When light interacts with metal surfaces, it excites electrons, which can form propagating excitation waves called surface plasmon polaritons. These collective electronic excitations can produce strong electric fields localized to subwavelength distance scales 1 , which makes surface plasmon polaritons interesting for several applications. Many of these potential uses, and in particular those related to quantum networks 2 , require a deep understanding of the fundamental quantum properties of surface plasmon polaritons. Remarkably, these collective electron states preserve many key quantum mechanical properties of the photons used to excite them, including entanglement 3,4 and sub-Poissonian statistics 5 . Here, we show that a single-photon source coupled to a silver nanowire excites single surface plasmon polaritons that exhibit both wave and particle properties, similar to those of single photons. Furthermore, the detailed analysis of the spectral interference pattern provides a new method to characterize the dimensions of metallic waveguides with nanometre accuracy.One of the most intriguing experiments of contemporary physics is the double-slit self-interference of single particles. Among the possible quantum systems, photons seem to be ideal for such demonstrations because of their ability to propagate long distances in ambient environment, yet still be efficiently detected. A key requirement for photon self-interference experiments is the availability of a true single-photon source. Light sources such as lasers show intrinsic fluctuation of photon numbers related to Poissonian statistics. Hence, the outcome of double-slit experiments with such light sources can be described classically without introducing the concept of photons (a quantized electromagnetic field) 6 . In the benchmark study realized two decades ago by Grangier and co-workers, true single-photon states emitted from an atomic cascade revealed a clear interference pattern 7,8 . Like photons, surface plasmon polaritons can be used for Young double-slit experiments 9 , and the recent generation of single plasmons by single quantum emitters opens the door for studying their fundamental quantum properties 5 . Here, both antibunching and self-interference are observed using single plasmons excited by a single-photon emitter, and this unambiguously shows that the concept of single-particle self-interference can be applied to surface plasmon polaritons. As this interference arises from an in situ interferometer wherein one beam splitter is the bi-directional emission into the nanowire, and the other beam splitter is the partially transmitting wire output end, it also provides a sensitive diagnostic method to determine nanowire properties. In addition, by choosing spin-selective nitrogen-vacancy colour centres in diamond as the single-photon emitters, we open the door to eventually achieving strong coupling between spins and plasmons, for which 1 3. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart, Germany, 2 Department of Electrical an...

Magneto-Raman-scattering experiments from the surface of graphite reveal novel features associated to purely electronic excitations which are observed in addition to phonon-mediated resonances. Graphene-like and graphite domains are identified through experiments with ∼1  μm spatial resolution performed in magnetic fields up to 32 T. Polarization resolved measurements emphasize the characteristic selection rules for electronic transitions in graphene. Graphene on graphite displays the unexpected hybridization between optical phonon and symmetric across the Dirac point inter Landau level transitions. The results open new experimental possibilities--to use light scattering methods in studies of graphene under quantum Hall effect conditions.