SpinScreen · Screening of an electron spin by an epitaxial superconducting island in a semiconductor nanowire

Europe’s leadership in quantum technologies can only be sustained by fundamental research. A challenge in this field is to choose the physical platform for qubits, the units of a quantum computer. Semiconductor nanowires coupled to superconductors offer a potential solution as a platform for a new type of qubit, which has the unique advantage of being inherently protected from decoherence.The physical ingredients of this qubit are Majorana modes. A common device geometry used to investigate transport through these modes is a superconducting island coupled to a nanowire.However, at this stage little is known about the interactions of islands with quantum dots, which are themselves commonplace near charge depletion. Whereas the interaction of a single dot strongly coupled to a superconductor lead at fixed potential is known to lead to anti-ferromagnetic screening by quasiparticles in the superconductor, the interaction of the dot with an isolated superconductor remains to be explored. If this gap in the knowledge is filled, we could learn to distinguish between subgap states related to screening of the dot, and those involving the island/nanowire hybrid. Crucially, only the latter ones are related to Majorana modes.At QDev, I will investigate screening by an epitaxial superconducting Al island of a single quantum dot defined in an InAs nanowire. To do this, I will tune the coupling between the island and the dot with a gate voltage and obtain the quantum phase diagram.While most of Majorana research is understandably focused on how to make a qubit, I will tackle three more fundamental questions, which will serve this purpose in the long run and go substantially beyond the state of the art. 1) How does a single quasiparticle in the island screen a spin? 2) How does screening occur when the island cannot accept quasiparticles? 3) Can novel non-Fermi liquid physics emerge when adding a superconducting lead to the dot-island system?