Nonlocal entanglement is crucial for quantum information processes and has been demonstrated for photons, while it is much more difficult to demonstrate for electrons. One approach is to use a superconductor since two electrons in a superconductor are bound to form a spin-entangled pair called Cooper pair, which is a natural source for nonlocal entangled electrons. For a three terminal normal metal/superconductor/normal-metal device, it is predicted that Cooper pairs can split into spin entangled electrons in two spatially separated normal-metals, resulting in a negative nonlocal resistance and a positive current-current correlation, the former of which has been observed, but the latter of which has not been measured. Here we show that both can be observed, consistent with Cooper pair splitting; moreover, the efficiency of splitting can be tuned by independently controlling the energy of electrons passing the two superconductor/normal-metal interfaces, which may lead to better understanding and control of nonlocal entanglement.
Nature Physics 6, 494 - 498 (2010)