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Posted: June 29, 2007

Can nuclear qubits point the way?

(Nanowerk News) Researchers from the London Centre for Nanotechnology and the US have published an article in Physical Review Letters (" Efficient Dynamic Nuclear Polarization at High Magnetic Fields") that describes a technique for making molecules a thousand times more useful for quantum computing.
Qubits might very well be the vehicle for the next revolution in computing. Silicon technology has made computers faster and faster, but now it seems that the limits of what is possible with ones and zeros are being reached. One of the answers could be the transition from the “good-old” bit to the flashy qubit.
The "qu" in qubit stands for quantum, and one way to realise such a qubit is to use the tiny magnetic fields (called spins) that are associated with the nucleus and the electrons of atoms. However, in order to build a “quantum computer” from these spin-qubits, scientists first need to learn how to effectively manipulate these spins.
The team of researches have added to the quantum-computing technology roadmap by developing a new technique that uses high magnetic fields and low temperatures to line up far more nuclear spins than has been possible so far.
The research team said the new technique will allow the initialisation of the nuclear qubit in schemes to exploit N@C60 molecules as components of a quantum information processing device.
A nitrogen atom trapped inside a buckyball
N@C60 is a nitrogen atom trapped inside a football-shaped carbon molecule called a buckyball.
“We suggest a new technique for dynamic nuclear polarization that is more than twice as effective as previous versions,” said lead author Gavin Morley, a researcher now at the London Centre for Nanotechnology. “Our N@C60 fullerene molecules have a nitrogen atom in their very centre with a nuclear spin and an electron spin. We want to use both spins to store quantum information as they can each point up or down. Then each molecule will be a quantum computer with two qubits.”
The researchers needed to have both of the spins in the molecule pointing up at the beginning of the quantum computation. They made the electron spin point up by using a strong magnetic field over 150,000 times the strength of the Earth’s magnetic field, (8.6 tesla) and a low temperature (–270°Celsius or 3° kelvin), but still the nuclear spin had about a fifty-fifty chance of pointing up or down.
“We used the fact that the electron was already pointing up to make the nuclear spin do the same,” said Morley. “This increased the number of useful molecules by over one thousand times.”
This technique has drawn great interest from scientists working on nuclear magnetic resonance, because one thousand times more useful molecules could greatly shorten the time it takes to perform an NMR experiment.
The co-authors on the paper are Johan van Tol from the National High Magnetic Field Lab in Florida, and Arzhang Ardavan, Kyriakos Porfyrakis, Jinying Zhang, and G. Andrew D. Briggs, all of the University of Oxford.
Source: London Centre for Nanotechnology