We report on the competing energy scales and spin selection rules between semiconductor quantum dots (QD) and bulk superconductors (SC). Quantum dots hold a small number of electrons with even-number (primarily) singlet ground states (GS) and odd-number (primarily) doublet states. Increasing the coupling between a doublet QD and singlet SC (geometry A) enforces a doublet-to-singlet GS transition when the tunnel coupling exceed the QD’s charging energy. Using circuit-QED, we have mapped out the spectrum of two phase-biased superconductors coupled via a QD while changing coupling strength and realised an Andreev qubit (geometry B). In an ‘inverse’ geometry, i.e. two QDs coupled via a superconductor (geometry C), we explore the different coupling QD-to-QD coupling mechanisms including direct tunnelling and crossed Andreev reflection. These couplings can be modified by Pauli spin blockade which in turn can be modified by varying an external magnetic field relative to a spin-orbit field. These ‘knobs’ allow to tune to a regime of triplet coupling between QDs and the realisation of a minimal Kitaev chain.
Delft University of Technology
QUANTUM DOTS COUPLED TO SUPERCONDUCTORS