A New Way to Build Single-Photon Sources Could Make Quantum Networks More Practical
Quantum communication has long promised a radically different way to transmit information, using the laws of physics to protect messages and connect quantum computers. Yet one of the field’s most basic requirements remains difficult to achieve: producing individual photons with the right properties, at the right frequency, and with enough consistency to be useful in a real network. Researchers at the Technical University of Munich (TUM) and the Munich Center for Quantum Science and Technology (MCQST) have now demonstrated a new strategy that could make such photon sources more flexible and easier to scale. Instead of forcing a light emitter to produce more photons at one selected frequency, they suppress the frequencies that are not wanted, allowing the desired photons to dominate the output.
Single photons are essential because quantum information can be encoded in properties such as a photon’s frequency, polarization, phase, or arrival time. Unlike a conventional light beam, which contains a continuous stream of many photons, a single-photon source is designed to emit light particles individually and in a controlled manner. This capability is central to quantum key distribution, quantum repeaters, photonic quantum computers, and future networks that could link distant quantum processors. In practice, however, light emitters rarely produce perfectly clean streams of identical photons. They often radiate across several frequencies, creating unwanted signals that can interfere with measurements and reduce the reliability of quantum communication.
The conventional solution has been to place the emitter inside or near a resonator, also known as an optical cavity. These tiny structures are engineered to interact strongly with light at a narrow range of frequencies. When the emitter is correctly aligned with the resonator, the cavity can enhance radiation at the selected frequency, increasing the rate at which useful photons are produced. The approach is powerful, but it comes with a demanding drawback: the resonator must be tuned very precisely to the particular emitter. Even small differences between individual emitters can shift their optical frequencies, making the fabrication and operation of large arrays technically challenging. The resonators also operate over a limited bandwidth, restricting how many sources can be integrated into one device.
The TUM and MCQST team has taken the opposite approach. Rather than amplifying one preferred frequency, the researchers engineered the environment around the emitter to inhibit the frequencies that are not useful. Their device is based on a photonic crystal waveguide, a nanostructure made from a material patterned with a regular arrangement of holes or other repeating features. This periodic structure changes the way light can propagate through the material. Depending on the geometry, certain optical frequencies fall into a photonic band gap, meaning that light at those frequencies cannot easily travel through the structure or escape through particular pathways. By designing the pattern carefully, the researchers were able to block unwanted emission while leaving the desired frequency available.
This effect changes the balance of the light produced by the emitter. In their initial experiments, the proportion of desired photons increased from approximately 23 percent to around 72 percent when the emitter was integrated with the photonic crystal waveguide. In other words, nearly three-quarters of the emitted light occupied the useful spectral range, compared with less than one-quarter before the engineered environment was applied. The result represents roughly a threefold improvement in the share of usable photons. Such spectral purification is important because quantum communication systems must distinguish individual photons with high precision. A cleaner optical signal can reduce filtering requirements, improve detection efficiency, and make it easier to synchronize photons traveling through a network.
The researchers also observed that photon generation became slightly slower with the new design. That may appear to be a disadvantage in systems where speed is highly valued, but it can provide an important benefit for quantum control. If an emitter releases photons too rapidly, there is less time to manipulate their properties or coordinate their emission with other components. Andreas Reiserer, a professor of quantum networks at TUM, explains that the more gradual emission process can be better suited to many types of emitters than the rapid enhancement typically associated with resonators. In a quantum network, reliability and control are often more important than simply maximizing the number of photons released per second.
The first demonstrations used erbium as the photon source. The element is especially attractive for quantum communication because its optical transitions are compatible with technologies already used in fiber-optic networks. Optical fibers transmit information most efficiently in particular telecommunications bands, where signal losses are relatively low over long distances. A photon source based on erbium can therefore provide a potential bridge between solid-state quantum systems and the existing communications infrastructure. Connecting these two worlds is one of the major engineering challenges facing quantum networking: information stored in a stationary quantum system must be converted into individual photons and sent through fiber without losing its quantum character.
The new architecture could also address two limitations of traditional resonators. Photonic crystal waveguides can accommodate comparatively larger emitters, allowing several emitters to operate within the same device. In a resonator, the tiny optical mode volume and narrow operating range can make the simultaneous integration of multiple sources difficult, especially when their emission frequencies are not identical. A waveguide-based structure offers more room and a broader usable bandwidth. It also reduces the need to individually tune the optical environment around each emitter. Instead of building a separate, precisely matched resonator for every source, researchers can design a waveguide that suppresses a wider set of unwanted frequencies while retaining flexibility over the selected output.
That flexibility could become crucial as quantum networks grow beyond laboratory demonstrations. A practical network may need to connect many quantum nodes, each containing different emitters and operating under slightly different conditions. Manufacturing variations, temperature changes, and local electromagnetic environments can all affect an emitter’s frequency. A system that depends on exact resonance with a single cavity may require extensive calibration and active stabilization. The photonic crystal approach does not eliminate the need for precision, but it shifts the design problem from maximizing one narrow emission line to controlling the surrounding optical density of states. In technical terms, the structure inhibits radiative decay into selected modes while preserving emission into the desired mode, giving engineers another way to shape light at the nanoscale.
The work, led in part by doctoral researcher Florian Burger, points toward a new class of quantum-light sources that prioritize spectral cleanliness, scalability, and controllability. The researchers describe the device as a foundation rather than a finished quantum network component, and further development will be needed to improve collection efficiency, integrate control electronics, and demonstrate operation with multiple synchronized emitters. Even so, the central result is striking: by blocking the wrong optical pathways instead of strengthening only the right one, a carefully patterned nanostructure can transform a noisy quantum emitter into a much more useful source of individual photons. As scientists work to connect quantum systems across laboratories, cities, and eventually continents, that change in perspective could prove as important as the photons themselves.
Subject of Research: Single-photon sources, photonic crystal waveguides, and quantum communication
Article Title: Inhibited radiative decay enhances single photon emitters
News Publication Date: 16-Jul-2026
Web References: https://doi.org/10.1038/s41467-026-75489-5
References: Nature Communications, DOI: 10.1038/s41467-026-75489-5
Image Credits: Christoph Hohmann / MCQST
Keywords
Quantum communication, single photons, quantum networks, photonic crystal waveguides, erbium, photon sources, nanostructures, quantum technology, optical fibers, TUM, MCQST
Tags: advances in quantum light source fabricationMunich Center for Quantum Science and Technology researchphoton frequency control in quantum systemsphotonic quantum computer componentspractical applications of single-photon emittersproperties of single photons in quantum infoquantum key distribution technologyquantum networks developmentquantum repeaters for secure communicationscalable quantum light emitterssingle photon sources for quantum communicationsuppression of unwanted photon frequencies
