McLuhan

When people think of science fiction, there come to their mind almost automatically certain devices: Faster-than-light spaceships; sentient robots; and everyone's favorite, teleportation. The Star Trek transporter often figured heavily in plots and is probably the one thing from Trek shows (after Spock's ears) that comes to the average person's mind. If you asked those same people which of the inventions I mentioned above is least likely to see the light of day, I think most would say, “Teleportation, of course.” And, they'd be wrong because, while no one has beamed people up to the ISS yet, scientists have been teleporting quantum particles since 1997.

I can give only you a very vague idea of how this works because, quite frankly, it's complicated and downright spooky. In fact, it involves something Einstein once called “spooky action at a distance.” I'm going to try to summarize Brian Greene's excellent discussion of teleportation from The Fabric of the Cosmos, but if you find it hard to follow, don't blame Dr. Greene. The whole subject is typical of any quantum mechanical discussion; that is, it sounds like swamp gas to anyone who hasn't done the math and the experiments. But, like most things that rely on quantum theory, somehow it works.

Teleportation relies on something called “quantum entanglement.” Two particles, say a couple of photons, can become couple such that anything that happens to one of them is reflected in the other, even if they are separated by vast distances. How such coupled particles come about is beyond the scope of this blog (actually this whole discussion is beyond the scope of this blog's author, but I'm giving it the old college try), but suffice it to say that the effect works.

Therefore, I could have a photon in my lab in Alabama while my able assistant has its coupled partner on Mars (how he got to Mars, we'll leave to your imagination). If I perform a measurement on the photon in my lab, thanks to the vagaries of the Uncertainty Principle, I will alter some characteristics of my photon. Those alterations will immediately show up on the photon in Ohio. I don't mean the alterations will show up 18 minutes later by traveling at the speed of light; I mean they will occur at the same instant in both locations. That is Einstein's “spooky action at a distance.”

Before you go buying stock in AT&T Hypertelecommunications Inc., Dr. Greene says there's a fly in the ointment. The entanglement can only be determined by comparing the results of my measurements with data on the Mars base. To send that information, I have to call Mars by conventional means so my able assistant can compare them. In other words, the instantaneously transmitted spooky action is “coded”, and the only way to decode the result is the get my data.

So, we're not going to communicate faster than light, according to Dr. Greene, but entanglement can be used to teleport particles at the speed of communications, which means, in our example, getting to Mars in 18 minutes (plus the time to receive and assess all the data, as we'll see). So, how does teleportation work?

To begin with, you've got to keep in mind that, in quantum mechanics, one particle of a given type is identical to any other particle of the same type. All electrons, for instance, are created equal, as are all protons, quarks, leptons, muons, and so on. So, armed with that intelligence, let's teleport a photon.

To do so, we'll need three photons: Photon A, the one we want to teleport to Mars; photon B, sitting in my lab next to A; and photon C, located on Mars which happens to be entangled with B. Now, I could measure a property of A, but all that would do is alter some characteristic of that photon. I can, however, measure a joint property of A and B, such as whether the direction of their spins is the same or different. I don't measure what the actual spin direction is, just whether or not they're the same.

So, now I know how A's spin is related to B's, which means I also know how A's spin is related to B's entangled Martian buddy, C. Now it gets complicated (as if it wasn't already). I send my data to Mars, where my able assistant can take that data and manipulate C. Before we go further, keep in mind that my act of measuring the relationship between A and B has, by quantum theory, disrupted A and, more importantly, B. C, then is also disrupted in exactly the same manner as B. Still with me?

Here's the magic. My able assistant utilizes my data and the observed disruption of C to put C into the quantum state A was in before I made my measurement! In other words, in my lab, photon A is no longer in the state that it was in when I wanted to teleport it, but C on Mars is. Therefore, photon C is now photon A. Or, to put it another way, A has been teleported to Mars.

Now, this sounds like a nice academic exercise, but it's not. You can argue that all I've done is change properties at my lab and at the Mars lab. But let's take it another step. Suppose we teleport YOU to Mars (you don't think I'm crazy enough to experiment on myself, do you?). Here's what we need to do.

In my lab and on Mars, we have chambers filled with enough electrons, protons, neutrons, and so on to construct whatever object we wish to teleport (in this case, you). All of the particles in my chamber are entangled with those in the chamber on Mars. I also have a device capable of taking measurements of the joint properties of all the particles in you with the particles in my chamber. In the process, I'm going to have to alter all the particles in your body, which could involve disassembling your constituent atoms (you see why I wanted someone else to test this). After taking those measurements, I send the results to Mars, where my able assistant uses the data to set the quantum states of all those particles back to what they were before I took my measurements, and voila! There you are, standing on Mars.

This does raise an interesting metaphysical question, namely, is the you on Mars the same being that used to be on Earth? According to Dr. Greene, since at the quantum level all particles are equal, and we have replicated the states of all those particles to be the same as they were here in Alabama, at the quantum level you are the same. If you're the same at that level, you're the same at the macro level. But we're a long way from having to deal with this issue since this is all horrifically complicated and well beyond our current capabilities. Just trying to take the trillions of measurements (not to mention disassembling your body into atoms) is a staggering concept, as is the idea of setting up two chambers containing millions upon millions of entangled particles.

But, unlike faster-than-light-drives, at least quantum teleportation is a fact, and it has ramifications for the future of computing. Quantum computing, which puts forth promise of incredibly fast and powerful computers, has a potential problem . Data is stored in quantum bits or qubits which, to be useful, have to be transported from the material in which they are held to other computers. A good way to do this is by sending the state information from one particle to another particle using a using a third particle. In other words, you have to use quantum teleportation to transport the state in one computer to another. The initial experiments, described above, used identical types of particles. Now, a team at USC has managed to use a photon to transport the state of one cesium atom to another, taking a major step forward in both quantum computing and teleportation.

Scotty may not be beaming us up any time soon, but computers may be beaming our data from one place to another in our lifetimes.

Reference:
The Fabric of the Cosmos, Brian Greene, Alfred Knopf, 2004

No comments: