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...

Rare-earth-doped laser materials show strong prospects for quantum information storage and processing, as well as for biological imaging, due to their high-Q 4f↔4f optical transitions. However, the inability to optically detect single rare-earth dopants has prevented these materials from reaching their full potential. Here we detect a single photostable Pr3+ ion in yttrium aluminium garnet nanocrystals with high contrast photon antibunching by using optical upconversion of the excited state population of the 4f↔4f optical transition into ultraviolet fluorescence. We also demonstrate on-demand creation of Pr3+ ions in a bulk yttrium aluminium garnet crystal by patterned ion implantation. Finally, we show generation of local nanophotonic structures and cell death due to photochemical effects caused by upconverted ultraviolet fluorescence of praseodymium-doped yttrium aluminium garnet in the surrounding environment. Our study demonstrates versatile use of rare-earth atomic-size ultraviolet emitters for nanoengineering and biotechnological applications.

Photonic structures in diamond are key to most of its application in quantum technology. Here, we demonstrate tapered nano-waveguides structured directly onto the diamond substrate hosting shallow-implanted nitrogen vacancy (NV) centers. By optimization based on simulations and precise experimental control of the geometry of these pillar-shaped nano-waveguides, we achieve a net photon flux up to ~ 1.7 × 10 6 /s. This presents the brightest monolithic bulk diamond structure based on single NV centers so far. We observe no impact on excited state lifetime and electronic spin dephasing time (T 2 ) due to the nanofabrication process. Possessing such high brightness with low background in addition to preserved spin quality, this geometry can improve the current nanomagnetometry sensitivity ~ 5 times. In addition, it facilitates a wide range of diamond defects-based magnetometry applications. As a demonstration, we measure the temperature dependency of T 1 relaxation time of a single shallow NV center electronic spin. We observe the two-phonon Raman process to be negligible in comparison to the dominant two-phonon Orbach process. KEYWORDS: shallow nitrogen vacancy center, diamond tapered nanopillar, nanofabrication, T 2 dephasing time, low temperature T 1 relaxation timeDiamond defect centers are exquisite nanoscale sensors for a variety of physical parameters like magnetic 1 and electric 2 fields and temperature 3 . Among other parameters their sensitivity relies on proximity and photon detection efficiency. Nuclear magnetic resonance (NMR) experiments were recently shown 4-8 using the negatively-charged nitrogen vacancy (NV) center positioned few nanometers below the diamond surface ("shallow" NV center). These experiments became possible by the ability to optically address and readout spins of the NV center 9 . Yet, a major drawback of all magnetometry-based experiments with shallow NV centers is the low number of collected photons which causes long measurement times. At the same time, the photon count rate (F) also limits the magnetometry sensitivity 10 as it scales with1/√F.A major reason for the low signal strength is the high refractive index mismatch 11 between diamond (n diamond ~ 2.4) and the collection medium (e.g. air; n air ~ 1). This causes most of the emitted photons from the NV center to be reflected back at the diamond-air interface into the diamond substrate. Even by benefiting from high NA microscope objective lenses, collection efficiencies are typically 12 below 10% resulting in total instrument detection efficiency on the order of 1%. Recently, efforts have been made to overcome this drawback by fabricating photonic structures directly onto the diamond surface in order to enhance the collection efficiency of either deep [12][13][14][15][16][17] or shallow NV centers 18 . Maletinsky et al. 18 demonstrated a monolithic diamond scanning tip based on nanopillars hosting NV centers ~ 10 nm below the nanopillars top surface. Such tips were fabricated from diamond nanopillars with a uniform di...

Imaging twisty magnets Twisting monolayers of graphene with respect to each other has led to a number of unusual correlated states. This approach has inspired researchers to try their hand at twisting two-dimensional (2D) magnets, but such experiments have proven a difficult challenge. Song et al . made structures out of layers of the 2D magnet chromium triiodide with a small twist angle (see the Perspective by Lado). Using nitrogen vacancy centers in diamond as a magnetometer, the authors imaged the magnetic domains in both twisted monolayer and twisted trilayer structures. For twisted trilayers, a periodic pattern of ferromagnetic and antiferromagnetic domains was revealed. —JS