I propose to unravel the quantum properties of Surface Plasmon Polaritons (SPPs), thanks to an experimental approach new in France and used by only a couple of groups in Europe. This ambitious project I propose will be conducted at the Institut Néel, in Grenoble (France), as the first project I would develop there as a newly appointed permanent CNRS researcher.SPPs, thanks to their ability to highly confine the electromagnetic field are expected to have applications in quantum optics, computing, and to foster photon-photon interactions at nanometric scales. However, whether they will truly lead to viable applications is controversial at present, because the benefits of SPPs come at the cost of high losses. Very little is known about SPPs in the quantum regime, and my project proposes to fully investigate their potential. The study will use a single photon source using nonlinear processes in a crystal, which produces phonton pairs entangled on the polarisation. The single photons are coupled into single SPPs thanks to a microscopy setup, and to appropriate design of the plasmonic devices. After undergoing the operation performed by the device, the single SPPs are then coupled back into photons and the measurements are performed on these photons. Devices will be produced to generate entanglement on ultrasmall scales and the quality of the entanglement quantified. Experiments will also be conducted under high confinement of the electromagnetic field, which is one of the conditions to reach the most challenging targets of the work: developing SPP-based quantum metrology and produce nanometric single photon switches. Success could lead to the development of novel optical quantum information processing devices of nanometric dimensions.