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applications in magnetic resonance, quantum computing, quantum optics, and broadband magnetometry., we apply a magnetic field 2 New J. Phys. 16 (2014) 093022 J Scheuer et al 7 New J. Phys. 16 (2014) 093022 J Scheuer et al

Colour centres in nanodiamonds are an important resource for applications in quantum sensing, biological imaging, and quantum optics. Here we report unprecedented narrow optical transitions for individual colour centres in nanodiamonds smaller than 200 nm. This demonstration has been achieved using the negatively charged silicon vacancy centre, which has recently received considerable attention due to its superb optical properties in bulk diamond. We have measured an ensemble of silicon-vacancy centres across numerous nanodiamonds to have an inhomogeneous distribution of 1.05 nm at 5 K. Individual spectral lines as narrower than 360 MHz were measured in photoluminescence excitation, and correcting for apparent spectral diffusion yielded an homogeneous linewidth of about 200 MHz which is close to the lifetime limit. These results indicate the high crystalline quality achieved in these nanodiamond samples, and advance the applicability of nanodiamond-hosted colour centres for quantum optics applications. Nanodiamonds (NDs) hosting optically active point defects ('colour centres') are an important technical material for applications in quantum sensing [1], biological imaging [2][3][4], and quantum optics [5]. One colour centre which has attracted recent attention is the negatively charged silicon vacancy (SiV − ) defect, which consists of a silicon atom taking the place of two adjacent carbon atoms in the lattice [6]. The SiV − centre in diamond has risen to prominence on the basis of its superb spectral properties, including a strong zero-phonon line (ZPL) at 737 nm which contains 70% of the fluorescence from this colour centre [7]. In low-strain bulk diamond, the SiV − centre has exhibited lifetime-limited spectral linewidths at 4 K with no spectral diffusion [8]. These ideal properties have enabled the efficient production of indistinguishable photons from distinct emitters [9]. Recent studies in bulk diamond have shown that the electronic spin coherence time in the SiV − centre is fundamentally limited by fast phonon-induced orbital relaxation in the ground state [10,11]. Small NDs should impose boundary conditions that prevent the availablilty of phonons at the critical frequency, thereby extending coherence time. This has increased the motivation to find well-behaved SiV − centres in the nanodiamond environment.Although SiV − centres have been observed to fluoresce in NDs as small as molecules (1.6 nm) [12], the ND host has always led to less homogeneous photon emission [13][14][15][16]. Some promising results have been recently reported for larger hybrid nanostructures [17], but the obstacle persists for SiV − applications requiring ND environments. Here we report unprecedented optical properties of SiV − colour centres hosted in nanodiamonds. Individual spectral lines close to the lifetime limit were measured for SiV − centres in OPEN ACCESS RECEIVED

Versatile biopolymer coatings for advancing fluorescent nanodiamonds are presented as unique magneto‐optic materials for biomedical applications by Y. Wu, F. Jelezko, T. Weil, and co‐workers on page 6576. The coatings—derived from a human protein—provide high biocompatibility and can be modified with various functional entities. Excellent colloidal and fluorescence stabilities allow monitoring of drug distribution and release processes, offering great prospects for theranostics.

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