No one knows whether Majorana particles—fermions that are their own antiparticles—exist in nature as elementary building blocks. When Italian particle theorist Ettore Majorana rewrote the complex Dirac equation in 1937 as a pair of real wave equations that would admit such exotic objects as solutions, he envisioned the neutrino as a likely suspect.1 But even today, whether the neutrino is its own antiparticle remains an important open question in particle physics. Experiments looking for telltale neutrinoless double beta decay signatures may resolve the issue (see PHYSICS TODAY, January 2010, page 20).

Even if Majorana fermions don’t exist as elementary particles, it’s possible to engineer one in a condensed-matter system. In a solid, excitations above the ground state often behave like elementary particles insofar as they can carry quantized amounts of energy, momentum, spin, and electric charge. They can also exhibit exotic properties. Magnetic monopoles that diffuse through a...

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