Physicists in the UK claim to have shown unambiguously that the high efficiency of photosynthesis is driven at least partly by a purely quantum-mechanical phenomenon. Their work could lead to discoveries of other quantum processes in biology, or help in the development of new and better technologies for harvesting solar energy.

Arguably the most important chemical reaction on Earth, photosynthesis allows a plant to harness sunlight by converting carbon dioxide and water into energy-rich carbohydrates. For the most part, this takes place in chlorophyll molecules, which are arranged such that neighbouring molecules have different energy levels. When light shines on one of these molecules, an electron is momentarily excited before passing its energy over to a nearby molecule with a slightly lower energy level. In this way, energy can flow "downhill" from energy level to energy level, via different routes, until it reaches a reaction centre where actual photosynthesis occurs.

Scientists had previously assumed that the energy moves downhill in a random walk – an incoherent "hopping" between energy levels. But this mechanism does not explain how solar energy is transferred so quickly to a reaction centre, which allows photosynthesis to proceed with energy efficiencies of 95% or more. In recent years, various theoretical and experimental studies have suggested that quantum mechanics plays a role, by transporting energy in a wave-like manner. But for all the results, an explanation based on classical physics could never be ruled out, according to Alexandra Olaya-Castro and Edward O'Reilly of University College London (UCL) in the UK.

Quantized vibrations

Olaya-Castro and O'Reilly claim to have uncovered the first unambiguous evidence for quantum effects by doing a theoretical study of the vibrational motion of chromophores – colour-producing molecules such as chlorophyll. Drawing inspiration from the field of quantum optics, where specialist techniques have been developed for characterizing the quantum-mechanical nature of light, the researchers showed that the absorption of a photon of sunlight generates an electronic excitation, the energy of which matches a collective vibration of two chromophores. So long as this vibrational energy is greater than the surrounding thermal energy, the researchers say, then a quantum of energy can be exchanged from one chromophore to the other.

Olaya-Castro and O'Reilly knew that this energy exchange was purely a quantum effect when they tried to plot a probability distribution of fluctuations in the occupation of the vibrational mode and found that these variations were too small to allow a classical description. "This unambiguously demonstrated that the phenomenon described has no classical analogue," says O'Reilly.

"I'm happy to see this paper published – it's a breakthrough," says Gregory Scholes, a chemist at the University of Toronto who has studied the quantum effects of photosynthesis. "There has been a lot of debate in the literature and at meetings lately about the interplay of vibrations – which [we] assumed to confer only classical effects – and electronic coherence in light harvesting. This new work takes the debate to a new level by showing that it is precisely this interplay that makes the system function quantum mechanically!"

"Non-trivial quantum effects"

Scholes adds that the UCL work "points the way" to experiments that directly detect the signatures of quantum effects. Moreoever, says Olaya-Castro, such quantum signatures might not only be found in photosynthesis: specific vibrational motions are also thought to be involved in other biological processes such as vision, smell and enzyme reactions. "Our results suggest that a careful inspection of the dynamics and fluctuations of these 'good vibrations' of molecules in their excited states could benchmark a common principle for non-trivial quantum effects in biology," she adds.

The understanding of photosynthesis is particularly important, however, because of the need to develop methods of harnessing solar energy. "The research on quantum effects in biology has the potential to provide invaluable insights on how to achieve robust, quantum-enhanced energy transfer," says Olaya-Castro.

The research is described in Nature Communications.