New Research Using Topology Opens a Path to Build a New Type of Quantum Bit

For the first time, Australian scientists have demonstrated the protection of correlated states between paired photons—packets of light energy—using the interesting physical concept of topology. This innovative breakthrough opens a new way to build a new type of quantum bit, which are the building blocks for quantum computers.

The research, developed in conjunction with Israeli coworkers, was recently published in the prestigious journal, Science, a recognition of how vital this foundational work is.

Dr. Andrea Blanco-Redondo from the University of Sydney Nano Institute and lead author of the study explained that they can now propose a pathway to building robust entangled states for logic gates by employing protected pairs of photons.

Logic gates are the switches required to operate algorithms written for quantum computers. When it comes to typical computational switches, they are in simple binary forms of zero or one. However, quantum switches exist in a state of ‘superposition’ that combines both zero and one.

In fact, one of the biggest hurdles in modern physics is protecting quantum information long enough for quantum machines to conduct necessary calculations. Useful quantum computers will need millions or billions of qubits to process information. To date, the best experimental devices only have about 20 qubits.

To harness the real potential of quantum technology, scientists need to determine how to protect the entangled superposition of quantum bits—or qubits—on the nanoscale. Efforts to achieve this using superconductors and trapped ions have been promising, but they are highly susceptible to electromagnetic interference, making them incredibly tricky to scale into useful machines.

The use of packets of light energy, known as photons, instead of electrons has been one suggested alternative upon which to build logic gates that can calculate quantum algorithms.

Unlike electrons, photons are well isolated from the thermal and electromagnetic environment. But, scaling quantum devices based on photonic qubits has been limited because of scattering loss and other errors. That is until now.

Dr. Blanco-Redondo, the Messel Research Fellow in the School of Physics, explained that what they did is develop a novel lattice structure of silicon nanowires, resulting in a particular symmetry that offers unusual strength to the photons’ correlation. The symmetry helps create and guide these correlated states, called ‘edge modes.

Dr. Blanco-Redondo added that this strength comes from the underlying topology, a global property of the lattice that stays unchanged against disorder.

The correlation this generates is required to build entangled states for quantum gates.

Channels, or waveguides, built using silicon nanowires just 500 nanometres wide, were lined up in pairs with a purposeful defect in symmetry through the middle, resulting in two lattice structures with different topologies and an intervening ‘edge.’

This topology enables the creation of unique modes where the photons can pair up—known as ‘edge modes.’ These modes permit information to be carried by the paired photons and transported robustly that otherwise would have been scattered and lost across a uniform lattice.

Dr. Blanco-Redondo engineered and experimented the Sydney Nanoscience Hub with Dr. Bryn Bell, previously at the University of Sydney and now at the University of Oxford.

The photons were created by high-intensity, ultra-short laser pulses, the same underlying technology that Donna Strickland and Gerard Mourou were awarded the 2018 Nobel Prize in Physics.

In that past ten years, there have been many breakthrough discoveries in topological states of matter. This research marks the latest findings in this field. These topological features protect classical and quantum information in areas as diverse as electromagnetism, condensed matter, acoustics and cold atoms.

Microsoft Quantum Laboratories, including the one in Sydney, are working on the development of electron-based qubits where quantum information is topologically protected through the knotting of quasiparticles known as Majorana fermions. This is similar to braiding half electron states induced through the interaction of superconductors and semiconducting metals.

Topologically protected states have been demonstrated for single photons before.

But, Dr. Blanco-Redondo explained that quantum information systems would depend on multiphoton states, highlighting the importance of this discovery for further development.”

She said the next step would be to improve protection of the photon entanglement to create robust, scalable quantum logic gates.

Professor Stephen Bartlett, a theoretical quantum physicist at Sydney Nano who is unconnected to the study, said that it is still unclear what this means for quantum computing since it is still in the nascent stages. But the hope is that the protection offered by these edge modes could be used to protect photons from the types of noise that cause issues for quantum applications.


Image Credit: Amin Van / Shutterstock


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