On-chip coexistence of classical optical communication and quantum key distribution on etchless thin-film lithium niobate

On-chip coexistence of classical optical communication and quantum key distribution on etchless thin-film lithium niobate | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article On-chip coexistence of classical optical communication and quantum key distribution on etchless thin-film lithium niobate Yonghui Tian, Mingrui Yuan, Boyu Xu, Xu Han, Zhuofan Cai, Lan-Tian Feng, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8525819/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Integrated photonics-based coexistence systems enable classical data channels and quantum channels to share the same optical waveguide by integrating quantum key distribution (QKD) into existing telecommunications infrastructure, reducing the costs and complexities of deploying quantum-secure links. Achieving this requires photonic components that efficiently reject intense classical light while preserving the integrity of ultra-weak single-photon quantum signals. Here, we demonstrate the first on-chip quantum-classical coexistence platform based on thin-film lithium niobate (TFLN). The chip integrates a polarization splitter-rotator (PSR) with two four-channel wavelength-division multiplexers, creating eight distinct channels encoded by wavelength and polarization in a highly compact millimeter-scale footprint. The device features ultra-low on-chip insertion loss ( IL ), high polarization isolation, and robust suppression of nonlinear noise, minimizing crosstalk from classical to quantum channels. Single-photon experiments verify its compatibility with QKD operations under realistic scenarios of simultaneous classical and quantum transmission. Its fabrication-robust design eases lithography constraints, enabling scalable and cost-effective manufacturing. Overall, this chip offers a viable path to integrate QKD into dense optical networks and advances the development of scalable quantum-secure communications. Physical sciences/Optics and photonics/Applied optics/Integrated optics Physical sciences/Optics and photonics/Applied optics/Optoelectronic devices and components quantum key distribution (QKD) classical optical communication coexistence system integrated photonic circuits thin-film lithium niobate (TFLN) Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Photonics provides the physical basis for quantum-secure communication by enabling high-speed, low-loss optical transmission with information-theoretic security in principle [ 1 , 2 ] . Quantum states of light, including superposition and entanglement, allow information to be encoded at the single-photon level [ 3 , 4 ] . These capabilities underpin practical QKD systems that already operate over optical fiber, free space, and satellite links [ 5 – 7 ] . As efforts to develop a quantum internet accelerate, a key challenge is to implement quantum channels compatible with existing high-capacity optical infrastructure [ 8 ] . Specifically, quantum channels must share hardware with high-power classical data channels while maintaining low noise levels and strict security guarantees. The success of this coexistence will determine whether quantum connectivity can progress from laboratory demonstrations to widespread deployment. Current coexistence studies focus mainly on fiber-based architectures, where mode-division multiplexing (MDM) [ 8 – 10 ] and wavelength-division multiplexing (WDM) [ 11 , 12 ] increase capacity by allowing multiple signals to share a common transmission medium [ 13 ] . These experiments show that with careful wavelength allocation, spectral filtering, and power management, quantum signals can coexist with intense classical traffic over tens to hundreds of kilometers. They also reveal key physical limits. In single-mode fiber, weak quantum signals coexisting with intense classical channels experience broadband noise from stimulated Raman scattering and four-wave mixing (FWM), which can obscure single-photon events and reduce secure key rates [ 14 – 16 ] . Polarization-mode dispersion and intermodal coupling further degrade link stability and often require active compensation over long distances [ 17 ] . In addition, many deployed systems rely on bulk multiplexers and discrete optical components that are not compatible with monolithic integration, complicating the dense and scalable deployment of on-chip quantum devices [ 18 ] . On the theoretical side, coexistence analyses often adopt simplified models with independent quantum links or ideal multiplexers. While these abstractions are useful for system-level design, they widen the gap between network models and the constraints of physically realizable integrated nodes. Integrated photonics offers a direct route to unifying classical and quantum optics by placing quantum-state generation, manipulation, and routing alongside classical signal processing on a single chip [ 19 , 20 ] . Integrated QKD has been explored on several material platforms, each with distinct strengths and limitations. Silicon offers high integration density but suffers from two-photon absorption and free-carrier effects under intense classical illumination, which generate noise and degrade single-photon states [ 21 ] . Indium phosphide (InP) enables on-chip lasers and detectors, but its electro-optic response is typically insufficient for high-fidelity quantum modulation [ 22 ] . Potassium titanyl phosphate (KTP) offers strong optical nonlinearity but faces challenges in thin-film growth and low-loss waveguide realization [ 23 ] . TFLN addresses many of these issues. It combines a large Pockels effect and strong second-order nonlinearity with low propagation loss and a high optical damage threshold, enabling high-fidelity single-photon operations, efficient electro-optic modulation, and robust nonlinear processes at high optical power [ 24 – 26 ] . On the TFLN platform, high-speed phase modulators provide precise spectral and temporal control of telecom-band single-photon pulses [ 27 ] , and directional couplers and multimode interferometers support efficient on-chip photonic routing [ 28 ] . These properties make TFLN a leading candidate for reconfigurable, high-performance QKD chips and integrated quantum-classical communication. Despite rapid progress at the device level, a fully integrated, chip-scale system demonstrating practical quantum-classical coexistence on TFLN has not yet been realized. Three gaps currently limit progress. First, no TFLN system has been experimentally validated under realistic coexistence conditions while simultaneously providing high-density multiplexing, low nonlinear noise, and robust single-photon performance. Second, the lack of complete physical implementations with comprehensive system-level measurements hinders reliable calibration of theoretical models and obscures true performance limits. Third, many existing TFLN platforms rely on direct etching of lithium niobate, and the resulting slanted sidewalls make it difficult to realize narrow coupling gaps for waveguides and subwavelength gratings. Closing these gaps is essential for assessing the engineering feasibility of high-density quantum and classical coexistence on the TFLN platform. In this work, we report, to our knowledge, the first on-chip quantum-classical coexistence system on TFLN. A single monolithic device integrates a PSR with two four-channel wavelength-division multiplexers, providing eight independent wavelength-polarization channels within a 2.7 × 0.68 mm 2 footprint. In classical transmission tests, the device sets a new benchmark for TFLN multiplexers, with on-chip IL s of 0.23 dB for the TE 0 mode and 0.33 dB for the TM 0 mode, polarization crosstalk below − 57.99 dB and − 56.40 dB, and same-polarization wavelength-channel crosstalk below − 20.96 dB across all channels. In quantum-classical coexistence experiments, the system strongly suppresses crosstalk from adjacent high-power classical channels while preserving QKD signals at the single-photon level. The multimode waveguide design also exhibits excellent fabrication tolerance: a width variation of ± 20 nm induces only ± 0.04 nm wavelength drift and negligible additional IL , defining a practical process window for large-scale manufacturing. These device- and system-level metrics provide realistic input parameters for network-level modeling and design. Together, the results establish TFLN as a unified quantum-classical platform and indicate that chip-scale quantum internet routing, reconfigurable access networks and distributed photonic processors are achievable through joint wavelength and polarization resource allocation in a scalable material system. 2. Results 2.1 Principles and design The realization of a practical on-chip system that supports the coexistence of classical and quantum signals critically depends on an architecture offering high channel isolation, low transmission loss, and a compact footprint. Figure 1 a schematically illustrates the coexistence system, where Alice (transmitter) and Bob (receiver) simultaneously exchange classical and quantum signals via a shared optical link. Classical data channels and QKD channels are jointly routed on-chip, while their mutual interference is carefully managed at the device level to preserve the integrity of fragile quantum states. In our design, coexistence is achieved by combining WDM with polarization-division multiplexing (PDM) on an integrated photonic platform, as shown in Fig. 1 b. At the core of the system are two four-channel wavelength-division multiplexers (Fig. 1 b, left inset) integrated with a PSR (Fig. 1 b, right inset). In each wavelength-division multiplexer, optical signals at four distinct wavelengths are combined into a single bus waveguide via reverse-coupled one-dimensional photonic crystal waveguides (1D-PhCWs). Light within the mini-stopband (MSB) of the photonic crystal couples efficiently to forward-propagating TE 0 modes, thereby enabling compact and low-loss wavelength multiplexing. The two 1D-PhCW-based multiplexers are connected to the cross and through ports of the PSR, which is implemented using an asymmetric directional coupler (ADC). The ADC is engineered such that TE-polarized light entering from the through port is transmitted directly, whereas TE light entering from the cross port is converted into TM polarization prior to wavelength multiplexing [ 29 ] . This configuration provides eight input channels in total: Channels 1–4 in TM polarization and Channels 5–8 in TE polarization. By allocating classical data and quantum channels across these wavelength-polarization resources, the system supports flexible coexistence strategies tailored to different noise and security requirements. The wavelength-division