Diamond has long been sought after as a gemstone without peer. Recently it has been shown to be the only known solid-state host of qubits and single photon sources that operates at room temperature. But although diamond colour centres have been great for scientific demonstrations and have pushed our understanding of quantum physics, they leave much to be desired for practical quantum devices. The most popular diamond single photon source, the nitrogen-vacancy centre (NV), emits light that is spectrally too broad for many practical fibre-based quantum applications, and is limited in its free-space applicability. This has hampered attempts to build quantum key distribution networks based on diamond. Now, thanks to discoveries at the University of Melbourne, ENS Cachan and Quantum Communications Victoria, all that may be about to change.

The team has discovered several new diamond single photon sources which show more than a million single photons per second, with far narrower spectral lines than NV [Simpson et al., Applied Physics Letters (2009) 94, 203107]. One of these very bright centres [Aharonovich et al., to appear in Nano Letters, DOI: 10.1021/nl9014167], believed to be based on chromium, shows ideal two-state emission, a vital milestone for efficient products based on it.

To fabricate the new centres, the team developed a new technique for growing nanodiamonds on a substrate containing the species of interest. “In this way the dopant incorporates during the growth, rather than afterwards by implantation – it gives us great flexibility to try new species in a reproducible way”, said Aharonovich. The technique has been patented, and has also been used to find what they believe to be a new class of hybrid dopants based on silicon-nickel complexes [Aharonovich et al., Physical Review B (2009) 79, 235316].

The new centres emit at around 750-800nm and are much more practical for fibre applications than NV, but there’s still a way to go. “The aim is to find an efficient two-state emitter in the low loss telecommunication window around 1550nm” explains researcher Simpson, “the discovery of these new centres indicates we’re on the right track”.