Quantum secure direct communication (QSDC) based on entanglement can directly transmit confidential information. However, the inability to simultaneously distinguish the four sets of encoded entangled states limits its practical application. Here, we explore a QSDC network based on time–energy entanglement and sum-frequency generation. In total,15 users are in a fully connected QSDC network, and the fidelity of the entangled state shared by any two users is >97%. The results show that when any two users are performing QSDC over 40 km of optical fiber, the fidelity of the entangled state shared by them is still >95%, and the rate of information transmission can be maintained above 1 Kbp/s. Our result demonstrates the feasibility of a proposed QSDC network and hence lays the foundation for the realization of satellite-based long-distance and global QSDC in the future.

Measuring topological invariants is an essential task in characterizing topological phases of matter. They are usually obtained from the number of edge states due to the bulk-edge correspondence or from interference since they are integrals of the geometric phases in the energy band. It is commonly believed that the bulk band structures could not be directly used to obtain the topological invariants. Here, we implement the experimental extraction of Zak phase from the bulk band structures of a Su-Schrieffer-Heeger (SSH) model in the synthetic frequency dimension. Such synthetic SSH lattices are constructed in the frequency axis of light, by controlling the coupling strengths between the symmetric and antisymmetric supermodes of two bichromatically driven rings. We measure the transmission spectra and obtain the projection of the time-resolved band structure on lattice sites, where a strong contrast between the non-trivial and trivial topological phases is observed. The topological Zak phase is naturally encoded in the bulk band structures of the synthetic SSH lattices, which can hence be experimentally extracted from the transmission spectra in a fiber-based modulated ring platform using a laser with telecom wavelength. Our method of extracting topological phases from the bulk band structure can be further extended to characterize topological invariants in higher dimensions, while the exhibited trivial and non-trivial transmission spectra from the topological transition may find future applications in optical communications.

Efficient wave mixers based on lithium-niobate-on-insulator (LNOI) hold great potential for next-generation photonic integrated circuits in both classical and quantum optics. However, achieving high-performance nonlinear photonic devices readily suitable for scalable, fiber-compatible applications remains challenging. Here, we report on the fabrication of LNOI micrometer waveguides, i.e., micro-waveguides, with a combination of ultraviolet lithography and deep dry etching technology for efficient nonlinear photonics applications. We fabricate periodically poled LNOI micro-waveguides with a cross section of ∼3×4µm² and demonstrate a fiber–chip–fiber second-harmonic generation conversion efficiency of 1320%/W with an insertion loss of 3.8 dB at the telecommunication band. We also demonstrate high-quality photon pair generation via spontaneous parametric downconversion with a flux of 178 MHz/mW at sub-mW pump power and coincidence-to-accidental ratio >8000 at microwatt pump power. The overall performance in both applications is on par with that of state-of-the-art counterparts using thin-film lithium niobate nano-waveguides. The technique would make micrometer-thick LNOI an attractive platform for ready applications in nonlinear and quantum optics.

Whispering-gallery-mode microresonators, one of the most important components of photonic integrated circuits, have the ability to dramatically boost light–matter interactions. In this Letter, we demonstrate effective sum-frequency generation via modal phase matching in a triply resonant lithium-niobate-on-insulator microdisk resonator through two individual continuous wave pumps working in the communication band. The sum-frequency conversion efficiency is measured to be 2.22 × 10 − 6 m W − 1 . This work shows a high-efficiency frequency convertor that may be applied in on-chip integrated optics in the future.

Lithium niobate on insulator (LNOI), by taking advantage of versatile properties of lithium niobate (LN) and a large refractive index contrast, provides an ideal on-chip platform for studying a broad range of optical effects as well as developing various superior photonic devices. It is a game-changer technology for traditional LN-based applications. Especially, with recent advances in the fabrication of high-quality micro-/nano-structures and devices on the LNOI platform, LN-based integrated photonics has been propelled to new heights. In this review, we summarize the latest research advances in lithium niobate thin film (LNTF), with a special focus on nonlinear wave mixing and their photonics applications. Different types of second- and third-order nonlinear processes in LNOI micro- and nano-structures are reviewed, including nonlinear frequency conversion, frequency comb generation and supercontinuum generation. Furthermore, perspectives for photonic integrated circuits (PICs) on the LNOI platform in nonlinear optics regime are predicted.

Whispering-gallery-mode (WGM) microcavities are very important in both fundamental science and practical applications, among which on-chip second-order nonlinear microresonators play an important role in integrated photonic functionalities. Here we demonstrate resonant second-harmonic generation (SHG) and cascaded third-harmonic generation (THG) in a lithium niobate-on-insulator (LNOI) microdisk resonator. Efficient SHG in the visible range was obtained with only several mW input powers at telecom wavelengths. THG was also observed through a cascading process, which reveals simultaneous phase matching and strong mode coupling in the resonator. Cascading of second-order nonlinear processes gives rise to an effectively large third-order nonlinearity, which makes on-chip second-order nonlinear microresonators a promising frequency converter for integrated nonlinear photonics.