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Physics + MathPhysics & Math

Beam Me Up, Schrodinger

ByYvonne Carts-PowellThe Nature of RealityThe Nature of Reality

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If the journey were really more important than the destination, then we wouldn’t dream about teleportation. Going from here to there, instantly, is a staple of science fiction and comic books. It is wish-fulfillment for anyone who has had a car break down in the middle of nowhere or waited in a crowded airport the day before Thanksgiving. Even “Star Trek”’s Captain Kirk, who had a starship and planetary shuttle at his disposal, bypassed trips by jumping into the ship’s teleporter.

The good news for would-be Captain Kirks is that teleportation is real. By exploiting quantum mechanical effects that are based on rigorous mathematics and bolstered by decades of laboratory experiments, physicists have demonstrated that teleportation works. The bad news: So far, it only works on very tiny objects. Teleporting an entire human being is still a long way off. Engineering complexities may stop humans from ever being teleported.

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Why can’t we teleport? After all, we can scan the surface of an object, transmit the data at the speed of the Internet, and recreate it as many times as we like using a 3D printer. Shouldn’t we be able to just improve the resolution of the scanners and printers (and the fidelity of the transmission) until we can print a living, breathing person from a data file?

Most of us think in terms of classical mechanics, the rules of reality that make sense at size scales humans can touch. And if reality were based on classical mechanics, then the print-me-to-Paris approach might work. But these familiar rules turn out to be just a special case of the much stranger principles of quantum mechanics. In the classical world, it is possible to have complete information about every atom that makes up your body, but quantum physics imposes restrictions on how much we can know. In the quantum world, there is a limit to the resolution with which we can measure an object, and measuring an object changes it. If we want to teleport, we have to play by the rules of quantum mechanics.

Teleportation has been demonstrated with tiny particles. The process is less glamorous than what you’ve seen in the Enterprise’s transporter room, though.

Here’s how it works. Imagine three particles, X, Y, and Z. We want to teleport particle Z from here to there . Each particle is in a particular quantum state, though we don’t know exactly what state—to measure it would be to destroy the information. So we have to take a gentler approach. We’ll start by putting particles X and Y into a bizarre sort of quantum co-dependency called “entanglement”—more on that in the next blog post. For now, just keep in mind that no matter how far they may travel from each other, particle X and particle Y retain an intimate connection that links their (still unknown) quantum states.

So, when researchers send particle Y someplace over there , it is still coupled to particle X. The next step in the experiment is to let particles Z and X interact. That gives the physicists a way to compare their quantum states. This measurement inevitably, unavoidably, changes the states of the particles: the original state of particle Z is destroyed. But the physicists can use the information revealed by that comparison to transform the distant particle Y into a perfect copy of particle Z. Particle Z has teleported from here to there.

But wait, you say! Particle Z and particle Y are different particles! You haven’t really transported Z—you’ve just turned Y into a facsimile. But according to the rules of quantum physics, a particle’s quantum state defines its identity. Two particles in the same quantum state are indistinguishable. They are, for all intents and purposes, the same particle.

This type of quantum teleportation was first proposed by an international group of physicists in 1993. A particle could be teleported, they concluded, as long as the original is destroyed. (This, presumably, is why there is only one Captain Kirk.) In 1997, researchers in Anton Zeilinger’s group in Innsbruck demonstrated quantum teleportation using photons .

Since then, researchers have increased both the teleportation distance and the size of the particles teleported. Zeilinger’s group demonstrated quantum teleportation using photons in free space across 144 km in the Canary Islands .

In 2004, Reiner Blatt’s group at the University of Innsbruck reported teleporting trapped calcium ions a distance of 5 microns and in 2009, researchers at the universities of Maryland and Michigan announced that they had successfully teleported ytterbium ions a full meter.

Next week we consider entanglement, superposition, and the technical challenges necessary before scientists can teleport living creatures, starting with a lowly virus and maybe someday culminating in a Star Fleet captain.