  We report on experimental evidences of the preservation of energy-time
entanglement for extraordinary plasmonic light transmission through
sub-wavelength metallic hole arrays, and for long range surface plasmon
polaritons. Plasmons are shown to coherently exist at two different times
separated by much more than the plasmons lifetime. This kind of entanglement
involving light and matter is expected to be useful for future processing and
storing of quantum information.
