  Quantum plasmonics lies at the intersection between nanophotonics and quantum
optics. Genuine quantum effects can be observed with non-classical states such
as Fock states and with entangled states. A N00N state combines both aspects:
it is a quantum superposition state of a Fock state with N excitations in two
spatial modes. Here we report the first observation of two-plasmon (N=2) N00N
state interferences using a plasmonic beamsplitter etched on a planar interface
between gold and air. We analyze in detail the role of losses at the
beamsplitter and during the propagation along the metal/air interface. While
the intrinsic losses of the beamsplitter are responsible for the emergence of
quantum nonlinear absorption, we note that N00N states decay N times faster
than classical states due to propagation losses.
