Plants Use Quantum Computing to Gain Efficiency—So Why Doesn’t This One?

When a photon of light hits a plant’s photosystem, it has to make a decision—fast. Which path would be the most efficient way to the molecular machinery that converts light into sugars? Like computer scientists who want to perform superfast calculations, they turn to quantum computing.

When light hits a plant cell, its chromophores—light-sensitive molecules—capture the photons and convert them to energy packets called excitons. The excitons bounce among the cell’s chromophores until they eventually arrive at the reaction center where water and carbon dioxide mingle to produce sugar and oxygen. The excitons could take any number of paths to get there—and indeed they do. Thanks to the quantum phenomenon known as coherence, excitons test out every pathway simultaneously, ultimately taking the most efficient route.

Cryptophytes under magnification

At least, we suspect those quantum calculations are made in the name of efficiency. Without knowing what happens in a cell when this quantum computing is turned off, it’s impossible to know exactly what function it’s performing.

Paul Curmi, a professor at the University of New South Wales, and his colleagues found a class of algae called the cryptophytes where quantum coherence is used in some but not all species. Cryptophytes are perfect candidates for testing quantum-enhanced photosynthesis because they inhabit dim environments underwater or beneath sheets of ice, where they subsist on meager scraps of light. To survive on this starvation diet, cryptophytes have to make the most of every photon they can get.

Curmi and his colleagues identified a genetic mutation in one group of cryptophytes that, puzzlingly, blocks quantum coherence, Ashley Yeager explains for Science News. The mutation slips in an extra amino acid, wrenching the two subunits of the algae’s main light-absorbing protein away from each other. That extra distance seems to be enough to keep the cell’s chromophores too far apart for the excitons to sample all the pathways between them.

Scientists still don’t know why some cryptophytes use coherence while others don’t, but learning how the non-coherence system works—and what advantages it might confer—could shed light on why plants became quantum computers in the first place.