Quantum sheds light on photosynthesis

Biological systems exhibit the same quantum effects as non-biological ones, according to scientists who identified superpositions in molecules inside light-harvesting green sulfur bacteria. The findings, published in the journal Nature Chemistry, simultaneously rule out an earlier claimed quantum-mechanical discovery in another species of bacteria while establishing for the first time that at one level photosynthesis is a quantum process.

A team of researchers, led by Thomas la Cour Jansen from the University of Groningen in the Netherlands, used light polarisations to examine the behaviour of seven light-sensitive molecules that comprise the photosynthetic apparatus inside the bacteria.

They used a photon to simultaneously excite two of the molecules and then engaged spectroscopy to determine whether the input had placed the targets into a superposition.

“In the case of such a superposition, spectroscopy should show a specific oscillating signal,” explains Jansen. “And that is indeed what we saw.”

Critical to the observation was the duration of the superposition.

A paper published in the journal Science Advances in 2017 claimed to have detected superpositions in a protein in another species of bacteria. The result, however, was questioned by some other researchers in the field, because the state was measured at lasting a fraction longer than a single picosecond. This is a tiny interval – just 0.000,000,000,001 of a whole second – but still much longer than quantum theory predicts.

Jansen says the work by his team proves the 2017 measurements weren’t a challenging anomaly – they were simply wrong.

“We have shown that the quantum effects they reported were simply regular vibrations of the molecules,” he adds.

Their own approach, he explains, produced results in line with theoretical predictions.

“We found quantum effects that lasted precisely as long as one would expect based on theory and proved that these belong to energy superimposed on two molecules simultaneously,” he says.

Although long suspected, the team’s findings constitute the first firm evidence that theoretically consistent quantum effects occur in biological systems.

“This is an interesting observation for anyone who is interested in the fascinating world of quantum mechanics,” says Jansen.

“Moreover, the results may play a role in the development of new systems, such as the storage of solar energy or the development of quantum computers.”

Related reading: Spotlight on photosynthesis