A key challenge for quantum science and technology is to realize large-scale, precisely controllable, practical systems for non-classical secured communications, metrology and, ultimately, meaningful quantum simulation and computation. Optical frequency combs represent a powerful approach towards this goal, as they provide a very high number of temporal and frequency modes that can result in large-scale quantum systems. The generation and control of quantum optical frequency combs will enable a unique, practical and scalable framework for quantum signal and information processing. Here, we review recent progress on the realization of energy–time entangled optical frequency combs and discuss how photonic integration and the use of fibre-optic telecommunications components can enable quantum state control with new functionalities, yielding unprecedented capability. This Review describes quantum frequency combs that operate via photon entanglement, beginning with mode-locked quantum frequency combs followed by energy–time entanglement methods. The use of photonic integration and fibre-optic telecommunications components in enabling the quantum state control are also discussed.

Quantum optical microcombs