Sydney scientists develop architecture for quantum computers

Xanthe Croot works in the Sydney Nanoscience Hub.

Australian scientists have developed an architecture for quantum computers that will help overcome interference caused by quantum bits being too close to each other.

There is a global scientific race to build a quantum computer – a machine that unlocks the strange behaviour of quantum mechanics to calculate completely new types of problems. Australia is recognised as a global leader in the emerging technology.

The problem is that the quantum bits, or qubits, needed to build the machines are finickity and susceptible to electromagnetic noise from the environment which decoheres their quantumness, reducing their calculations to normal, classical switches.

The qubits need to be entangled to work as quantum switches, or logic gates.

READ: Goal to build silicon-based quantum computer ramps up

This requires proximity at the order of 10s of nanometres, thousands of times smaller than the width of human hair.

Due to this tight packing, controlling individual qubits without interfering with the operation of nearby qubits remains an open challenge.

Research team leader and director of the Sydney Microsoft Quantum Laboratory, David Reilly, said building quantum computers with single electrons in semiconductors has an advantage that the devices can be incredibly small – packing a lot in a small area. But that’s also a challenge, he said.

“If you have the luxury of spacing things out a bit, scaling the devices is much more straightforward. So, what’s described in this paper is a way of coupling qubits that aren’t direct neighbours,’ said Reilly.

Xanthe Croot, now at Princeton University, and her colleague Sebastian Pauka, a doctoral candidate at the School of Physics at the University of Sydney, have designed a work-around to separate entangled electrons while still allowing them to remain coupled.

The technique uses trapped electrons in tiny nanoscale semiconductors called quantum dots for semiconductor-based qubits.

The entangled electrons can be separated while remaining correlated via puddles of other electrons.

This indirect coupling process should allow increased design flexibility and reduce the crowding on quantum chips for scaled-up devices.

Working with Professor Reilly at the ARC Centre of Excellence for Engineered Quantum Systems, Croot and Pauka have developed a design that uses a ‘puddle of electrons’ through which the qubit electrons can interact.

“This large puddle of electrons can have unoccupied energy levels through which the qubit electrons can virtually interact,” said Croot.

The proposed architecture is designed to be tailored depending on the material used to make the quantum bits.