  Quantum superpositions and entanglement are at the heart of the quantum
information science. There have been only a few investigations of these
phenomena at the mesoscopic level, despite the fact that these systems are
promising for quantum state storage and processing. Here we present two novel
experiments with surface plasmons propagating on cm-long metallic stripe
waveguides. We demonstrate that two plasmons can be entangled at remote places.
In addition, we create a single plasmon in a temporal superposition state: it
exists in a superposition of two widely separated moments. These quantum
states, created using photons at telecom wavelength, are collectively held by a
mesoscopic number of electrons coding a single quantum bit of information; They
are shown to be very robust against decoherence.
