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March 15, 2007

Possible New state of matter could be useful for Quantum computing

Xiao-Gang Wen at the Massachusetts Institute of Technology and Michael Levin at Harvard University have come up with a prediction for a new state of matter (string-net liquid) and even a tantalising picture of the nature of space-time itself.

Herbertsmithite could be the new silicon - the building block for quantum computers. In theory, quantum computers are far superior to classical computers. In practice, they are difficult to construct because quantum bits, or qubits, are extremely fragile. Even a slight knock can destroy stored information. In the late 1980s, mathematician Michael Freedman, then at Harvard University, and Alexei Kitaev, then at the Landau Institute for Theoretical Physics in Russia, independently came up with a radical solution to this problem. Instead of storing qubits in properties of particles, such as an electron's spin, they suggested that qubits could be encoded into properties shared by the whole material, and so would be harder to disrupt.
If the Herbertsmithite material were a string-net liquid with elementary and quasi-particles at the end of each string. Physicists could manipulate quasi-particles with electric fields, braiding them around each other, encoding information in the number of times the strings twist and knot, says Freedman. A disturbance might knock the whole braid, but it won't change the number of twists - protecting the information. "The hardware itself would correct any errors," says Miguel Angel Martin-Delgado of Complutense University in Madrid, Spain.



Herbertsmithite (pictured) is unusual because its electrons are arranged in a triangular lattice. Normally, electrons prefer to line up so that their spins are in the opposite direction to that of their immediate neighbours, but in a triangle this is impossible - there will always be neighbouring electrons spinning in the same direction. Wen and Levin's model shows that such a system would be a string-net liquid.

Although herbertsmithite exists in nature, the mineral contains impurities that disrupt any string-net signatures, says Lee. So Helton's team made a pure sample in the lab. "It was painstaking," says Lee. "It took us a full year to prepare it and another year to analyse it."

The team measured the degree of magnetisation in the material, in response to an applied magnetic field. If herbertsmithite behaves like ordinary matter, they argue, then below about 26 °C the spins of its electrons should stop fluctuating - a condition called magnetic order. But the team found no such transition, even down to just a fraction above absolute zero.

They measured other properties, too, such as heat conduction. In conventional solids, the relationship between their temperature and their ability to conduct heat changes below a certain temperature, because the structure of the material changes. The team found no sign of such a transition in herbertsmithite, suggesting that, unlike other types of matter, its lowest energy state has no discernible order. "We could have created something in the lab that nobody has seen before," says Lee.

The team plans further tests to visualise the position of individual electrons, looking for long-range entanglement by firing neutrons at the crystal and observing how they scatter. "We want to see the dynamics of the spin," says Lee. "If we tweak one [electron], we can see how the others are affected."

Even if herbertsmithite is not a new state of matter, we shouldn't be surprised if one is found soon, as many teams are hunting for them, says Freedman. He says people wrongly assume that particle accelerators are the only places where big discoveries about matter can be made. "Accelerators are just recreating conditions after the big bang and repeating experiments that are old hat for the universe," he says. "But in labs people are creating [conditions] that are colder than anywhere that has ever existed in the universe. We are bound to stumble on something the universe has never seen before

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