Key component for quantum computers miniaturized by 1000 times

The Microwave circulator is a key part of many quantum computer implementations and researchers has been miniaturized by 1000 times. Making parts like these smaller will help enable quantum computers with millions of qubits.

Above – Lead author of the study, PhD candidate Alice Mahoney, in the quantum science laboratories at the Sydney Nanoscience Hub.

The Sydney team’s component, coined a microwave circulator, acts like a traffic roundabout, ensuring that electrical signals only propagate in one direction, clockwise or anti-clockwise, as required. Similar devices are found in mobile phone base-stations and radar systems, and will be required in large quantities in the construction of quantum computers. A major limitation, until now, is that typical circulators are bulky objects the size of your hand.

They used the properties of topological insulators to slow the speed of light in the material. This miniaturization paves the way for many circulators to be integrated on a chip and manufactured in the large quantities that will be needed to build quantum computers.

The work to scale-up quantum computing is driving breakthroughs in related areas of electronics and nanoscience.

“It is not just about qubits, the fundamental building blocks for quantum machines. Building a large-scale quantum computer will also need a revolution in classical computing and device engineering,” Professor Reilly said.

“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.”

Lead author of the paper and PhD candidate Alice Mahoney said: “Such compact circulators could be implemented in a variety of quantum hardware platforms, irrespective of the particular quantum system used.”

A practical quantum computer is still some years away. Scientists expect to be able to carry out currently unsolvable computations with quantum computers that will have applications in fields such as chemistry and drug design, climate and economic modeling, and cryptography.

Professor David Reilly is director of the University of Sydney’s Microsoft Quantum Laboratory, a multimillion dollar partnership, which is part of a global effort by Microsoft to build the world’s first practical quantum computer. The partnership is housed in the Sydney Nanoscience Hub, the flagship building of the University of Sydney Nano Institute.

Nature Communications – Zero-field edge plasmons in a magnetic topological insulator.


Incorporating ferromagnetic dopants into three-dimensional topological insulator thin films has recently led to the realisation of the quantum anomalous Hall effect. These materials are of great interest since they may support electrical currents that flow without resistance, even at zero magnetic field. To date, the quantum anomalous Hall effect has been investigated using low-frequency transport measurements. However, transport results can be difficult to interpret due to the presence of parallel conductive paths, or because additional non-chiral edge channels may exist. Here we move beyond transport measurements by probing the microwave response of a magnetised disk of Cr-(Bi,Sb)2Te3. We identify features associated with chiral edge plasmons, a signature that robust edge channels are intrinsic to this material system. Our results provide a measure of the velocity of edge excitations without contacting the sample, and pave the way for an on-chip circuit element of practical importance: the zero-field microwave circulator.