Min-Kyo Seo

Ultra-high- Q , single-spatial-mode micro-resonators are essential for high-coherence integrated photonics because they combine long photon lifetimes, clean modal spectra, and predictable phase matching for tailored nonlinear and quantum optical applications. In birefringent thin-film lithium niobate (TFLN), conventional traveling-wave resonators face intrinsic trade-offs between maintaining high Q factors and controlling multimode excitation, polarization rotation, and dispersion. Standing-wave resonators offer a promising alternative, but reaching ultra-high Q remains challenging. Here, a photonic-crystal Fabry–Perot micro-resonator on TFLN achieves a milestone intrinsic quality factor of up to 4.8×10 ⁷ with single-spatial-mode standing-wave resonances. The bending-free, polarization-fixed resonator with mode-selective photonic-crystal mirrors minimizes optical loss and leverages alignment with the nonlinear-optical axis of the material. A wide-core geometry suppresses sidewall scattering to a record-low, to our knowledge, propagation loss of 0.25±0.26 dB/m, approaching the material-absorption limit of TFLN. Length-dependent analysis reveals a photonic-crystal mirror insertion loss of ∼0.03 dB per reflection. The one-dimensional footprint further facilitates denser planar integration than two-dimensional resonator layouts. The scalable architecture of mode-pure, standing-wave TFLN micro-resonators opens opportunities for compact photon-pair sources, power-efficient electro-optic frequency combs, and reconfigurable quantum photonic circuits.

Integrated visible photonic engines for solid‐state quantum defects provide a foundation for scalable quantum networks. While miniaturization is advancing, active manipulation remains limited by the difficulty of achieving simultaneous milliwatt‐scale visible light generation and high‐contrast modulation. Despite extensive efforts, the concurrent chip‐scale realization of nonlinear frequency conversion and fast temporal gating for high‐fidelity quantum control has remained elusive. Here, we demonstrate a monolithic thin‐film lithium niobate (TFLN) platform integrating periodically poled frequency conversion with GHz‐bandwidth electro‐optic (EO) switching. The device delivers off‐chip green‐light power exceeding 1 mW with an extinction ratio (ER) of 42.2 dB, enabling coherent spin control and time‐resolved lifetime measurements of individual nitrogen‐vacancy (NV) centers in diamond through nanosecond gating. System performance is validated through pulsed optically detected magnetic resonance (ODMR), Rabi oscillations, and Ramsey interference, supported by time‐tagged photon counting with nanosecond resolution. By unifying sufficient nonlinear light generation with high‐speed active manipulation, this platform establishes a scalable framework for the realization of high‐rate quantum communication nodes.

Encoding quantum information in high‐dimensional photonic states as a qudit provides a powerful route to resource‐efficient quantum simulation. Among various integrated photonic platforms, lithium niobate on insulator is particularly attractive because it combines low optical loss, strong optical nonlinearity, and high‐speed electro‐optic modulation. Here, we demonstrate an electro‐optically controlled variational quantum eigensolver (VQE) on an integrated lithium niobate ququart processor. Using ququart encoding in four path modes and electro‐optic modulation, the processor enables reconfigurable high‐fidelity state preparation and projective measurements. To reduce the number of measurement groups, we implement entangled‐basis‐emulating ququart projective measurements that reproduce the measurement‐grouping role of two‐qubit entangled‐basis measurements for fully commuting Pauli operators, without requiring genuine two‐qubit entanglement or entangling gates. Using this approach, we estimate molecular ground‐state energies within the chemical‐accuracy threshold over the measured interatomic‐distance range. We further extend the platform to a chip that integrates a periodically poled lithium niobate photon‐pair source with a ququart photonic processor. These results highlight LNOI photonics as a promising platform for reconfigurable photonic quantum simulation with on‐chip photon‐pair sources.

Quantum photonics has rapidly advanced as a key area for developing quantum technologies by harnessing photons’ inherent quantum characteristics, particularly entanglement. Generation of entangled photon pairs, known as Bell states, is crucial for quantum communications, precision sensing, and quantum computing. While bulk quantum optical setups have provided foundational progress, integrated quantum photonic platforms now offer superior scalability, efficiency, and integrative potential. In this study, we demonstrate a compact and bright source of polarization-entangled Bell state utilizing continuous-wave pumping on thin film lithium niobate (TFLN) integrated photonics. Our periodically poled lithium niobate device achieves on-chip brightness of photon pair generation rate of 518.2 MHz/mW, comparable to representative integrated photon-pair sources reported across various integrated photonic platforms. This demonstration marks the first realization, to our knowledge, of polarization entanglement on TFLN platforms. Experimentally measured metrics confirm high-quality entangled photon pairs with a purity of 0.901, a concurrence of 0.9, and a fidelity of 0.941. We expect our compact quantum devices to have great potential for advancing quantum communication systems and photonic quantum technologies.

Integrated quantum photonic technologies require scalable devices with high-visibility quantum interference. Recent studies have demonstrated that lithium niobate on insulator enables efficient nonlinear interactions, via quasi-phase matching in periodically poled lithium niobate (PPLN) waveguides. However, the on-chip control of two-photon interference utilizing path entanglement remains limited. In this study, we demonstrate on-chip generation of single-photon sources via quantum interference using path-entangled N00N states in two ways. Using the basic structure of a reverse Hong–Ou–Mandel (RHOM) configuration, we directly observe high on-chip visibility of quantum interference. We achieve a visibility of 97.7% ± 1.1% using a dual-PPLN waveguide as the basic RHOM configuration. To improve the compactness, efficiency, and stability of on-chip operation, we further incorporate a PPLN ring resonator with a Sagnac loop. This allows counter-propagating paths to share a single periodically poled waveguide, achieving a visibility of 84.4% ± 1.6%. These results demonstrate that lithium niobate on insulator (LNOI)-based quantum photonic systems provide a robust foundation for the development of future integrated quantum technologies.

Magnetic skyrmions, as topological quasiparticles, have shown great potential as information carriers for next-generation spintronic technologies, including memory, logic, and computing devices. While their non-trivial topological dynamics and interactions have been extensively explored, experimental control of individual skyrmions has so far relied mainly on global stimuli such as electric currents, magnetic fields, and surface acoustic waves. Efforts to achieve precise skyrmion positioning are significantly hampered by the skyrmion Hall effect and defect pinning. Here, we demonstrate high-precision trapping and transport of individual skyrmions using a photothermal potential well, thereby extending optical manipulation into the quasiparticle realm. Moreover, by integrating deterministic optical skyrmion creation and annihilation with photothermal and conventional electrical transport methods, we establish a fully programmable platform for systematic investigation of skyrmion dynamics and interactions. This approach opens new avenues towards reconfigurable and scalable architectures for future skyrmionic device applications and enriches studies of light–quasiparticle interactions in solid-state systems.