New quantum optics technique sheds light on polariton interactions

Could future computers be powered by light particles? A new quantum optics technique developed by EQUS researchers at Macquarie University could help make this a reality.

An international scientific collaboration led by EQUS Chief Investigator Prof Thomas Volz (Macquarie University) and his team has introduced a new quantum optics technique that can provide unprecedented access to the fundamental properties of light–matter interactions in semiconductors.

This breakthrough represents a significant step in quantum photonics.

Understanding light–matter interactions

The research, published in Nature Physics, uses a novel spectroscopic technique to explore interactions between photons and electrons at the quantum scale.

Thomas Volz, senior author of the study, says the work has the potential to drive a breakthrough in the global quest for accessible quantum photonic technologies.

“We have developed a new technique that uses the radiative quantum cascade, where photons stored in a material journey down a ladder of energy levels generated when light and matter interact,” says Thomas.

“This applies even when the interactions are so weak that the resulting energy levels involved were previously too close to distinguish by standard optical spectroscopy techniques.”

This innovative approach addresses one of the key challenges in quantum optics: observing and measuring extremely weak interactions in a controlled manner.

This ability to peer closer into the quantum realm holds immense potential to unlock pathways to novel quantum photonic technologies, such as ultrafast light-matter transistors and single-photon switches.

“By understanding the timing of light particles emissions, we gain valuable insights into the quantum properties of solid materials, such as semiconductors,” Thomas says.

Time-based observations reveal light interactions

The team’s technique, which they dubbed ‘photon-cascade correlation spectroscopy’, combines spectral filtering and photon-correlation analysis to reveal interactions between semiconductor exciton-polaritons, which are quasi-particles made up of both photons (light) and matter (excitons).

Lead author Dr Lorenzo Scarpelli, previously an EQUS Research Fellow at Macquarie University and now postdoctoral researcher at Delft University of Technology in the Netherlands, says, “Photon-cascade correlation spectroscopy operates a little bit like a time-based microscope”.

“We create an image-in-time of the photons and this tells us if they tend to travel together or not, and also allows us to extract information about the strength of their interaction.”

He says that the team’s new technique enabled them to detect interactions that involved complex bound states of three particles, which had previously only been theorised.

This finding is important in quantum optics because it enables scientists to directly excite and measure specific single-photon transitions, allowing them to characterise subtle few-particle quantum effects in solid-state systems and identify materials that could work well in new applications.

“There’s a worldwide search to find materials that allow us to control how light particles interact in these materials, so we can build optical transistors, very fast optical switches and do information processing with single particles of light rather than with electrons.”

“Our experiments use gallium arsenide as the hosting material, but the technique can be easily applied to other materials as well, where we can expect to see similar physics effects or behaviour.”

This adaptability is crucial as researchers search for the ideal platforms for next-generation quantum technologies.

“This technique will allow us to gain valuable insights into the quantum properties of a wide range of solid materials.”

Engineering quantum materials for future technologies

This research sits under the EQUS theme of Designer Quantum Materials, geared at developing new, specially designed and engineered materials that would not naturally exist otherwise.

Such research not only enriches our fundamental understanding of quantum systems but also supports the design of materials and devices optimised for specific quantum behaviours in future technologies.

Portions of this case study were first published by Macquarie University.

This story was first published in the 2024 EQUS annual report.