Don't Entangle the Messenger

Acting as a go-between, a particle such as an electron can create a strange quantum-mechanical connection between two others without getting entangled itself. The counterintuitive finding raises questions about the fundamental nature of that link.

Physicists hope to develop superfast quantum computers and uncrackable quantum communications by exploiting a bizarre phenomenon known as entanglement. When two particles become entangled, a measurement made on one instantly affects the other, even if it's light-years away. For example, two electrons can be entangled so that if one spins clockwise the other must spin counterclockwise, and vice versa. Strangely, both electrons spin both ways at the same time--until one of the particles is measured, at which point it must turn one way or the other, and its counterpart flops into the opposite state.

Researchers can entangle two distant particles by entangling a third, intermediary particle with the first and then with the second. For example, two scientists, Alice and Bob, might begin by combining several quantum states: one in which all three electrons spin clockwise, one in which all three spin counterclockwise, one in which only Alice's spins clockwise, and so on, to construct a complicated "pure" state of the three electrons. The combination is called pure because the various quantum waves in the sum oscillate in definite lockstep. By flipping their own electron and the intermediary in the right way, Alice and Bob entangle their particles. This approach necessarily also entangles the messenger particle.

But there's a way to keep the messenger particle from getting entangled, say theoretical physicists Ignacio Cirac, Frank Verstraete, and colleagues at the Max Planck Institute for Quantum Optics in Garching, Germany. The trick is to add a dash of randomness to the process, they report in a paper to be published in Physical Review Letters. Alice and Bob could begin with a "mixed state," in which the oscillations of the various quantum waves are completely random. With the right set of starting conditions, they then flip their own electron and the intermediary in such a way that, in the end, their two electrons will entangle, but the go-between electron remains forever untangled.

"It's a very counterintuitive result," says Maciej Lewenstein, a theoretical physicist at the University of Hannover in Germany. It calls into question physicists' assumption that entanglement is something that flows from particle to particle, he says. Seth Lloyd, a quantum-mechanical engineer at the Massachusetts Institute of Technology, says the finding suggests that it may be easier than previously believed to entangle distant quantum computers, should they ever prove practical. "It's wacky but lovely," he says.

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