Topology yields miniaturisation breakthrough for quantum computing

Exploiting an exotic state of matter has led to a major step forward in the race to revolutionise computing. Andrew Masterson reports.

A prototype microwave circulator developed by a team at Sydney University, shown next to a five cent coin.
A prototype microwave circulator developed by a team at Sydney University, shown next to a five cent coin.
Alice Mahoney

The research that led to the 2016 Nobel Prize in Physics has found concrete expression and practical application, thanks to research by the University of Sydney and Microsoft.

The prize that year was awarded to American physicists David Thouless, Duncan Haldane and Michael Kosterlitz, ”for theoretical discoveries of topological phase transitions and topological phases of matter”.

By using a mathematical construct called topology – an approach that focusses on object properties preserved despite shape being deformed – Thouless and his colleagues offered fresh insight into exotic phases of matter, including superconductors and superfluids.

Now, researchers have used that research to refine an exotic phase of matter known as a topological insulator. The result is a component that is critical for the effective development of quantum computing.

Topological insulators, first discovered in 2006, are materials that have interiors that function as insulators, but exteriors that support the flow of electrons and thus function as conductors. They also exhibit what’s known as “time-reversal symmetry” – physics that is independent whether time is flowing forwards or backwards.

Researchers working on developing quantum computing have long acknowledged that the unique qualities of topological insulators mean that they can function as a connector between quantum and classical systems. Such a connection is essential for the practical realisation of quantum computers.

In a paper in the journal Nature Communications, researchers led by David Reilly, director of the Microsoft Quantum Laboratory at Sydney Uni, demonstrate how topological insulating materials can be manipulated to produce a miniaturised version of an electronic engineering component called a microwave circulator.

These devices effectively shunt microwave signals in through one port and funnel them, as in a roundabout, to a subsequent exit, ensuring everything moves in a consistent clockwise (or anti-clockwise) direction.

Microwave circulators are already commonplace in large structures such as mobile phone relay stations and radar systems, but until now no one has been able to build one any smaller than roughly the size of a human hand – thus excluding them from use in computers.

All that is now about to change. By exploiting the properties of topological insulators, Reilly and his colleagues have succeeded in building a microwave circulator that is 1000 times smaller than its predecessors.

So small is the new device that potentially hundreds can be integrated onto a single computer chip – paving the way for the kind of precise manipulation of the large numbers of quantum bits (“qubits”) that are an essential prerequisite for quantum computing.

“It is not just about qubits, the fundamental building blocks for quantum machines,” Reilly says.

“Building a large-scale quantum computer will also need a revolution in classical computing and device engineering.

“Even if we had millions of qubits today, it is not clear that we have the classical technology to control them. Realising a scaled-up quantum computer will require the invention of new devices and techniques at the quantum-classical interface.”

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Andrew Masterson is news editor of Cosmos.
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