Working in close collaboration, researchers from Paderborn University, the University of Basel and Ruhr University Bochum have made a significant breakthrough in the field of quantum communication. In their recently published paper in the prestigious journal Physical Review Letters, they demonstrate how special semiconductor nanostructures can be used to generate individual photons and pairs of photons that are almost perfectly identical. These 'indistinguishable' particles form the basis for quantum entanglement and quantum interference.
Biexciton decay
In quantum information processing, photons are the ideal carriers of information. However, in order to use these light particles for complex calculations, they must possess exactly the same properties – an aspect known as 'indistinguishability'. Until now, such sources have suffered from the fact that the photons generated were temporally correlated or out of focus, which greatly reduced their indistinguishability and thus their quality. A team of doctoral candidates from Basel and Paderborn has now solved this problem using a process known as 'biexciton decay' in semiconductor quantum dots within an optical resonator. This is a process in which a molecule consisting of two bound excitons (each a pair comprising an electron and an electron hole) decays, leaving behind a single exciton and 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," explains lead author Timon Baltisberger from the 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 specialised optical cavity – similar to that found in a laser – the light emission process was specifically accelerated and controlled in this study," explains Prof. Dr Stefan Schumacher, head of the 'Theory of Functional Photonic Structures' research group at the Department of Physics and the Institute for Photonic Quantum Systems (PhoQS) at Paderborn University.
Over the past few years, Prof. Richard Warburton's research group at the University of Basel has conducted intensive research into the interaction of these quantum dots with optical cavities and has made groundbreaking progress in various fields. The researchers have now been able to apply this expertise to the bi-exciton in order to accelerate its decay in a controlled manner. This results in photons of much higher quality: they are 90 per cent indistinguishable, whereas without this effect the figure is merely 60 per cent – a striking improvement. "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," said Prof. Warburton.
Purity is adjustable
The joint work with Dr Arne Ludwig from Ruhr University Bochum also provided deep insights into the underlying physical mechanisms. "We have found that the purity of the photons generated can also be optimised 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 minimised even further," explains 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 indistinguishable and purer the photons generated are, the lower the error rate in data processing. Quantum dots are regarded as a promising technology for the mass production of such photons.
About the paper: https://doi.org/10.1103/t8sk-b2w4
Initial detailed findings on the single-photon source application described in this paper can be found in a further article, which was published in the journal *Physical Review Applied* as an "Editors' Suggestion": https://doi.org/10.1103/78c9-j817
