Computers are everywhere these days. They play us music, tell us when to wake up, remind us that we’re late for an appointment, and provide us with entertainment. Even if we don’t realise it, so ingrained in our lives are computers that the world would be a very different place without them. Computing is also an incredibly fast moving field of technology, and research is finally taking us towards the exciting world of quantum computing!
Quantum computers will work using quantum bits, or qubits for short, which are analogous to the digital bits used in computers like the one which you’re using to read this article. Recently, a team of engineers at the University of New South Wales (UNSW) has successfully demonstrated, for the first time ever, how a single atom can be act as a qubit, effectively showing the first step in building an ultra fast quantum computer. And they might just have created the best qubit ever made.
A quantum computer is, simply, a computer which makes use of quantum mechanical phenomena to perform calculations. Well, I say “simply”… Let’s step back a moment. The simplest form of computers involve actual moving objects, and using the positions of those objects to perform calculations. This is essentially how an abacus works, if you’ve ever used one. The earliest computers to be designed, automated this process, using mechanisms. Charles Babbage’s famous, albeit never built, Analytical Engine worked on exactly this basic principle, and if it had been constructed it would have truly been the world’s first computer.
Essentially, the way these old mechanical computers work is to use the positions of their mechanical parts to perform mathematical and logic functions. This is actually the fundamental way in which all computers work. Since the discovery of electricity and the invention of electronics, computers have worked using electric circuits – effectively using the position of electrons instead of the position of actual moving parts. As technology has progressed, computers have become faster, smaller, and more reliable, until the world around us today.
In modern electronics, silicon is king. Silicon-based electronics are the standard used everywhere, though they’re reaching the limit of what they’re capable of. For the next generation of electronics, some people are beginning to advocate new materials, such as graphene, over silicon. But ultimately, others have a higher goal. Proponents of quantum computing believe that in the future, the most vital components of computers will not be electronics at all, but single atoms.
In quantum mechanics, any single particle, from an electron to an atomic nucleus, has a set of properties which can often be changed quite easily. Where past computers used motion of mechanical parts and modern computers use motion of electrons, quantum computers will use changes in the properties of these particles to perform their calculations.
One such quantum property is known as spin (the same property behind magnetism), and this is what the UNSW engineers managed to manipulate. They based their qubit on a single silicon atom and demonstrated how they used changes in the nuclear spin of the nucleus to store and retrieve information. Andrea Morello at UNSW’s School of Electrical Engineering and Telecommunications described how; “We have adapted magnetic resonance technology, commonly known for its application in chemical analysis and MRI scans, to control and read-out the nuclear spin of a single atom in real time.”
Many atomic nuclei are essentially very weak magnets due to their spin, which can have a value of either “up” or “down” (electrons and many other particles possess similar properties). However, because quantum mechanics bends the rules, there’s also a third state which is actually both up and down simultaneously. This state is known as a quantum superposition and makes everything a little more complicated.
Using the up and down positions of spin in the same way that binary code uses ones and zeroes, researchers manipulated a single atomic nucleus, writing a value to its spin and then retrieving that value. “We achieved a read-out fidelity of 99.8 per cent, which sets a new benchmark for qubit accuracy in solid-state devices,” commented Andrew Dzurak, Director of the Australian National Fabrication Facility at UNSW.
Using the technology they’re developing, qubits which work using nuclear spin in this way could be integrated into quantum computers to provide memory functions or to implement logic gates. This is perfectly in line with existing ideas about how quantum computers may work, using the spin of electrons as processors.
The previous best qubit was effective but rather impractical. A single atom trapped in an electromagnetic field inside a vacuum chamber. While this managed to win a Nobel prize in physics, its potential applications are rather limited. The team at UNSW have suceeded in creating qubit which is much more readily useable in technology.
“Our nuclear spin qubit operates at a similar level of accuracy, but it’s not in a vacuum chamber – it’s in a silicon chip that can be wired up and operated electrically like normal integrated circuits,” explained Morello. “Silicon is the dominant material in the microelectronics industry, which means our qubit is more compatible with existing industry technology and is more easily scalable.”
Working with atomic nuclei isn’t easy. While most of the mass of an atom is contained in its nucleus, it’s difficult to visualise exactly how small they are. A nucleus of an atom has a diameter roughly one millionth the size of the atom itself – imagine a pea sitting in the centre of a sports arena and you’re thinking along the right lines.
“This means it’s more challenging to measure, but it’s almost completely immune to disturbances from the outside world, which makes it an exceptional quantum bit,” explains Jarryd Pla, lead author on the study. “Our nuclear spin qubit can store information for longer times and with greater accuracy. This will greatly enhance our ability to carry out complex quantum calculations once we put many of these qubits together.”
Emboldened by their success, the team at UNSW are presently working on demonstrations for both quantum memories an two-qubit logic gates, as well as looking at ways to further improve their existing technology.
Image: Composition showing theoretical representations of qubits superimposed onto an electron microscope image of a sheet of atoms. Credits: Glosser.ca/jbw2 & White Timberwolf/Erwinrossen/Wikimedia Commons