Abstract
Quantum technology is a developing field of sciencewhere devices possess novel and superior functionalities thanks to their quantummechanical 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 speedups 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, faulttolerant implementations of quantumcomputing need to be developed.
Topological quantum computing has been proposed to achieve faulttolerance since its significant robustness to decoherence is inherent in the quantummechanical nature of topological qubits. Building units of a topological qubit are Majorana zero modes (MZMs) – zeroenergy quasiparticles that possess the nonAbelian 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 nonlocal fermions. By braiding nonoverlapping MZMs, the information stored in the nonlocal 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, IIIV semiconductor nanowires proximitized by swave 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 spinorbit interaction, swave 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...
Topological quantum computing has been proposed to achieve faulttolerance since its significant robustness to decoherence is inherent in the quantummechanical nature of topological qubits. Building units of a topological qubit are Majorana zero modes (MZMs) – zeroenergy quasiparticles that possess the nonAbelian 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 nonlocal fermions. By braiding nonoverlapping MZMs, the information stored in the nonlocal 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, IIIV semiconductor nanowires proximitized by swave 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 spinorbit interaction, swave 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...
Original language  English 

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Award date  20 Nov 2023 
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Publication status  Published  2023 