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Induced photon correlations are directly demonstrated by exploring two coupled nonlinear processes in an integrated device. Using orthogonally polarized modes within an integrated microring cavity, phase matching of two different nonlinear four‐wave mixing processes is achieved simultaneously, wherein both processes share one target frequency mode, while their other frequency modes differ. The overlap of these modes leads to the coupling of both nonlinear processes, producing photon correlations. The nature of this process is confirmed by means of time‐ and power‐dependent photon correlation measurements. These findings are relevant to the fundamental understanding of spontaneous parametric effects as well as single‐photon‐induced processes, and their effect on optical quantum state generation and control.

The investigation of integrated frequency comb sources characterized by equidistant spectral modes was initially driven by considerations towards classical applications, seeking a more practical and miniaturized way to generate stable broadband sources of light. Recently, in the context of scaling the complexity of optical quantum circuits, these on-chip approaches have provided a new framework to address the challenges associated with non-classical state generation and manipulation. For example, multi-photon and high-dimensional states were to date either inaccessible, lacked scalability, or were difficult to manipulate, requiring elaborate approaches. The emerging field of quantum frequency combs studying spectral multimode Manuscript

The development of quantum technologies for quantum information (QI) science demands the realization and precise control of complex (multipartite and high dimensional) entangled systems on practical and scalable platforms. Quantum frequency combs (QFCs) represent a powerful tool towards this goal. They enable the generation of complex photon states within a single spatial mode as well as their manipulation using standard fiber-based telecommunication components. Here, we review recent progress in the development of QFCs, with a focus on results that highlight their importance for the realization of complex quantum states. In particular, we outline recent work on the use of integrated QFCs for the generation of high-dimensional multipartite optical cluster states -lying at the basis of measurement-based quantum computation. These results confirm that the QFC approach can provide a stable, practical, low-cost, and established platform for the development of quantum technologies, paving the way towards the advancement of QI science for out-of-the-lab applications, ranging from practical quantum computing to more secure communications. Index Terms-coherent control of photon states, computing and information science, fiber-based telecommunications, highdimensional multipartite entanglement, photon cluster states, photonic integrated circuits, practical and scalable quantum technology, quantum frequency combs.

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