Quantum Physics

15
Feb

Tangling with Teleportation

Will we ever realize the sci-fi dream of human teleportation? Physicists have already successfully teleported tiny objects. (See Beam Me Up, Schrödinger for more on the mechanics of quantum teleportation.) What will it take to extend the technique to a living, breathing human being?

Quantum teleportation is possible because of two quantum phenomena that are utterly foreign to our everyday experience: entanglement and superposition. Entanglement is the connection that links the quantum states of two particles, even when they are separated: The two particles can be described only by their joint properties.

Though there is no classical analogue for entanglement, in his book Dance of the Photons Zeilinger imagined how entanglement might work if it could be applied to a pair of ordinary dice instead of a pair of subatomic particles: “The science fiction Quantum Entanglement Generator produces pairs of entangled dice. These dice do not show any number before they are observed.” In other words, they are in a superposition of states where there is an equal chance of producing any number between one and six. “When one die is observed, it randomly chooses to show a number of dots. Then, the other distant die instantly shows the same number.”

This works no matter how far apart the dice are. They can be sitting beside each other or on opposite ends of the universe. In either case, when the particle over here is measured to be in one of many possible states, then we can infer the state of the particle over there, even though no energy, no mass, and no information travels between A and B when the first one is observed. The state of particle B simply is what it is. The difficult concept is that B’s state corresponds with the state of the measured particle A.

Entanglement is so confounding that in the early days of quantum theory, when entanglement was supported only by thought experiments and math on paper, Einstein famously derided it as “spooky action at a distance.” Today, though, entanglement has been thoroughly tested and verified. In fact, entangling particles isn’t even the hard part: For physicists, the most difficult task is maintaining the entanglement. An unexpected particle from the surrounding environment—something as insubstantial as a photon—can jostle one of the entangled particles, changing its quantum state. These interactions must be carefully controlled or else this fragile connection will be broken.

If entanglement is one gear in the quantum machinery of teleportation, the second critical gear is superposition. Remember the thought experiment about Schrödinger’s cat? A cat, a flask of poison, and a radioactive source are all placed in a sealed box. If the source decays and emits a particle, then the flask breaks and the cat dies. While the box is closed, we can’t know whether the cat is living or dead. Moreover, the cat can be considered both alive and dead until the box is opened: The cat will stay in a superposition of the two states until a “measurement is made—that is, until we look in the box and observe that the cat is either alive or dead.

Schrödinger never tried this on a real cat—in fact, he drew up the thought experiment just to demonstrate the apparently preposterous implications of quantum theory, and to force theorists to examine what constitutes a “measurement”—but today scientists have demonstrated that superposition is real using systems that are increasingly large (albeit still much smaller than a cat). In 2010, a group of researchers at the University of California, Santa Barbara demonstrated superposition in a tiny mechanical resonator—like a tuning fork, it vibrates at a characteristic frequency, but just like the cat it doesn’t exist in a single position until measured. Last year, another group of researchers demonstrated quantum superposition in systems of as many as 430 atoms.

Before superposition and entanglement appear in a human-scale teleporter, if ever, they will be harnessed for multiple applications in computing. Quantum cryptography uses entanglement to encode messages and detect eavesdropping. Because observation perturbs entanglement, eavesdropping destroys information carried by entangled particles. And if two people each receive entangled particles, they can generate an entirely secure key. Quantum cryptography is an active area of research and some systems are already on the market.

Quantum mechanical superposition and entanglement could also be exploited to make faster and more powerful computers that store information in quantum states, known as “qubits,” instead of traditional electronic bits. Quantum computers could solve problems that are intractable for today’s computers. Whether it’s possible to make a working quantum computer is still in question, but roughly two dozen research groups around the world are avidly investigating methods and architectures.

So we know how to teleport one particle. But what if we want to make like Captain Kirk and teleport an entire human being?

Remember that we wouldn’t be moving Kirk’s molecules from one place to another. He would interact with a suite of previously-entangled particles, and when we read the quantum state we would destroy the complex quantum information that makes his molecules into him while instantly providing the information required to recreate his quantum state from other atoms in a distant location.

Quantum mechanics doesn’t forbid it. The rules of quantum mechanics still apply whether you’re talking about a system of two particles or human being made of 1027 atoms. “The size doesn’t matter in and of itself,” says Andrew Cleland, a physicist at the University of California, Santa Barbara. Macroscopic systems like superconductors and Bose-Einstein condensates show quantum effects while arbitrarily large.

From an engineering standpoint, though, teleporting larger objects becomes an increasingly tough problem. Cleland comments, “Taking any object and putting it in a quantum state is hard. Two is multiply hard.” Maintaining entanglement between particle requires isolating them from interactions that would break their entanglement. We don’t want Captain Kirk to end up like The Fly, so we need to keep the particles absolutely isolated.

What if we start with something simpler: Instead of teleporting a person, can we teleport a much smaller living thing—like a virus?

In 2009, Oriol Romero-Isart of the Max-Planck-Institut fur Quantenoptik in Germany and his colleagues proposed just such an experiment. Using current technology, it should be possible to demonstrate superposition in a virus, they argued. They didn’t try it, but laid out a procedure: First, store the virus in a vacuum to reduce interactions with the environment, and then cool it to its quantum ground state before pumping it with enough laser light to create a superposition of two different energy states.

This is possible in theory because some viruses can survive cold and vacuum. But humans are hot, and that thermal energy is a problem. “We have quadrillions of quantum states superimposed at the same time, dynamically changing,” says Cleland. Not only are we hot, but we interact strongly with our environment: We touch the ground, we breathe. Ironically, our need to interact with our environment, our sheer physicality, could come between us and the dream of human teleportation.

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Yvonne Carts-Powell

    Yvonne Carts-Powell is a science writer and the author of The Science of Heroes (Berkeley Press, 2008). Her articles have appeared in such publications as New Scientist, ScienceNOW, and Editorial Humor. She has mostly uses her B.S. in physics to write about light. Visit her homepage at the National Association of Science Writers.