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Samples - Gaining control over a single electron spin (Splendid Science 2008)

 

The ultimate dream of quantum physicists is building a quantum computer. This challenge requires an ultimate control over single electrons. Delftnanoscientists are making significant progress.

Nienke Beintema

An everyday computer works on the basis of information units, or bits, that can assume a value of either 0 or 1. Physicists worldwide are dreaming of a computer based on bits that can assume both these values at the same time. Such a computer would be able to perform multiple calculations simultaneously, solving mathematical problems that are too complicated for regular computers simply because they would require too much time.  

The key to making such a computer is quantum mechanics – the theory that describes the behaviour of the absolute smallest: atoms, and even sub-atomic particles such as electrons. Quantum theory predicts that particles, or even tiny electric currents in a superconducting ringlet, can spin leftward and rightward at the same time. This implies that any given time, a quantum information unit, or qubit, can be both 0 and 1. Quantum scientists often illustrate this mysterious concept with the analogy of the ring-shaped wave that forms when you throw a stone into a pond. The wave, which represents the information, reaches multiple places at the same time. If you assume, for instance, that the wave represents 0 if it is one side of the pond, and 1 if it is on the other side, you can imagine that 0 and 1 can coexist at the same time. That analogy is not entirely random: after all, quantum theory assumes that small particles behave like waves.

Electron puddles

“The concept of quantum mechanics is mysterious indeed,” says prof. dr. Lieven Vandersypen at the Department of Nanoscience. “It is hard to imagine, let alone believe it, or even understand it. Yet our experimental measurements do show that these mysterious things actually happen.” For their measurements, Vandersypen and his colleagues use an experimental set-up that was proposed only as recently as ten years ago. They trap individual electrons in semiconductor structures called quantum dots, and use the electrons’ spins as qubits. These quantum dots typically have dimensions of a few nanometers to a few micrometers. “To make a quantum dot that is suitable for a quantum computer,” explains Vandersypen, “you start with a half-millimeter-thick wafer of gallium arsenide and cover it with a 100 nanometer-thick layer of silicon-doped aluminium-gallium-arsenide. Free electrons will concentrate at the interface between the two materials, forming a thin electron sheet. Next, you attach a set of gold electrodes to the top layer and apply negative voltages to them. The electrodes will repel electrons in the sheet underneath and create small islands of electrons isolated from the rest.”

After having created such electron puddles, Vandersypen points out, the challenge lies in ‘reading out’ and even manipulating the spin of the electrons contained in them. To explain how this can be done, he shows the experimental set-up in one of the Department’s laboratory rooms. “The core of the machine is a single chip, which sits inside an ultra-cold receptacle,

called a dilution refrigerator, to reduce the thermal movement of the electrons to an absolute minimum,” he indicates. “This construction itself is encircled by a powerful superconducting magnet.” Whereas a conventional microchip is packed with transistors, the quantum-computing chip will be packed with quantum dots. Ideally, each dot holds one electron. Electrodes near each dot are used to measure and manipulate the spin of the electrons that it holds.

The set-up used in the lab, as Vandersypen underlines, is far from perfect. It is merely used to test the general principles and techniques – but actual quantum computing is still far away, for a variety of reasons. Firstly, the microchips that are available today are not nearly sophisticated enough to hold more than a few quantum dots. Secondly, even though many of the basic principles have been demonstrated in individual experiments, integrating them into a single, usable system still remains to be done.

Single spin control and detection

Isolating single electrons in a quantum dot, as Vandersypen highlights, is a challenge of its own. His Delftcolleague Leo Kouwenhoven was the first to accomplish this together with a Japanese colleague in 1996. By applying negative voltages to electrodes near a quantum dot, they managed to expel electrons from a quantum dot one by one until just a single electron remained. The next challenge was to measure the spin of this single electron. This too was first accomplished by a Delftteam, which included Kouwenhoven and Vandersypen. “The trick is to measure the spin indirectly,” he says. “To do that, we pulsed the electrodes near a quantum dot with tiny electric signals. These pulses give the electron just enough energy to make it slop out of the dot if its spin is down, but not if its spin is up. That’s because in a magnetic field, a spin-down electron has a higher energy than a spin-up one. The presence or absence of a single electron in a dot in turn slightly changes a nearby electric current. We measure this tiny current using highly sensitive electronics.” Two years later, in 2006, the Delftscientists were able to controllably rotate the spin of this one electron, by applying carefully calibrated radio-frequency magnetic fields – another breakthrough.

Another bump in the road towards quantum computing is a phenomenon called decoherence. Any quantum dot, no matter how well it is isolated from its environment, will slowly ‘leak’ information. “Isolation, in fact, contrasts with another characteristic that you need,” says Vandersypen, “which is access. If you want to read out the electron and manipulate it, you need to have access to it – but at the same time you want to shield it from external influences.”

The atoms of the semiconducting material itself, in this case gallium arsenide, also exert an undesirable influence. To circumvent this problem, Vandersypen and his colleagues are exploring the possibilities offered by other materials. One of their options is graphene: a material made entirely of carbon atoms arranged hexagonally in a grid. “The nuclei of carbon atoms hardly have any spin of their own,” he indicates, “and thus hardly exert any disturbing influence.” So far it has not been possible to create suitable quantum dots in graphene, and scientists are working on improving the material to enable this. “Whether this will work out,” says Vandersypen, “is still an open question.”

Even if graphene eventually fails to measure up to the expectations, the Delftnanoscientists will have discovered new phenomena that are interesting from a fundamental point of view. They are gaining new insights, for instance, into how electrons behave in graphene. “I really expect that this kind of information will prove to be useful in the future,” says Vandersypen.

Among other things, Vandersypen would love to be able to prove experimentally that an electron really can exhibit two opposite spin states simultaneously. And he would like to see how far he can get in making the spin keep its information for a long time. “Our experiments,” he says, “may yield totally unexpected outcomes, which don’t even answer our original questions. That makes it scientifically interesting, but also quite hard. You need a certain degree of vision – or perhaps bravery – to decide whether to push on or to turn around. You won’t reach a breakthrough if you don’t take risks, but some risks are more promising than others.”

A burning question, of course, is whether quantum computers will ever become a reality. “I am convinced that it should be possible,” states Vandersypen, “if we work hard enough. But that, at the same time, is the limitation. It depends on how long it takes and how long the financing partners will remain interested. It is a delicate trade-off. But in any case, along the way, we learn invaluable new things about the possibilities and impossibilities of quantum systems.”


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