Detecting nonlocal Cooper pair entanglement by optical Bell inequality   violation
  Based on the Bardeen Cooper Schrieffer (BCS) theory of superconductivity, the coherent splitting of Cooper pairs from a superconductor to two spatially separated quantum dots has been predicted to generate nonlocal pairs of entangled electrons. In order to test this hypothesis, we propose a scheme to transfer the spin state of a split Cooper pair onto the polarization state of a pair of optical photons. We show that the produced photon pairs can be used to violate a Bell inequality, unambiguously demonstrating the entanglement of the split Cooper pairs. 
Simon E. Nigg, Rakesh P. Tiwari, Stefan Walter, Thomas L. Schmidt
cond-mat.mes-hall cond-mat.supr-con