Electron spin shielded by microwaves

Delft researchers succeeded in substantially prolonging an electron spin’s quantum state by using microwave pulses to shield it from surrounding perturbations.

Dr. Ronald Hanson and Gijs de Lange from the Kavli Institute of Nanoscience (Applied Sciences) had a busy day last Friday. On Thursday, the article they co-authored with researchers from the Ames Lab (U.S. Department of Energy) was published in Science magazine. Almost immediately colleagues from all over the world began mailing and phoning to pass on their congratulations. “Our finding has opened up a whole new field of research”, Dr. Hanson says.
Their achievement had already been causing quite a stir, as maintaining a quantum state of an electron spin over anything longer than a microsecond is a big – possibly the biggest – hurdle in the development of quantum electronics. This is because other electron spins or nuclear spins usually disturb the quantum state under study before anything practical can be done with it. Or, as quantum physicists say, its coherence is broken by external perturbation.

What Dr. Hanson and his colleagues revealed in Science is that a series of ten nanosecond microwave pulses effectively shield an electron spin from external perturbations, just as was predicted some ten years ago. But Dr. Hanson and others were the first to actually demonstrate the effect. The more pulses, the longer the protection lasts. They showed that 136 pulses prolonged the coherence by a factor of 26. “Theoretically, there is no limit to it”, Hanson declares. Practically, what counts is that the coherence is long, compared to the time needed to manipulate the spin. In this case, the coherence lasts 10,000 times longer than setting the spin. In quantum computer talk, the microwave shield would allow researchers to perform some 10,000 operations before the quantum state collapses. This is all the more surprising because, contrary to most such experiments, which are performed in cryostats near 0° Kelvin, the researchers comfortably worked at room temperature.

Physically, the electron spin consists of an atomic defect in the grid of a synthetic diamond layer. If a nitrogen atom replaces one of the grid’s carbon atoms with an empty space opposite the nitrogen atom, the resulting nitrogen-vacancy (NV) centre will harbour a free electron. This electron’s spin can be set and read out by polarised laser pulses, which makes it, together with its ability to function at room temperature, a very user-friendly medium for quantum electronics.

The first application that Dr. Hanson is working on is not the much talked about quantum computer, but rather an atomic magnetic probe. If microwaves stabilise the electron spin sufficiently, researchers can perhaps also use the spin to measure magnetic fields on an atomic scale, or so the reasoning goes.

Universal dynamical decoupling of a single solid-state spin from a spin bath, G. de Lange, Z. H. Wang, D. Ristè, V. V. Dobrovitski, and R. Hanson,
Science, 9 September 2010.

To article in Delta