Sydney Uni follows the light and steps closer to quantum computing

A significant barrier to quantum computing - the fragility of the link between entangled electrons exhibiting Einstein’s “spooky action at a distance” - could be overcome by using paired particles of light (photons) instead.

Researchers at the University of Sydney have demonstrated that biphoton states could be a good candidate on which to build quantum bits (qubits), since they are less affected by electromagnetic interference than electrons.

“We can now propose a pathway to build robust entangled states for logic gates using protected pairs of photons,” said lead author Dr Andrea Blanco-Redondo at the University of Sydney Nano Institute.

Logic gates are the swtiches needed to operate algorithms written for quantum computers.

Whereas classical computational switches operate in the binary forms of zero or one, quantum switches exist in a “superposition” of zero and one combined - a state so fragile it must be kept as close to absolute zero as possible.

So far the biggest electron-based experimental devices have about 20 qubits, although a number of companies and researchers unveiled plans to build devices with qubits in the 50-to-100 ballpark.

While photons are relatively well isolated from thermal and electromagnetic disturbances, efforts to build quantum computers out of photonic qubits have been limited to scattering loss in the optical fibres used to transmit the photons.

Blanco-Redondo and her team have overcome this by developing a new lattice structure of silicon nanowires, creating “a particular symmetry that provides unusual robustness to the photons’ correlation,” she said.

“The symmetry both helps create and guide these correlated states, known as ‘edge modes’.

“This robustness stems from the underlying topology, a global property of the lattice that remains unchanged against disorder.”

The correlation between photons produced by the lattice is needed to build entangled states for the quantum logic gates.

The silicon nanowires, only 500 nanometres wide, were formed into channels or waveguides and lined up in pairs with a deliberate defect in symmetry through the middle, creating two lattice structures with different topologies and an intervening ‘edge’.

These topologies all for the creation of the ‘edge mods’ in which the photons can pair up, allowing information to be carried by the paired photons rather than being lost and scattered across a uniform lattice.

Scaling up the system from single-photon set-ups will be the next step for the team.

“Quantum information systems will rely on multiphoton states, highlighting the importance of this discovery for further development,” Blaco-Redondo added.

Theoretical quantum physicist at Sydney Nano professor Stephen Bartlett, who was not involved in the study, said Blanco-Redondo’s result is “exciting at a fundamental level because it shows the existence of protected modes attached to the boundary of a topologically ordered material”.

“What it means for quantum computing is unclear as it is still early days. But the hope is that the protection offered by these edge modes could be used to protect photons from the types of noise that are problematic for quantum applications.”