All That Matters

Quantum computers are so highly sought after because they can solve complex mathematical problems and parallel computer operations such as code breaking really fast. Attempts to build quantum computers come in many flavours and use different kinds of quantum states, ranging from trapped atoms, superconductors to semiconductors such as gallium arsenide or diamond. The approach with diamond is particularly promising as it uses the spin of an electron to perform quantum computing operations, which means that such devices could be nicely tied in with conventional electronics and its control of electron transport itself.

Reading single spins. The information of a single spin in a reservoir can be read by a silicon single electron transistor island. Reprinted by permission from Macmillan Publishers Ltd. Nature (2010). doi:10.1038/nature09392

Now,  Andrea Morello, Andrew Dzurak and colleagues from the University of New South Wales in Sydney in collaboration with researchers from Melbourne and Aalto University in Finland have achieved a major step towards such quantum computers using the most straightforward electronic material available: silicon. In a paper in Nature they demonstrate the reading of a single electron’s spin with a silicon electronic circuit. “Until this experiment, no-one had actually measured the spin of a single electron in silicon in a single-shot experiment,” says Morello. Ronald Hanson from the TU Delft, who works on the competing approach using impurities within diamond, agrees: “this is a result that the silicon quantum computing community has been waiting for for a long time.”

The principle of how the electron spin is read with a silicon circuit is relatively straightforward. It uses a charge reservoir, where the electrons are stored in phosphorus impurities that are implanted into the silicon. Next to the reservoir is a single electron transistor, which is a tiny silicon transistor that can detect the presence of a single electron.

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Scanning tunnelling microscopes (STM) are a wonderful instruments that can not only image but also manipulate individual atoms on a surface. Developed by Gerd Binning and Heinrich Rohrer in 1981, STM and their derivatives revolutionized our understanding of what happens on the surface of materials.

The famous quantum corral, fabricated and imaged in 1993 by Don Eigler at IBM. Atoms (blue) are placed on a circle and consequently modify the electron density on the surface (red ripples). (c) IBM

Expanding on Binning and Rohrer’s work at IBM in Zurich, Don Eigler at IBM’s Almaden lab in particular pioneered the modification of individual atoms on a surface. In 1989 he famously accomplished moving atoms on a surface to form the letters IBM. For these accomplishments Eigler received this year’s Kavli Prize in Nanoscience.

IBM continues to pioneer STM research at Almaden. The group of Andreas Heinrich, for example, who started at IBM in 1998 as a postdoc with Don Eigler, is working on the study of magnetic atoms. He and Eigler have now accomplished another major advance in microscopy technology. So far, STM measurements were mainly static and little information of dynamic properties was available. In a study published in Science, they now have developed a method that is able to measure how long it takes for atoms to flip their electron spin. This technique opens the door to measurements of the dynamics of a number of properties, for example vibrational states.

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The nuclear reactor at ILL. The nuclear fuel is underneath the steel shield. The blue glow of that is partly visible is caused by Cerenkov radiation.

You look down into a clear pool of water. The water has an appealing blue glow to it that makes you want to dive into it. But this isn’t a swimming pool, it is a nuclear reactor. And the soothing blue glow is not due to the blue paint of the pool walls but caused by the Cerenkov radiation, emitted as a result of the electrons created by the fission process that move faster than the speed of light in water.

The electrons that are ejected from the nuclear fuel elements are fast than the speed of light in water (about 75% of the speed of light in vacuum). Similar to the supersonic bang of jets that fly faster than the speed of sound, Cerenkov radiation is emitted by water as the fast electrons pass through it. The blue shimmer of the Cerenkov radiation is visible on the right of the photo, showing the pool containing spent reactor fuel.

The reactor I am visiting is that at the Institut Laue-Languevin (ILL) in Grenoble. As a research reactor it is generating up to 58 megawatts of power, about 25 times less than that of commercial reactors. Still, I am nervous holding my camera directly above the pool to take pictures, afraid I might be dropping it into the running reactor. But there is no need to worry, it is safe to stand there, the water is a perfect shield from the radiation, it absorbs all the neutrons and electrons created by the nuclear reaction. And there is of course plenty of security and radiation monitoring before, during and after my visit. Even if I would drop the camera into the pool, there is a steel construction in the water that would catch larger objects. And stuff that would slip through that grid would probably lie harmlessly at the bottom of the pool until the reactor is decommissioned.

Not many people are allowed inside the reactor and I am lucky enough to be invited to the ILL along with a few British colleagues. It is only the second time I am inside a nuclear reactor. It is an awesome feeling, certainly for a physicist, to see an operating reactor and to admire the technology that keeps the nuclear chain reaction under control. The impressions from my visit not only reinforce what I know about the benefits of neutron research, but the variety of research to me also underlines the dangerous implications of possible recession-related government budget cuts to facilities like ILL.

ILL was founded by France and Germany in 1967, with the UK becoming a third major partner in 1973. Initially, the UK did not join the institute because it wanted to build its own reactor, tells us our tour guide, Andrew Harrison, an Associate Director of ILL. Nowadays it seems almost unbelievable to me that the UK abstains from a European research project because it intends to invest more money into a certain area of research, not less. In any case, today, Germany, France, and the UK still share 75% of ILL’s operating budget of 88 million Euros, the rest being distributed amongst its other international partners. Their continuing support has made ILL one of the leading research institutions that uses neutrons for experiments in life sciences (18% share of experiments), environment (11%), materials science (29%) and fundamental sciences (35%).

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