Editorial: Quantum biology has come in from the cold
WHILE physicists struggle to get quantum computers to function at cryogenic temperatures, other researchers are saying that humble algae and bacteria may have been performing quantum calculations at life-friendly temperatures for billions of years.
The evidence comes from a study of how energy travels across the light-harvesting molecules involved in photosynthesis. The work has culminated this week in the extraordinary announcement that these molecules in a marine alga may exploit quantum processes at room temperature to transfer energy without loss. Physicists had previously ruled out quantum processes, arguing that they could not persist for long enough at such temperatures to achieve anything useful.
Photosynthesis starts when large light-harvesting structures called antennas capture photons. In the alga called Chroomonas CCMP270, these antennas have eight pigment molecules woven into a larger protein structure, with different pigments absorbing light from different parts of the spectrum. The energy of the photons then travels across the antenna to a part of the cell where it is used to make chemical fuel.
The route the energy takes as it jumps across these large molecules is important because longer journeys could lead to losses. In classical physics, the energy can only work its way across the molecules randomly. "Normal energy transfer theory tells us that energy hops from molecule to molecule in a random walk, like the path taken home from the bar by a drunken sailor," says Gregory Scholes at the University of Toronto, Canada, one of the co-authors of the paper published in Nature this week (DOI: 10.1038/nature08811).
But Scholes and his colleagues have found that the energy-routeing mechanism may actually be highly efficient. The evidence comes from the behaviour of pigment molecules at the centre of the Chroomonas antenna. The team first excited two of these molecules with a brief laser pulse, causing electrons in the pigment molecules to jump into a quantum superposition of excited states. When this superposition collapses, it emits photons of slightly different wavelengths which combine to form an interference pattern. By studying this pattern in the emitted light, the team can work out the details of the quantum superposition that created it.
This is going to change the way we think about photosynthesis and quantum computing The results are a surprise. Not only are the two pigment molecules at the centre of the antenna involved in the superposition; so are the other six pigment molecules. This "quantum coherence" binds them together for a fleeting 400 femtoseconds (4 × 10-13 seconds). But this is long enough for the energy from the absorbed photon to simultaneously "try out" all possible paths across the antenna. When the shared coherence ends, the energy settles on one path, allowing it to make the journey without loss.
The discovery overturns some long-held beliefs about quantum mechanics, which held that quantum coherence cannot occur at anything other than cryogenic temperatures because a hot environment would destroy the effect. However, the Chroomonas algae perform their work at 21 °C.
"Scholes's work is fantastic," says Gregory Engel at the University of Chicago. "The difficulty of this experiment is extraordinary." Engel demonstrated the same principle in 2007 at the University of California, Berkeley, though at a frigid -196 °C. His team examined a bacteriochlorophyll complex found in green sulphur bacteria and discovered that the pigment molecules were similarly wired together in a quantum mechanical network. His experiment showed that the quantum superposition allows the energy to explore all possible routes and settle on the most efficient one (DOI: 10.1038/nature05678). In a sense, he says, the antenna performs a quantum computation to determine the best way to transfer energy.
Engel and his group at Chicago have just repeated the experiment at a more life-friendly 4 °C. They found the duration of the coherence to be about 300 femtoseconds (arxiv.org/abs/1001.5108v1).
Exactly how these molecules remain coherent for so long, at such high temperatures and with relatively large gaps between them, is a mystery, says Alexandra Olaya-Castro of University College London, who has been collaborating with Scholes to understand the underlying mechanisms and apply them elsewhere. She believes that the antenna's protein structure plays a crucial role. "Coherence would not survive without it," she says.
The hope is that quantum coherence could be used to make solar cells more efficient. The work is going to change the way we think about photosynthesis and quantum computing, Engel says. "It's an enormous result."
Editorial
QUANTUM biology has come in from the cold. First came news that birds may see magnetic fields, thanks to quantum effects. Now it seems that pigments used in photosynthesis use quantum calculations to harness light (see "Hot green quantum computers revealed"). Physicists had ruled this out at life-friendly temperatures because heat disrupts an effect called quantum coherence. The implication is that we, too, could possess quantum computers. We may only need to look into our own eyes to find the evidence, in the form of the pigment rhodopsin.