A Solid Case for Majorana Fermions
A year before he mysteriously disappeared at sea, Italian physicist Ettore Majorana posed a puzzle for future researchers. Quantum pioneer Paul Dirac had in 1928 predicted the existence of antimatter — mirror particles that annihilate with their matter counterparts. All known fermions — particles with half-integer spin, such as electrons — obey Dirac’s rules, but in 1937 Majorana varied Dirac’s equation to predict a class of particle that is its own antiparticle: the Majorana fermion.
Whether anything in nature fits Majorana’s prediction has remained unclear. But Leo Kouwenhoven, a nanoscientist at Delft University of Technology in the Netherlands, unveiled the most promising evidence so far for the existence of Majorana fermions, at a meeting of the American Physical Society in Boston, Massachusetts, on 27 February. The result is not just a vindication for the vanished theorist: Majoranas might also turn out to be the ideal information carriers in certain schemes for quantum computing.
What Kouwenhoven’s group spotted are not real particles, but quasiparticles formed from the collective movement of electrons in a semiconducting nanowire. Other researchers have invoked Majoranas to explain various observed behaviours in solid materials, but Kouwenhoven’s data are the first to clearly demonstrate a predicted Majorana signature. “What he showed is extremely compelling,” says Jason Alicea, a theoretical physicist at the University of California, Irvine.
The Delft group tested a 2010 proposal that a pair of Majorana fermions could form at the interface between a superconductor and a semiconducting nanowire in a magnetic field (R. M. Lutchyn et al. Preprint at arxiv.org 2010). Majoranas are electrically neutral, and the mass of free-floating electrical charge in the superconductor allows electrons and absences of electrons — known as holes — to form neutral entities at the interface with the nanowire. Kouwenhoven reported a peak in the conductance through the nanowire at zero voltage: a signature of a spatially separated pair of Majoranas forming.