In Nature, elementary charged particles emerge in pairs: for each particle with a given charge, there is another particle (an “antiparticle”) with the opposite charge. Neutral particles (i.e., particles with no net charge) however, do not obey this rule. In 1937, the brilliant italian physicist Ettore Majorana, in his research about neutrinos (i.e., neutral particles travelling close to the speed of light), proposed a special kind of particles with the exotic property of being their own antiparticles. However, after 80 years of many theoretical and experimental efforts, and despite the elegant derivation of Majorana, it is not yet clear whether neutrinos are Majorana particles or not. Fortunately, sometimes happens that when one door of physics closes, another opens.

Recently, it has been theoretically proposed that, in certain kinds of superconducting materials (i.e., materials with no electrical resistance below a certain critical temperature) Majorana particles could be artificially produced and experimentally detected. Since, strictly speaking, these would not be the particles originally proposed by E. Majorana (they are actually special excitations in the superconductor), in this context they are known as “Majorana modes” or “Majorana quasi-particles”.

In addition to having a lot of interest for fundamental physics (since they would constitute the first confirmation of Majorana predictions), producing and controlling Majorana quasi-particles are important for another, unsuspected reason: they would allow to build a new kind of computers, much more powerful than the ones we have nowadays. This new type of computation is called “topological quantum computation”. Since 2012, renowned experimental groups around the world performed the proposed experiment, which involved superconducting wires 1000 times thinner than a human hair, and observed preliminary signatures of Majorana quasiparticles in transport measurements. However, none of those experiments are actually done in ideal conditions (the conditions assumed in the theoretical predictions), and although very encouraging, the so-far existing experimental signatures do not constitute a conclusive proof for the observation of the elusive Majorana quasiparticle.

Our work is actually a theoretical proposal for an experiment which would allow to elucidate and interpret better the experimental data. The main idea is to exploit a unique feature of Majorana quasiparticles: 1) they always appear in pairs, localizing at the end of the superconducting wire, and 2) exactly at the point when they emerge (upon the application of an external magnetic field), important correlations among them are established. In our work, we specifically propose a transport experiment in a configuration called “normal-superconductor-normal” (or “NSN” contact) in order to measure such correlations. In particular, the idea is to measure the electrical conductance through one of the interfaces (say, the left NS interface), and testing its sensitivity to a perturbation on the other (i.e., right) end. Any change in the electrical conductance on the left NS contact due to such a non-local perturbation could be interpreted as a signature of the Majorana quasiparticles in the system.

We hope this proposal, o a similar one, could be soon implemented by the experimental groups, therefore contributing to the understanding of these interesting novel systems.
Tunneling transport in NSN Majorana junctions across the topological quantum phase transition
Alejandro M Lobos1,2 and S Das Sarma2
Published 15 June 2015 • © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
New Journal of Physics, Volume 17, June 2015