multiplexer consists of a TE 0 –TE 2 mode converter, a 1D-PhCW, and tapered waveguides to ensure low-loss interconnections. The mode converter is implemented as an ADC consisting of a single-mode waveguide and a multimode waveguide. The single-mode waveguide has a width of W 1 and a bend radius of L 2 , while the multimode waveguide has a width of W 2 . The coupling length is L 1 , and the coupling gap is W g . The 1D-PhCW is formed by periodically etching N air holes with radius R and period Λ into a multimode waveguide. To suppress passband sidelobes and minimize inter-channel crosstalk, a Gaussian-apodized hole-radius profile is adopted to achieve a high sidelobe suppression ratio. For the TE 0 –TE 2 mode converter, W 1 is fixed at 1 µm. Based on the phase-matching condition, W 2 is set to 4.059 µm (see Supplementary Note 1). To ensure sufficient spatial separation between the two waveguides, L 2 is chosen as 150 µm, and W g is set to 0.2 µm. The coupling length L 1 is optimized to 26.5 µm through finite-difference time-domain (FDTD) simulations. To minimize transition loss between waveguides with different widths, tapered waveguides with a length of 150 µm are inserted between adjacent multimode waveguides. The performance of the mode converter is evaluated numerically. Figure 2 a shows the normalized transmission spectrum from 1500 to 1600 nm, and Fig. 2 b presents the corresponding electric field distribution at 1550 nm. The conversion efficiency remains better than − 1.01 dB across a 100 nm wavelength range. At 1550 nm, the crosstalk to the TE 0 and TE 1 modes is below − 34.20 dB and − 22.39 dB, respectively. The 1D-PhCW is designed by calculating the photonic band structure using the finite element method. Figure 2 c shows the band structure of TE-polarized Bloch modes in the multimode waveguide. With the chosen parameters, an MSB opens between the TE 0 and TE 2 modes, as indicated by the orange-shaded region. This MSB enables efficient coupling from the incident TE 2 mode to a backward-propagating TE 0 mode (see Supplementary Note 2). Figure 2 d presents the three-dimensional FDTD (3D-FDTD) simulation results. The input TE 0 mode is first converted into a backward-propagating TE 2 mode by the mode converter. It is then reflected by the 1D-PhCW and converted back into a forward-propagating TE 0 mode. To improve the performance of the 1D-PhCW filter and ensure high channel isolation across the C-band for the coexistence of quantum and classical signals, we further optimize the photonic crystal parameters. Using 3D-FDTD simulations, we systematically study the effects of key structural parameters on the center wavelength and bandwidth, and identify configurations that achieve a high extinction ratio ( ER ), low crosstalk, and appropriate channel bandwidth. The transmission spectra are shown in Fig. 2 e– 2 h. By varying the hole period, hole radius, number of periods, and apodization index, we obtain the corresponding trends. Based on this optimization, the hole periods Λ 1 – Λ 4 are set to 412, 416, 420, and 424 nm, corresponding to center wavelengths of 1540, 1552, 1564, and 1576 nm, respectively. A hole radius of 60 nm, a period number of 500, and an apodization index of 1 are selected to achieve high-performance bandpass filtering. Under these conditions, the filter exhibits an IL below 1.6 dB, a sidelobe suppression ratio above 18.8 dB, a channel spacing of 12 nm, and crosstalk below − 26.9 dB and − 33.3 dB for adjacent and non-adjacent channels, respectively. To enable the coexistence of quantum and classical optical signals on a single link, we leverage two orthogonal and strongly isolated polarization modes, TE and TM, supported by a single-mode polarization-maintaining platform. A PSR based on mode hybridization is employed as the polarization multiplexer. The device comprises a cascaded TE 0 to TE 1 mode coupler followed by a polarization rotator. Two TE 0 polarized inputs are injected from the through port and the cross port, respectively. The TE 0 mode launched from the cross port is phase-matched to the adjacent waveguide, where it is efficiently coupled and converted into the TE 1 mode. By contrast, the TE 0 mode injected from the through port remains in the original waveguide with negligible mode conversion. The generated TE 1 mode is subsequently converted into the fundamental TM 0 mode via mode hybridization in the rotator section and is finally multiplexed with the TE 0 mode in the bus waveguide. 3D-FDTD simulations predict low IL and robust mode and polarization conversion at 1550 nm, which are preserved over a broad bandwidth of 100 nm spanning 1500 to 1600 nm. In the mode coupler, the through port TE 0 mode undergoes negligible conversion, while the cross-port input achieves a TE 0 -to-TE 1 conversion efficiency of − 0.35 dB, remaining better than − 0.43 dB across the full bandwidth. In the rotator, the TE 0 channel exhibits almost no transmission loss, and the TE 1 to TM 0 conversion efficiency reaches − 0.08 dB, remaining better than − 0.54 dB over the entire spectral range. The design rationale, optimization procedure, and the corresponding simulated spectra and field profiles are detailed in the Supplementary Information (see Supplementary Note 3). 2.2 Performance characterization of the coexistence system The device is fabricated on an x -cut TFLN wafer supplied by NANOLN, adopting an etchless approach for the lithium niobate film itself. A 300 nm-thick silicon nitride (Si 3 N 4 ) film is deposited on the TFLN wafer by reactive sputtering, and device patterns are defined in the silicon nitride layer via electron-beam lithography (EBL) followed by inductively coupled plasma (ICP) etching. Figure 3 a shows a microscope image of the fabricated coexistence system (overall footprint: 2.7 × 0.68 mm 2 ), along with magnified scanning electron microscope (SEM) images of the coupling region in the TE 0 -to-TE 2 mode multiplexer and the 1D-PhCW structure. The input ports of the coexistence system are labeled Channel 1–8, corresponding to coupling of optical signals at wavelengths λ 1 – λ 4 in TM and TE polarization into the input bus waveguide. The device interfaces with grating couplers to enable efficient coupling of optical signals into and out of the chip. Because the grating couplers are polarization-selective, a PSR with the same design is integrated at the output port of the bus waveguide to separate the TE and TM modes. When a TM 0 -mode signal exits the bus waveguide and enters the PSR, it is converted to TE polarization and routed to the cross port O 1 . In contrast, a TE 0 -mode signal propagates through the PSR without polarization rotation and is routed to the through port O 2 . To eliminate the influence of the PSR and the grating couplers on the measured device performance, a reference device is fabricated in close proximity on the same chip. The transmission spectra are normalized as T = 10 × log 10 ( P / P r ), where T is the normalized transmittance, P and P r are the measured output powers of the tested device and the reference device, respectively. The schematic of the experimental setup is shown in Fig. 3 b and consists of a tunable semiconductor laser (TSL), a polarization controller (PC), and an optical spectrum analyzer (OSA). During the measurement, the laser output from the TSL is sent to the PC, coupled into and out of the chip through the input and output grating couplers, and analyzed by the OSA to obtain the transmission spectra. To achieve high coupling efficiency, a TE-polarized grating coupler with a period of 940 nm, a duty cycle of 0.4, and an etching depth of 300 nm is employed. The measured transmission spectra of the fabricated PSR are shown in Fig. 3 c. The IL for the TE 0 mode is below 0.23 dB, with crosstalk lower than − 57.99 dB at 1550 nm. For the TM 0 mode, the IL is below 0.33 dB and the crosstalk is below − 56.40 dB. These results confirm that the PSR provides low-loss and highly isolated polarization (de)multiplexing suitable for quantum-classical coexistence. Figures 3 d and 3 e show the normalized transmission spectra of the fabricated coexistence system. For TM polarization, the measured IL s of the four channels centered at 1538, 1550, 1562, and 1574 nm are below 1.79, 2.55, 2.50, and 2.81 dB, respectively, and the corresponding inter-channel crosstalk levels are better than − 28.75, − 24.88, − 23.18, and − 23.42 dB. For TE polarization at the same center wavelengths, the IL s are below 1.41, 1.59, 1.86, and 1.74 dB, with crosstalk below − 29.18, − 22.98, − 20.96, and − 21.88 dB, respectively. Overall, the maximum measured IL among all channels is 2.81 dB, while the crosstalk remains below − 20.96 dB. Compared with simulation results, the experimentally measured crosstalk is slightly higher. This discrepancy is likely due to the coupling and reflection of non-target fundamental modes in the cascaded 1D-PhCW structures. Since the stopband frequency of the fundamental mode is lower than the MSB between the TE 0 and TE 2 modes, the additional crosstalk mainly appears at wavelengths above the designed operating band. In the simulated non-cascaded structures, the crosstalk induced by reflection is negligible. The relatively higher IL measured in the experiment is attributed to waveguide sidewall roughness, which has a stronger impact on higher-order modes because of their larger field overlap with the etched silicon nitride sidewalls. To further reduce IL , the etching process can be optimized in future work, for example by reducing the etch rate or applying resist reflow before etching, which is expected to mitigate sidewall roughness in the silicon nitride layer. In addition, owing to fabrication tolerances, the measured channel wavelengths exhibit an approximately 2 nm blue-shift relative to the simulated values, while maintaining a uniform and narrow channel spacing of 12 nm (see Supplementary Note 4). Given the linear relationship between the hole period of the 1D-PhCW and the center wavelength, this offset can be compensated through iterative optimization of the fabrication process. Notably, all channels demonstrate excellent center-wavelength alignment under both TE and TM polarizations. For future improvements requiring tighter wavelength control, thermal or electrical tuning could be incorporated to finely adjust the center wavelength by modulating the refractive index of the photonic crystal material. Quantum signals are much weaker than classical signals and can approach the single-photon level. When QKD coexists with conventional data in the same transmission channel, system performance can degrade substantially due to noise. In such coexistence systems, noise in the quantum channel can be broadly categorized into two types. The first is crosstalk from the