Research team achieves breakthrough in quantum communication, successfully generates photon pairs with 90% similarity

Researchers from the Universities of Basel, Ruhr, and Paderborn have joined forces to achieve a major breakthrough in quantum communication, successfully generating photon pairs that are 90% identical. These nearly indistinguishable particles can be used to realize quantum entanglement or interference, phenomena crucial to quantum communication. Understanding of quantum physics opens new frontiers for computing and sensing. Scientists are using different materials to store and process information at the quantum level, employing various methods including trapped ions, neutral atoms, superconducting circuits, and photons as quantum bits or qubits. Photons are among the easiest information carriers to manipulate.

However, to use them in quantum communication, they must possess completely identical characteristics or be "indistinguishable" from one another. Previous attempts to generate such photons met with limited success, because the photons were either poorly focused or only temporarily correlated.

Breakthrough Photon Generation Technology

The collaborative effort of researchers from Basel, Ruhr, and Paderborn has overcome this obstacle through semiconductor quantum dots. The researchers used semiconductor quantum dots with optical cavities, employing a process known as "biexciton decay" to generate nearly indistinguishable photons. In this process, a molecule composed of two bound excitons decays, releasing an exciton and a photon. "Quantum dots are typically described as artificial atoms in semiconductors that can generate single photons," explains Stefan Schumacher, professor of functional photonic structures at Paderborn University. "By integrating them into a specialized optical cavity — similar to the cavity in a laser — this work particularly accelerates and controls the light emission process."

An exciton consists of an electron and an electron hole. In this process, the quantum dot is doubly excited, after which the excitation decays. Consequently, the biexciton cascade generates two photons, one after the other. Importantly, the photon pairs can be generated at the push of a button, which is highly useful for the deployment of next-generation technologies.

Optimization Methods for Photon Purity

The research team further investigated the interaction between quantum dots and optical cavities, work led by Richard Warburton, professor of experimental condensed matter physics at the University of Basel. While this work helped achieve breakthrough progress in various areas, the joint research team utilized the science of biexciton decay to make it proceed in a controllable manner. This helped the researchers achieve 90% indistinguishability, an improvement over the previously achieved 60%. Higher photon indistinguishability means fewer errors in data processing. "The excellent agreement of the results with theoretical predictions points the way toward generating photons with even higher indistinguishability," Warburton added in a press release.

"They proved that biexciton decay can produce very high-quality photons — provided the system is well controlled using a cavity."

Klaus Jöns, head of hybrid quantum photonic devices at Paderborn University, said: "We found that the purity of the generated photons can also be optimized via the cavity, limited only by vibrations in the semiconductor lattice — phonons. This phenomenon, known as 'cavity feeding,' must be taken into account in future designs and can be systematically minimized further." The research results have been published in the journal Physical Review Applied.

How New Technology Advances Quantum Communication

This research demonstrates the potential of semiconductor quantum dots in quantum communication, particularly through biexciton decay to generate high-quality photons. As photon indistinguishability improves, this will help enhance the reliability of quantum communication and reduce errors in data processing. The research team's achievements not only advance quantum communication technology but may also influence future quantum computing and sensing technologies, as these fields all rely on efficient qubit operations.