Quantum transport in hybrid semiconductor-superconductor nanostructures

Abstract

Quantum technology is a developing field of sciencewhere devices possess novel and superior functionalities thanks to their quantum-mechanical behaviour at the nanometer scale. A typical example is a quantum computer, where information is stored in quantum states of its quantum bits. By manipulating entangled and superposition states of these qubits, quantumcomputers can achieve exponential speed-ups in calculation and therefore solve currently unsolvable problems within polynomial computational times. This powerful advantage of quantum computers is particularly difficult to achieve in practice, due to decoherence - a tendency of quantum objects to lose their quantummechanical properties when interacting with their environment. Obviously, qubit decoherence cannot be avoided because the control of a quantum computer inevitably causes couplings to the environment. To mitigate decoherence, fault-tolerant implementations of quantumcomputing need to be developed.

Topological quantum computing has been proposed to achieve fault-tolerance since its significant robustness to decoherence is inherent in the quantum-mechanical nature of topological qubits. Building units of a topological qubit are Majorana zero modes (MZMs) – zero-energy quasiparticles that possess the non-Abelian anyonic exchange statistics and are localized at the boundaries of a topological superconductor. In sufficiently large topological superconductors, MZMs exhibit no overlap and therefore can in pairs host non-local fermions. By braiding non-overlapping MZMs, the information stored in the non-local fermions is manipulated while being insensitive to local noise. In this way one can perform computation that is topologically protected against local sources of decoherence.

In 2010, III-V semiconductor nanowires proximitized by s-wave superconductorswere proposed as a suitable candidate platform for the realization of topological superconductors. Topological superconducting phase occurs in such a hybrid nanowire due to an interplay among the large spin-orbit interaction, s-wave superconductivity, controllable electron density and large Zeeman energy introduced by an externalmagnetic field. Consequently, the nanowire bulk undergoes a band inversion and two MZMs appear at the two nanowire ends. First signatures of MZMs were reported in 2012 and since then a lot of effort has been put in fully demonstrating them. Despite huge improvements in the materials and measurement techniques, conclusive evidence of MZMs in hybrid nanowires is still missing. This is because disorder in hybrid nanowires can also cause the observed signatures of MZMs and make the topological scenario indistinguishable from the trivial ones. Therefore, further improvements and more detailed studies are needed and this thesis shows some recent examples of these...