Solid-state-based quantum electronics is a paradigm that promises on chip scalable quantum processors that can outperform any classical computer. The most natural quantum degree of freedom tight to the basic mobile entity of charge in electronics is the spin of the electron. The recent advances in nanotechnology made the coherent manipulation of a single electron spin as well as coherent operations between pairs of spins possible [1]. Operations of this kind form the basis of novel quantum algorithms. The power of quantum computing as well as quantum communication strongly roots in a property called entanglement. Here, two (or more) quantum states, also called quantum bits, share information between themselves in a intricate way that is classically not possible.

On the roadmap to a full electron spin based quantum computer the controlled generation of entangled electron pairs is an important milestone. Superconductors provide a natural source of entangled electrons since electrons pair together in spin-singlet states in its condensate. Recher et al. worked out a theoretical proposal [2] on how single Cooper pairs can be extracted and spatially separated with a double quantum dot circuit connected to a superconductor. Recently, the first realizations of such Cooper pair splitters (CPS) were demonstrated in InAs nanowire and carbon nanotube based hybrid nanodevices [3,4]. These initial experiments were followed by an experimental proof of principle that very high splitting efficiencies beyond 90% are possible. Although this result is very promising, it is not possible to obtain Cooper-pair splitters working in the correct parameter regime from start by design. There is more work needed in order to understand the microscopics of the splitting process within the potential landscape of the double quantum dot system.

The aim of the present project is to design, fabricate and analyze a new generation of Cooper pair splitters. Unlike to the first generation we aim to form quantum dots by a controlled confinement potential using several fine local gates and we will be using superconductors with higher gap energies. The in-situ tunability of the dot-electrode coupling and a larger energy window would allow to access the theoretically desired parameter range and lead to high and predictable CPS efficiencies. Besides higher efficiency, the smaller dimensions of the quantum dots would lead to a larger level spacing and thereby open the way to investigate the influence of the splitting process on the quantum dot orbital states. Such measurements open the possibility to measure the spin correlations of spatially separated electrons originating from a single spin-singlet pair. Comparing the splitting rate for several dot states with different spin orientations could provide evidence for the spin entanglement [5].