Lithium niobate (LN), an outstanding and versatile material, has influenced our daily life for decades: from enabling high-speed optical communications that form the backbone of the Internet to realizing radio-frequency filtering used in our cell phones. This half-century-old material is currently embracing a revolution in thin-film LN integrated photonics. The success of manufacturing wafer-scale, high-quality, thin films of LN on insulator (LNOI), accompanied with breakthroughs in nanofabrication techniques, have made high-performance integrated nanophotonic components possible. With rapid development in the past few years, some of these thin-film LN devices, such as optical modulators and nonlinear wavelength converters, have already outperformed their legacy counterparts realized in bulk LN crystals. Furthermore, the nanophotonic integration enabled ultra-low-loss resonators in LN, which unlocked many novel applications such as optical frequency combs and quantum transducers. In this Review, we cover -- from basic principles to the state of the art -- the diverse aspects of integrated thin-film LN photonics, including the materials, basic passive components, and various active devices based on electro-optics, all-optical nonlinearities, and acousto-optics. We also identify challenges that this platform is currently facing and point out future opportunities. The field of integrated LNOI photonics is advancing rapidly and poised to make critical impacts on a broad range of applications in communication, signal processing, and quantum information.

We demonstrate a new approach to supercontinuum generation and carrier-envelope-offset detection in dispersion-engineered nanophotonic waveguides based on saturated second-harmonic generation of femtosecond pulses. In contrast with traditional approaches based on self-phase modulation, this technique simultaneously broadens both harmonics by generating rapid amplitude modulations of the field envelopes. The generated supercontinuum produces coherent carrier-envelope-offset beatnotes in the overlap region that remain in phase across 100’s of nanometers of bandwidth while requiring <10 picojoules of pulse energy.

A high-resolution broad-spectral-bandwidth spectrometer on a chip would create new opportunities for gas-phase molecular fingerprinting, especially in environmental sensing. A resolution high enough to observe transitions at atmospheric pressure and the simultaneous sensitive detection of multiple atoms or molecules are the key challenges. Here, an electro-optic microring-based dualcomb interferometer, fabricated on a low-loss lithium-niobate-on-insulator nanophotonic platform, demonstrates significant progress towards such an achievement. Spectra spanning 1.6 THz (53 cm^-1) at a resolution of 10 GHz (0.33 cm^-1) are obtained in a single measurement without requiring frequency scanning or moving parts. The frequency agility of the system enables spectrally-tailored multiplexed sensing, which allows for interrogation of non-adjacent spectral regions, here separated by 6.6 THz (220 cm^-1), without compromising the signal-to-noise ratio.

Just as “classical” information technology rests on a foundation built of interconnected information-processing systems, quantum information technology (QIT) must do the same. A critical component of such systems is the “interconnect,” a device or process that allows transfer of information between disparate physical media, for example, semiconductor electronics, individual atoms, light pulses in optical fiber, or microwave fields. While interconnects have been well engineered for decades in the realm of classical information technology, quantum interconnects (QuICs) present special challenges, as they must allow the transfer of fragile quantum states between different physical parts or degrees of freedom of the system. The diversity of QIT platforms (superconducting, atomic, solid-state color center, optical, etc.) that will form a “quantum internet” poses additional challenges. As quantum systems scale to larger size, the quantum interconnect bottleneck is imminent, and is emerging as a grand challenge for QIT. For these reasons, it is the position of the community represented by participants of the NSF workshop on “Quantum Interconnects” that accelerating QuIC research is crucial for sustained development of a national quantum science and technology program. Given the diversity of QIT platforms, materials used, applications, and infrastructure required, a convergent research program including partnership between academia, industry, and national laboratories is required.

Lithium niobate (LN) is an excellent nonlinear optical and electro-optic material that has found many applications in classical nonlinear optics, optical fiber communications and quantum photonics. Here we review the recent development of thin-film LN technology that has allowed the miniaturization of LN photonic devices and microcavities with ultra-high quality factors. We discuss the design principle of LN devices that makes use of the largest nonlinear coefficients, various device fabrication approaches and resulting device performances, and the current and potential applications of LN microcavities.