classical channels into the quantum channel, caused by non-ideal multiplexing and insufficient channel isolation. The second comprises noise generated by light-medium interactions during propagation, including scattering (Rayleigh, Brillouin, Raman) and nonlinear effects (FWM, self-phase modulation, cross-phase modulation) [ 30 ] . Here, we focus on classical-signal power leakage into the quantum channel resulting from non-ideal multiplexing performance. Denoting the classical launch power before multiplexing as P in , the leaked power coupled into the quantum channel, P out , is: $$\:{\text{P}}_{\text{out}}\text{}\text{=}{\text{}\text{P}}_{\text{in}}{\text{t}}_{\text{mux}}{\text{t}}_{\text{leak}}\text{}\text{=}{\text{}\text{P}}_{\text{in}}{\text{10}}^{\text{-}\text{δ/10}}{\text{10}}^{\text{-}\text{ζ/10}}$$ 1 , where δ and ζ are the IL and the isolation of the multiplexing system (in dB), respectively. The classical-signal transmission coefficient is t mux = 10 − δ /10 , and the crosstalk ratio leaking into the quantum channel is t leak = 10 − ζ /10 . Four wavelength-division-multiplexed classical channels with TM (or TE) polarization are selected. The channels correspond to Channel 1–Channel 4 (or Channel 5–Channel 8), spanning 1538–1574 nm with a channel spacing of 12 nm. Figure 4 a and Fig. 4 b present the leakage power of these classical signals into the coexistence quantum channel, where the blue bars denote the leakage contribution from each individual channel. For TM and TE polarizations, the maximum leakage among the wavelength channels is approximately − 24.226 dB and − 27.478 dB relative to the input power, respectively. By further leveraging the excellent polarization isolation of the coexistence system, quantum-classical coexistence can be substantially improved. Specifically, when classical and quantum signals are launched in orthogonal polarizations and assigned to different wavelength channels, the leakage can be suppressed to more than 100 dB below the input power, enabling robust coexistence in practical networks. In addition, because the crosstalk power in QKD increases linearly with the launch power of the classical channels, lowering the transmitted power of classical data channels is an effective way to enhance QKD performance. Finally, incorporating tunable bandpass filters can further suppress residual leakage crosstalk and improve the overall robustness of quantum-classical coexistence. After comprehensive device characterization, we establish a coexistence test system to evaluate the transmission quality of quantum signals in the presence of classical optical channels, as shown in Fig. 4 c. In this setup, light from one TSL passes through a PC and a C13 filter (centered at 1567 nm) and is then coupled into chip Channels 4 and 5 with a pre-grating power of 1.1 mW to emulate classical data transmission. Simultaneously, light from a second TSL is routed through another PC, a C34 filter (centered at 1550.12 nm), and a spiral waveguide, where nondegenerate photon pairs are generated via spontaneous FWM. Owing to energy conservation, their frequencies satisfy \(\:\text{2}{\text{ω}}_{\text{p}}\text{=}{\text{ω}}_{\text{s}}\text{+}{\text{ω}}_{\text{i}}\text{.}\) A dense wavelength-division multiplexer (DWDM) separates the photons, and the signal photons are coupled into chip Channels 3 and 6. At the receiver side, the TM- and TE-polarized outputs are collected from ports O 1 and O 2 , respectively, to emulate quantum signal transmission. Both signal and idler photons are detected by superconducting nanowire single-photon detectors (SNSPDs) and analyzed using a time-correlated single-photon counting (TCSPC) system. The TCSPC module converts detected photon events into electrical pulses and assigns a timestamp to each event with picosecond resolution, enabling accurate measurement of the arrival-time difference between the signal and idler photons. During measurement, the coincidence time window is set to 1.6 ns. Using this experimental system, we realize a heralded single-photon source and deploy it as a quantum light source for chip testing. Performance tests of the quantum channels yield the following results. For the Channel 3– O 1 path, the 20-s coincidence counts without classical light, with classical light injected into Channel 4, and with classical light injected into Channel 5 are 149, 129, and 128, corresponding to relative losses of 1.3 dB, 1.9 dB, and 1.9 dB, respectively. For the Channel 6– O 2 path, the corresponding 20-s coincidence counts are 137, 117, and 132, with relative losses of 1.7 dB, 2.4 dB, and 1.8 dB, respectively. These results show that the additional transmission loss in the quantum channels remains small and well-controlled, even when adjacent channels carry relatively high-power classical signals. This confirms that the proposed wavelength-polarization hybrid multiplexer effectively suppresses crosstalk between classical and quantum light, thereby preserving the integrity of QKD signals in coexistence scenarios. 3. Discussion Integrating quantum and classical communication on a single photonic platform is critical for scalable quantum-secure networks, as this integration is a prerequisite for bridging laboratory prototypes to real-world deployment. In such networks, QKD must coexist with high-capacity classical data streams without dedicated infrastructure. This study demonstrates, for the first time, on-chip coexistence of classical optical transport network (OTN) signals and QKD channels using an etchless thin-film lithium niobate (TFLN) platform. It directly addresses the longstanding challenge of quantum-classical interference in shared physical infrastructure. By integrating a wavelength-polarization hybrid (de)multiplexer with breakthrough performance, this chip-scale system establishes TFLN as a transformative platform for unified quantum-classical photonics, with applications spanning metropolitan networks, data centers, and future quantum internet architectures. The core innovation resides in the monolithic integration of wavelength and polarization multiplexing, enabling 8 independent wavelength-polarization channels within a compact 2.7 × 0.68 mm 2 footprint. This hybrid design delivers exceptional performance that directly meets quantum-classical coexistence requirements: on-chip IL is as low as 0.23 dB for the TE₀ mode and 0.33 dB for the TM₀ mode; polarization crosstalk is below − 57.99 dB (TE 0 ) and − 56.40 dB (TM 0 ); same-polarization wavelength-channel crosstalk is below − 20.96 dB across all channels. These metrics are summarized in Table 1 , which provides a comprehensive comparison with existing quantum-classical coexistence systems. Previous quantum-classical coexistence systems primarily relied on fiber-end multiplexing, which lack the miniaturization and reconfigurability required for dense network integration [ 8 – 11 ] . While these fiber-based systems offered advantages like scalable transmission capacity or small channel spacing, their key multiplexing and isolation capabilities depended on discrete devices rather than monolithic integration. Although a silicon photonics chip was reported to support continuous-variable QKD (CV-QKD) with a local oscillator (LO), where quantum signals and strong LO light from the same laser co-propagated via polarization multiplexing [ 19 ] , it did not systematically compare critical metrics such as channel count, scalability, or classical link loss. In contrast, this work achieves on-chip coexistence of classical optical communication and QKD using etchless TFLN, integrating dynamically reconfigurable multi-channel quantum/classical paths on a compact chip while balancing low classical IL and high isolation. Its advantage extends beyond mere “on-chip integration”: leveraging TFLN’s inherent low loss and strong polarization/multiplexing capabilities, it realizes a scalable, multi-channel, reconfigurable architecture that provides a more miniaturized, integrable pathway for quantum network nodes compatible with existing optical infrastructure. The coexistence system features exceptional fabrication tolerance, ensuring compatibility with large-scale manufacturing. The uniform 12 nm channel spacing is well-matched to existing C-band spectral resources, eliminating the need to reconfigure deployed telecommunications infrastructure. These parameters translate to tangible application advantages: 12 nm wavelength spacing enables efficient C-band utilization, critical for fiber-constrained metropolitan networks, while low IL ensures minimal signal degradation for both classical and quantum channels. The multimode waveguide design exhibits excellent robustness: a width variation of ± 20 nm induces only ± 0.04 nm wavelength drift and negligible additional IL , defining a practical process window for large-scale manufacturing and addressing a key commercialization barrier for integrated photonic devices. Importantly, high channel isolation prevents high-power classical signals from overwhelming quantum channels, preserving QKD transmission quality even with adjacent classical traffic. Together, these features make the TFLN chip an ideal choice for real-world networks requiring simultaneous reliability, spectral efficiency, and security. Table 1 Performance comparison of quantum-classical coexistence systems. Year Core Platform On-Chip Integration Number of Channels (Quantum/Classical) Channel Spacing (nm) IL (Classical) (dB) Isolation (Classical) (dB) Chip Size (mm 2 ) 2023 (ref. 8) Single-mode fiber TDM system No 2(1/1) 15.0 ~ 6.00 10.86 NA 2020 (ref. 9) Weakly-coupled few-mode fiber No 2(1/1) 5.0 2.60 (LP 01 ) 3.70 (LP 02 ) 23.58 (LP 01 ) 23.20 (LP 02 ) NA 2020 (ref. 10) 7-core multicore fiber No 8 (1/7) 0.4 5.00 44.00 NA 2024 (ref. 11) Single-mode fiber DWDM system No 61(1/60) 0.4 1.70 NA NA 2019 (ref. 19) Silicon photonic chip Yes 2(1/1) NA NA 35.00 NA This Work TFLN-based integrated photonics Yes 8 (Dynamically Reconfigurable) 12.0 2.81 (TM) 1.86 (TE) 56.40 2.7 × 0.68 Note: IL : Insertion Loss; TDM: Time Division Multiplexing; DWDM: Dense Wavelength Division Multiplexing; LP: Linear Polarization; TFLN: Thin-Film Lithium Niobate; NA: Not Available. The unique material properties of TFLN underpin system performance by addressing key limitations of alternative platforms. TFLN combines ultra-low propagation loss, strong electro-optic modulation, and a broad transparency window, making it inherently suitable for quantum-classical coexistence. Silicon nitride cladding enhances mode confinement and reduces bending loss, enabling compact footprints without sacrificing optical performance. These material advantages, combined with hybrid wavelength-polarization multiplexing, achieve a balanced combination of channel density, isolation, and loss. Supporting