Sydney scientists develop work-around for quantum architecture

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: they are 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. 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, Professor 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.

“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.”

Dr 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 (EQuS), Dr Croot and Mr Pauka have developed a design that uses a ‘puddle of electrons’ through which the qubit electrons can interact.

Dr Croot completed her PhD at Sydney this year and has won a Dickie Fellowship at Princeton University. She said: “This large puddle of electrons can have unoccupied energy levels through which the qubit electrons can virtually interact.”

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

Co-author Mr Pauka said: "One of the challenges for spin qubits is to figure out how to scale up the number of dots from one or two, to much larger numbers. The ability to couple qubits over longer distances is an important part of this goal. By creating a structure that enables long-distance coupling, we can now begin creating much larger arrays of devices in a scaleable way."

The research appeared in Physical Review Applied. It was done in collaboration with scientists at Purdue University.