Probably no technology from the Star Trek universe is more fervently wished for than the transporter. After all, who wouldn't want to be able to get from Point a to Point B without all the tedious waiting in lines, sitting in traffic, and shoving through crowds that comes with travel through the physical world? It'd be so much more convenient to simply step up onto a platform, see some pretty sparkling lights, and rematerialize exactly at your destination.
Of course, as much as we might like to have the ability to teleport from place to place, it's probably not possible. The laws of physics cannot be surpassed by even the most talented Starfleet engineers, alas, and the laws of physics mean it's probably impossible to teleport real objects the way they do on Star Trek.
There is, however, a very real process known as "quantum teleportation" (type that into Google and you'll find several hundred thousand hits). The name is a bit of a stretch, but it's a fascinating application of the non-local properties of quantum physics, so it's worth talking about how it does and does not resemble its fictional counterpart.
The Basics of Quantum Teleportation
The name "teleportation" obviously raises certain expectations, namely that you make a device that takes a thing at point A and re-creates it at point B. An old-school, low-tech analogue of this is a fax machine: you take a document, scan it to determine the information needed to reproduce it, send that information over telecommunications lines, and print out a copy at the far end. (These days, you would probably use a scanner and email the document, or just snap a picture with a phone and email that...). You can imagine extending this to three-dimensional objects using an MRI scanner or something to determine the interior structure, and a 3-d printer at the far end. In all these cases, the operating principle is the same: you determine the information needed to reproduce the object you want at the destination, send that information, and make a copy.
This works fine for a classical system, but the quantum randomness that protects against faster-than-light communication complicates the situation when you talk about trying to send quantum states. If you have a single quantum object whose state you need to convey to a distant location, the classical recipe won't work-- a quantum object can be in an arbitrary superposition of multiple states, but when you measure it, you'll find only one outcome. You can't determine the probability of the different outcomes from a single measurement, and the no-cloning theorem shows that you can't make and measure a bunch of copies to determine the probability that way.
Quantum entanglement, however, provides a way of moving the state of one quantum object to another in a distant location without transporting the original object. The original scheme was proposed in 1993 by a team including one of my undergrad professors, Bill Wootters. It's been demonstrated many times since, mostly with photons, but it's been used to teleport a state between two atoms in different vacuum chambers, as well.
Schematic illustration of quantum teleportation between Truman the Boston terrier and RD the... [+]
I explained this at length a few years ago, using SteelyKid's stuffed animals, and also wrote about it in How to Teach Physics to Your Dog, so I'll just give a brief outline here. The most basic scheme involves three identical quantum systems-- usually three photons-- two of them in an entangled state. The two individuals wanting to communicate-- Truman the Boston terrier and RD the Labrador retriever in the figure above-- share the entangled pair between them (photons 2 and 3 in the figure), and Truman has a third photon whose state (some superposition of vertical and horizontal polarizations at the same time) he would like to transmit to RD.
The quantum teleportation protocol is surprisingly simple. All Truman needs to do is make a joint measurement on photons 1 and 2, essentially asking "Do you have the same polarization, or different polarizations?" This is done in a way that doesn't reveal the actual polarization-- he doesn't know if they're both vertical or both horizontal, just that they're the same-- which puts photons 1 and 2 into one of four possible entangled states.
After Truman's measurement, his two photons are entangled, which leaves RD's photon 3 in one of four definite polarization states. None of these is the exact superposition of vertical and horizontal polarization that Truman started with, but they're all simply related to it. Truman then tells RD which of the four measurement outcomes he got, which lets RD know the operations he needs to do to convert photon 3 into an exact copy of Truman's initial state.
As noted above, this isn't restricted to photons-- you can use entangled photons to transfer the state of material objects like atoms, it just adds a step to the process at either end. If Truman wants to send the state of an atom to RD, he just does an operation to encode the superposition of two atomic states into the polarization of his photon 1, then proceeds as above. RD uses Truman's measurement outcome to put photon 3 in the right state, then reverses Truman's operation to transfer the polarization superposition onto his atom.
Quantum Teleportation Versus Star Trek's Transporters
The most important difference between this scheme and the fictional teleportation of Star Trek and other science fiction stories is that nothing material moves from one place to another. The only thing transferred from Truman to RD is the state of a photon-- RD has to have a photon 3 to take on that state, and at the end of the process Truman still has the photon he started with. There's no way to use this to "beam down" information to a planet that doesn't have some infrastructure in place to receive that.
Another critical difference is that while there's some non-local quantum business involved in getting the state from one dog to the other, the process is not complete until after RD receives the result of Truman's measurement, and takes the appropriate action. Which wouldn't be a big issue when beaming from an orbiting starship down to the ground, but is somewhat less useful when, as happens in the recent movies, characters use long-distance transporters to move between star systems. Even assuming there was something there to receive it, the teleportation would take years to complete, as the light carrying the measurement results made its slow way across the intervening space.
But the biggest difference between the schemes is that real-world quantum teleportation only works on single particles, not the vast number of particles needed to move real objects. This is the aspect that makes Star Trek style teleportation impractical in the real world. Not only would teleportation need to move the individual state of each atom from Truman to RD, but it would need to transfer any entanglement between the different atoms, which vastly increases the scale of the problem.
To get a sense of the scale, let's think about a relatively optimistic case where we restrict the number of states involved artificially. Say we only care about the quantum information in the brain of the person to be teleported (since there are some claims that quantum processes play a role in the brain, though I've never personally found them all that convincing), and want to map that onto the brain of a copy body waiting at the other end. There are something like a hundred billion neurons in a human brain, and about a hundred trillion connections between them. That's about 2100,000,000,000,000 possible states to worry about, or roughly 1030,000,000,000,000. That's considerably more states than there are particles in the known universe, and if you need one entangled pair to teleport each of those (as a ballpark estimate), well, let's just say the odds aren't very good.
So what good is quantum teleportation, if it doesn't get us closer to Star Trek? Well, even if it can't move objects around instantaneously, it might prove useful for quantum computing. If you want to make a computer that takes advantage of quantum effects to do specialized calculations faster than could be managed with a classical computer, those calculations require preserving superposition states and entanglement between "qubits" involved in the calculation. Quantum teleportation might be an essential component of a data bus to move information between parts of such a computer, or to connect distant quantum computers together to make a "quantum Internet."
For the moment, though, it's mostly of interest as a window on the deep structure of quantum mechanics, letting physicists explore the ways the universe restricts the flow of information. Which won't do anything to shorten your morning commute, but does at least provide some fun stuff to think about on a boring trip through physical space.
Want more Star Trek science? Check out this look at geology in Star Trek.
