TUM Creates Single-Photon Sources for Quantum Tech

Instead of amplifying the desired frequency, the researchers selectively suppress unwanted frequencies. Until now, so-called resonators have mostly been used to produce single photons-tiny optical structures that influence the photon sources in such a way that they emit light predominantly at a specific frequency. However, these resonators only function within a narrow frequency range and must be precisely tuned to the respective photon source.

Researchers at TUM and MCQST have therefore developed a new approach and are taking the opposite route. Instead of causing the emitters to emit more light at a specific frequency, they adapt the emitter's environment so that less light is emitted at unwanted frequencies. To do this, they use photonic crystal waveguides. These are nanostructures that, through regularly arranged patterns, block pathways through which a photon source can emit light. The team designs the photonic crystal waveguides so that they suppress only unwanted light frequencies while preserving the desired ones.

Initial experiments confirm the technology's effectiveness: using photonic crystal waveguides, they were able to increase the proportion of desired photons in the emitted light from about 23% to around 72%. This means the new method achieves results that were previously only attainable with significantly more complex resonator approaches.

With the new approach, photon generation also occurs slightly more slowly than before. This, too, is important for quantum communication: "If photons are generated too quickly, it's difficult for us to control their properties," explains Andreas Reiserer, professor of quantum networks at TUM. "Our approach is therefore significantly better suited for many emitters than the resonators used to date."

The researchers conducted their initial experiments using erbium as the photon source-an element that is already used in fiber-optic technologies today.

Because the crystal waveguides have larger emitters and a broader bandwidth, this results in two key advantages. First, multiple photon sources-known as emitters-can be used simultaneously within a single device. With resonators, this is only possible to a limited extent due to their very small size. Second, the desired frequency can be selected more flexibly, since photonic crystal waveguides do not need to be precisely tuned to each emitter, unlike classical resonators.

Florian Burger, doctoral student and the first author of the publication, offers a look ahead:

"Quantum networks are expected to connect many quantum systems with one another one day. This requires interfaces that can reliably transfer information from a quantum system to individual photons and then transmit them, for example, via optical fibers. Our work lays the foundation for this."