In a cooperative effort, scientists from Paderborn University, the University of Basel, and Ruhr University Bochum have achieved a major advancement in quantum communication. Their new study, published in Physical Review Letters, shows that unique semiconductor nanostructures can produce single photons and nearly identical photon pairs. These ‘indistinguishable’ particles are fundamental for quantum entanglement and quantum interference.
In quantum information processing, photons are an ideal way to transmit information. Nonetheless, to employ these light particles for intricate calculations, they must exhibit identical characteristics. This principle is called ‘indistinguishability.’
Up to this point, the sources used have been hindered because the emitted photons were either temporally correlated or unfocused, significantly reducing their indistinguishability and, consequently, their overall quality.
A team of PhD students in Basel and Paderborn has addressed this issue using a process known as biexciton decay in semiconductor quantum dots inside an optical resonator. In this approach, a biexciton, a state made up of two bound excitons (each an electron–hole pair), decays to a single exciton while emitting a photon.
The so-called ‘biexciton cascade’ in a semiconductor quantum dot emits photons at the push of a button, which are of great interest for modern applications. This is a process in which a quantum dot is doubly excited and the excitation then decays. This generates two photons, one after the other.
Timon Baltisberger, Study Lead Author, University of Basel
Identical Photons at the Push of a Button
A quantum dot is often described as an artificial atom within a semiconductor that can generate individual particles of light. By integrating it into a specialized optical cavity – similar to that found in a laser – the light emission process was specifically accelerated and controlled in this study.
Dr Stefan Schumacher, Head, ‘Theory of Functional Photonic Structures’ Research Group, Department of Physics, Paderborn University
In recent years, Professor Richard Warburton’s research team at the University of Basel has conducted extensive investigations into the interaction between quantum dots and optical cavities, achieving significant advances across multiple disciplines. The researchers have used this knowledge to manipulate the biexciton, enabling controlled acceleration of its decay.
This produces markedly higher-quality photons: they are 90% indistinguishable, compared with 60% without the effect.
The results show excellent agreement with the theoretical prediction and point the way towards generating photons with even higher indistinguishability. They demonstrate that biexciton decay can produce very high-quality photons – provided the system is properly controlled using a cavity.
Richard Warburton, Professor, University of Basel
Purity is Adjustable
Collaborative efforts with Dr. Arne Ludwig from Ruhr University Bochum provided a deeper understanding of the underlying physical mechanisms.
“We have found that the purity of the photons generated can also be optimized using the resonator and is limited only by vibrations in the semiconductor’s crystal lattice (phonons). This phenomenon, known as ‘cavity feeding’, must be taken into account in future designs and can then be systematically minimized even further,” explained Prof. Dr. Klaus Jöns, head of the ‘Hybrid Quantum Photonic Devices’ research group at the Department of Physics and the Institute for Photonic Quantum Systems (PhoQS) at Paderborn University.
The more similar and higher-quality the generated photons are, the lower the error rate in data processing. Quantum dots are viewed as a potential technology for the large-scale production of these types of photons.
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Journal Reference:
Baltisberger, T. L., et al. (2026) Hi Indistinguishable Photons from a Two-Photon Cascade. Physical Review Letters. DOI:10.1103/t8sk-b2w4. https://journals.aps.org/prl/abstract/10.1103/t8sk-b2w4.