  The state of a two-particle system is called entangled when its quantum
mechanical wave function cannot be factorized in two single-particle wave
functions. Entanglement leads to the strongest counter-intuitive feature of
quantum mechanics, namely nonlocality. Experimental realization of quantum
entanglement is relatively easy for the case of photons; a pump photon can
spontaneously split into a pair of entangled photons inside a nonlinear
crystal. In this paper we combine quantum entanglement with nanostructured
metal optics in the form of optically thick metal films perforated with a
periodic array of subwavelength holes. These arrays act as photonic crystals
that may convert entangled photons into surface-plasmon waves, i.e.,
compressive charge density waves. We address the question whether the
entanglement survives such a conversion. We find that, in principle, optical
excitation of the surface plasmon modes of a metal is a coherent process at the
single-particle level. However, the quality of the plasmon-assisted
entanglement is limited by spatial dispersion of the hole arrays. This spatial
dispersion is due to the nonlocal dielectric response of a metal, which is
particularly large in the plasmonic regime; it introduces "which way" labels,
that may kill entanglement.
