  Surface plasma waves are collective oscillations of electrons that propagate
along a metal-dielectric interface. In the last ten years, several groups have
reproduced fundamental quantum optics experiments with surface plasmons.
Observation of single-plasmon states, waveparticle duality, preservation of
entanglement of photons in plasmon-assisted transmission, and more recently,
two-plasmon interference have been reported. While losses are detrimental for
the observation of squeezed states, they can be seen as a new degree of freedom
in the design of plasmonic devices, thus revealing new quantum interference
scenarios. Here we report the observation of two-plasmon quantum interference
between two freely-propagating, non-guided SPPs interfering on lossy plasmonic
beamsplitters. As discussed in the article "Quantum optics of lossy beam
splitters" by Barnett et al. (Phys. Rev. A 57, 2134 (1998)) , the presence of
losses (scattering or absorption) relaxes constraints on the reflection and
transmission factors of the beamsplitter, allowing the control of their
relative phase. By using this degree of freedom, we are able to observe either
coalescence or anticoalescence of identical plasmons.
