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  1. Materials with strong second-order (χ<#comment/>(2)) optical nonlinearity, especially lithium niobate, play a critical role in building optical parametric oscillators (OPOs). However, chip-scale integration of low-lossχ<#comment/>(2)materials remains challenging and limits the threshold power of on-chipχ<#comment/>(2)OPO. Here we report an on-chip lithium niobate optical parametric oscillator at the telecom wavelengths using a quasi-phase-matched, high-quality microring resonator, whose threshold power (∼<#comment/>30µ<#comment/>W) is 400 times lower than that in previousχ<#comment/>(2)integrated photonics platforms. An on-chip power conversion efficiency of 11% is obtained from pump to signal and idler fields at a pump power of 93 µW. The OPO wavelength tuning is achieved by varying the pump frequency and chip temperature. With the lowest power threshold among all on-chip OPOs demonstrated so far, as well as advantages including high conversion efficiency, flexibility in quasi-phase-matching, and device scalability, the thin-film lithium niobate OPO opens new opportunities for chip-based tunable classical and quantum light sources and provides a potential platform for realizing photonic neural networks.

  2. The absence of the single-photon nonlinearity has been a major roadblock in developing quantum photonic circuits at optical frequencies. In this paper, we demonstrate a periodically poled thin film lithium niobate microring resonator (PPLNMR) that reaches 5,000,000%/W second-harmonic conversion efficiency—almost 20-fold enhancement over the state-of-the-art—by accessing its largestχ<#comment/>(2)tensor componentd33via quasi-phase matching. The corresponding single-photon coupling rateg/2π<#comment/>is estimated to be 1.2 MHz, which is an important milestone as it approaches the dissipation rateκ<#comment/>/2π<#comment/>of best-available lithium niobate microresonators developed in the community. Using a figure of merit defined asg/κ<#comment/>, our device reaches a single-photon nonlinear anharmonicity approaching 1%. We show that, by further scaling of the device, it is possible to improve the single-photon anharmonicity to a regime where photon blockade effect can be manifested.