Non-reciprocity in quantum technologies

Abstract

Non-reciprocity—the ability to transmit signals in one direction while blocking them in the reverse—has become an important resource for quantum technologies, enabling directional amplification, routing of quantum information and topologically protected quantum states. Recent experimental advances have demonstrated non-reciprocal behaviour in low-loss, fully integrated devices operating with weak or no magnetic bias fields. Many of the most mature demonstrations so far have been realized in microwave-frequency and superconducting circuits, but progress continues in parallel in optical and nanophotonic systems. These achievements overcome the limitations of more traditional approaches that use stronger magnetic bias fields. This makes non-reciprocity compatible with superconducting circuits and scalable quantum photonic architectures, as well as an integral part of the next generation of modular quantum computers, distributed quantum networks and precision metrology. Here we highlight the key concepts for engineering non-reciprocity in quantum systems and describe how this functionality can be used for high-fidelity qubit readout, robust quantum state transfer and boosting the sensitivity of quantum sensors. Non-reciprocity in the quantum regime promises unidirectional transport and enhanced sensing, but traditional approaches hinder device integration. This Review highlights magnet-free routes for engineering non-reciprocity in quantum systems.