杏吧原创

In 1998, just after he won a share of the Nobel prize for physics, Robert Laughlin of Stanford University in California was asked how his discovery of 鈥減articles鈥� with fractional charge, now called quasi-particles, would affect the lives of ordinary people. 鈥淚t probably won鈥檛,鈥� he said, 鈥渦nless people are concerned about how the universe works.鈥�

Well, people were. Xiao-Gang Wen at the Massachusetts Institute of Technology and Michael Levin at Harvard University ran with Laughlin鈥檚 ideas and have come up with a prediction for a new state of matter, and even a tantalising picture of the nature of space-time itself. Levin presented their work at the Topological Quantum Computing conference at the University of California, Los Angeles, early this month.

The first hint that a new type of matter may exist came in 1982. 鈥淭wenty five years ago we thought we understood everything about how matter changes phase,鈥� says Wen. 鈥淭hen along came an experiment that opened up a whole new world.鈥�

In the experiment, electrons moving in the interface between two semiconductors behaved as though they were made up of particles with only a fraction of the electron鈥檚 charge. This so-called fractional quantum hall effect (FQHE) suggested that electrons may not be elementary particles after all. However, it soon became clear that electrons under certain conditions can congregate in a way that gives them the illusion of having fractional charge 鈥� an explanation that earned Laughlin, Horst St枚rmer and Daniel Tsui the Nobel prize (New 杏吧原创, 31 January 1998, p 36).

Wen suspected that the effect could be an example of a new type of matter. Different phases of matter are characterised by the way their atoms are organised. In a liquid, for instance, atoms are randomly distributed, whereas atoms in a solid are rigidly positioned in a lattice. FQHE systems are different. 鈥淚f you take a snapshot of the position of electrons in an FQHE system they appear random and you think you have a liquid,鈥� says Wen. But step back, and you see that, unlike in a liquid, the electrons dance around each other in well-defined steps.

鈥淭he position of the electrons in this material appears random like in a liquid, but they also move in well-defined steps鈥�

It is as if the electrons are entangled. Today, physicists use the term to describe a property in quantum mechanics in which particles can be linked despite being separated by great distances. Wen speculated that FQHE systems represented a state of matter in which entanglement was an intrinsic property, with particles tied to each other in a complicated manner across the entire material.

This led Wen and Levin to the idea that there may be a different way of thinking about matter. What if electrons were not really elementary, but were formed at the ends of long 鈥渟trings鈥� of other, fundamental particles? They formulated a model in which such strings are free to move 鈥渓ike noodles in a soup鈥� and weave together into huge 鈥渟tring-nets鈥�.

鈥淲hat if electrons were not elementary, but were formed at the ends of long strings of other, fundamental particles?鈥�

Light and matter unified

The pair ran simulations to see if their string-nets could give rise to conventional particles and fractionally charged quasi-particles. They did. They also found something even more surprising. As the net of strings vibrated, it produced a wave that behaved according to a very familiar set of laws 鈥� Maxwell鈥檚 equations, which describe the behaviour of light. 鈥淎 hundred and fifty years after Maxwell wrote them down, here they emerged by accident,鈥� says Wen.

That wasn鈥檛 all. They found that their model naturally gave rise to other elementary particles, such as quarks, which make up protons and neutrons, and the particles responsible for some of the fundamental forces, such as gluons and the W and Z bosons.

From this, the researchers made another leap. Could the entire universe be modelled in a similar way? 鈥淪uddenly we realised, maybe the vacuum of our whole universe is a string-net liquid,鈥� says Wen. 鈥淚t would provide a unified explanation of how both light and matter arise.鈥� So in their theory elementary particles are not the fundamental building blocks of matter. Instead, they emerge from the deeper structure of the non-empty vacuum of space-time.

鈥淲en and Levin鈥檚 theory is really beautiful stuff,鈥� says Michael Freedman, 1986 winner of the Fields medal, the highest prize in mathematics, and a quantum computing specialist at Microsoft Station Q at the University of California, Santa Barbara. 鈥淚 admire their approach, which is to be suspicious of anything 鈥� electrons, photons, Maxwell鈥檚 equations 鈥� that everyone else accepts as fundamental.鈥�

Other theories that try to explain the same phenomena abound, of course; Wen and Levin realise that the burden of proof is on them. It may not be far off. Their model predicts specific arrangements of atoms in the new state of matter, which they dub the 鈥渟tring-net liquid鈥�, and Young Lee鈥檚 group at MIT might have found it.

Lee was aware of Wen鈥檚 work and decided to look for such materials. Trawling through geology journals, his team spotted a candidate 鈥� a dark green crystal that geologists stumbled across in the mountains of Chile in 1972. 鈥淭he geologists named it after a mineralogist they really admired, Herbert Smith, labelled it and put it to one side,鈥� says Lee. 鈥淭hey didn鈥檛 realise the potential herbertsmithite would have for physicists years later.鈥�

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 鈥� there will always be neighbouring electrons spinning in the same direction. Wen and his colleagues propose 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 Lee鈥檚 team made a pure sample in the lab. 鈥淚t was painstaking,鈥� says Lee. 鈥淚t took us a full year to prepare it and another year to analyse it.鈥�

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 掳C the spins of its electrons should stop fluctuating 鈥� 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. 鈥淲e could have created something in the lab that nobody has seen before,鈥� 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. 鈥淲e want to see the dynamics of the spin,鈥� says Lee. 鈥淚f we tweak one [electron], we can see how the others are affected.鈥�

This intrigues Paul Fendley, a quantum computing specialist at the University of Virginia, Charlottesville (see 鈥淪ilicon for a quantum age鈥�). 鈥淚t鈥檚 reasonable to hope that we are seeing something exotic here,鈥� he says. 鈥淧eople are getting very excited about this.鈥�

Even if herbertsmithite is not a new state of matter, we shouldn鈥檛 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. 鈥淎ccelerators are just recreating conditions after the big bang and repeating experiments that are old hat for the universe,鈥� he says. 鈥淏ut 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.鈥