During photosynthesis, a symphony of chemicals transforms light into the energy needed for plants, algae and some bacteria to live. Scientists now know that this amazing reaction requires the least amount of light possible, just a single photon to get started.

A US team of researchers in quantum optics and biology has shown that a single photon can start photosynthesis in purple bacterium Rhodobacter sphaeroidesand they are hopeful that it works in plants and algae as all photosynthetic organisms share a similar evolutionary ancestor and processes.

The team says their findings strengthen our understanding of photosynthesis and will lead to a better understanding of the intersection of quantum physics in a wide range of complex biological, chemical and physical systems, including renewable fuels.

“A tremendous amount of work, theoretically and experimentally, has been done around the world to try to understand what happens after a photon has been absorbed,” says Graham Fleming, a biochemist at the University of California, Berkeley.

“But we realized that no one was talking about step one. That was still a question that needed to be answered in detail.”

Chlorophyll molecules receive photons from the sun, where the chlorophyll electron is excited, jumping to different molecules to form the building blocks of sugar, feeding plants and releasing oxygen.

The Sun doesn’t shower us with an overly generous number of photons on a sunny day, only about 1,000 photons reach a chlorophyll molecule per second, so the efficiency of photosynthesis in harnessing sunlight to produce energy-rich molecules has led scientists believe that a single photon could initiate this reaction.

“Nature has come up with a very clever trick,” says Fleming.

The researchers focused on a well-studied structure of proteins in purple bacteria, called light-harvesting complex 2 (LH2), which can absorb photons at a particular wavelength.

Using specialized tools, they created a photon source that created a photon pair from a higher energy photon using spontaneous parametric down conversion.

A diagram of the experiment
A simplified diagram of the experimental procedure in the LH2 complex. (Janis et al., Nature2023)

During one pulse, the first photon, called ‘the herald’, was observed with a highly sensitive detector, signaling the arrival of the partner photon, which interacted with LH2 molecules in a laboratory sample of the purple bacteria.

When a photon with a wavelength of 800 nanometers strikes a ring of molecules in LH2, energy goes to a second ring, which emits fluorescent photons with a wavelength of 850 nanometers.

In nature, this energy transfer would continue until the process of photosynthesis has started. Finding a photon with a wavelength of 850 nanometers in the lab was a clear sign that that process had begun, especially since the LH2 structures were separated from other parts of the cell.

The challenge was to deal with single photons, which are easy to miss. To get around this problem, the scientists used the photon herald as a guide.

“I think the first thing is that this experiment showed that you can actually do things with single photons,” says chemical physicist Birgitta Whaley of Berkeley. “So this is a very, very important point.”

Using a probability distribution model and computer algorithm, the team analyzed more than 17.7 billion herald photon detection events and 1.6 million fluorescent photon detection events.

The thorough analysis means the researchers are confident the findings were caused by the absorption of a single photon alone and that no other factors could have had an effect.

Much of the previous research conducted on the subsequent post-light absorption stages of photosynthesis has involved sending powerful, ultra-fast laser pulses to photosynthetic molecules.

“There is a huge difference in intensity between a laser and sunlight—a typical focused laser beam is a million times brighter than sunlight,” explains Quanwei Li, a quantum physicist and engineer at Berkeley.

By demonstrating how single photons behave during photosynthesis, this research provides us with important insights into how the energy conversion process works in nature. Artificial photosynthesis techniques could one day hold the key to sustainably surviving and thriving in space.

“Just as it is necessary to understand each particle to build a quantum computer,” adds Li, “we must study the quantum properties of living systems to truly understand them and create efficient artificial systems that generate renewable fuels.”

This study was a unique opportunity for two scientific fields that don’t usually work together to apply and combine the techniques of quantum optics and biology.

“Next thing is, what else can we do?” says Whaley.

‘Our goal is to study the transfer of energy from single photons through the photosynthetic complex at the shortest possible time and space scales.’

The research was published in Nature.

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