Quantum Character of Light Transport Observed in Photosynthesis

Photosynthetic organisms, such as plants and some bacteria can whisk away 95 percent of the sunlight they absorb in less than a couple of trillionths of a second to drive the metabolic reactions that provide them with energy.

Various research groups around the world have found indications that this highly efficient energy transport is connected to a quantum-mechanical phenomenon. However, until now, no one had directly observed the possible impacts of such a quantum transport mechanism at work at room temperature.

In an article published in Science, researchers from the group led by ICREA Professor at ICFO Niek van Hulst, in collaboration with biochemists from the University of Glasgow, have been able to show for the first time at ambient conditions that the quantum mechanisms of energy transfer do make photosynthesis more robust in the face of environmental influences. Quantum coherence is manifested in so-called photosynthetic antenna proteins that are responsible for absorption of sunlight and energy transport towards the photochemical reaction centers where the energy is stored.

In order to observe this process, researchers send ultrafast femtosecond light flashes to capture a high-speed series of ‘pictures’ of the states of individual antenna proteins after light absorption. With these "snapshots", they are able to understand how solar energy is transported through single proteins and observe how energy flows through sunlight absorbing photosynthetic systems with unprecedented spatial and temporal resolution.

Van Hulst and his group have traced the energy transport pathways at the level of individual antenna proteins, and shown that each protein uses a distinct pathway. The most surprising discovery was that, while the transport paths within single proteins vary over time due to changes in the environmental conditions, the protein uses the quantum character to adapt for optimal efficiency. These results show that coherence, a genuine quantum effect of superposition of states, is responsible for maintaining high levels of transport efficiency in biological systems, even while they adapt their energy transport pathways due to environmental influences.