both classical and quantum signals on-chip with realistic spacing and power levels bridges the gap between laboratory demonstrations and real-world quantum-secure networks. Beyond device-level performance, the system-level architecture offers transformative capabilities for optical network design: by leveraging wavelength and polarization degrees of freedom, the chip provides 8 dynamically allocable channels between classical and quantum signals, enabling flexible resource management for time-varying network demands. This flexibility suits diverse scenarios: in metropolitan networks, the chip can function as a compact coexistence node that adds or drops QKD channels from existing high-capacity fiber links without dedicated dark fibers; in data centers and short-reach scenarios, its small size and low power consumption make it ideal for high-bandwidth-density links requiring both throughput and security. Despite these significant advances, key challenges persist for TFLN-based quantum-classical coexistence. The fixed wavelength-polarization mapping limits adaptability to dynamic networks. Future iterations could integrate thermo-optic and electro-optic tuning to enable real-time channel reconfiguration, thereby suppressing FWM noise and enhancing overall efficiency. Long-haul transmission requires hybrid amplification schemes: erbium-doped fiber amplifiers (EDFAs) for classical signals, and quantum repeaters or heralded single-photon sources for QKD. The chip’s low IL reduces reliance on quantum amplification, lowering security vulnerability risks. Finally, system-level integration with on-chip light sources and detectors will eliminate fiber pigtailing losses and enable fully integrated transceivers with smaller form factors and improved reliability. Looking forward, this study redefines the paradigm of optical networks with intrinsic quantum security. By demonstrating on-chip coexistence of high-intensity classical signals and quantum channels with realistic spacing, low loss, and high isolation, it advances the vision of a quantum-enhanced internet, where classical infrastructure simultaneously transmits high-speed data and unbreakable encryption based on quantum mechanics. With continued advances in dynamic resource allocation and long-haul optimization, TFLN-based integrated QKD will provide high-security encryption for scenarios ranging from metropolitan networks to global quantum backbones. These chip-scale results validate that quantum channels can share a platform with high-intensity classical signals while maintaining strict security guarantees, paving the way for the seamless transition of existing networks to quantum-secure communications via deliberate photonic hardware design. 4. Methods Device simulation TE-polarized photonic band structures are simulated using periodic boundary conditions on photonic crystal unit cells with the finite element method. The dependence of the effective refractive index on waveguide width is calculated using a finite-difference eigenmode (FDE) solver. For all other simulation tasks, a 3D-FDTD method is employed. Lithium niobate and silicon nitride are retrieved from the default material database. At 1550 nm, LN has refractive indices of ~ 2.14 (extraordinary, n e ) and ~ 2.22 (ordinary, n o ), while silicon nitride has a refractive index of ~ 1.99. To ensure reliable results, the FDTD simulation time is set to 10,000 fs, and an auto-shutoff level of 1 × 10 − 4 is used. An automatic mesh with an accuracy level of 4 is adopted to balance computational cost and numerical precision. A mode expansion monitor is introduced to enhance the accuracy of the 3D-FDTD simulations, which significantly improves the evaluation of IL , ER , and crosstalk. All light propagation profiles are presented as continuous distributions to facilitate physical interpretation and analysis. Device fabrication The system is fabricated on a commercial x -cut TFLN wafer incorporating a 300 nm-thick lithium niobate film supplied by NANOLN. First, a 300 nm-thick silicon nitride layer is deposited on top of the TFLN wafer by reactive sputtering. Device structures are then defined in the silicon nitride layer using electron-beam lithography (EBL), followed by inductively coupled plasma (ICP) etching. Declarations Acknowledgements This work was supported by the National Natural Science Foundation of China (W2411059, 62405125, T2325022, U23A2074), the Gansu Provincial Science and Technology Major Special Project (25ZDWA001, 25ZDGA005), the Key Research and Development Program of Gansu Province (24YFGA007), the Joint Research Fund of Gansu Province (25JRRA1126), the CAS Project for Young Scientists in Basic Research (YSBR-049), the Australian Research Council (CE230100006), the Fundamental Research Funds for the Central Universities, and the Talent Scientific Fund of Lanzhou University. We thank the Micro Nano Research Facility (MNRF) and the Australian Microscopy & Microanalysis Research Facility at RMIT University for access to facilities and for scientific and technical assistance. Parts of this work were carried out at the Melbourne Centre for Nanofabrication (MCN), the Victorian node of the Australian National Fabrication Facility (ANFF), and at the OptoFab node of ANFF, with support from NCRIS and the Victorian state government. Author contributions M.Y. and Y.T. conceived the project and optimized the research plan. M.Y. designed and performed numerical simulations of the hybrid (de)multiplexer, with contributions from H.L. and X.Z. M.X.L., T.G.N., and G.R. fabricated and characterized the devices. M.Y. performed the transmission measurements and analyzed the data, with additional analysis support from B.X., X.H., Z.C., and L.F., who characterized the classical-quantum coexistence system. M.Y. and Y.T. wrote the manuscript with input from all authors. Y.T. supervised the overall project. G.R., A.M., X.R., and Y.T. provided guidance and feedback. All authors discussed the results and approved the final manuscript. Competing Interests Statement The authors declare no competing interests. Data Availability Statement All the data supporting the findings in this study are available in the paper and Supplementary Information. Source data are provided with this paper. Additional data related to this paper are available from the corresponding authors upon request. References O’Brien, J. L., Furusawa, A., & Vučković, J. Photonic quantum technologies. Nature Photonics 3 , 687–695 (2009). Pittaluga, M., et al. Long-distance coherent quantum communications in deployed telecom networks. Nature 640 , 911–917 (2025). Bernstein, D. J., & Lange, T. Post-quantum cryptography. Nature 549 , 188–194 (2017). Ding, X., et al. High-efficiency single-photon source above the loss-tolerant threshold for efficient linear optical quantum computing. Nature Photonics 19 , 387–391 (2025). Zhang, X., et al. Polarization-encoded quantum key distribution with a room-temperature telecom single-photon emitter. 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Journal of Lightwave Technology 42 , 1321–1327 (2024). Du, S., Tian, Y., & Li, Y. Impact of four-wave-mixing noise from dense wavelength-division-multiplexing systems on entangled-state continuous-variable quantum key distribution. Physical Review Applied 14 , 024013 (2020). Gisin, N., & Thew, R. Quantum communication. Nature Photonics 1 , 165–171 (2007). Patel, K. A., et al. Coexistence of high-bit-rate quantum key distribution and data on optical fiber. Physical Review X 2 , 041010 (2012). Comandar, L. C., et al. Quantum key distribution without detector vulnerabilities using optically seeded lasers. Nature Photonics 10 , 312–315 (2016). Schuck, C., Pernice, W. H. P., & Tang, H. X. Quantum interference in heterogeneous superconducting-photonic circuits on a silicon chip. Nature Communications 7 , 10352 (2016). Poole, C. D. Statistical treatment of polarization dispersion in single-mode fiber. Optics Letters 13 , 687–689 (1988). Richardson, D. J., Fini, J. M., & Nelson, L. E. Space-division multiplexing in optical fibres. Nature Photonics 7 , 354–362 (2013). Zhang, G., et al. An integrated silicon photonic chip platform for continuous-variable quantum key distribution. Nature Photonics 13 , 839–842 (2019). Zhang, Y., et al. Classical-decisive quantum internet by integrated photonics. Science 389 , 940–944 (2025). Goswami, A., Raj, Y., & Das, B. K. Four-wave mixing in silicon nanophotonic waveguides and microring resonators: influence of two-photon absorption. Journal of Optical Microsystems 4 , 041404 (2024). Aldama, J., et al. Integrated InP-based transmitter for continuous-variable quantum key distribution. Optics Express 33 , 8139–8149 (2025). Tanzilli, S., et al. On the genesis and evolution of integrated quantum optics. Laser & Photonics Reviews 6 , 115–143 (2012). Boes, A., et al. Lithium niobate photonics: unlocking the electromagnetic spectrum. Science 379 , eabj4396 (2023). Xie, Z., et al. Recent development in integrated lithium niobate photonics. Advanced Physics X 9 , 2322739 (2024). Labbé, F., et al. Thin-film lithium niobate quantum photonics: review and perspectives. Advanced Photonics 7 , 044002 (2025). Zhu, D., Chen, C., Yu, M., et al. Spectral control of nonclassical light pulses using an integrated thin-film lithium niobate modulator. Light: Science & Applications 11 , 327 (2022). Desiatov, B., Shams-Ansari, A., Zhang, M., et al. Ultra-low-loss integrated visible photonics using thin-film lithium niobate. Optica 6 , 380–384 (2019). Han, X., et al. Mode and polarization-division multiplexing based on silicon nitride loaded lithium niobate on insulator platform. Laser & Photonics Reviews 16 , 2100529 (2022). Mehdizadeh, P., et al. Quantum-classical coexistence in multi-band optical networks: a noise analysis of QKD. IEEE Communications Letters 28 , 488–492 (2024). Additional Declarations There is NO Competing Interest. