Is photosynthesis a quantum phenomenon?

The quirks of quantum physics are something you might expect to find under exotic conditions in a laboratory, but not in a meadow. Yet in recent years, a blossoming idea called quantum biology proposes that life’s molecular mechanisms deploy some of those notoriously counterintuitive behaviours. Ten years ago, researchers reported evidence that photosynthesis – the process by which green plants and some bacteria turn sunlight into chemical energy – gains light-harvesting efficiency by exploiting the phenomenon of “quantum coherence”. This involves the superpositions of electronic quantum states, which seem able to explore many energy-transmitting pathways at once. If so, quantum mechanics is assisting the fundamental energetic process that drives all life on the surface of the Earth.

It was a remarkable claim. But was it true? The question has been hotly debated over the past decade. Early talk of “quantum coherence” in photosynthesis being akin to that in quantum computers, where it underpins the faster and more efficient computation that those devices achieve, has now largely evaporated in favour of a more nuanced picture. And some researchers insist that such coherence plays no useful role in photosynthesis at all.

On the one hand, Greg Engel of the University of Chicago, who was involved in the initial work suggesting quantum coherence as a new design principle of nature, says that “the general notion that the language and mathematics of quantum information, including coherence, can be used to understand photosynthetic dynamics in ultrafast spectroscopy experiments seems to be growing in acceptance”. But biophysical chemist Sebastian Westenhoff of the University of Gothenburg in Sweden says that ever more scientists in the field regard the earlier work as a misinterpretation, and that “there is no such thing as quantum coherence in [natural] photosynthesis”. So what’s the argument all about?