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Quantum computing becomes more than just spin

The building blocks of a quantum computer have been created and tested in a high tech basement at the University of NSW, and within a few years Andrea Morello and his colleagues expect to have a small working prototype.

People have speculated about the potential of quantum computers for decades—how they would make child’s play of constructing and testing new drugs, searching through huge amounts of data and ensuring that information was fundamentally secure.

But it all seemed like science fiction. No-one really knew how to build one, despite lots of clever ideas for using exotic materials and light. But 15 years of work at the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology and its predecessors have changed everything. The building blocks of a quantum computer have been created and tested in a high tech basement at the University of New South Wales (UNSW). And within a few years Andrea Morello and his colleagues expect to have a small working prototype.

What’s more, the device will be constructed out of the same base material used to build classical computers—silicon, that inexpensive, abundant material which a trillion-dollar global industry is already equipped to handle and manipulate. This should make gearing up to produce quantum computers relatively seamless, and gives Australia a solid lead in one of the game-changing technologies of the 21st century.

For his intellectual leadership in developing the silicon components to make quantum computing possible, Associate Professor Andrea Morello has been awarded the 2013 Malcolm McIntosh Prize for Physical Scientist of the Year.

Quantum computers promise exponential increases in processing speed over today’s computers through their use of the spin, or magnetic orientation, of individual electrons or atomic nuclei to represent data in their calculations. The work depends on quantum physics, the field of science that describes what happens at the level of elementary particles, atoms and molecules. At that tiny scale, materials show infamously strange properties—this, says Andrea, makes quantum physics very difficult to explain in plain language. “It doesn’t connect to any prior experience. It’s something you have to grow into, like riding a bicycle.”

The quantum characteristics that have most relevance to computing are known as “superposition” and “entanglement”. Superposition means that a quantum system can be in several different states at the same time—and resolves into one particular state only when someone examines it. Entanglement describes a special type of correlation between multiple quantum particles.

In normal computers the basic unit of information, the bit, exists either as a 1 or a 0 (power on or power off). But a quantum bit, or qubit, can be in both states at once. This means that a computer operation using a single qubit can give results for both of its values at the same time. The quantum computer actually performs the operation on the two different values.

Using two qubits, the operation can be performed on four values, three qubits on eight values, and so on. As you add qubits, the computer’s capacity to perform operations, and hence its power, increases exponentially.

Andrea Morello, the man who is leading the effort to turn this promise into reality, came to Australia via Italy, France, the Netherlands and Canada. At each stop, he added to a unique set of skills in ultra-low temperature physics and the quantum behaviour of spin. By the time he reached UNSW, his experience fitted hand in glove with that of Professor Andrew Dzurak, an expert in the nanofabrication of semiconductors and director of the NSW node of the Australian National Fabrication Facility.

They became close collaborators, great friends, and colleagues in the School of Electrical Engineering and Telecommunications at UNSW. Together they have constructed the means to read and write information in terms of the spin of an individual electron or nucleus of a single phosphorus atom in a silicon chip. The phosphorus atom is implanted in the chip by a team led by Professor David Jamieson at the University of Melbourne, using techniques developed for the electronics industry.

Ultra-low temperatures are important to harness the quantum behaviour of spin. Just as you can change the natural orientation of a compass from north to south by adding energy—physically moving it against the grain—you can change the spin of an electron from up to down or vice versa. But the amount of energy it takes, even under a strong magnetic field, is very small—the equivalent of a temperature rise of about 1 °C. So under normal conditions electrons flip back and forth spontaneously. But at a temperature of less than one degree above absolute zero, the spin can be frozen in place. And that is what Andrea has spent many years doing.

Andrea grew up in the historic town of Cumiana near Turin at the foot of the Alps between Italy and France. He was the first member of his family to go to university, and undertook a degree in electrical engineering at the Politecnico di Torino. The solid grounding this provided enabled him to do an honours year equivalent at the Max Planck High Magnetic Field Laboratory on the other side of the Alps in Grenoble, France. “It was the sort of lab where you have coffee with visiting Nobel Laureates. So I got into hardcore fundamental research at the highest possible level, and I found I was kind of good at it.”

He had discovered his career—and he was good at it. As Professor Gerhard Klimeck, Director of nanoHUB.org, the Network for Computational Nanotechnology, at Purdue University in the US, writes, “The depth and breadth of his understanding of spin physics, and his ability to devise experimental methods to observe and control their quantum nature, has few parallels in the world.”

Andrea studied for his PhD at the world’s foremost ultra-low temperature laboratory, the Kamerlingh Onnes at Leiden in the Netherlands. This was the place where helium was first liquefied and superconductors discovered. Morello performed breakthrough experiments to study nuclear spins inside magnetic molecules that behave in a quantum mechanical manner.

From Leiden he moved on to a postdoctoral fellowship at the University of British Columbia (UBC) in Vancouver, Canada, where he went from working on nuclear spin to electron spin. At UBC he developed a theory of the quantum dynamics of electron spins, and attempted a challenging experiment to test it. The equipment to which he had access could not make conditions cold enough to succeed, but when his theory finally was tested in the US, it proved entirely correct.

In 2006, he was attracted to Australia both by the quest of building a quantum computer and the setting. He understood that the ambitious idea of quantum computing would not only draw on his expertise of low temperature physics and spin, but would also expand his horizons into the world of engineered nanostructures. And he found the cosmopolitan society of Sydney very attractive.

The quantum computer Andrea and his team set out to build will use the spin of individual electrons for calculations. But in order to employ electron spin, the computer needed both a way of changing the spin state (writing information) and of measuring that change (reading information). Together these two form the heart of the qubit.

Things moved quickly. By Christmas 2008 they were well on the way to developing a single electron reader. “Our device detects the spin state of a single electron in a single phosphorus atom implanted in a block of pure silicon. You can watch the spin orientation directly, in real time, because it controls the flow of electrical current in a nearby single electron transistor circuit,” Andrea says. The paper announcing their success was published in Nature in September 2010.

In a second Nature paper last year—of which the first author was his PhD student, Jarryd Pla—Andrea announced that they could control the spin state of an electron, by using a special nano-antenna. The antenna comes extremely close to the phosphorous atom allowing fast and accurate encoding of quantum information. That result was also covered in The New York Times.

This year, the team has been able to create an even more reliable qubit using a single nuclear spin, a very challenging feat because of the extremely weak magnetism of the nucleus. That achievement resulted in a third Nature paper in three years. And the UNSW group now has the ingredients of both an electron and a nuclear silicon-based qubit.

The student involvement in Andrea’s research is no accident. Students are integral to the success of the research program, Andrea says. “One day my research may change the world of information, but in the meantime teaching and motivating outstanding students is a tangible and immediate outcome of my work.”

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