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Algae master quantum mechanics
Some algae rely on quantum mechanics to rapidly and efficiently convert sunlight into energy they can use, scientists have found.

The findings have implications for research into more efficient solar cells and quantum computing.

Physicist Professor Paul Curmi of the University of New South Wales in Sydney and colleagues report their research in today's issue of the journal Nature.

"Nobody expected to see quantum effects in biology," says Curmi.

Certain species of algae that live in low-light underwater conditions are known for their highly efficient, almost instantaneous conversion of sunlight into electrical energy to power photosynthesis.

Scientists have long wondered how these algae are so good converting the Sun's energy.

One recent suggestion has been that they have evolved to use "quantum coherence".

This phenomenon, used in quantum computing, involves an object or an amount of energy being in more than one place, or in more than one state, simultaneously.

In a quantum computer, quantum coherence allows what's called "parallel processing", in which many calculations can be carried out at the same time, enabling faster solving of problems.

In the algae, says Curmi, quantum coherence is used to ensure the fastest possible transfer of sunlight energy, absorbed by light harvesting proteins, to reaction centres that convert the energy into something more useful.
The best path

"Imagine you are trying to find your way through a maze," says Curmi.

"Classically, you have to randomly try every path until you eventually find your way through the maze. This can take a long time."

In this analogy, quantum coherence, allows you to travel all routes at the same time so you can find the right path quickly.

In the case of the algae's light-harvesting system, the researchers used a technique called two-dimensional photon echo spectroscopy to study the behaviour of energy in chromophores, the molecules in the light harvesting protein that transfer the light to the reaction centre.

"We are observing the light energy taking all paths through the maze of chromophore molecules simultaneously," says Curmi.

Curmi says researchers had previously thought that the energy searched for the reaction centre in a random way.

But by using quantum coherence it can find the shortest route to the reaction centre immediately.

"It does exactly the same thing that physicists are trying to do by building a quantum computer," says Curmi.
'Lengthy' coherence

Curmi says the findings may help scientists who are developing solar cells or quantum computers, which normally only achieve the required quantum coherence at super-cooled temperatures.

The quantum coherence documented by Curmi and colleagues occurred at normal temperatures and held for around 200 femtoseconds.

Curmi says quantum coherence usually occurs at much smaller scales and for much shorter times, which is why we are not normally familiar with them.

He says what exactly enables the algae proteins to make quantum coherence last so long is still a mystery.

"There's something very special happening," says Curmi.

But he says understanding this could help in the development of different kinds of "biological" quantum computers.

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