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Hot green quantum computers revealed
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Hot green quantum computers revealed 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 energyrouteing 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
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, 12 | NewScientist | 6 February 2010
AMANA IMAGES/ALAMY
Kate McAlpine
algae perform their work at 21 ˚C. emitted light, the team can work “Scholes’s work is fantastic,” out the details of the quantum says Gregory Engel at the superposition that created it. University of Chicago. “The The results are a surprise. difficulty of this experiment Not only are the two pigment is extraordinary.” Engel molecules at the centre of demonstrated the same principle the antenna involved in the in 2007 at the University of superposition; so are the other California, Berkeley, though at a six pigment molecules. This frigid -196 ˚C. His team examined “quantum coherence” binds a bacteriochlorophyll complex them together for a fleeting 400 found in green sulphur bacteria femtoseconds (4 x 10-13 seconds). But this is long enough for the and discovered that the pigment energy from the absorbed photon “This is going to change to simultaneously “try out” all possible paths across the antenna. the way we think about When the shared coherence ends, photosynthesis and quantum computing” the energy settles on one path, allowing it to make the journey without loss. molecules were similarly The discovery overturns some wired together in a quantum long-held beliefs about quantum mechanical network. His mechanics, which held that experiment showed that the quantum coherence cannot occur quantum superposition allows at anything other than cryogenic the energy to explore all possible temperatures because a hot routes and settle on the most environment would destroy the efficient one (DOI: 10.1038/ effect. However, the Chroomonas 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. –Super-efficient, naturally– “It’s an enormous result.” ■
Hot green quantum computers revealed 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 energyrouteing 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
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, 12 | NewScientist | 6 February 2010
AMANA IMAGES/ALAMY
Kate McAlpine
algae perform their work at 21 ˚C. emitted light, the team can work “Scholes’s work is fantastic,” out the details of the quantum says Gregory Engel at the superposition that created it. University of Chicago. “The The results are a surprise. difficulty of this experiment Not only are the two pigment is extraordinary.” Engel molecules at the centre of demonstrated the same principle the antenna involved in the in 2007 at the University of superposition; so are the other California, Berkeley, though at a six pigment molecules. This frigid -196 ˚C. His team examined “quantum coherence” binds a bacteriochlorophyll complex them together for a fleeting 400 found in green sulphur bacteria femtoseconds (4 x 10-13 seconds). But this is long enough for the and discovered that the pigment energy from the absorbed photon “This is going to change to simultaneously “try out” all possible paths across the antenna. the way we think about When the shared coherence ends, photosynthesis and quantum computing” the energy settles on one path, allowing it to make the journey without loss. molecules were similarly The discovery overturns some wired together in a quantum long-held beliefs about quantum mechanical network. His mechanics, which held that experiment showed that the quantum coherence cannot occur quantum superposition allows at anything other than cryogenic the energy to explore all possible temperatures because a hot routes and settle on the most environment would destroy the efficient one (DOI: 10.1038/ effect. However, the Chroomonas 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. –Super-efficient, naturally– “It’s an enormous result.” ■