One of the main goals in semiconductor quantum optics is to make efficient interfaces between photons and atom-like quantum emitters for applications such as quantum memories and single-photon sources. The quantum emitters themselves also need to be positioned near the so-called mode-maximum of nanophotonics devices with nanoscale precision.

“We have now succeeded in directly implanting silicon ions into diamond photonic crystal nanocavities with a precision of about 50 nm and have created SiV centres with nearly perfectly stable optical transitions,” says team leader Dirk Englund of MIT. “This value compares well to the best ‘naturally’ growth-incorporated SiVs. What is more, the emission wavelength of many SiVs has a small spread of just 50 GHz, meaning that all emitters are within 0.01% of each other in the optical spectrum. This implantation method brings us a big step closer to a nearly perfect quantum emitter system that can be positioned where needed on scalable quantum photonic circuits.”

Millions of quantum emitters over a wafer-scale sized sample

A silicon-vacancy centre is a special defect in diamond that strongly interacts with light. The nanophotonic diamond cavity, which is essentially made of special nanoscale mirrors, guides photons using the same principle as optical fibres to increase the interaction strength even more by bouncing the light back and forth over the SiV centre thousands of times. The internal states of the centre in such a cavity can be controlled by the flow of light.

“The technique we employed is able to create millions of these quantum emitters over a wafer-scale-sized sample,” says team member and lead author of the new study, Tim Schroder, “and these emitters generate coherent, indistinguishable single photons.”

Focused ion-beam nanoImplanter

The technique relies on using a focused ion-beam “nanoImplanter” to generate a tight beam of silicon ions in a way that is similar to how electrons are guided in a scanning electron microscope, he explains. “We then steer this beam of silicon ions to precisely target a diamond nanostructure,” explains Schröder. “The silicon atoms penetrate a controlled distance of about 100 nm into the diamond and after implantation we anneal the sample at about 1000°C. This causes the lattice defects in the material to diffuse around the sample and they can then merge with the Si ions to form the SiV centres. Since the Si ions stay exactly where we deposited them, we can create SiVs with a spatial precision of about 40 nm.”

According to the researchers, the SiVs could be integrated into optical nanostructures and photonic crystal cavities as an interface between photons and quantum memories. This would further increase their applicability for quantum applications, including quantum secure communications, metrology and computing, says Schröder.

On-chip quantum entanglement

“Recent years have seen tremendous progress in quantum information processing systems based on such atomic quantum memories connected by photons,” he tells nanotechweb.org. “In particular, atom-like quantum emitters in semiconductors have made great advances and researchers recently succeeded in making the first ever quantum repeater links and demonstrated the first quantum entanglement on-chip.”

The MIT–Sandia–Harvard team says that it would now like to try and couple several SiVs in a quantum network-like architecture. “By connecting several photonic crystal cavities with embedded SiVs via hybrid waveguides, we might also be able to realize on-chip entanglement of more than two SiVs,” says Schröder.

The research will be detailed in Nature Communications. A pre-print of the paper is on the arXiv server.