The on-chip generation of large and complex optical quantum states will enable low-cost and accessible advances for quantum technologies, such as secure communications and quantum computation.Integrated frequency combs are on-chip light sources with a broad spectrum of evenly-spaced frequency modes, commonly generated by four-wave mixing in optically-excited nonlinear micro-cavities, whose recent use for quantum state generation has provided a solution for scalable and multi-mode quantum light sources.Pulsed quantum frequency combs are of particular interest, since they allow the generation of singlefrequency-mode photons, required for scaling state complexity towards, e.g., multi-photon states, and for quantum information applications.However, generation schemes for such pulsed combs have, to date, relied on micro-cavity excitation via lasers external to the sources, being neither versatile nor power-efficient, and impractical for scalable realizations of quantum technologies.Here, we introduce an actively-modulated, nested-cavity configuration that exploits the resonance pass-band characteristic of the micro-cavity to enable a mode-locked and energy-efficient excitation.We demonstrate that the scheme allows the generation of high-purity photons at large coincidence-to-accidental ratios (CAR).Furthermore, by increasing the repetition rate of the excitation field via harmonic modelocking (i.e.driving the cavity modulation at harmonics of the fundamental repetition rate), we managed to increase the pair production rates (i.e.source efficiency), while maintaining a high CAR and photon purity.Our approach represents a significant step towards the realization of fully on-chip, stable, and versatile sources of pulsed quantum frequency combs, crucial for the development of accessible quantum technologies.

Practical system for the generation of pulsed quantum frequency combs