No one knows whether Majorana fermions—particles that are their own antiparticles—exist in nature. But one can construct the exotic entities as elementary excitations, or quasiparticles, in a solid. For example, the electron and hole excitations of a superconductor naturally play roles of particle and antiparticle, but with their distinct identities blurred because the charge difference can be absorbed as a Cooper pair in a condensate. The challenge is to construct and reveal the presence of a quasiparticle at the Fermi level—the zero-energy state in the middle of the superconducting gap. Researchers led by Leo Kouwenhoven from Delft University of Technology now offer compelling evidence for having done just that in a nanowire. The Delft researchers connected the wire, made of indium antimonide, to a circuit with a gold contact at one end (N) and a superconductor (S) on the other. By coupling a gate to the wire, they created a tunneling barrier (green) through which they spectroscopically probed the wire's density of states in the presence of a magnetic field B. Measurements of the electrical conductance showed a telltale peak consistent with the presence of a Majorana in the wire. The excitement over Majoranas lies in their unusual statistics and potential as building blocks of an eventual topological computer. (V. Mourik et al., Science, in press, doi:10.1126/science.1222360 .)—R. Mark Wilson