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8525819","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":579323592,"identity":"d9920bc6-dcc3-425a-af26-50181f07805c","order_by":0,"name":"Yonghui Tian","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxUlEQVRIiWNgGAWjYDACZiBmbLCRAXN4iNHBA9GSxkOCFgawlsMkaLFnZ372mHfHeR75GQmMD962McibE3YYm7kx75nbPIwzEpgN57YxGO5sIKiFwUyat+02D7NEAhuQwZBgcICgFvZvQJXneNgkEth/E6mFB2TLAR4eoC3MxGk5zFMmObctmUeC52Gz5JxzEoYbCGlh7z++TeJtm52cfHvywQ9vymzkCdoCAkyQ6GBsABISRKgHqf1BnLpRMApGwSgYqQAA/vQyF0/pxkMAAAAASUVORK5CYII=","orcid":"","institution":"Lanzhou University","correspondingAuthor":true,"prefix":"","firstName":"Yonghui","middleName":"","lastName":"Tian","suffix":""},{"id":579323593,"identity":"ab4bb3cf-729c-4bd3-b3a3-708b0c56c2bc","order_by":1,"name":"Mingrui Yuan","email":"","orcid":"https://orcid.org/0009-0005-0332-6254","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Mingrui","middleName":"","lastName":"Yuan","suffix":""},{"id":579323594,"identity":"907ae76d-a901-4396-8223-38bb3e16d83f","order_by":2,"name":"Boyu 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China","correspondingAuthor":false,"prefix":"","firstName":"Lan-Tian","middleName":"","lastName":"Feng","suffix":""},{"id":579323598,"identity":"aca20041-5ded-425e-9863-3b33eb515ec7","order_by":6,"name":"Hao Li","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"Hao","middleName":"","lastName":"Li","suffix":""},{"id":579323599,"identity":"9d58b391-e052-460f-b77e-c2fe2aee49e9","order_by":7,"name":"XuDong Zhou","email":"","orcid":"","institution":"Lanzhou University","correspondingAuthor":false,"prefix":"","firstName":"XuDong","middleName":"","lastName":"Zhou","suffix":""},{"id":579323600,"identity":"ae25e66b-7fb8-4e64-b0fc-f43d3185ebbb","order_by":8,"name":"Mei Xian Low","email":"","orcid":"","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Mei","middleName":"Xian","lastName":"Low","suffix":""},{"id":579323601,"identity":"24bab2d5-833e-426b-89ad-c18dae6d7c0c","order_by":9,"name":"Thach Nguyen","email":"","orcid":"https://orcid.org/0000-0002-8409-5638","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Thach","middleName":"","lastName":"Nguyen","suffix":""},{"id":579323602,"identity":"29459ff4-093a-4c9e-a321-ef0ec91b13a9","order_by":10,"name":"Guanghui Ren","email":"","orcid":"https://orcid.org/0000-0002-9867-8279","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Guanghui","middleName":"","lastName":"Ren","suffix":""},{"id":579323603,"identity":"89addd44-21e0-4641-8b0f-b65bf1514eae","order_by":11,"name":"Arnan Mitchell","email":"","orcid":"https://orcid.org/0000-0002-2463-2956","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Arnan","middleName":"","lastName":"Mitchell","suffix":""},{"id":579323604,"identity":"17ac4270-ce52-47b6-90f1-021fd93ed34c","order_by":12,"name":"Xi-Feng Ren","email":"","orcid":"https://orcid.org/0000-0001-6559-8101","institution":"University of science and technology of China","correspondingAuthor":false,"prefix":"","firstName":"Xi-Feng","middleName":"","lastName":"Ren","suffix":""}],"badges":[],"createdAt":"2026-01-06 02:30:50","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8525819/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8525819/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101363478,"identity":"6af983a5-8d79-4de9-ac18-65ebf8a82d7a","added_by":"auto","created_at":"2026-01-29 00:37:43","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":240838,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic of the compact coexistence system for classical optical communication and QKD.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e, Schematic of the coexistence system, where Alice (transmitter) and Bob (receiver) simultaneously exchange classical and quantum signals over a shared optical link. \u003cstrong\u003eb\u003c/strong\u003e, Illustration of the proposed wavelength-polarization hybrid (de)multiplexer. Left inset: structure of the four-channel wavelength-division multiplexer. Right inset: structure of the PSR. OTN, optical transport network; QKD, quantum key distribution; MUX, multiplexer; WDM, wavelength-division multiplexing; PSR, polarization splitter-rotator.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8525819/v1/7f90514fc4ae22bde2ad2513.png"},{"id":101363480,"identity":"8fccd898-41be-472e-a88e-054665093494","added_by":"auto","created_at":"2026-01-29 00:37:43","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":463984,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDesign principle of the wavelength-division multiplexer. a\u003c/strong\u003e Simulated transmission spectra for TE\u003csub\u003e0\u003c/sub\u003e–TE\u003csub\u003e2\u003c/sub\u003e mode conversion. \u003cstrong\u003eb \u003c/strong\u003eElectric-field distribution of the TE\u003csub\u003e0\u003c/sub\u003e–TE\u003csub\u003e2\u003c/sub\u003e mode conversion at 1550 nm.\u003cstrong\u003e c\u003c/strong\u003e Band structure of the TE-polarized Bloch mode in the multimode waveguide, showing the mini-stopband (MSB, orange shaded region) between the TE\u003csub\u003e0\u003c/sub\u003e and TE\u003csub\u003e2\u003c/sub\u003e modes. \u003cstrong\u003ed\u003c/strong\u003e Electric-field profile illustrating coupling of a backward-propagating TE\u003csub\u003e2\u003c/sub\u003e mode to a forward-propagating TE\u003csub\u003e0\u003c/sub\u003e mode within the 1D-PhCW. \u003cstrong\u003ee–h\u003c/strong\u003e Simulated transmission spectra of the 1D-PhCW filter under variations in structural parameters: e hole period (\u003cem\u003eΛ\u003c/em\u003e), f hole radius (\u003cem\u003eR\u003c/em\u003e), g number of periods (\u003cem\u003eN\u003c/em\u003e), and h apodization index (\u003cem\u003eG\u003c/em\u003e).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8525819/v1/a59cea52a883beaa01744817.png"},{"id":101397973,"identity":"9e835c66-d50d-445d-b1a0-3aa073eff903","added_by":"auto","created_at":"2026-01-29 09:38:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":509580,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExperimental characterization of the fabricated TFLN chip. a \u003c/strong\u003eOptical micrograph of the integrated quantum-classical coexistence system. Insets: SEM images of the TE\u003csub\u003e0\u003c/sub\u003e-to-TE\u003csub\u003e2\u003c/sub\u003e mode-multiplexer coupling region and the 1D-PhCW structure. \u003cstrong\u003eb\u003c/strong\u003e Schematic of the measurement setup comprising a tunable semiconductor laser (TSL), polarization controller (PC), and optical spectrum analyzer (OSA). \u003cstrong\u003ec\u003c/strong\u003e Normalized transmission spectra of the fabricated PSR. \u003cstrong\u003ed,e\u003c/strong\u003e Normalized transmission spectra of the coexistence system for TM- and TE-polarized inputs, respectively.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8525819/v1/3b44ec097a40d29438a2ee53.png"},{"id":101751333,"identity":"5e690e2b-652a-46b8-9833-30b38c27603e","added_by":"auto","created_at":"2026-02-03 10:19:22","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":229831,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eClassical-quantum coexistence performance. a \u003c/strong\u003eLeakage power from four classical channels into the co-polarized quantum channel for TM polarization. \u003cstrong\u003eb\u003c/strong\u003e Leakage power from four classical channels into the co-polarized quantum channel for TE polarization. \u003cstrong\u003ec\u003c/strong\u003e Experimental setup for coexistence performance characterization. TSL, tunable semiconductor laser; PC, polarization controller; DWDM, dense wavelength-division multiplexer; SNSPD, superconducting nanowire single-photon detector; TCSPC, time-correlated single-photon counting.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8525819/v1/2cdebe17c6b7d0eebf1a1637.png"},{"id":103507340,"identity":"4716588b-e0ef-430e-86ef-2c2b17bf1925","added_by":"auto","created_at":"2026-02-26 13:41:03","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2204923,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8525819/v1/7a1c361e-93d9-412b-8fa0-d9fc2ed63dbb.pdf"},{"id":101363482,"identity":"f8ec3345-e1d7-4cba-9328-c22ff7e43033","added_by":"auto","created_at":"2026-01-29 00:37:43","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":307369,"visible":true,"origin":"","legend":"Supplementary information","description":"","filename":"Supplementary.docx","url":"https://assets-eu.researchsquare.com/files/rs-8525819/v1/2c2518d510bd2100eeb5a817.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"On-chip coexistence of classical optical communication and quantum key distribution on etchless thin-film lithium niobate","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePhotonics provides the physical basis for quantum-secure communication by enabling high-speed, low-loss optical transmission with information-theoretic security in principle \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. Quantum states of light, including superposition and entanglement, allow information to be encoded at the single-photon level \u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e. These capabilities underpin practical QKD systems that already operate over optical fiber, free space, and satellite links \u003csup\u003e[\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. As efforts to develop a quantum internet accelerate, a key challenge is to implement quantum channels compatible with existing high-capacity optical infrastructure \u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. Specifically, quantum channels must share hardware with high-power classical data channels while maintaining low noise levels and strict security guarantees. The success of this coexistence will determine whether quantum connectivity can progress from laboratory demonstrations to widespread deployment. Current coexistence studies focus mainly on fiber-based architectures, where mode-division multiplexing (MDM) \u003csup\u003e[\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e and wavelength-division multiplexing (WDM) \u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e increase capacity by allowing multiple signals to share a common transmission medium \u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. These experiments show that with careful wavelength allocation, spectral filtering, and power management, quantum signals can coexist with intense classical traffic over tens to hundreds of kilometers. They also reveal key physical limits. In single-mode fiber, weak quantum signals coexisting with intense classical channels experience broadband noise from stimulated Raman scattering and four-wave mixing (FWM), which can obscure single-photon events and reduce secure key rates \u003csup\u003e[\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. Polarization-mode dispersion and intermodal coupling further degrade link stability and often require active compensation over long distances \u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. In addition, many deployed systems rely on bulk multiplexers and discrete optical components that are not compatible with monolithic integration, complicating the dense and scalable deployment of on-chip quantum devices \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. On the theoretical side, coexistence analyses often adopt simplified models with independent quantum links or ideal multiplexers. While these abstractions are useful for system-level design, they widen the gap between network models and the constraints of physically realizable integrated nodes.