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 largest χ ( 2 ) tensor component d 33 via quasi-phase matching. The corresponding single-photon coupling rate g / 2 π is estimated to be 1.2 MHz, which is an important milestone as it approaches the dissipation rate κ / 2 π of best-available lithium niobate microresonators developed in the community. Using a figure of merit defined as g / κ , 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.

Hybrid quantum systems are essential for the realization of distributed quantum networks. In particular, piezo-mechanics operating at typical superconducting qubit frequencies features low thermal excitations, and offers an appealing platform to bridge superconducting quantum processors and optical telecommunication channels. However, integrating superconducting and optomechanical elements at cryogenic temperatures with sufficiently strong interactions remains a tremendous challenge. Here, we report an integrated superconducting cavity piezo-optomechanical platform where 10 GHz phonons are resonantly coupled with photons in a superconducting cavity and a nanophotonic cavity at the same time. Taking advantage of the large piezo-mechanical cooperativity (C em~7) and the enhanced optomechanical coupling boosted by a pulsed optical pump, we demonstrate coherent interactions at cryogenic temperatures via the observation of efficient microwave-optical photon conversion. This hybrid interface makes a substantial step towards quantum communication at large scale, as well as novel explorations in microwave-optical photon entanglement and quantum sensing mediated by gigahertz phonons.

Materials with strong second-order ( χ ( 2 ) ) optical nonlinearity, especially lithium niobate, play a critical role in building optical parametric oscillators (OPOs). However, chip-scale integration of low-loss χ ( 2 ) materials remains challenging and limits the threshold power of on-chip χ ( 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 ( ∼ 30 µ W ) is 400 times lower than that in previous χ ( 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.

We demonstrate waveguide-integrated superconducting nanowire single-photon detectors on thin-film lithium niobate (LN). Using a 250 µm-long NbN superconducting nanowire lithographically defined on top of a 125 µm-long LN nanowaveguide, on-chip detection efficiency of 46% is realized with simultaneous high performance in dark count rate and timing jitter. As LN possesses high second-order nonlinear χ (2) and electro-optic properties, an efficient single-photon detector on thin-film LN opens up the possibility to construct small-scale fully-integrated quantum photonic chip which includes single-photon sources, filters, tunable quantum gates and detectors. Waveguide-integratedsuperconducting nanowire single-photon detectors (SNSPDs) are powerful components that can be exploited to analyze photonic quantum states in an integrated quantum photonic circuit 1,2 . Originally, such detectors have been proposed as an alternative to traditional normal incident planar SNSPDs 3-6 . Waveguide-integrated SNSPDs rival traditional detectors in terms of efficiency, compactness, dark count rate and timing characteristics 7-10 with added benefit of being compatible with photonic circuit fabrication 1,11 . In waveguide-integrated SNSPDs, photons are absorbed by thin-film superconducting nanowires situated atop the waveguide through the evanescent coupling 7-10 . Such detectors approach unity efficiency by increasing the nanowire length 8 . Although various material platforms such as GaAs 7,12 , silicon 13 and SiN 8,11 have been employed to demonstrate such integrated detectors, a much anticipated material platform has been lithium niobate (LiNbO 3 , LN) which is among the most desirable photonic materials for classical and quantum integrated optics because of its strong second-order nonlinear χ (2) and electro-optic properties. Only recently has it been possible to fabricate integrated thin-film LN photonic devices such as modulators 14,15 , high-Q optical resonators 16,17 , low loss non-linear waveguides 18 , high-efficiency on-chip periodically poled waveguides 19,20 and rings 21,22 . With the recent experimental demonstration of ultra-high efficiency second harmonic generation 19,21,22 , it is expected that highly efficient spontaneous parametric down-conversion (SPDC) process on an integrated chip with very large signal(idler) to pump photon ratio is now feasible with current fabrication technology. As a result, it opens up the possibility of multi-qubit integration in a single nano-photonic chip 1,23-25 . The achievement of such a feat, however, is dependent on efficient singlephoton detectors on thin-film LN wavguides which has not been demonstrated so far.In this Letter, we demonstrate efficient waveguideintegrated SNSPDs on thin-film LN with on-chip detection efficiency (OCDE) of 46%, dark count rate of 13 Hz, timing jitter of 32 ps and noise equivalent power (NEP) a) Electronic mail: hong.tang@yale.edu of 1.42 × 10 −18 W/ √ Hz. Together with the ultralow loss characteristics of thin-film LN waveguides, large secondorder non-...

Superconducting cavity electro-optics presents a promising route to coherently convert microwave and optical photons and distribute quantum entanglement between superconducting circuits over long-distance. Strong Pockels nonlinearity and high-performance optical cavity are the prerequisites for high conversion efficiency. Thin-film lithium niobate (TFLN) offers these desired characteristics. Despite significant recent progresses, only unidirectional conversion with efficiencies on the order of 10−5 has been realized. In this article, we demonstrate the bidirectional electro-optic conversion in TFLN-superconductor hybrid system, with conversion efficiency improved by more than three orders of magnitude. Our air-clad device architecture boosts the sustainable intracavity pump power at cryogenic temperatures by suppressing the prominent photorefractive effect that limits cryogenic performance of TFLN, and reaches an efficiency of 1.02% (internal efficiency of 15.2%). This work firmly establishes the TFLN-superconductor hybrid EO system as a highly competitive transduction platform for future quantum network applications.