  We investigate theoretically the simultaneous tunneling of two electrons from
a superconductor into a normal metal at low temperatures and voltages. Such an
emission process is shown to be equivalent to the Andreev reflection of an
incident hole. We obtain a local tunneling Hamiltonian that permits to
investigate transport through interfaces of arbitrary geometry and potential
barrier shapes. We prove that the bilinear momentum dependence of the
low-energy tunneling matrix element translates into a real space Hamiltonian
involving the normal derivatives of the electron fields in each electrode. The
angular distribution of the electron current as it is emitted into the normal
metal is analyzed for various experimental setups. We show that, in a full
three-dimensional problem, the neglect of the momentum dependence of tunneling
causes a violation of unitarity and leads to the wrong thermodynamic (broad
interface) limit. More importantly for current research on quantum information
devices, in the case of an interface made of two narrow tunneling contacts
separated by a distance $r$, the assumption of momentum-independent hopping
yields a nonlocally entangled electron current that decays with a prefactor
proportional to $r^{-2}$ instead of the correct $r^{-4}$.
