Highly efficient Cooper pair splitting with carbon nanotube quantum dots

An elegant idea for the creation of entangled electrons in a solid-state device is to split spin-singlet Cooper pairs by coupling a superconductor to two parallel quantum dots (QDs) in a Y-junction geometry. Such Cooper pair splitting (CPS) was successfully detected in recent transport experiments on devices based on InAs nanowires and Carbon nanotubes (CNTs). Here we present experiments on a CNT based Cooper pair splitter device with low inter-dot tunnel coupling. We find an unprecedented splitting efficiency, often much larger than 50%, the upper limit in devices with large inter-dot coupling. Such CPS rates allow us to investigate this effect as a function of a variety of external parameters. In addition, we discuss Fano line-shapes as a feature of our QD resonances. A high CPS efficiency is a prerequisite for Bell state measurements, a clear way of proving that Cooper pairs can be extracted coherently and lead to spatially separated entangled electron pairs.