Exploring the Secret of Plants

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Plants can convert sunlight into chemical energy with a high degree of efficiency. How this is achieved is still not entirely clear. ETH physicists have now constructed a quantum physical model that aims to answer this question.

Image caption: Three-qubit circuit built for simulating light-harvesting models: Three qubits (in red, blue and green) harvest microwave radiation from the purple waveguide. With the help of specifically engineered classical noise, applied through pink flux-lines, the qubits channel the harvested energy to the output resonator shown in orange. (Illustration: ETH Zurich, Quantum Device Lab, A. Potočnik)

Chlorophyll is the crucial molecule. Thanks to this green pigment, plants are able to convert sunlight directly into chemical energy. The fact that the ATP molecule – the central building block of energy supply in plants – is created within the plant cells using light can now be read in any good biology textbook. However, this process is still a scientific mystery. Researchers are astonished in particular by the high degree of efficiency with which plants convert sunlight into chemical energy.

Contrasting Worlds

Several experiments conducted over the last few years have suggested that quantum physical effects play an important role in energy conversion. Thanks to these effects, the energy collected by chlorophyll molecules can be transferred to the areas where ATP is formed without significant loss. “We have a paradoxical situation,” explains Anton Potočnik, postdoc in Andreas Wallraff’s group in the Quantum Device Lab at the Department of Physics. “On the one hand, molecular processes are governed by quantum  physical effects, but on the other hand, photosynthesis takes place in a wet and warm environment where the rules of classical physics apply.”

However, the key could be hidden in this apparent contradiction: several theoretical models support the suggestion that the interplay between these two worlds might explain the high efficiency of photosynthesis. Whether this is actually the case, however, has not been verified experimentally.

A Model System with Three Qubits

Potočnik worked with Arno Bargerbos and his fellow researchers to close this particular gap. As he reports in the current issue of the journal Nature Communications, he has developed a test procedure with researchers from the University of Cambridge and Princeton University that allows experimental verification of the different theoretical models.

It involves a simple, fully controlled model quantum system that replicates basic structures, which appear in a plant cell. At its core are three superconducting quantum bits (qubits), which are coupled to each other. They represent chlorophyll molecules that absorb the light energy and transfer it to the ATP-forming enzyme complex.

“Our test process provides accurate insights into how light is converted into chemical energy, as we are able to selectively control the various parameters,” explains Potočnik. “This understanding is important, as it may help us to convert light into electricity in photovoltaic cells more efficiently in the future.”

It Comes Down to Vibration

Potočnik’s experiments confirm the suggestion that the natural vibrations of the chlorophyll molecules play a key role during energy transfer. Depending on how rapidly the molecules move, energy is transported more or less efficiently.

However, with these three coupled qubits, the researchers have developed a set-up that only rudimentarily replicates the real conditions within the plant cells. “After tour conceptual demonstration that our system replicates the processes realistically, our next step will be to build more complex systems with more qubits in order to finally uncover the secret of photosynthesis,” explains Potočnik.

Quantum Physics in Everyday Life

The researchers’ experimental approach could also provide new insights in other areas. For example, scientists suspect that our sense of smell is also based on a combination of quantum physics and classical physics. After all, classical physics alone cannot explain why we are able to distinguish between so many different smells. “Whether this is the case could be experimentally verified using a model such as ours,” says Potočnik.