How Plants Use Quantum Physics to Achieve Perfect Energy Efficiency in Photosynthesis

🔬 An Unexpected Discovery in Chicago

It was autumn 2007 when Gregory Engel, a young physicist at the University of Chicago, stared at the data on his screen in disbelief. Using ultrafast laser pulses and two-dimensional electronic spectroscopy, he had just revealed something nobody expected: photosynthetic bacteria appeared to be using quantum coherence to transfer energy. The publication in Nature sent shockwaves through the scientific world.

The idea was not entirely new. As early as 1938, scientists had suspected that quantum phenomena could explain the extraordinary efficiency of photosynthesis. And in 1944, Erwin Schrödinger in his book What Is Life? had argued that quantum mechanics is fundamental to understanding life. But sixty years passed before experimental proof materialized.

🦠 The Bacterium That Solved the Mystery

The protagonist bacterium of this story is a green sulfur bacterium (Chlorobaculum tepidum), an organism living in hot springs that photosynthesizes in near-total darkness. Inside it lies the FMO (Fenna-Matthews-Olson) complex — a protein that functions as a bridge, transferring energy from light-harvesting antennae to the reaction center.

Engel's team observed something astonishing. When a photon is absorbed by the FMO, the energy does not “hop” randomly from molecule to molecule, as classical Förster theory predicted. Instead, the energy appeared to explore multiple pathways simultaneously, exactly like a quantum particle in superposition. And it selected the optimal one.

🕵️ The First Suspect: Don DeVault, 1966

The story, however, begins much earlier. In 1966, Don DeVault and Britton Chase at the University of Pennsylvania were working with the photosynthetic bacterium Chromatium. Using a pulsed laser, they discovered something paradoxical: at temperatures below 100 K (-173°C), cytochrome oxidation was temperature-independent. This was a purely quantum phenomenon — quantum electron tunneling.

But the era was not ready. In 1966, the tools to study the phenomenon in depth did not exist. Four decades had to pass until 2007, when ultrafast laser techniques allowed Engel to see clearly what happens at the femtosecond scale.

🌿 The Algorithm of Leaves

Inside every leaf, the process resembles a quantum computation. When a photon strikes chlorophyll, an exciton is created — a quantum energy packet in superposition, delocalized across multiple chromophores simultaneously. This allows the system to simultaneously explore all possible pathways to the reaction center and, through wave packet interference, select the optimal one.

In 2008, Mohseni, Lloyd, and Aspuru-Guzik showed that this process is described as an "environment-assisted quantum walk." It is neither purely quantum nor classical — it is a combination where thermal noise from the environment helps the exciton avoid getting trapped.

📖 Read more: Microsoft's Topological Qubits: The Majorana Revolution

🌊 2010: Ocean Algae Confirm

Three years after Engel, Gregory Scholes's team at the University of Toronto took it one step further. They didn't work with bacteria in cryogenic conditions, but with marine photosynthetic algae (cryptophytes) at ambient temperature. The results, published in Nature in 2010, showed quantum coherence lasting 300 femtoseconds at biologically relevant temperatures.

The scientific community was abuzz. Headlines asked: “Do plants perform quantum computations?” The answer, as is usually the case in science, was more complex than it seemed.

📊 The Revision: 2017-2020

In 2017, a team led by R. J. Dwayne Miller repeated Engel's original experiment, but under ambient conditions. Their publication in the Proceedings of the National Academy of Sciences was illuminating: "Nature does not rely on long-lived electronic quantum coherence for photosynthetic energy transfer." Electronic quantum coherence lasted only 60 femtoseconds.

In 2020, an extensive review in Science Advances reached a new synthesis: quantum coherence dominates in the first femtoseconds, but long-term transport requires a semi-quantum, semi-classical explanation. The interaction between electronic and vibrational exciton states requires new theoretical tools.

🧠 Do They Really Compute?

The answer depends on the definition of “computation.” If computation means “processing information for optimal decisions,” then yes: plants “compute” the best energy transfer pathway, exploiting quantum coherence in the first femtoseconds and then classical mechanisms. They have no “consciousness” of what they do — but 3.5 billion years of evolution optimized the system to an astonishing degree.

In 1984, Hartmut Michel, Johann Deisenhofer, and Robert Huber had deciphered the three-dimensional structure of the bacterial reaction center, earning the Nobel Prize in Chemistry in 1988. It was the first three-dimensional membrane protein structure ever solved — and revealed that charge separation occurs in just 10 picoseconds.

From Schrödinger in 1944 to Engel in 2007, from DeVault in 1966 to Dwayne Miller in 2017, the story of quantum photosynthesis is a story of people who dared to ask: “Do plants understand quantum physics better than we do?” The answer, for now, is a cautious “perhaps.”