\u003c/p\u003e \u003cp\u003eIntegrated photonics offers a direct route to unifying classical and quantum optics by placing quantum-state generation, manipulation, and routing alongside classical signal processing on a single chip \u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. Integrated QKD has been explored on several material platforms, each with distinct strengths and limitations. Silicon offers high integration density but suffers from two-photon absorption and free-carrier effects under intense classical illumination, which generate noise and degrade single-photon states \u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. Indium phosphide (InP) enables on-chip lasers and detectors, but its electro-optic response is typically insufficient for high-fidelity quantum modulation \u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. Potassium titanyl phosphate (KTP) offers strong optical nonlinearity but faces challenges in thin-film growth and low-loss waveguide realization \u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. TFLN addresses many of these issues. It combines a large Pockels effect and strong second-order nonlinearity with low propagation loss and a high optical damage threshold, enabling high-fidelity single-photon operations, efficient electro-optic modulation, and robust nonlinear processes at high optical power \u003csup\u003e[\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. On the TFLN platform, high-speed phase modulators provide precise spectral and temporal control of telecom-band single-photon pulses \u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e, and directional couplers and multimode interferometers support efficient on-chip photonic routing \u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. These properties make TFLN a leading candidate for reconfigurable, high-performance QKD chips and integrated quantum-classical communication. Despite rapid progress at the device level, a fully integrated, chip-scale system demonstrating practical quantum-classical coexistence on TFLN has not yet been realized. Three gaps currently limit progress. First, no TFLN system has been experimentally validated under realistic coexistence conditions while simultaneously providing high-density multiplexing, low nonlinear noise, and robust single-photon performance. Second, the lack of complete physical implementations with comprehensive system-level measurements hinders reliable calibration of theoretical models and obscures true performance limits. Third, many existing TFLN platforms rely on direct etching of lithium niobate, and the resulting slanted sidewalls make it difficult to realize narrow coupling gaps for waveguides and subwavelength gratings. Closing these gaps is essential for assessing the engineering feasibility of high-density quantum and classical coexistence on the TFLN platform.\u003c/p\u003e \u003cp\u003eIn this work, we report, to our knowledge, the first on-chip quantum-classical coexistence system on TFLN. A single monolithic device integrates a PSR with two four-channel wavelength-division multiplexers, providing eight independent wavelength-polarization channels within a 2.7 \u0026times; 0.68 mm\u003csup\u003e2\u003c/sup\u003e footprint. In classical transmission tests, the device sets a new benchmark for TFLN multiplexers, with on-chip \u003cem\u003eIL\u003c/em\u003es of 0.23 dB for the TE\u003csub\u003e0\u003c/sub\u003e mode and 0.33 dB for the TM\u003csub\u003e0\u003c/sub\u003e mode, polarization crosstalk below \u0026minus;\u0026thinsp;57.99 dB and \u0026minus;\u0026thinsp;56.40 dB, and same-polarization wavelength-channel crosstalk below \u0026minus;\u0026thinsp;20.96 dB across all channels. In quantum-classical coexistence experiments, the system strongly suppresses crosstalk from adjacent high-power classical channels while preserving QKD signals at the single-photon level. The multimode waveguide design also exhibits excellent fabrication tolerance: a width variation of \u0026plusmn;\u0026thinsp;20 nm induces only\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 nm wavelength drift and negligible additional \u003cem\u003eIL\u003c/em\u003e, defining a practical process window for large-scale manufacturing. These device- and system-level metrics provide realistic input parameters for network-level modeling and design. Together, the results establish TFLN as a unified quantum-classical platform and indicate that chip-scale quantum internet routing, reconfigurable access networks and distributed photonic processors are achievable through joint wavelength and polarization resource allocation in a scalable material system.\u003c/p\u003e"},{"header":"2. Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Principles and design\u003c/h2\u003e \u003cp\u003eThe realization of a practical on-chip system that supports the coexistence of classical and quantum signals critically depends on an architecture offering high channel isolation, low transmission loss, and a compact footprint. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea schematically illustrates the coexistence system, where Alice (transmitter) and Bob (receiver) simultaneously exchange classical and quantum signals via a shared optical link. Classical data channels and QKD channels are jointly routed on-chip, while their mutual interference is carefully managed at the device level to preserve the integrity of fragile quantum states. In our design, coexistence is achieved by combining WDM with polarization-division multiplexing (PDM) on an integrated photonic platform, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. At the core of the system are two four-channel wavelength-division multiplexers (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, left inset) integrated with a PSR (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, right inset). In each wavelength-division multiplexer, optical signals at four distinct wavelengths are combined into a single bus waveguide via reverse-coupled one-dimensional photonic crystal waveguides (1D-PhCWs). Light within the mini-stopband (MSB) of the photonic crystal couples efficiently to forward-propagating TE\u003csub\u003e0\u003c/sub\u003e modes, thereby enabling compact and low-loss wavelength multiplexing. The two 1D-PhCW-based multiplexers are connected to the cross and through ports of the PSR, which is implemented using an asymmetric directional coupler (ADC). The ADC is engineered such that TE-polarized light entering from the through port is transmitted directly, whereas TE light entering from the cross port is converted into TM polarization prior to wavelength multiplexing \u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. This configuration provides eight input channels in total: Channels 1\u0026ndash;4 in TM polarization and Channels 5\u0026ndash;8 in TE polarization. By allocating classical data and quantum channels across these wavelength-polarization resources, the system supports flexible coexistence strategies tailored to different noise and security requirements.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe wavelength-division multiplexer consists of a TE\u003csub\u003e0\u003c/sub\u003e\u0026ndash;TE\u003csub\u003e2\u003c/sub\u003e mode converter, a 1D-PhCW, and tapered waveguides to ensure low-loss interconnections. The mode converter is implemented as an ADC consisting of a single-mode waveguide and a multimode waveguide. The single-mode waveguide has a width of \u003cem\u003eW\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e and a bend radius of \u003cem\u003eL\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e, while the multimode waveguide has a width of \u003cem\u003eW\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e. The coupling length is \u003cem\u003eL\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e, and the coupling gap is \u003cem\u003eW\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e. The 1D-PhCW is formed by periodically etching \u003cem\u003eN\u003c/em\u003e air holes with radius \u003cem\u003eR\u003c/em\u003e and period \u003cem\u003eΛ\u003c/em\u003e into a multimode waveguide. To suppress passband sidelobes and minimize inter-channel crosstalk, a Gaussian-apodized hole-radius profile is adopted to achieve a high sidelobe suppression ratio. For the TE\u003csub\u003e0\u003c/sub\u003e\u0026ndash;TE\u003csub\u003e2\u003c/sub\u003e mode converter, \u003cem\u003eW\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e is fixed at 1 \u0026micro;m. Based on the phase-matching condition, \u003cem\u003eW\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e is set to 4.059 \u0026micro;m (see Supplementary Note 1). To ensure sufficient spatial separation between the two waveguides, \u003cem\u003eL\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e is chosen as 150 \u0026micro;m, and \u003cem\u003eW\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e is set to 0.2 \u0026micro;m. The coupling length \u003cem\u003eL\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e is optimized to 26.5 \u0026micro;m through finite-difference time-domain (FDTD) simulations. To minimize transition loss between waveguides with different widths, tapered waveguides with a length of 150 \u0026micro;m are inserted between adjacent multimode waveguides. The performance of the mode converter is evaluated numerically. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the normalized transmission spectrum from 1500 to 1600 nm, and Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb presents the corresponding electric field distribution at 1550 nm. The conversion efficiency remains better than \u0026minus;\u0026thinsp;1.01 dB across a 100 nm wavelength range. At 1550 nm, the crosstalk to the TE\u003csub\u003e0\u003c/sub\u003e and TE\u003csub\u003e1\u003c/sub\u003e modes is below \u0026minus;\u0026thinsp;34.20 dB and \u0026minus;\u0026thinsp;22.39 dB, respectively.\u003c/p\u003e \u003cp\u003eThe 1D-PhCW is designed by calculating the photonic band structure using the finite element method. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the band structure of TE-polarized Bloch modes in the multimode waveguide. With the chosen parameters, an MSB opens between the TE\u003csub\u003e0\u003c/sub\u003e and TE\u003csub\u003e2\u003c/sub\u003e modes, as indicated by the orange-shaded region. This MSB enables efficient coupling from the incident TE\u003csub\u003e2\u003c/sub\u003e mode to a backward-propagating TE\u003csub\u003e0\u003c/sub\u003e mode (see Supplementary Note 2). Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed presents the three-dimensional FDTD (3D-FDTD) simulation results. The input TE\u003csub\u003e0\u003c/sub\u003e mode is first converted into a backward-propagating TE\u003csub\u003e2\u003c/sub\u003e mode by the mode converter. It is then reflected by the 1D-PhCW and converted back into a forward-propagating TE\u003csub\u003e0\u003c/sub\u003e mode. To improve the performance of the 1D-PhCW filter and ensure high channel isolation across the C-band for the coexistence of quantum and classical signals, we further optimize the photonic crystal parameters. Using 3D-FDTD simulations, we systematically study the effects of key structural parameters on the center wavelength and bandwidth, and identify configurations that achieve a high extinction ratio (\u003cem\u003eER\u003c/em\u003e), low crosstalk, and appropriate channel bandwidth. The transmission spectra are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee\u0026ndash;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh. By varying the hole period, hole radius, number of periods, and apodization index, we obtain the corresponding trends. Based on this optimization, the hole periods \u003cem\u003eΛ\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026ndash;\u003cem\u003eΛ\u003c/em\u003e\u003csub\u003e4\u003c/sub\u003e are set to 412, 416, 420, and 424 nm, corresponding to center wavelengths of 1540, 1552, 1564, and 1576 nm, respectively. A hole radius of 60 nm, a period number of 500, and an apodization index of 1 are selected to achieve high-performance bandpass filtering. Under these conditions, the filter exhibits an \u003cem\u003eIL\u003c/em\u003e below 1.6 dB, a sidelobe suppression ratio above 18.8 dB, a channel spacing of 12 nm, and crosstalk below \u0026minus;\u0026thinsp;26.9 dB and \u0026minus;\u0026thinsp;33.3 dB for adjacent and non-adjacent channels, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo enable the coexistence of quantum and classical optical signals on a single link, we leverage two orthogonal and strongly isolated polarization modes, TE and TM, supported by a single-mode polarization-maintaining platform. A PSR based on mode hybridization is employed as the polarization multiplexer. The device comprises a cascaded TE\u003csub\u003e0\u003c/sub\u003e to TE\u003csub\u003e1\u003c/sub\u003e mode coupler followed by a polarization rotator. Two TE\u003csub\u003e0\u003c/sub\u003e polarized inputs are injected from the through port and the cross port, respectively. The TE\u003csub\u003e0\u003c/sub\u003e mode launched from the cross port is phase-matched to the adjacent waveguide, where it is efficiently coupled and converted into the TE\u003csub\u003e1\u003c/sub\u003e mode. By contrast, the TE\u003csub\u003e0\u003c/sub\u003e mode injected from the through port remains in the original waveguide with negligible mode conversion. The generated TE\u003csub\u003e1\u003c/sub\u003e mode is subsequently converted into the fundamental TM\u003csub\u003e0\u003c/sub\u003e mode via mode hybridization in the rotator section and is finally multiplexed with the TE\u003csub\u003e0\u003c/sub\u003e mode in the bus waveguide. 3D-FDTD simulations predict low \u003cem\u003eIL\u003c/em\u003e and robust mode and polarization conversion at 1550 nm, which are preserved over a broad bandwidth of 100 nm spanning 1500 to 1600 nm. In the mode coupler, the through port TE\u003csub\u003e0\u003c/sub\u003e mode undergoes negligible conversion, while the cross-port input achieves a TE\u003csub\u003e0\u003c/sub\u003e-to-TE\u003csub\u003e1\u003c/sub\u003e conversion efficiency of \u0026minus;\u0026thinsp;0.35 dB, remaining better than \u0026minus;\u0026thinsp;0.43 dB across the full bandwidth. In the rotator, the TE\u003csub\u003e0\u003c/sub\u003e channel exhibits almost no transmission loss, and the TE\u003csub\u003e1\u003c/sub\u003e to TM\u003csub\u003e0\u003c/sub\u003e conversion efficiency reaches \u0026minus;\u0026thinsp;0.08 dB, remaining better than \u0026minus;\u0026thinsp;0.54 dB over the entire spectral range. The design rationale, optimization procedure, and the corresponding simulated spectra and field profiles are detailed in the Supplementary Information (see Supplementary Note 3).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Performance characterization of the coexistence system\u003c/h2\u003e \u003cp\u003eThe device is fabricated on an \u003cem\u003ex\u003c/em\u003e-cut TFLN wafer supplied by NANOLN, adopting an etchless approach for the lithium niobate film itself. A 300 nm-thick silicon nitride (Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e) film is deposited on the TFLN wafer by reactive sputtering, and device patterns are defined in the silicon nitride layer via electron-beam lithography (EBL) followed by inductively coupled plasma (ICP) etching. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows a microscope image of the fabricated coexistence system (overall footprint: 2.7 \u0026times; 0.68 mm\u003csup\u003e2\u003c/sup\u003e), along with magnified scanning electron microscope (SEM) images of the coupling region in the TE\u003csub\u003e0\u003c/sub\u003e-to-TE\u003csub\u003e2\u003c/sub\u003e mode multiplexer and the 1D-PhCW structure. The input ports of the coexistence system are labeled Channel 1\u0026ndash;8, corresponding to coupling of optical signals at wavelengths \u003cem\u003eλ\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026ndash;\u003cem\u003eλ\u003c/em\u003e\u003csub\u003e4\u003c/sub\u003e in TM and TE polarization into the input bus waveguide. The device interfaces with grating couplers to enable efficient coupling of optical signals into and out of the chip. Because the grating couplers are polarization-selective, a PSR with the same design is integrated at the output port of the bus waveguide to separate the TE and TM modes. When a TM\u003csub\u003e0\u003c/sub\u003e-mode signal exits the bus waveguide and enters the PSR, it is converted to TE polarization and routed to the cross port \u003cem\u003eO\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e. In contrast, a TE\u003csub\u003e0\u003c/sub\u003e-mode signal propagates through the PSR without polarization rotation and is routed to the through port \u003cem\u003eO\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e. To eliminate the influence of the PSR and the grating couplers on the measured device performance, a reference device is fabricated in close proximity on the same chip. The transmission spectra are normalized as \u003cem\u003eT\u003c/em\u003e\u0026thinsp;=\u0026thinsp;10 \u0026times; log\u003csub\u003e10\u003c/sub\u003e (\u003cem\u003eP\u003c/em\u003e/\u003cem\u003eP\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e), where \u003cem\u003eT\u003c/em\u003e is the normalized transmittance, \u003cem\u003eP\u003c/em\u003e and \u003cem\u003eP\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e are the measured output powers of the tested device and the reference device, respectively.\u003c/p\u003e \u003cp\u003eThe schematic of the experimental setup is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and consists of a tunable semiconductor laser (TSL), a polarization controller (PC), and an optical spectrum analyzer (OSA). During the measurement, the laser output from the TSL is sent to the PC, coupled into and out of the chip through the input and output grating couplers, and analyzed by the OSA to obtain the transmission spectra. To achieve high coupling efficiency, a TE-polarized grating coupler with a period of 940 nm, a duty cycle of 0.4, and an etching depth of 300 nm is employed. The measured transmission spectra of the fabricated PSR are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec. The \u003cem\u003eIL\u003c/em\u003e for the TE\u003csub\u003e0\u003c/sub\u003e mode is below 0.23 dB, with crosstalk lower than \u0026minus;\u0026thinsp;57.99 dB at 1550 nm. For the TM\u003csub\u003e0\u003c/sub\u003e mode, the \u003cem\u003eIL\u003c/em\u003e is below 0.33 dB and the crosstalk is below \u0026minus;\u0026thinsp;56.40 dB. These results confirm that the PSR provides low-loss and highly isolated polarization (de)multiplexing suitable for quantum-classical coexistence.\u003c/p\u003e \u003cp\u003eFigures \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee show the normalized transmission spectra of the fabricated coexistence system. For TM polarization, the measured \u003cem\u003eIL\u003c/em\u003es of the four channels centered at 1538, 1550, 1562, and 1574 nm are below 1.79, 2.55, 2.50, and 2.81 dB, respectively, and the corresponding inter-channel crosstalk levels are better than \u0026minus;\u0026thinsp;28.75, \u0026minus;\u0026thinsp;24.88, \u0026minus;\u0026thinsp;23.18, and \u0026minus;\u0026thinsp;23.42 dB. For TE polarization at the same center wavelengths, the \u003cem\u003eIL\u003c/em\u003es are below 1.41, 1.59, 1.86, and 1.74 dB, with crosstalk below \u0026minus;\u0026thinsp;29.18, \u0026minus;\u0026thinsp;22.98, \u0026minus;\u0026thinsp;20.96, and \u0026minus;\u0026thinsp;21.88 dB, respectively. Overall, the maximum measured \u003cem\u003eIL\u003c/em\u003e among all channels is 2.81 dB, while the crosstalk remains below \u0026minus;\u0026thinsp;20.96 dB. Compared with simulation results, the experimentally measured crosstalk is slightly higher. This discrepancy is likely due to the coupling and reflection of non-target fundamental modes in the cascaded 1D-PhCW structures. Since the stopband frequency of the fundamental mode is lower than the MSB between the TE\u003csub\u003e0\u003c/sub\u003e and TE\u003csub\u003e2\u003c/sub\u003e modes, the additional crosstalk mainly appears at wavelengths above the designed operating band. In the simulated non-cascaded structures, the crosstalk induced by reflection is negligible. The relatively higher \u003cem\u003eIL\u003c/em\u003e measured in the experiment is attributed to waveguide sidewall roughness, which has a stronger impact on higher-order modes because of their larger field overlap with the etched silicon nitride sidewalls. To further reduce \u003cem\u003eIL\u003c/em\u003e, the etching process can be optimized in future work, for example by reducing the etch rate or applying resist reflow before etching, which is expected to mitigate sidewall roughness in the silicon nitride layer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn addition, owing to fabrication tolerances, the measured channel wavelengths exhibit an approximately 2 nm blue-shift relative to the simulated values, while maintaining a uniform and narrow channel spacing of 12 nm (see Supplementary Note 4). Given the linear relationship between the hole period of the 1D-PhCW and the center wavelength, this offset can be compensated through iterative optimization of the fabrication process. Notably, all channels demonstrate excellent center-wavelength alignment under both TE and TM polarizations. For future improvements requiring tighter wavelength control, thermal or electrical tuning could be incorporated to finely adjust the center wavelength by modulating the refractive index of the photonic crystal material.