Quantum Teleportantion

Toby Howard

This article first appeared in Personal Computer World magazine, October 2000.

TELEPORTATION is a Science Fiction staple. People and objects are disintegrated, beamed across the universe in the blink of an eye, and reconstructed on a remote planet, often in a landscape strewn with styrofoam boulders. The trouble is, fundamental physics has always said teleportation cannot possibly ever work. Until recently, that is.

The idea of teleportation is simple. You first record all possible information about the object you want to teleport, then transmit the information to the new location. Then, you read back the information, decode it, and reconstruct the object. But the laws of quantum mechanics scupper you before you can even get started.

Let's consider teleporting a single atom. Quantum mechanics says that the very act of measuring an atom's state changes that state. And the more accurate you try to make your measurement, the more you disrupt the atom. So this rules out the possibility of ever extracting enough information about an atom to make a perfect replica it. But researchers have recently found a way around the problem, using a quantum property called "entanglement".

Quantum particles have states which can be used to represent binary digits. If a particle spins one way, for example, it can represent a 0; if it spins the other way, it's a 1. Particles like this are called "qubits", and form the basis for building quantum computers. But you can only ever tell what state a particle is in by performing a measurement on it. When you're not looking, it exists in a weird mixture of 0/1 states called a "superposition".

It's possible to prepare two particles – P and Q, say – in such a way that they're intimately related, like quantum twins. Let's separate P and Q, and send them as far apart as we like. They're still both in the indeterminate 0-ish/1-ish state. Now let's measure P, and say we discover it's in state 0. Instantaneously, this means that particle Q, which may be light-years away on the other side of the universe, instantly assumes the opposite state – it's now definitely spinning as a 1. Entanglement has been known theoretically for decades, butit's only recently been proven by experiment. "Entanglement means if you tickle one the other one laughs", says Caltech physicist Jeff Kimble, one of the researchers to first demonstrate entanglement in the lab.

Although entanglment effect isn't itself teleportation, it provides the key to it. Let's imagine Alice wants to teleport a particle (say, A) to Bob. First, she creates two entangled particles, P and Q, keeping P, and sending Q to Bob through some ordinary data channel like the Internet. Now, Alice allows particles P and A to interact, measures the resulting state, and sends that data to Bob. Bob then applies that data to Q, which – and this is the magic bit – then turns into an exact copy of particle A! The original particle A is still with Alice, but its state is scrambled.

So you can teleport an atom, and people are made of atoms so... well, the answer to the obvious question is at best "who knows?" – but what is emerging as a possibility is the use of teleportation to send qubits between quantum computers, linked into a quantum internet. Quantum computers already exist (see Futures, May 1999), and the current largest machine computes with words seven qubits long. It's also possible to prepare the entangled particles such that the entanglement itself actually encodes an algorithm.

You can already try quantum computation for yourself, by running a simulator on your PC. As for teleporting yourself across the universe, you might have to wait a little longer.

Toby Howard teaches at the University of Manchester.