  We discuss several scenarios for the creation of nonlocal spin-entangled
electrons which provide a source of electronic Einstein-Podolsky-Rosen (EPR)
pairs. The central idea is to exploit the spin correlations naturally present
in superconductors in form of Cooper pairs. We show that nonlocal
spin-entanglement in form of an effective Heisenberg spin interaction is
induced between electron spins residing on two quantum dots with no direct
coupling between them but each of them being tunnel-coupled to the same
superconductor. We then discuss a nonequilibrium setup where mobile and
nonlocal spin-entanglement can be created by coherent injection of two
electrons in an Andreev tunneling process into two spatially separated quantum
dots and subsequently into two Fermi-liquid leads. The current for injecting
two spin-entangled electrons into different leads shows a resonance whereas
tunneling via the same dot into the same lead is suppressed by the Coulomb
blockade effect of the quantum dots. The Aharonov-Bohm oscillations in the
current are shown to contain h/e and h/2e periods. Finally we discuss a
structure consisting of a superconductor weakly coupled to two separate
Luttinger liquid leads. We show that strong correlations again suppress the
coherent subsequent tunneling of two electrons into the same lead, thus
generating again nonlocal spin-entangled electrons.
