Beneath many cities are complex networks of optical fibres that carry data, encoded in pulses of light, to offices and homes. Researchers from the National University of Singapore (NUS) and Singtel, Asia’s leading communications technology group, have demonstrated a technique that will help pairs of light particles smoothly navigate these networks, a breakthrough that will enable stronger cyber security. The demonstration was performed over 10 km of Singtel’s fibre network. This project, conducted in Singapore, is driven by the NUS-Singtel Cyber Security Research & Development Laboratory, a public-private partnership supported by the National Research Foundation, Prime Minister’s Office, Singapore. It relies on the expertise from the Centre for Quantum Technologies (CQT) at NUS.