Apr 23, 2012
First hint of Majorana fermions spotted in nanowires
Physicists in the Netherlands say that they have found the first evidence for the existence of "Majorana fermions" – particles that are their own antiparticles. The researchers claim to have spotted what they call "signatures" of these elusive particles, which were first predicted by the Italian physicist Ettore Majorana in 1937, at the interface between a tiny semiconductor wire and a superconducting electrode. The Majorana fermions spotted in the Netherlands are not, however, fundamental particles but quasiparticles – particle-like entities that emerge from the collective behaviour of electrons in a solid.
As well as backing Majorana's original prediction, the discovery also agrees with more recent theoretical work that the particle could be lurking within solid-state devices. The latter could be important for the development of quantum computers because Majorana fermions – unlike more familiar "Dirac" fermions, such as electrons – obey "non-Abelian statistics" and so should be resistant to environmental noise. Majorana fermions could, therefore, be able to store and transmit quantum information without being perturbed by the outside world, which is the bane of anyone trying to build a practical quantum computer.
Half and half
The new evidence for Majorana fermions has been obtained by a team led by Leo Kouwenhoven at the Delft University of Technology and the Eindhoven University of Technology that has studied materials known as "topological insulators". These are materials that are insulating in the bulk but can conduct electricity on their surface via special surface electronic states. Theory predicts that quasiparticle Majorana fermions could be made by combining an ordinary superconductor with a topological insulator.
If a superconductor is placed in contact with a topological insulator, the surface states become superconducting. Since the surface states are "half" an ordinary 2D electron system, their superconducting state is "half" an ordinary superconductor. This is the situation that physicists believe will lead to the emergence of quasipartcle Majorana fermions.
For its topological insulator the team used a nanowire of indium strontium, which bridged the gap between a superconducting electrode made of niobium titanium nitride and a normal electrode made of gold. The device is cooled to temperatures of tens of millikelvin and a magnetic field is applied along the direction of the nanowire.
The team then measured the current flowing through the nanowire as a function of voltage – and, in particular, how the current changed in response to changes in voltage. At zero applied magnetic field, two small peaks were observed on either side of zero applied voltage. When the applied magnetic field was increased, the position of these peaks remained in the same position. This also occurred when an electric field was applied to the nanowire.
According to the team, this lack of response by the peaks to magnetic and electric fields can only be explained by the presence of pairs of Majorana fermions at one end of the nanowire. "What is magical about quantum mechanics is that a Majorana particle created in this way is similar to the ones that may be observed in a particle accelerator, although that is very difficult to comprehend," says Kouwenhoven.
The team acknowledges that its measurements do not confirm the expected topological properties of the Majorana fermions that it has seen – something that would make the particles useful for quantum-computing applications. To do so, the team suggests a number of new experiments to measure other properties of the quasiparticles to establish their non-Abelian nature.
The research is described in Science.
About the author
Hamish Johnston is editor of nanotechweb.org's sister website, physicsworld.com