Marko Lončar

Diamond photonics has enabled efficient interfaces for quantum memories and is predicted to be a critical component of quantum networks. However, scalable network architectures require spatial, temporal, and spectral control of photons, which relies on nonlinear and electro-optic functionalities that diamond alone cannot provide. Here, we demonstrate the heterogeneous integration of a thin-film lithium niobate (TFLN) platform, which has strong chi-2 nonlinearity and electro-optic effects, with thin diamond films. We demonstrate high-Q diamond photonic crystal cavities (Q factors exceeding 5 × 104 at 735 nm) that are lithographically aligned with the TFLN photonic backbone and critically coupled to it. This allows us to realize low-loss diamond-TFLN “escalators” (loss ∼1 dB/coupler) that support efficient light transfer between them. At cryogenic temperatures (5 K), we can collect photons emitted from silicon vacancies embedded within the diamond structure via the TFLN photonic circuit. This approach establishes a scalable route toward integrated photonic circuits for practical quantum networking and other technologies.

Thin-film lithium niobate (TFLN) has emerged as a versatile integrated photonics platform, combining strong electro-optic and nonlinear effects. Among TFLN devices, ring resonators play a central role in filtering, modulation, and nonlinear optical processes. However, intrinsic loss—which ultimately limits ring performance—is most often summarized by single-valued metrics, and its statistical variability across resonances has received limited attention. Here, we show that intrinsic loss rates in monolithic TFLN ring resonators follow a statistical distribution, comprising a baseline loss and a tail arising from discrete loss events. This behavior is revealed by characterizing 2233 resonances, using an adiabatic waveguide-ring coupling architecture that selectively excites the fundamental mode and yields clean spectra in the ultrahigh-Qi regime. We find the most probable intrinsic loss rate κi = 2π × 10.4 MHz, indicating operation in a low-loss regime comparable to state-of-the-art thick silicon nitride platforms.

The radiative properties of emitters are inherently linked to their surrounding environment¹. Placing an electromagnetic resonator around emitters can enhance spontaneous emission, as shown by Purcell in the 1940s². This approach is now routinely used in quantum computing and communication to channel photons emitted by atoms into well-defined modes and control atom–photon interactions^{3,4,5,6,7,8,9}. For solid-state emitters, such as colour centres, the host lattice introduces an acoustic environment, allowing excited atoms to relax by emitting phonons^10,11. Here we observe the acoustic Purcell effect by constructing a specially engineered, microwave-frequency nanomechanical resonator around a colour-centre spin qubit in diamond. Using a co-localized optical mode of the structure that strongly couples to the excited state of the colour centre, we perform single-photon-level laser spectroscopy at millikelvin temperatures and observe a 10-fold faster spin relaxation when the spin qubit is tuned into resonance with a 12 GHz acoustic mode. Moreover, we use the colour centre as an atomic-scale probe to measure the broadband phonon spectrum of the nanostructure up to 28 GHz. Our work establishes a new regime of control for quantum defects in solids and paves the way for interconnects between atomic-scale quantum memories¹² and qubits encoded in acoustic and superconducting devices¹³.

Integrated coherent sources of ultra-violet (UV) light are essential for a wide range of applications, from ion-based quantum computing and optical clocks to gas sensing and microscopy. Recently, approaches that use frequency upconversion have received considerable attention. Among these, the integrated thin-film lithium niobate (TFLN) photonic platform shows particular promise. However, to date, the high propagation losses and lack of reliable techniques for consistent poling of cm-long waveguides with small poling periods have impeded progress. Here, we present a sidewall poled lithium niobate (SPLN) waveguide approach that overcomes these obstacles and results in a two-orders-of-magnitude increase in generated UV power. We demonstrate SPLN waveguides featuring record-low propagation losses of 2.3 dB/cm, complete domain inversion across the waveguide cross-section, and an optimum 50% duty cycle, resulting in a record-high normalized conversion efficiency of 5050%W ⁻¹ cm ⁻² , and 4.2 mW of generated on-chip power at 390 nm wavelength. This advancement makes the TFLN platform a viable option for high-quality on-chip UV generation, benefiting emerging applications.

