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Advertising and content can be personalised based on your profile. Your activity on this service can be used to build or improve a profile about you for personalised advertising and content. Advertising and content performance can be measured. Reports can be generated based on your activity and those of others. Your activity on this service can help develop and improve products and services. Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Room-temperature MASERs (Microwave Amplification by Stimulated Emission of Radiation) amplify electromagnetic waves at microwave frequencies with minimal noise. We demonstrate a cost-effective LED-pumped maser using pentacene-doped para-terphenyl as the gain medium. Here, we show that LED light, which is brightness-enhanced and guided via a cerium-doped yttrium aluminium garnet luminescent concentrator, achieves persistent maser emission at 1.45 GHz with a duration of 200 µs and a microwave output power of 0.014 mW, surpassing previous non-laser pumped systems. Operating at low voltage, the LED-pumped maser ensures safety, reduced costs, and simple integration. Potential applications include sensitive magnetic resonance imaging, portable atomic clocks, quantum technologies, and enhanced deep-space radio astronomy. Masers can detect and amplify weak microwave signals while introducing minimal noise1 (Deep Space Communication and Navigation Series, Jet Propulsion Laboratory, Caltech, 2008).") . Although they have many potential applications, their usage has been limited to only a few specialised areas due to the requirement for vacuum, high magnetic fields, and extremely low temperatures2.") ,3.") . The essential components of a maser include a gain medium, an excitation source (pump), and a resonant cavity. Recent advances have enabled room-temperature masers using organic crystalline materials, such as pentacene-doped para-terphenyl (PcPTP), coupled with cylindrical resonators and laser or lamp pumping4.") ,5.") ,6.") . Additionally, progress in miniaturised laser technology has facilitated the creation of compact laser-pumped masers7.") . Here, we report an alternative pumping system based on light-emitting diodes (LEDs), which offers several distinct advantages over traditional laser or lamp-based excitation sources. By directly coupling the LED output to the gain medium via a cerium-doped yttrium aluminium garnet (Ce:YAG) luminescent concentrator (LC), the design eliminates the need for complex optical alignment, enhancing simplicity and reproducibility. Moreover, the LED-based approach is safer, operating at low voltage and intensity, and is more cost-effective, utilising widely available components. Specifically, LED pumping offers an energy-efficient approach, minimises thermal loads in the gain medium compared to laser pumping, and ensures sufficient spectral overlap and photon density to drive quantum transitions in the pentacene-doped gain medium. By eliminating the need for high-power lasers and flashlamps, LED-pumped masers simplify both design and operation, broadening the practical applications of room-temperature maser technology. These features make LED-pumped masers particularly attractive for applications requiring scalability and cost-effectiveness. We address issues related to efficiency, thermal management, quantum mechanical considerations, and limitations of the existing pump systems. Driven by the lighting market, visible LEDs have been making steady progress for 25 years. Now, LEDs feature an even longer operating lifetime than laser diodes ( 50,000 hrs) and a drastically lower cost. Considering LED costs are below 0.5 $/W and continuously decreasing13.") , LED pumping of masers appea