  Nonlocal entanglement is crucial for quantum information processes. While
nonlocal entanglement has been realized for photons, it is much more difficult
to demonstrate for electrons. One approach that has been proposed is to use
hybrid superconducting/normal-metal devices, where a Cooper pair splits into
spin-entangled electrons in two spatially separated normal-metal leads. This
process of nonlocal Andreev reflection is predicted to lead to a negative
nonlocal resistance and positive current-current correlation. By
cross-correlation measurements as well as measurements of the local and
nonlocal resistance, we present here experimental evidence showing that by
independently controlling the energy of electrons at the
superconductor/normal-metal interfaces, nonlocal Andreev reflction, the
signature of spin-entanglement, can be maximized.
