An optical circuit based on the principles of topology could provide protection for propagation of biphoton states, which will be needed for quantum computing. Researchers at the University of Sydney have experimentally demonstrated topological protection of biphoton states, and have shown that topological design could provide the robustness required for quantum optical circuitry. For the experiment, waveguides made using 500-nm-wide silicon nanowires were lined up in pairs with a deliberate defect in symmetry through the middle, creating two lattice structures with different topologies and an intervening “edge.” This topology allowed for the creation of special modes — called “edge modes” — in which the photons could pair up. The edge modes allowed information carried by the paired photons to be transported in a robust fashion. Otherwise the photons, and the information they carry, would likely be scattered and lost. “What we have done is develop a novel lattice structure of silicon nanowires, creating a particular symmetry that provides unusual robustness to the photons’ correlation,” said researcher Andrea Blanco-Redondo. “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 correlated states are needed to build entangled states for quantum gates. Andrea Blanco-Redondo in her photonics laboratory at the Sydney Nanoscience Hub at the University of Sydney. Courtesy of Jayne Ion/University of Sydney. Quantum computers will require millions of qubits to process information, and a way to protect the entangled superposition of these qubits is required. Use of photons (rather than electrons, which can be affected by electromagnetic interference) to build logic gates that can calculate quantum algorithms is one possible solution. However, scaling quantum devices based on photonic qubits has been limited due to scattering loss and other errors. “We can now propose a pathway to build robust entangled states for logic gates using protected pairs of photons,” Blanco-Redondo said. The next step would be to improve protection of the photon entanglement to create scalable, reliable quantum logic gates, she said. Topologically protected states have previously been demonstrated for single photons. However, quantum information systems will rely on multiphoton states, Blanco-Redondo said. Professor Stephen Bartlett said, “Dr. 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 in the 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.” The research was published in Science (https://doi.org/10.1126/science.aau4296). An artist’s rendering of correlated photons of different wavelengths on a nanowire lattice structure with a topological defect. Courtesy of Sebastian Zentilomo/University of Sydney.