Quanta compute differently

Text: Manuela Kuhar

David DiVincenzo is one of the leading minds in the young research field of quantum information technology. He investigates the theoretical and experimental potential of applying the rules of quantum mechanics to information processing. A physicist of outstanding international renown, he will teach and conduct research at RWTH Aachen University and Forschungszentrum Jülich.

All over the world scientists are searching for alternatives to today’s silicon chip technology. Users need ever more computing capacity, vast amounts of electricity are required, and we are reaching the limits of miniaturisation. In the near future, novel materials and switching elements could ease the situation. But David DiVincenzo is taking the longer-term, revolutionary view: “A quantum computer is very different from a digital computer in that it uses quantum mechanics at the most basic level,” he explains. Since 1993, the American scientist has been researching into what was then the completely new field of quantum information processing.

Computing with qubits

The classical PC uses bits for computing, which can either have a value of 0 or 1 – just like a switch that is either on or off, but cannot be both at once. Quantum objects on the other hand, such as single electrons, can exist in more than one state simultaneously. Quantum mechanical bits, known as qubits, can be found in superposition 0 and 1 and, in addition, exhibit so-called entanglement: the way in which quantum objects are able to interact means that they have to be considered as an entity, irrespective of how far apart they may be.

Due to these properties, quantum computers may be able to speed up certain computing processes enormously. “With ten qubits you can perform 210 computing operations at the same time. Potential processing power increases exponentially with the number of qubits,” explains DiVincenzo. Basically, quantum computers could break down numbers of any size into their prime factors at speed. This would pull the carpet out from under modern encryption technology which is based on there being no known efficient technique for doing so. Apart from this, they would also facilitate the realisation of completely bug-proof communication. Another application is the simulation of the tiniest particles in gases and solids to enhance our understanding of the nature of phenomena like magnetism or superconductivity – a task which is well beyond the capabilities of today’s computers. However, not all the algorithms used by current-day computers can be easily transferred to qubits. And so far, no-one can say whether there ever actually will be industrial strength quantum computers. Be that as it may, there are certainly enough good reasons for thoroughly investigating the laws of the tiniest particles: in nanotechnology, even today, we regularly work with such minuscule objects that the effects of quantum mechanics are starting to play an increasingly important role in many areas of technology. Who knows what undreamt of applications might still emerge? “We’re at the stage in quantum computing where conventional computing was in about 1948,” explains DiVincenzo. “Many basic algorithms didn’t even exist at that time. They were only stimulated by the actual possession of a reasonable-size computer in later decades.”

The challenge of implementation

There are many ways of creating a quantum system, whether with the aid of single atoms or molecules, liquid or solid states. The role of the qubits could be taken on by electron spins or photons, for example. But for a quantum computer to function properly it has to fulfil five criteria, and it was for drawing up these criteria that DiVincenzo became famous in the quantum community. One of the items on “DiVincenzo’s checklist” says that the superposition has to be maintained for much longer than a single computing operation. In practice, this is a huge hurdle because as soon as the quantum state interacts with its environment, the superposition is lost; so-called decoherence occurs and computation is interrupted. In order to minimise contact with the environment, up to now, high-level cooling, magnetic fields or vacuums have been required. DiVincenzo thinks it is possible that superconductors may soon carry out this task: “We’re only about a factor of 10 away from the necessary small decoherence. Over the last 10 years, the level has been decreased by about a factor of 1000.”

DiVincenzo has also developed ideas on how to use what are known as quantum dots – tiny structures of a few nanometres – for computing processes. In Germany, he will work together with experimental physicists to drive forward implementation.

Plans for the future

Since January 2011, DiVincenzo has been setting up the Institute for Quantum Information at RWTH Aachen University and the Institute of Theoretical Nanoelectronics at Forschungszentrum Jülich. In his spare time he prepares his lectures – an unusual activity after 26 years of working in industrial research. “When I started at IBM, it was not necessary to write applications to initiate research; you could work quite freely,” he enthuses. But a great deal has changed since then. “Today, work is much more programme-oriented.” This may have influenced his decision to accept an appointment in Germany. The plans of the institutes at Jülich and Aachen to employ a cross-disciplinary approach particularly appealed to him: “Here we’re going to combine solid state science and nanoscience with information processing.” Together with a team of top researchers, DiVincenzo will develop new ideas and experiments and be instrumental in establishing the two research locations.