The sensitivity of non-local optical measurements at low light intensities, such as those involved in long-baseline telescope arrays^1,2, is limited by fundamental quantum noise and photon losses³. Distributed quantum entanglement has been proposed as a route towards overcoming these limitations and accessing new regimes of non-local optical sensing^4-6. Here we demonstrate the use of entangled quantum memories in a quantum network of silicon-vacancy centres in diamond nanocavities^7-9 to experimentally perform such non-local phase measurements. Specifically, we combine the generation of event-ready remote quantum entanglement, photon mode erasure that hides the 'which-path' information of temporally and spatially separated incoming optical modes and non-local, non-destructive photon heralding enabled by remote entanglement to perform a proof-of-concept entanglement-assisted differential phase measurement of weak incident light between two spatially separate stations. Demonstrating successful operation of the remote phase sensing protocol with a fibre link baseline up to 1.55 km, our results provide an opportunity for a new class of quantum-enhanced optical imaging methods with potential applications ranging from long-baseline interferometry and astronomy to microscopy^10,11.

The ability to generate efficient and coherent frequency combs using photonic integrated circuits offers tremendous potential for a range of applications. X-cut thin-film lithium niobate (TFLN) is promising for developing next-generation microcomb-driven photonic systems, providing a diversity of functionalities through combined χ ( 3 ) and electro-optic effects. Normal-dispersion Kerr combs are critically needed because of their standout advantages, yet this dispersion regime remains unexplored on x-cut TFLN. Here, we design and demonstrate microresonators suitable for the robust generation of normal-dispersion Kerr combs. We show Kerr combs that substantially surpass state-of-the-art microcombs on x-cut TFLN in key performance metrics and an ultrabroad frequency comb whose existence is underpinned by both normal-dispersion Kerr dynamics and stimulated Raman scattering. Our work will unlock high-speed and low-energy-consumption photonic circuits for communications and signal processing enabled by a monolithic microcomb technology while also stimulating investigations of previously unknown nonlinear states that may synergize hybrid material nonlinearities.

Optical cavities are widely used in modern science and technology to enable a wide range of both quantum and classical applications. Recently, the growing demand for miniaturization and high performance has fueled the exploration of new fabrication methods beyond traditional polishing techniques and macroscopic mirrors. Visible and near-infrared (NIR) wavelengths are particularly important for quantum applications, where achieving low-loss resonators is also more challenging than in the telecom range, presenting unique challenges and opportunities for microscopic cavity systems. Here, we present a fabrication method for making NIR microcavities using buckled dielectric membrane mirrors, achieving a record finesse of 0.9 million at 780 nm for microcavities. We demonstrated flexible device geometries—including singular mirrors and mirror arrays, featuring radii of curvature ranging from 1 to 10 mm. The fabrication process offers high uniformity, high yield, and robust performance across a wide range of cavity lengths. Additionally, we can produce easy-to-assemble microcavity packages, with a total volume of ∼2(4)mm ³ , featuring optical modes with a linewidth of 5.16 MHz (570 kHz) and a free spectral range (FSR) of 3.18 THz (150 GHz). Our results extend the frontier of microcavity fabrication for classical and quantum photonic technologies.

Thermo-optic (TO) control is crucial for thin-film lithium niobate (TFLN) photonic integrated circuits (PICs), offering a simple and practical method for low-frequency and DC tuning while remaining compatible with high-frequency electro-optic (EO) modulation. In x-cut TFLN, the TO response is inherently anisotropic, depending on both waveguide propagation angle and polarization due to the mode-specific overlap of the electric field with the ordinary and extraordinary refractive index axes of the crystal. Despite its significance, a systematic and quantitative analysis of this anisotropy has remained elusive. Here, we present the first generalized analytical model that describes the anisotropic TO response as a function of polarization and arbitrary waveguide orientation, and rigorously validate it through numerical simulations and experiments. This study provides foundational insight into anisotropic thermal tuning and enables new opportunities for engineering energy-efficient and scalable photonic design in next-generation TFLN PICs.