Quantum Theory Goes Green

The key to quantum computing may lie in the mechanism that drives photosynthesis.

There is no shortage of solar energy on earth, but our technological inability to convert it efficiently into electricity is what still makes the dream of cheap clean energy a distant goal. Commercial photovoltaic cells convert less than 15 per cent of the incident sunlight into electricity, and under the best controlled conditions in laboratories scientists have achieved an efficiency of just over 40 per cent. In contrast, plants convert 95 per cent of the incident energy into electricity, which then powers photosynthesis. The mechanism has remained a mystery, but more and more evidence is emerging that the answer lies in quantum mechanics.

In 2007 scientists had shown this for a sulfur bacterium. They found that the flow of energy within the bacterium photosynthetic system cannot be thought of as energy particles travelling from one point to another, but resembles far more closely a wave like pattern. This wave particle duality is a key feature of quantum mechanics but is only observed in the realm of the very small. At the dimensions that exist in the bacterium this kind of duality is expected to collapse, but somehow quantum coherence is preserved. Now, the same effect has been observed in marine algae. In a paper in Nature, scientists have noted that their observations ‘provide compelling evidence for quantum-coherent sharing of electronic excitation across the 5-nanometre-wide proteins under biologically relevant conditions…’

This ability to retain quantum coherence seems to be the key to efficient utilisation of sunlight. Particles have to search for the most efficient path through the photosynthetic network, as we would, one at a time. But a wave smeared out in space can do so simultaneously, which is far more efficient. This ability to test several paths simultaneously is also the key to quantum computing, which can tackle several problems that are intractable through ordinary computing. The failure to maintain quantum coherence is one of the main problems in realising quantum computing, but it seems plants hold a secret which may help.

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Hartosh Singh Bal