On-chip quantum state generation by means of integrated frequency combs

Entangled photon-pair sources are key building blocks towards the realization of applications in quantum information processing [1], quantum communications [2], as well as imaging and sensing with resolutions exceeding the classical limit [3]. The generation of, e.g. polarization, time-energy and time-bin entangled photon-pairs has been demonstrated using spontaneous parametric down-conversion (SPDC) in nonlinear second-order media, as well as spontaneous four-wave mixing (SFWM) in third-order nonlinear media. Specifically, nonlinear (third-order) interactions in on-chip microring resonators have been widely used to achieve classical frequency combs [4], mode-lock lasers [5], signal processing [6], etc. Integrated photonics can also find applications for quantum state generation in compact, scalable and efficient devices, required for future optical quantum circuits. In particular, solutions focusing on an integrated (on-chip) approach have been recently investigated and developed, including integrated quantum circuits, sources and detectors [7]. In contrast to waveguides, microring resonators [8] with narrow resonances and high Q-factors, offer an improvement in photon-pair generation efficiency, as well as a narrow photon-pair bandwidth, making them compatible with quantum optical devices (e.g. high temporal-resolution single-photon detectors and quantum memories). Most importantly, in contrast to non-resonant waveguides, where individuals photon-pairs, featured by one signal/idler frequency pair, are generally produced, resonant nonlinear cavities (e.g., microring resonators) allow the generation of correlated photon-pairs on multiple signal/idler frequency channels [9], due to their periodic and equidistant resonance structure.