A Bristol-led team of physicists has found a way to operate chip-scale photonic sensors at the quantum limit. The key to this breakthrough is the use of ring resonators – tiny racetrack structures that guide light in a loop and maximize its interaction with the sample under study. Importantly, ring resonators can be mass manufactured using the same processes as the chips in our computers and smartphones. Employing this technology to sense absorption or refractive index changes can be used to identify and characterise a wide range of materials and biochemical samples, with topical applications from monitoring greenhouse gases to cancer detection.