# Wystąpienie ustne Roland Wiesendanger (NEF-Sr)

Bottom-Up Design of Topological Qubits for Robust Quantum Computing

Dept. of Physics, University of Hamburg, Jungiusstraße 11a, 20355 Hamburg, Germany

Majorana states in atomic-scale magnet-superconductor hybrid systems have recently become of great interest because they can encode topological qubits and ultimately provide a new direction in topological quantum computation [1,2]. However, an unambiguous identification of Majorana states requires well defined model-type platforms and appropriate experimental tools for their investigation.

Our experimental approach is based on the use of STM-based single atom manipulation techniques in order to fabricate well-defined defect-free 1D atomic chains as well as 2D arrays of magnetic adatoms on s-wave superconductor substrates with high spin-orbit coupling [3-5]. Spin-polarized STM measurements [6,7] allow us to reveal the presence of collinear [7] or non-collinear [3] spin textures, i.e. spin spiral ground states, of the 1D chains. Simultaneously performed scanning tunneling spectroscopy on the magnetic atom chains proximity-coupled to the superconducting substrates reveal the evolution of the spatially and energetically resolved local density of states as well as the emergence of zero-energy bound states at both chain ends above a critical chain length for Fe chains on Re(0001) [3] as well as Mn chains on Nb(110) [8]. Based on the exact knowledge of the geometrical, electronic, and spin structure of the magnetic chain – superconductor hybrid system, the experimental results can be compared rigorously with ab-initio and model-type tight-binding calculations supporting the interpretation of the spectroscopic signatures at the ends of the chains as Majorana states [3,8,9]. Moreover, by making use of Bogoliubov quasiparticle interference (QPI) mapping of the 1D magnet-superconductor hybrid systems, the non-trivial band structure of the topological phases as well as the bulk-boundary correspondence can directly be probed [10], constituting the ultimate test and rigorous proof for the existence of topologically non-trivial zero-energy modes [9].

[1] J. Alicea et al., Nature Phys. 7, 412 (2011).

[2] S. Nadj-Perge et al., Phys. Rev. B 88, 20407 (2013).

[3] H. Kim et al., Science Advances 4, eaar5251 (2018).

[4] L. Schneider et al., Nature Commun. 11, 4707 (2020).

[5] A. Kamlapure et al., Nature Commun. 9, 3253 (2018).

[6] R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).

[7] L. Schneider et al., Science Advances 7, eabd7302 (2021).

[8] L. Schneider et al., Nature Nanotechnol. 17, 384 (2022).

[9] D. Crawford et al., arXiv:2109.06894.

[10] L. Schneider et al., Nature Physics 17, 943 (2021).