Majorana zero-modes-a type of localized quasiparticle-hold great promise for topological quantum computing. Tunnelling spectroscopy in electrical transport is the primary tool for identifying the presence of Majorana zero-modes, for instance as a zero-bias peak in differential conductance. The height of the Majorana zero-bias peak is predicted to be quantized at the universal conductance value of 2e/h at zero temperature (where e is the charge of an electron and h is the Planck constant), as a direct consequence of the famous Majorana symmetry in which a particle is its own antiparticle. The Majorana symmetry protects the quantization against disorder, interactions and variations in the tunnel coupling. Previous experiments, however, have mostly shown zero-bias peaks much smaller than 2e/h, with a recent observation of a peak height close to 2e/h. Here we report a quantized conductance plateau at 2e/h in the zero-bias conductance measured in indium antimonide semiconductor nanowires covered with an aluminium superconducting shell. The height of our zero-bias peak remains constant despite changing parameters such as the magnetic field and tunnel coupling, indicating that it is a quantized conductance plateau. We distinguish this quantized Majorana peak from possible non-Majorana origins by investigating its robustness to electric and magnetic fields as well as its temperature dependence. The observation of a quantized conductance plateau strongly supports the existence of Majorana zero-modes in the system, consequently paving the way for future braiding experiments that could lead to topological quantum computing.

Semiconductor nanowires provide an ideal platform for various low-dimensional quantum devices. In particular, topological phases of matter hosting non-Abelian quasiparticles can emerge when a semiconductor nanowire with strong spin-orbit coupling is brought in contact with a superconductor 1,2 . To fully exploit the potential of non-Abelian anyons for topological quantum computing, they need to be exchanged in a wellcontrolled braiding operation 3-8 . Essential hardware for braiding is a network of singlecrystalline nanowires coupled to superconducting islands. Here, we demonstrate a technique for generic bottom-up synthesis of complex quantum devices with a special focus on nanowire networks having a predefined number of superconducting islands.Structural analysis confirms the high crystalline quality of the nanowire junctions, as well as an epitaxial superconductor-semiconductor interface. Quantum transport measurements of nanowire "hashtags" reveal Aharonov-Bohm and weak-antilocalization effects, indicating a phase coherent system with strong spin-orbit coupling. In addition, a 2 proximity-induced hard superconducting gap is demonstrated in these hybrid superconductor-semiconductor nanowires, highlighting the successful materials development necessary for a first braiding experiment. Our approach opens new avenues for the realization of epitaxial 3-dimensional quantum device architectures.Majorana Zero Modes (MZMs) are predicted to emerge once a superconductor (SC) is coupled to a semiconductor nanowire (NW) with a strong spin-orbit interaction (SOI) in an external magnetic field 1,2 . InSb NWs are a prime choice for this application due to the large Landé g-factor (~50) and strong Rashba SOI 9 , crucial for realization of MZMs. In addition, InSb nanowires generally show high mobility and ballistic transport [10][11][12] . Indeed, signatures of Majorana zero modes (MZMs) have been detected in hybrid superconductor-semiconductor InSb and InAs NW systems 11,[13][14][15] . Multiple schemes for topological quantum computing based on braiding of MZMs have been reported, all employing hybrid NW networks 3-8 .Top-down fabrication of InSb NW networks is an attractive route towards scalability 16 , however, the large lattice mismatch between InSb and insulating growth substrates limits the crystal quality. An alternative approach is bottom-up synthesis of out-of-plane NW networks which, due to their large surface-to-volume ratio, effectively relieve strain on their sidewalls, enabling the growth of single-crystalline NWs on highly lattice-mismatched substrates [17][18][19] .Recently, different schemes have been reported for merging NWs into networks [20][21][22] .Unfortunately, these structures are either not single-crystalline, due to a mismatch of the crystal structure of the wires with that of the substrate (i.e. hexagonal NWs on a cubic substrate) 22 , or the yield is low due to the limited control over the multiple accessible growth directions (the yield decreases with the number of junctions in the network) 23 ....

Semiconductor nanowires have opened new research avenues in quantum transport owing to their confined geometry and electrostatic tunability. They have offered an exceptional testbed for superconductivity, leading to the realization of hybrid systems combining the macroscopic quantum properties of superconductors with the possibility to control charges down to a single electron. These advances brought semiconductor nanowires to the forefront of efforts to realize topological superconductivity and Majorana modes. A prime challenge to benefit from the topological properties of Majoranas is to reduce the disorder in hybrid nanowire devices. Here we show ballistic superconductivity in InSb semiconductor nanowires. Our structural and chemical analyses demonstrate a high-quality interface between the nanowire and a NbTiN superconductor that enables ballistic transport. This is manifested by a quantized conductance for normal carriers, a strongly enhanced conductance for Andreev-reflecting carriers, and an induced hard gap with a significantly reduced density of states. These results pave the way for disorder-free Majorana devices.

Group IV semiconductor optoelectronic devices are now possible by using strain-free direct band gap GeSn alloys grown on a Ge/Si virtual substrate with Sn contents above 9%. Here, we demonstrate the growth of Ge/GeSn core/shell nanowire arrays with Sn incorporation up to 13% and without the formation of Sn clusters. The nanowire geometry promotes strain relaxation in the GeSn shell and limits the formation of structural defects. This results in room-temperature photoluminescence centered at 0.465 eV and enhanced absorption above 98%. Therefore, direct band gap GeSn grown in a nanowire geometry holds promise as a low-cost and high-efficiency material for photodetectors operating in the short-wave infrared and thermal imaging devices.

Topological superconductivity is a state of matter that can host Majorana modes, the building blocks of a topological quantum computer. Many experimental platforms predicted to show such a topological state rely on proximity-induced superconductivity. However, accessing the topological properties requires an induced hard superconducting gap, which is challenging to achieve for most material systems. We have systematically studied how the interface between an InSb semiconductor nanowire and a NbTiN superconductor affects the induced superconducting properties. Step by step, we improve the homogeneity of the interface while ensuring a barrier-free electrical contact to the superconductor and obtain a hard gap in the InSb nanowire. The magnetic field stability of NbTiN allows the InSb nanowire to maintain a hard gap and a supercurrent in the presence of magnetic fields (∼0.5 T), a requirement for topological superconductivity in one-dimensional systems. Our study provides a guideline to induce superconductivity in various experimental platforms such as semiconductor nanowires, two-dimensional electron gases, and topological insulators and holds relevance for topological superconductivity and quantum computation.