The search for stable, pure, coherent quantum sources is an increasingly active area of research for use in photonic quantum computing, such as qubit transmission nodes and quantum logic gates^[1]. In the past decade, several types of single emitters have been studied, ranging from colloidal quantum dots^[2], embedded quantum dots in nanowires^[3], laser written defects in bulk crystals^[4] to name a few. Yet, quantitative comparison between these materials remains challenging due to lack of a suitable reference when considering the many variables of experimental equipment. This can be addressed by using reference single emitters as a benchmark material with known properties – specifically the second order degree of coherence, g⁽²⁾(τ) – which exhibits a g⁽²⁾(0) < 0.5 and emits at a known countrate at saturation to act as a robust source of brightness, i.e. a ground truth standard candle.

An ideal candidate for this application is the nitrogen-vacancy (NV^-) colour centre in diamond, due to their stability, ease of fabrication and room-temperature single emitter emission.^[5] This work aims to explore the feasibility of using NV centres in nanodiamonds (NDs) in a cross-laboratory study to identify single emitting NV centres indicated by a g⁽²⁾(0) < 0.5 that emit at a known countrate at saturation. Thousands of NDs were initially identified with a high-throughput Hanbury Brown-Twiss (HBT) confocal microscopy setup, where the search was narrowed down to candidates containing single NV^‑ centres with g⁽²⁾(0) < 0.5. The fingerprint character of these candidate NDs were recovered at a second laboratory site through correlating g⁽²⁾(0), saturation power, countrates and other parameters using different experimental apparatus, thus verifying the approach of using NV^- centres as reference emitters. As a result, we propose the use of stable NV^- centres in nanodiamonds as a reference material for quantum metrology and as a benchmark for measurements on less stable and/or novel quantum materials.

[1] L. Zhai, Nat. Nanotechnol. 17, 829–833 (2022)
[2] A. H. Proppe, Nat. Nanotechnol. 18, 993–999 (2023)
[3] M. E. Reimer, Nat Commun 3, 737 (2012)
[4] B. D. Wood, Phys. Rev. B 105, 205401 (2022)
[5] P. R. Dolan, Opt. Express 26, 7056-7065 (2018)