Entangled Andreev pairs and collective excitations in nanoscale superconductors

Nanoscale superconductors connected to normal metallic electrodes provide a potential source of entangled electron pairs(1-5). Such states would arise from the splitting of Cooper pairs in the superconductor into two electrons with opposite spins, which then tunnel into different leads by means of a process known as crossed Andreev reflection (refs 6-8). In an actual system, the detection of these processes is hindered by the elastic transmission of individual electrons between the leads, which yields an opposite contribution to the non-local conductance. Here we demonstrate that low-energy collective excitations, which appear in superconducting structures of reduced dimensionality(9), can have a significant influence on the transport properties of this type of hybrid nanostructure. When an electron tunnels into the superconductor it can excite such low-energy excitations that alter the balance between the different electronic processes, leading to a dominance of one over the other depending on the spatial symmetry of these excitations. These findings help to clarify some intriguing experimental results and provide future strategies for the detection of entangled electron pairs in solid-state devices for quantum computation.