Quantum transport in hybrid semiconductor-superconductor nanostructures

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...QRD/Kouwenhoven La

Supercurrent in the Presence of Direct Transmission and a Resonant Localized State

We study the current-phase relation (CPR) of an InSb-Al nanowire Josephson junction in parallel magnetic fields up to 700 mT. At high magnetic fields and in narrow voltage intervals of a gate under the junction, the CPR exhibits π shifts. The supercurrent declines within these gate intervals and shows asymmetric gate voltage dependence above and below them. We detect these features sometimes also at zero magnetic field. The observed CPR properties are reproduced by a theoretical model of supercurrent transport via interference between direct transmission and a resonant localized state.QRD/Kouwenhoven LabQN/Nazarov GroupQRD/Wimmer GroupQN/Kouwenhoven La

Supercurrent parity meter in a nanowire Cooper pair transistor

We study a Cooper pair transistor realized by two Josephson weak links that enclose a superconducting island in an InSb-Al hybrid nanowire. When the nanowire is subject to a magnetic field, isolated subgap levels arise in the superconducting island and, because of the Coulomb blockade, mediate a supercurrent by coherent cotunneling of Cooper pairs. We show that the supercurrent resulting from such cotunneling events exhibits, for low to moderate magnetic fields, a phase offset that discriminates even and odd charge ground states on the superconducting island. Notably, this phase offset persists when a subgap state approaches zero energy and, based on theoretical considerations, permits parity measurements of subgap states by supercurrent interferometry. Such supercurrent parity measurements could, in a series of experiments, provide an alternative approach for manipulating and protecting quantum information stored in the isolated subgap levels of superconducting islands. Electrical Engineering, Mathematics and Computer ScienceQuTechQRD/Kouwenhoven LabBUS/Quantum DelftQN/Kouwenhoven La

Subgap spectroscopy along hybrid nanowires by nm-thick tunnel barriers

Tunneling spectroscopy is widely used to examine the subgap spectra in semiconductor-superconductor nanostructures when searching for Majorana zero modes (MZMs). Typically, semiconductor sections controlled by local gates at the ends of hybrids serve as tunnel barriers. Besides detecting states only at the hybrid ends, such gate-defined tunnel probes can cause the formation of non-topological subgap states that mimic MZMs. Here, we develop an alternative type of tunnel probes to overcome these limitations. After the growth of an InSb-Al hybrid nanowire, a precisely controlled in-situ oxidation of the Al shell is performed to yield a nm-thick AlOx layer. In such thin isolating layer, tunnel probes can be arbitrarily defined at any position along the hybrid nanowire by shadow-wall angle-deposition of metallic leads. In this work, we make multiple tunnel probes along single nanowire hybrids and successfully identify Andreev bound states (ABSs) of various spatial extension residing along the hybrids.QRD/Kouwenhoven LabBUS/Quantum DelftQN/AfdelingsbureauQN/Kouwenhoven La

Impact of Junction Length on Supercurrent Resilience against Magnetic Field in InSb-Al Nanowire Josephson Junctions

Semiconducting nanowire Josephson junctions represent an attractive platform to investigate the anomalous Josephson effect and detect topological superconductivity. However, an external magnetic field generally suppresses the supercurrent through hybrid nanowire junctions and significantly limits the field range in which the supercurrent phenomena can be studied. In this work, we investigate the impact of the length of InSb-Al nanowire Josephson junctions on the supercurrent resilience against magnetic fields. We find that the critical parallel field of the supercurrent can be considerably enhanced by reducing the junction length. Particularly, in 30 nm long junctions supercurrent can persist up to 1.3 T parallel field─approaching the critical field of the superconducting film. Furthermore, we embed such short junctions into a superconducting loop and obtain the supercurrent interference at a parallel field of 1 T. Our findings are highly relevant for multiple experiments on hybrid nanowires requiring a magnetic-field-resilient supercurrent.QRD/Kouwenhoven LabQRD/Wimmer GroupQCD/Veldhorst LabBUS/Quantum DelftQN/Kouwenhoven La

Full parity phase diagram of a proximitized nanowire island

We measure the charge periodicity of Coulomb blockade conductance oscillations of a hybrid InSb-Al island as a function of gate voltage and parallel magnetic field. The periodicity changes from to at a gate-dependent value of the magnetic field, , decreasing from a high to a low limit upon increasing the gate voltage. In the gate voltage region between the two limits, which our numerical simulations indicate to be the most promising for locating Majorana zero modes, we observe correlated oscillations of peak spacings and heights. For positive gate voltages, the transition with low is due to the presence of nontopological states whose energy quickly disperses below the charging energy due to the orbital effect of the magnetic field. Our measurements highlight the importance of a careful exploration of the entire available phase space of a proximitized nanowire as a prerequisite to define future topological qubits.Architecture and the Built EnvironmentQuTechQCD/Veldhorst LabBUS/Quantum DelftQRD/Kouwenhoven LabOptical and Laser Remote SensingQN/Kouwenhoven La

Single-Shot Fabrication of Semiconducting–Superconducting Nanowire Devices

Semiconducting–superconducting hybrids are vital components for the realization of high-performance nanoscale devices. In particular, semiconducting–superconducting nanowires attract widespread interest owing to the possible presence of non-abelian Majorana zero modes, which are quasiparticles that hold promise for topological quantum computing. However, systematic search for Majoranas signatures is challenging because it requires reproducible hybrid devices and reliable fabrication methods. This work introduces a fabrication concept based on shadow walls that enables the in situ, selective, and consecutive depositions of superconductors and normal metals to form normal-superconducting junctions. Crucially, this method allows to realize devices in a single shot, eliminating fabrication steps after the synthesis of the fragile semiconductor/superconductor interface. At the atomic level, all investigated devices reveal a sharp and defect-free semiconducting–superconducting interface and, correspondingly, a hard induced superconducting gap resilient up to 2 T is measured electrically. While the cleanliness of the technique enables systematic studies of topological superconductivity in nanowires, it also allows for the synthesis of advanced nano-devices based on a wide range of material combinations and geometries while maintaining an exceptionally high interface quality.QCD/Veldhorst LabQRD/Kouwenhoven LabBUS/Quantum DelftQN/Kouwenhoven La