Surface plasma waves are collective oscillations of electrons that propagate along a metaldielectric interface [1]. In the last ten years, several groups have reproduced fundamental quantum optics experiments with surface plasmons. Observation of single-plasmon states [2,3], waveparticle duality [4,5], preservation of entanglement of photons in plasmon-assisted transmission [6][7][8], and more recently, two-plasmon interference have been reported [3,[9][10][11][12]. 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 Refs. [13,14], 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.Two-particle interference, as a fundamental quantum feature, has been extensively studied with photons through the Hong-Ou-Mandel [15] dip and has been recently observed with guided plasmons in a large variety of plasmonic circuits. It showed the possibility to generate pairs of indistinguishable single plasmons (SPPs), which is an important requirement for potential quantum information applications [12,16,17]. Despite the presence of losses, these experiments have shown that quantum effects remain observable. In these experiments, the propagation paths were lossy, but the beamsplitters were non-lossy. The presence of losses on the beamsplitter was studied in Refs [13,14], where novel effects were predicted, including coherent absorption of single photon and N00N states [18,19]. In our work, we designed several plasmonic beamsplitters with different sets of reflection and transmission factors, that were used in a plasmonic version of the Hong-Ou-Mandel experiment. Depending on the samples, coincidences detection measurements lead either to a HOM-like dip, i.e. a signature of plasmon coalescence, or a HOM-peak, that we associate to plasmon anti-coalescence. Let first begin with a brief description of the experimental setup. It is based on a source of photon pairs. The photons of a given pair are sent to two photon-to-SPP converters, located at the surface of a plasmonic test platform. It has been shown recently that the photon arXiv:1610.07479v1 [quant-ph]
