Inside materials, the interactions between groups of electrons and atoms in the crystal lattice can give rise to a variety of interesting phenomena. Their collective behavior, especially at low temperatures, can give rise to quasiparticles: particle-like excitations that have strikingly different properties than the electrons that form them. Quasiparticles have been discovered that have behaviors predicted by particle physics, but have not been observed in particle collidors.

Researchers in the Netherlands have now produced quasiparticles that act like Majorana fermions: electrically-neutral particles that are their own antiparticles, such that if two collide, they annihilate. The existence of Majorana fermions was first predicted in the 1930s, but no individual particles are known to behave that way. V. Mourik et al. found a quasiparticle version by constructing a very thin wire—a nanowire—of semiconductor material and connected it to a superconductor. The specific electronic properties of the hybrid system gave rise to a pair of zero-velocity quasiparticles at two positions in the nanowire, and these showed behavior consistent with Majorana fermions. Some researchers suggest that quasiparticles of this type would be very useful in quantum computing applications.

Fermions vs. Bosons Particles and quasiparticles come in two basic types, fermions and bosons, depending on the type of spin they have. The elementary particles of matter (electrons, quarks, and neutrinos) are fermions, while photons and other force carriers are bosons. Particles are paired with antiparticles—antimatter electrons are positrons, etc.—but photons are their own antiparticles. To annihilate, particles and antiparticles must have opposite charge, so Majorana fermions, which are their own antiparticles, need to be electrically-neutral. At present, no fermion is known to be its own antiparticle, although neutrinos may have this property (we don't yet know).



Theorists predicted the existence of Majorana fermion quasiparticles in a materials known as topological superconductors, in which the interior of the material has zero electrical resistance, but the outside behaves like an ordinary conductor. To create a topological superconductor, Mourik et al. connected a semiconducting indium-antimony nanowire (InSb) between a gold electrode and the edge of a superconductor (NbTiN). They deposited the whole system onto a silicon substrate, which itself was printed with set of logic circuits that read the electronic properties of the wire.

By measuring the relationship between current and voltage at various positions along the nanowire, the researchers found a strong response at two points where the Majorana fermions are expected to appear. These quasiparticles didn't move under the influence of either a magnetic field or an additional current, indicating that they are electrically neutral and trapped in place.

This effect was strongest at 60 millikelvins (60 mK, which is 0.06 degrees above absolute zero) and vanished entirely at temperatures higher than 300 mK. Additionally, Mourik et al. confirmed that these Majorana quasiparticles failed to appear when the superconductor was replaced with another gold electrode, showing that the combination of the nanowire with the superconductor was necessary to create the fermions.

As the researchers themselves note, these results are consistent with Majorana fermions, but they have not been able to test for the presence of some of the predicted properties. Specifically, while the quasiparticles in the nanowire are electrically neutral and trapped at the expected positions, they should also behave in a certain way if their positions are swapped. While that can't be directly tested in this device, this fundamental property of Majorana fermions can be tested using a superconducting device known as a Josephson junction, a standard technique.

Since the quantum states of Majorana quasiparticles in topological superconductors are not independent of each other, the total system represents a qubit (quantum bit), which has been proposed as another way to achieve working quantum computers (although that may be overselling them). Apart from that, from a pure physics point of view, this result is very important: if these quasiparticles indeed turn out to be Majorana fermions, that will be the first confirmed detection in any physical system.

Science, 2012. DOI: 10.1126/science.1222360 (About DOIs).