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Ettore Majorana and his enigmatic fermions

Along with the Higgs boson, the Majorana fermion is an important quantum particle predicted by theory that has proved elusive. While the Higgs boson is the quest of the hugely expensive and gigantic CERN hadron collider, the Majorana fermion now appears to have been found in a desk-top experiment involving a nanowire and a magnetic field. In the April 12 online issue of Science Express, Leo Kouwenhoven and his colleagues from Delft University of Technology report observing the "signatures" of Marjorana fermions in a hybrid semiconductor/superconductor nanowire device.

Ettore Majorana was an Italian physicist who worked with the great Enrico Fermi (after whom fermions are named) as well as Werner Heisenberg and Niels Bohr. Marjorana is supposedly the first to propose the idea of a neutral particle the size of a proton. Though Fermi suggested that he write an article about the new particle, he never got around to it, and so the Nobel Prize for the neutron went to James Chadwick. Majorana did publish a paper in 1937 predicting the particle now called the Majorana fermion, as a solution to a wave equation similar to the Dirac equation. The neutral particle would have a spin of ½ but would be identical to its own antiparticle. Protons and electrons have anti-particles with the opposite charge, and would be annihilated should they chance to collide with one. But a Majorana fermion exists in a kind of purgatory midway between particle and anti-particle. 

One theory holds that dark matter is composed of Majorana fermions, so Kouwenhoven's experiment may bring us closer to understanding the 70 percent of the mass in the universe that we can now detect only by its gravitational field.

Majorana fermions may also serve as the quantum particle required to create a still-theoretical type of computer, called a topological quantum computer. This computer is, in fact, one the objects of Kouwenhouven's research, which is funded, in part, by Microsoft. Such a computer uses quantum particles that form two dimensional "braids" in three-dimensional space that are protected from the decoherence of qubits that limits the usefulness of an "ordinary" (as if there was anything ordinary about a) quantum computer. 

Majorana was as enigmatic as the particles that bear his name. Returning from Germany in 1933, he became estranged from his family and friends, and even his appearances in his lab became less frequent. He made a trip to Palermo in 1938, prior to which he withdrew all of the money from his bank account. On the return trip to Naples, he disappeared at sea and was never seen again in Italy. Though he was presumed dead at the time, many alternate theories have been proposed as to what really happened to him. At least one witness reported meeting him in Buenos Aires many years later.

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Steven A. Edwards, Ph.D.

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