Innsbruck develops new technique to improve multi-photon state generation

19 Aug 2025

Described as “significant progress in making quantum dots more practical for real applications”.

Quantum dots – semiconductor nanostructures that can emit single photons on demand – are considered among the most promising sources for photonic quantum computing.

However, every quantum dot is slightly different and may emit a slightly different color, according to a team at the University of Innsbruck, Austria, which has developed a technique to improve multi-photon state generation. The Innsbruck team states that, “the different forms of quantum dot means that, to produce multi-photon states we cannot use multiple quantum dots.”

Usually, researchers use a single quantum dot and multiplex the emission into different spatial and temporal modes, using a fast electro-optic modulator. But a contemporary technological challenge: faster electro-optic modulators are expensive and often require very customized engineering. To add to that, it may not be very efficient, which introduces unwanted losses in the system.

‘Sidestepping the limitations’

The international research team, led by Vikas Remesh from the Photonics Group at the Department of Experimental Physics at Innsbruck and involving researchers from the University of Cambridge, Johannes Kepler University Linz, and others, has now demonstrated what they call “an elegant solution that sidesteps these limitations”.

The achievement is described in Nature (NPJ Quantum Information).

The Innsbruck approach uses a purely optical technique called stimulated two-photon excitation to generate streams of photons in different polarization states directly from a quantum dot without requiring any active switching components. The team demonstrated their technique by generating high-quality two-photon states with excellent single-photon properties.

“The method works by first exciting the quantum dot with precisely timed laser pulses to create a biexciton state, followed by polarization-controlled stimulation pulses that deterministically trigger photon emission in the desired polarization,” said Yusuf Karli and Iker Avila Arenas, the study’s first authors.

“It was a fantastic experience for me to work in the photonics group for my master’s thesis, said Iker Avila Arenas, who was part of 2022-2024 cohort of the Erasmus Mundus Joint Master’s program in Photonics for Security Reliability and Safety and spent six months in Innsbruck.

“What makes this approach particularly elegant is that we have moved the complexity from expensive, loss-inducing electronic components after the single photon emission to the optical excitation stage, and it is a significant step forward in making quantum dot sources more practical for real-world applications,” said Vikas Remesh, the study’s lead researcher. Looking ahead, the researchers envision extending the technique to generate photons with arbitrary linear polarization states using specially engineered quantum dots.

‘Immediate applications’

“The study has immediate applications in secure quantum key distribution protocols, where multiple independent photon streams can enable simultaneous secure communication with different parties, and in multi-photon interference experiments which are very important to test even the fundamental principles of quantum mechanics,”said Gregor Weihs, head of the photonics research group in Innsbruck.

The research represents a collaborative effort involving expertise in quantum optics, semiconductor physics, and photonic engineering. The work was supported by the Austrian Science Fund (FWF), the Austrian Research Promotion Agency (FFG), and the European Union’s research programs.