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...

This article reports stable photoluminescence and high-contrast optically detected electron spin resonance (ODESR) from single nitrogen-vacancy (NV) defect centers created within ultrasmall, disperse nanodiamonds of radius less than 4 nm. Unexpectedly, the efficiency for the production of NV fluorescent defects by electron irradiation is found to be independent of the size of the nanocrystals. Fluorescence lifetime imaging shows lifetimes with a mean value of around 17 ns, only slightly longer than the bulk value of the defects. After proper surface cleaning, the dephasing times of the electron spin resonance in the nanocrystals approach values of some microseconds, which is typical for the type Ib diamond from which the nanoparticle is made. We conclude that despite the tiny size of these nanodiamonds the photoactive nitrogen-vacancy color centers retain their bulk properties to the benefit of numerous exciting potential applications in photonics, biomedical labeling, and imaging.

We investigate the effect of surface termination on the charge state of nitrogen vacancy centers, which have been ion-implanted few nanometers below the surface of diamond. We find that, when changing the surface termination from oxygen to hydrogen, previously stable NV − centers convert into NV 0 and, subsequently, into an unknown non-fluorescent state. This effect is found to depend strongly on the implantation dose. Simulations of the electronic band structure confirm the dissappearance of NV − in the vicinity of the hydrogen-terminated surface. The band bending, which induces a p-type surface conductive layer leads to a depletion of electrons in the nitrogenvacancies close to the surface. Therefore, hydrogen surface termination provides a chemical way for the control of the charge state of nitrogen-vacancy centers in diamond. Furthermore, it opens the way to an electrostatic control of the charge state with the use of an external gate electrode.

The nitrogen-vacancy (NV) centre in diamond is a promising candidate for a solid-state qubit. However, its charge state is known to be unstable, discharging from the qubit state NV− into the neutral state NV0 under various circumstances. Here we demonstrate that the charge state can be controlled by an electrolytic gate electrode. This way, single centres can be switched from an unknown non-fluorescent state into the neutral charge state NV0, and the population of an ensemble of centres can be shifted from NV0 to NV−. Numerical simulations confirm the manipulation of the charge state to be induced by the gate-controlled shift of the Fermi level at the diamond surface. This result opens the way to a dynamic control of transitions between charge states and to explore hitherto inaccessible states, such as NV+.

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