Placing single-photon emitters where they are needed

Quantum computers are expected to provide new approaches to database searches, AI systems, simulations and more. To achieve such novel quantum technology applications, photonic integrated circuits which can effectively control photonic quantum states — the so-called qubits — are needed. Physicists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), TU Dresden and Leibniz-Institut für Kristallzüchtung (IKZ) have demonstrated the controlled creation of single-photon emitters in silicon at the nanoscale, as reported in Nature Communications.

Photonic integrated circuits (PICs) utilise particles of light, better known as photons, as opposed to electrons that run in electronic integrated circuits. The main difference between the two is that a photonic integrated circuit provides functions for information signals imposed on optical wavelengths typically in the near infrared spectrum.

Dr Georgy Astakhov, Head of Quantum Technologies at HZDR’s Institute of Ion Beam Physics and Materials Research, said PICs with many integrated photonic components are able to generate, route, process and detect light on a single chip. “This modality is poised to play a key role in upcoming future technology, such as quantum computing. And PICs will lead the way,” Astakhov said.

Before, quantum photonics experiments used ‘bulk optics’ that were distributed across the optic table and occupied the entire lab. Now, photonic chips are changing this landscape. Miniaturisation, stability and suitability for mass production might make them the workhorse of modern-day quantum photonics. Monolithic integration of single-photon sources in a controllable way would give a resource-efficient route to implement millions of photonic qubits into PICs. To run quantum computation protocols, these photons must be indistinguishable. With this, industrial-scale photonic quantum processor production would become feasible. However, the currently fabrication method stands in the way of the compatibility of this concept with today’s semiconductor technology.

In a first attempt reported two years ago, the researchers generated single photons on a silicon wafer, but only in a random and non-scalable way. Physicist Dr Nico Klinger said the recent research demonstrated how focused ion beams from liquid metal alloy ion sources are used to place single-photon emitters at desired positions on the wafter while obtaining a high creation yield and high spectral quality.

The scientists at HZDR subjected the same single-photon emitters to a material testing program. After several cooling-down and warming-up cycles, they did not observe any degradation of their optical properties. These findings meet the preconditions required for mass production later on. To translate these findings into a widespread technology, and allow for wafer-scale engineering of individual photon emitters on the atomic scale compatible with established foundry manufacturing, the researchers implemented broad-beam implantation in a commercial implanter through a lithographically defined mask.

“This work really allowed us to take advantage of the state-of-the-art silicon processing cleanroom and electron beam lithography machines at the Nano Fabrication facility Rossendorf,” said Dr Ciarán Fowley, Cleanroom group leader.

Using both methods, the team can create dozens of telecom single-proton emitters at predefined locations with a spatial accuracy of about 50 nm. They emit in the telecommunication O-band and exhibit stable operation over days under continuous-wave excitation.

This realisation of controllable fabrication of single-photon emitters in silicon makes it a promising candidate for photonic quantum technologies, with a fabrication pathway compatible with large-scale integration. These single-photon emitters are now technologically ready for production in semiconductor fabrication plants and incorporation into the existing telecommunications infrastructure.

Top image caption: Controlled generation of single-photon emitters in silicon (red) by broad-beam implantation of ions (blue) through a lithographically defined mask (left) and by a scanned focused ion beam (right). Symbolically shown: The emission of two single photons at locations defined for this purpose by the process. In the background: An electron beam creates holes in the lithographic mask made of acrylate. Image credit: M. Hollenbach, B. Schröder/HZDR