Sunlight is like a continuous stream of balls flying at a wall. Antenna proteins in photosynthetic bacteria catch these "balls" (photons), but sometimes the impact is too strong, and a dangerous vibration, a triplet state, arises in the "wall," a chlorophyll molecule. If not quenched, it can generate aggressive singlet oxygen that would burn down the whole structure. Carotenoid molecules act as fire extinguishers here: they take the dangerous energy onto themselves and dissipate it as heat.

Until now, it was unclear exactly how the carotenoid receives this energy — whether directly from chlorophyll or by generating triplets itself through a process akin to splitting one strong pulse into two weak ones (singlet fission). Spectral noise obscured the answer: signals from different processes overlapped like voices in a crowded room. The authors used a method comparable to tuning a radio to different frequencies. Using femtosecond stimulated Raman spectroscopy under different resonance conditions, they separated the overlapping signals. It turned out that in the LH2 protein of Rhodoblastus acidophilus, the carotenoid triplet is born almost exclusively via energy transfer from bacteriochlorophyll, not through singlet fission.

Interestingly, the protein hosted an entangled triplet pair that lived eight times longer than in liquid solution. But this delay did not lead to separation into two independent triplets or trigger a harmful reaction with chlorophyll. Nature has built a reliable safety catch: the energy escapes along a single safe route, like water through an emergency spillway of a dam, without damaging the station.