Single-photon sources form the basis of quantum communication. Stephan Rinner and Florian Burger are developing nanostructures for this purpose that block unwanted frequencies, thereby significantly improving efficiency. In the experimental setup, laser light is guided through optical fibers to a microscope, where the tiny structures can be visualized.
Photo Credit: Christoph Hohmann / MCQST
Scientific Frontline: Extended "At a Glance" Summary: Single-Photon Sources for Quantum Communication
The Core Concept: A novel method for generating single photons by utilizing photonic crystal waveguides to selectively block unwanted light frequencies, ensuring only the required frequencies are emitted.
Key Distinction/Mechanism: Traditional resonators work by amplifying a desired frequency within a narrow range and must be tuned precisely to individual emitters. In contrast, this new approach uses nanostructures (photonic crystal waveguides) to shape the emitter's environment, blocking the pathways for unwanted frequencies and thus increasing the proportion of desired photons from approximately 23% to 72%.
Major Frameworks/Components:
- Single-Photon Sources (Emitters): The fundamental basis for transmitting data in quantum communication systems.
- Photonic Crystal Waveguides: Micrometer-sized nanostructures with regularly arranged patterns that block specific pathways for light emission, suppressing unwanted frequencies.
- Resonators (Previous Method): Tiny optical structures previously used to force an emitter to produce more light at a specific frequency, limited by small size and narrow bandwidth.
- Erbium: The chemical element used as the photon source in the initial experiments, notable for its existing use in fiber-optic technologies.
Branch of Science: Quantum Physics, Nanotechnology, Photonics, and Optical Engineering.
Future Application: The development of robust interfaces capable of transferring information from complex quantum systems into individual photons for transmission over long distances via optical fibers, forming the backbone of future quantum networks.
Why It Matters: Generating single photons reliably is exceedingly difficult but strictly necessary for quantum communication. This new method is far more scalable than traditional resonators because it allows multiple emitters to be used simultaneously within a single device and does not require precise tuning for each emitter. Additionally, the slightly slower generation of photons allows for better control over their properties, paving the way for the absolutely secure transmission of large amounts of data.
Absolutely secure transmission of large amounts of data: Quantum communication promises many advantages over today’s standard technologies. However, it requires single photons, and generating them is very difficult. Researchers at the Technical University of Munich (TUM) and the Munich Center for Quantum Science and Technology (MCQST) have developed a new method that overcomes the problems of previous approaches.
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. These tiny optical structures 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, 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 nanostructures, through regularly arranged patterns, block pathways through which a photon source can emit light. The team designs the photonic crystal waveguides to suppress only unwanted light frequencies while preserving the desired ones.
The photonic crystal waveguides are only a few micrometers in size. They surround the emitter, thereby preventing the emission of photons at unwanted frequencies and ensuring that only photons of the required frequency are generated.
Threefold Increase in the Proportion of Photons in Emitted Light
Initial experiments confirm the technology’s effectiveness: using photonic crystal waveguides, researchers 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 previously attainable only 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 already used in today's fiber-optic technologies.
Multiple Emitters and Customizability
Because the crystal waveguides can accommodate larger emitters and offer a broader bandwidth, they provide 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, a 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.”
Funding: The project was funded by the Federal Ministry of Education and Research (BMBF) and the Free State of Bavaria as part of the federal and state Excellence Strategy (www.exzellenz.tum.de). This project was also funded by the High-Tech Agenda Bavaria (HTA).
Published in journal: Nature Communications
Title: Inhibited radiative decay enhances single-photon emitters
Authors: Florian Burger, Stephan Rinner, Andreas Gritsch, Kilian Sandholzer, and Andreas Reiserer
Source/Credit: Technische Universität München
Edited by: Scientific Frontline
Reference Number: qs081826_02
