Discover, January 31, 1994
Quantum mechanics has many assaults on common sense to answer for–telepathic particles, for instances, and wormholes to other universes. These bits of weirdness are now old hat. But last March a team of physicists from the United States, Canada, France, and Israel added a new one: teleportation.
At first, it might seem as if teleportation is merely a problem of technology rather than fundamental physics. With a few technical breakthroughs, you might imagine, you’d be able to teleport over to a friend’s house for dinner simply by stepping into a scanner that would record all the pertinent information about the atoms making you up–their positions, their bonds with other atoms, their energy levels, and so on.
With all the data collected, the machine would then vaporize your body and relay the information to your friend’s teleporter, which would rebuild you from a stock of on-hand hydrogen, carbon, and other materials.
The problem with this scheme, unfortunately, begins with the very first particle a teleporter might try to scan. The act of measurement always affects the thing that is measured. If you try to locate a fuse box with a flashlight, you actually change the box, in that a few of its atoms absorb photons from the flashlight. To be sure, the effect on the fuse box is negligible. But if you’re trying to measure something on the subatomic scale, the change can be profound.
Say you want to know how a photon is polarized (that is, whether it is vibrating horizontally, vertically, or in between). You can send the photon into a filter that blocks horizontally vibrating light and see if it passes through. But Heisenberg’s uncertainty principle–one of the pillars of quantum mechanics–says that even if the photon is vibrating at some angle away from the horizontal, it has a certain probability of switching to a horizontal polarization as a result of your measurement. There’s no way to work back and discover whether your original photon was horizontally polarized or not.
Last fall several physicists got together at a conference to toy with ways to get information around Heisenberg’s wall of confusion (and, they would later realize, make teleportation possible). It dawned on them that a quantum phenomenon first recognized by Einstein almost 60 years ago might do the trick. In seeking to expose what he saw as absurdities inherent in quantum mechanics, Einstein, along with Boris Podolsky and Nathan Rosen, demonstrated by means of a thought experiment that quantum particles should be able to communicate telepathically. This “absurdity” later turned out to be true; Einstein’s thought experiment has been executed in the lab. And, worse irony, his concept of quantum telepathy is now the foundation for the further absurdity of quantum teleportation.
To understand Einstein’s idea, consider a cobalt atom occasionally spitting out pairs of photons. Quantum mechanics requires that the total polarization of the two photons add up to zero; in other words, they are polarized in opposite directions. But unlike the photon in the previous example, which had a definite polarization, these photons, Einstein realized, had no fixed polarization at all. Only when someone tried to measure one of the photons would it choose a polarization. At the same time (here’s where the telepathy comes in), the other photon would emerge from a quantum haze to assume the opposite polarization. The fates of the two photons are entangled–even when they are some distance apart.
The entangled pair serves as the perfect intermediary for quantum teleportation. Say that Alice has a photon–call it T–that she’d like to teleport to her friend Bob. She can’t measure it and then call Bob with the information, because she can’t trust a direct measurement. (The measurement might make the photon flip into a new state.) Instead, Alice and Bob go to the local cobalt atom shop and get a pair of entangled photons. Alice puts one of the photons (call it A) in a light trap. Bob does the same with the other one (photon B), and they each go back home. Neither has looked at the particles, which thus remain in their entangled state.
When Alice gets to her house she can now teleport T–her original photon. While she still can’t accurately measure its polarization, she can measure the difference between the polarization of A and T–by firing both photons at an atom, say, and measuring how they affect it. That very measurement also forces A and T into an entangled state. And since A was already entangled with B, it instantaneously forces Bob’s photon into an opposite polarization. Of course, Alice has to destroy both photons to get her answer; both are absorbed by the atom and vanish. Yet now photon T can be reconstructed at Bob’s house.
It works like this: Alice calls Bob and tells him what difference she measured between A and T. Bob knows that his part of the entangled pair–photon B–has to have a polarization opposite A’s. Those two things tell him the difference in polarization between B and T. He then simply rotates photon B through that angle (for instance, by passing it through sugar water) and voilà! His photon is now identical, in its polarization anyway, to the dear departed photon T.
This procedure is as simple as teleportation can get. And you’re still transferring only one characteristic of one particle to another location. Still, it may be theoretically possible to teleport all characteristics of a particle–and thus the particle itself. Researchers at the University of Innsbruck in Austria are currently trying to build an apparatus that will teleport a photon in this way. In principle the procedure should work for individual electrons and protons as well; they too can be entangled.
A whole human body, of course, is another story. The complexity of teleportation increases exponentially with every new particle you want to teleport at the same time, and the human body contains some 10^(27) atoms, making it all but a practical impossibility. But some of the physicists who dreamed up the teleportation scheme remain open-minded about the possibility of practical applications. “Who knows?” says one of them, William Wootters of Williams College. “Maybe it’s just a lack of imagination on our part. It’s hard to predict what kind of teleportation might be going on hundreds of years from now.”
Copyright 1994 Discover Magazine. Reprinted with permission.