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Finding Majorana - Update

Condensed matter physicists including researchers in Sankar Das Sarma’s group* at the University of Maryland, have been in hot pursuit of Majorana fermions. Originally predicted in 1937 by Ettore Majorana, these exotic particles serve as their own anti-particles. Quantum information scientists believe that the condensed matter realization of Majorana fermions represent robust ‘topological’ qubits and would open new possibilities in quantum computation.

Now a group at Delft University in the Netherlands led by L.P. Kouwenhoven has published experimental signatures of the elusive particle. The research appeared inScience Express1 on April 12, 2012, and precisely follows the theoretical proposals2-4 made in 2010 by Sankar Das Sarma and his collaborators at JQI.

The scientists observed evidence of the particle at the ends of one-dimensional (1D) nanowires. The wires are made of the semiconducting material indium antimonide. This substance has one of the necessary ingredients for supporting the Majorana fermions: strong spin-orbit coupling.

What is spin-orbit coupling? An electron, which can be roughly thought of as a tiny spinning top, lives in a natural environment of electric fields. These fields force a charged particle into motion. Due to the laws of electromagnetism, the moving charge gives rise to a magnetic field, which can in turn affect the behavior of the electron. Heavier elements are likely candidates for having strong spin-orbit interactions.

The wires are placed near a superconductor and the “proximity effect” causes a region of superconductivity to also form in the wire. The experimentalists combine the nanowire and superconductor on a microchip and begin the search at temperatures just above absolute zero. Das Sarma’s theory established that such a nanowire in the presence of an external magnetic field along the wire would lead to the Majorana fermions at low (~1K) temperatures, exactly as observed in the Delft experiment.

The JQI/CMTC research group has predicted different ways to observe these particles in semiconductor/superconductor systems. For instance, in a variation on their original 1D nanowire proposals, they showed the surprising result that the Majorana fermions in the wire are not so delicate and would survive even if the strict 1D restrictions were relaxed. In fact, the Majorana fermions can be stable, even in the presence of the imperfections and disorder that often exist in solid state materials. A very recent work5 from the group, which appeared on the condensed matter archive on April 15, provided a detailed theoretical analysis of the Delft data, further enhancing the claim that the elusive Majorana particle may have finally been found in nature.

When asked for comments on this development, Das Sarma said “This is certainly very exciting news, it is not often that a theoretical prediction for something totally new actually works out in the laboratory. One, however, has to be cautious because while this experiment from Delft has provided the likely necessary evidence for the existence of the Majorana, the sufficient conditions are more difficult to achieve and may take more time.” He added that the theoretical success of his group in this highly competitive research area has only been possible because he has been able to recruit outstanding postdocs to Maryland because of the high reputation of the Maryland physics department and the Joint Quantum Institute.