  The two electrons of a Cooper pair in a conventional superconductor form a
singlet and therefore a maximally entangled state. Recently, it was
demonstrated that the two particles can be extracted from the superconductor
into two spatially separated contacts via two quantum dots (QDs) in a process
called Cooper pair splitting (CPS). Competing transport processes, however,
limit the efficiency of this process. Here we demonstrate efficiencies up to
90%, significantly larger than required to demonstrate interaction-dominated
CPS, and on the right order to test Bell's inequality with electrons. We
compare the CPS currents through both QDs, for which large apparent
discrepancies are possible. The latter we explain intuitively and in a
semi-classical master equation model. Large efficiencies are required to detect
electron entanglement and for prospective electronics-based quantum information
technologies.
