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Thursday, March 1, 2012

Peacable Matter-Antimatter Pairs

The first result is purely theoretical and suggests that something like the elusive Higgs – the particle thought to give all others mass – might be glimpsed in crystals known as spin ice. The second is more concrete – and may be our first peek at a fabled particle first dreamed up more than 70 years ago.

Spin ice gets its name from the ordering of quantum-mechanical spins in these crystals, which mimic the hydrogen atoms in ordinary water ice. It shot to prominence in 2008 when these crystals were found to harbour magnetic monopoles – single magnetic "charges" that could move around the spin-ice crystal just like an electric charge. That was a shock: we are used to magnetic charge coming locked in overall neutral pairs of north and south poles.

Now theorists, including Stephen Powell of the University of Maryland in College Park, who gave a talk at the Boston meeting, suggest that spin ice could have another trick up its sleeve. Under certain circumstances, it undergoes a transition to a more ordered state that is described by the same mathematics as a transition the early universe underwent when the Higgs boson gave the other elementary particles mass. If so, an entity very similar to the Higgs boson could be lurking within spin ice.

Because of its theoretical nature, this revelation is unlikely to distract the Higgs hunters at the LHC, based at CERN, near Geneva, Switzerland. The second laboratory result reported in Boston, however, is practical – and provides another chapter to a strange human narrative that now spans almost a century.

More than 70 years ago, Italian physicist Ettore Majorana was investigating the theory behind fermions, the class of particle that includes electrons and quarks, the basic building blocks of matter. Each of these particles has an antimatter equivalent, which is, to all intents and purposes, identical, but has the opposite electric charge. An electron's antiparticle, for instance, is the positively charged positron.

A particle and its antimatter counterpart normally annihilate on contact. But in 1937, Majorana proposed what at the time seemed a mathematical curiosity: that a fermion with no electric charge might also have an antiparticle – that would also have zero charge. Such a particle would be indistinguishable from its antiparticle, and represent matter and antimatter peacefully co-existing.

On 27 February, Leo Kouwenhoven of the Delft University of Technology in the Netherlands and colleagues produced tentative evidence for such a twosome. Their Majorana particles are not free agents of the sort that might wander into a particle detector on their own, but collective excitations of electrons and "hole" states – absences of electrons – within nanoscale wires made of the semiconductor indium antimonide.

Kouwenhoven and his team saw a suggestive blip in the spectrum of energies in the nanowire consistent with the formation of an object of precisely zero energy – exactly the signature that a pair of Majorana fermions would be expected to produce. The clincher came when the team applied a magnetic field to the nanowire. Had the signal come from anything else but a Majorana pair, its energy would have changed in response to the field. But it didn't.
Delicate creatures

If the sighting is confirmed, Majorana particles would be perfect candidates to act as the quantum bits, or qubits, in a quantum computer. Unlike ordinary bits, qubits can exist in multiple states at once, known as a superposition, and can also be entangled with each other. Together, this means quantum computers could perform multiple calculations simultaneously, making them much faster than ordinary computers at some tasks.

There are many schemes for creating qubits, including photons and trapped ions, but Majorana particles could have unique advantages. Qubits are delicate creatures, susceptible to losing their information when buffeted by their environment. Majorana pairs would encode the same information spread over two sites, making them far more robust against local fluctuations.

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