Quantum Light Experiment Proves Photosynthesis Starts with a Single Photon

The process that powers much of life on Earth, photosynthesis, is so finely tuned that just one photon is enough to kick it off.

Scientists have long suspected that photosynthesis must be sensitive to individual photons, or particles of light, because despite the way it dominates our days, the sun’s light is surprisingly sparse at the level of individual plant cells. But only now, with the help of quantum physics, have researchers been able to watch a single packet of light begin the process in an experiment described on June 14 in the journal Nature.

“It makes sense that photosynthesis only requires a single photon, but to actually be able to measure that … is really groundbreaking,” says Sara Massey, a physical chemist at Southwestern University in Texas, who was not involved with the new research. “Being able to actually see that hands-on with the data from these experiments is very valuable.”

A key hurdle is that it’s surprisingly difficult to produce a single photon at a time. So in the experiment, the researchers created individual pairs of entangled photons, which affected each other regardless of their separation, according to the laws that govern the quantum realm. The scientists beamed one photon from each pair toward a detector to establish that only its single matching photon was in the system. They simultaneously sent that matching photon into a bacterium’s so-called light-harvesting 2 (LH2) complex, which is involved in the very first step of photosynthesis. The LH2 structure was extracted from the purple bacterium Rhodobacter sphaeroides, where it gathered light (absorbed energy) and fed it into photosynthesis machinery. In the experimental setup, the LH2 complex released light after the energy was absorbed, alerting the scientists that the photon had passed through the system.

“We make a pair of entangled photons, we detect one—we call that the herald—then we put the other one on our sample and look for a signal,” says Graham Fleming, a physical chemist at the University of California, Berkeley, and Lawrence Berkeley National Laboratory and a co-author of the new study.

Or rather that’s the theory. In reality, the approach is a little harder because the fluorescence can occur in any direction, making it difficult for basic detectors with a limited field of view to spot. “It’s easy enough to detect the herald; it’s a heck of a lot harder to detect the fluorescence,” Fleming says. “Most of the time, we’re not seeing anything.”

So the scientists repeated the process over and over and over—producing more than 17 billion herald photons and detecting a fluorescence from LH2 for about one out of every 10,000 heralds. That number of runs was enough for the researchers to statistically analyze the results. Study co-author Quanwei Li, a quantum physicist at the U.C. Berkeley, says this is a crucial step in quantum optics, given how finicky individual photons can be. The analysis confirmed that both the input and output of the experiment were single quanta of energy.

“They actually use quantum light to show that the energy absorption is a quantum event that occurs one photon at a time,” says Jianshu Cao, a physical chemist at the Massachusetts Institute of Technology, who was not involved with the research. “I think it’s very intriguing they were able to apply the new quantum technology, which is the quantum light, to a very large and complex and messy biological system.”

Fleming, Cao and Massey say that the success of the experiment paves the way for other investigations using quantum light—in the context of photosynthesis and beyond.

And in the meantime, the new research gives scientists their sharpest look ever at a process that’s vital to life on Earth. “This is an experiment where you’re actually looking at one single particle of light, and that’s just amazing and mind-blowing to think about, that we’re zooming in at that level,” Massey says.