\u003c/p\u003e \u003cp\u003eQuantum signals are much weaker than classical signals and can approach the single-photon level. When QKD coexists with conventional data in the same transmission channel, system performance can degrade substantially due to noise. In such coexistence systems, noise in the quantum channel can be broadly categorized into two types. The first is crosstalk from the classical channels into the quantum channel, caused by non-ideal multiplexing and insufficient channel isolation. The second comprises noise generated by light-medium interactions during propagation, including scattering (Rayleigh, Brillouin, Raman) and nonlinear effects (FWM, self-phase modulation, cross-phase modulation) \u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e. Here, we focus on classical-signal power leakage into the quantum channel resulting from non-ideal multiplexing performance. Denoting the classical launch power before multiplexing as \u003cem\u003eP\u003c/em\u003e\u003csub\u003ein\u003c/sub\u003e, the leaked power coupled into the quantum channel, \u003cem\u003eP\u003c/em\u003e\u003csub\u003eout\u003c/sub\u003e, is:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{\\text{P}}_{\\text{out}}\\text{}\\text{=}{\\text{}\\text{P}}_{\\text{in}}{\\text{t}}_{\\text{mux}}{\\text{t}}_{\\text{leak}}\\text{}\\text{=}{\\text{}\\text{P}}_{\\text{in}}{\\text{10}}^{\\text{-}\\text{\u0026delta;/10}}{\\text{10}}^{\\text{-}\\text{\u0026zeta;/10}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eδ\u003c/em\u003e and \u003cem\u003eζ\u003c/em\u003e are the \u003cem\u003eIL\u003c/em\u003e and the isolation of the multiplexing system (in dB), respectively. The classical-signal transmission coefficient is \u003cem\u003et\u003c/em\u003e\u003csub\u003emux\u003c/sub\u003e = 10\u003csup\u003e\u0026minus;\u003cem\u003eδ\u003c/em\u003e/10\u003c/sup\u003e, and the crosstalk ratio leaking into the quantum channel is \u003cem\u003et\u003c/em\u003e\u003csub\u003eleak\u003c/sub\u003e = 10\u003csup\u003e\u0026minus;\u003cem\u003eζ\u003c/em\u003e/10\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFour wavelength-division-multiplexed classical channels with TM (or TE) polarization are selected. The channels correspond to Channel 1\u0026ndash;Channel 4 (or Channel 5\u0026ndash;Channel 8), spanning 1538\u0026ndash;1574 nm with a channel spacing of 12 nm. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb present the leakage power of these classical signals into the coexistence quantum channel, where the blue bars denote the leakage contribution from each individual channel. For TM and TE polarizations, the maximum leakage among the wavelength channels is approximately \u0026minus;\u0026thinsp;24.226 dB and \u0026minus;\u0026thinsp;27.478 dB relative to the input power, respectively. By further leveraging the excellent polarization isolation of the coexistence system, quantum-classical coexistence can be substantially improved. Specifically, when classical and quantum signals are launched in orthogonal polarizations and assigned to different wavelength channels, the leakage can be suppressed to more than 100 dB below the input power, enabling robust coexistence in practical networks. In addition, because the crosstalk power in QKD increases linearly with the launch power of the classical channels, lowering the transmitted power of classical data channels is an effective way to enhance QKD performance. Finally, incorporating tunable bandpass filters can further suppress residual leakage crosstalk and improve the overall robustness of quantum-classical coexistence.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter comprehensive device characterization, we establish a coexistence test system to evaluate the transmission quality of quantum signals in the presence of classical optical channels, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. In this setup, light from one TSL passes through a PC and a C13 filter (centered at 1567 nm) and is then coupled into chip Channels 4 and 5 with a pre-grating power of 1.1 mW to emulate classical data transmission. Simultaneously, light from a second TSL is routed through another PC, a C34 filter (centered at 1550.12 nm), and a spiral waveguide, where nondegenerate photon pairs are generated via spontaneous FWM. Owing to energy conservation, their frequencies satisfy \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{2}{\\text{\u0026omega;}}_{\\text{p}}\\text{=}{\\text{\u0026omega;}}_{\\text{s}}\\text{+}{\\text{\u0026omega;}}_{\\text{i}}\\text{.}\\)\u003c/span\u003e\u003c/span\u003e A dense wavelength-division multiplexer (DWDM) separates the photons, and the signal photons are coupled into chip Channels 3 and 6. At the receiver side, the TM- and TE-polarized outputs are collected from ports \u003cem\u003eO\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e and \u003cem\u003eO\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e, respectively, to emulate quantum signal transmission. Both signal and idler photons are detected by superconducting nanowire single-photon detectors (SNSPDs) and analyzed using a time-correlated single-photon counting (TCSPC) system. The TCSPC module converts detected photon events into electrical pulses and assigns a timestamp to each event with picosecond resolution, enabling accurate measurement of the arrival-time difference between the signal and idler photons. During measurement, the coincidence time window is set to 1.6 ns. Using this experimental system, we realize a heralded single-photon source and deploy it as a quantum light source for chip testing.\u003c/p\u003e \u003cp\u003ePerformance tests of the quantum channels yield the following results. For the Channel 3\u0026ndash;\u003cem\u003eO\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e path, the 20-s coincidence counts without classical light, with classical light injected into Channel 4, and with classical light injected into Channel 5 are 149, 129, and 128, corresponding to relative losses of 1.3 dB, 1.9 dB, and 1.9 dB, respectively. For the Channel 6\u0026ndash;\u003cem\u003eO\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e path, the corresponding 20-s coincidence counts are 137, 117, and 132, with relative losses of 1.7 dB, 2.4 dB, and 1.8 dB, respectively. These results show that the additional transmission loss in the quantum channels remains small and well-controlled, even when adjacent channels carry relatively high-power classical signals. This confirms that the proposed wavelength-polarization hybrid multiplexer effectively suppresses crosstalk between classical and quantum light, thereby preserving the integrity of QKD signals in coexistence scenarios.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Discussion","content":"\u003cp\u003eIntegrating quantum and classical communication on a single photonic platform is critical for scalable quantum-secure networks, as this integration is a prerequisite for bridging laboratory prototypes to real-world deployment. In such networks, QKD must coexist with high-capacity classical data streams without dedicated infrastructure. This study demonstrates, for the first time, on-chip coexistence of classical optical transport network (OTN) signals and QKD channels using an etchless thin-film lithium niobate (TFLN) platform. It directly addresses the longstanding challenge of quantum-classical interference in shared physical infrastructure. By integrating a wavelength-polarization hybrid (de)multiplexer with breakthrough performance, this chip-scale system establishes TFLN as a transformative platform for unified quantum-classical photonics, with applications spanning metropolitan networks, data centers, and future quantum internet architectures. The core innovation resides in the monolithic integration of wavelength and polarization multiplexing, enabling 8 independent wavelength-polarization channels within a compact 2.7 \u0026times; 0.68 mm\u003csup\u003e2\u003c/sup\u003e footprint. This hybrid design delivers exceptional performance that directly meets quantum-classical coexistence requirements: on-chip \u003cem\u003eIL\u003c/em\u003e is as low as 0.23 dB for the TE₀ mode and 0.33 dB for the TM₀ mode; polarization crosstalk is below \u0026minus;\u0026thinsp;57.99 dB (TE\u003csub\u003e0\u003c/sub\u003e) and \u0026minus;\u0026thinsp;56.40 dB (TM\u003csub\u003e0\u003c/sub\u003e); same-polarization wavelength-channel crosstalk is below \u0026minus;\u0026thinsp;20.96 dB across all channels. These metrics are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, which provides a comprehensive comparison with existing quantum-classical coexistence systems. Previous quantum-classical coexistence systems primarily relied on fiber-end multiplexing, which lack the miniaturization and reconfigurability required for dense network integration \u003csup\u003e[\u003cspan additionalcitationids=\"CR9 CR10\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. While these fiber-based systems offered advantages like scalable transmission capacity or small channel spacing, their key multiplexing and isolation capabilities depended on discrete devices rather than monolithic integration. Although a silicon photonics chip was reported to support continuous-variable QKD (CV-QKD) with a local oscillator (LO), where quantum signals and strong LO light from the same laser co-propagated via polarization multiplexing \u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, it did not systematically compare critical metrics such as channel count, scalability, or classical link loss. In contrast, this work achieves on-chip coexistence of classical optical communication and QKD using etchless TFLN, integrating dynamically reconfigurable multi-channel quantum/classical paths on a compact chip while balancing low classical \u003cem\u003eIL\u003c/em\u003e and high isolation. Its advantage extends beyond mere \u0026ldquo;on-chip integration\u0026rdquo;: leveraging TFLN\u0026rsquo;s inherent low loss and strong polarization/multiplexing capabilities, it realizes a scalable, multi-channel, reconfigurable architecture that provides a more miniaturized, integrable pathway for quantum network nodes compatible with existing optical infrastructure.\u003c/p\u003e \u003cp\u003eThe coexistence system features exceptional fabrication tolerance, ensuring compatibility with large-scale manufacturing. The uniform 12 nm channel spacing is well-matched to existing C-band spectral resources, eliminating the need to reconfigure deployed telecommunications infrastructure. These parameters translate to tangible application advantages: 12 nm wavelength spacing enables efficient C-band utilization, critical for fiber-constrained metropolitan networks, while low \u003cem\u003eIL\u003c/em\u003e ensures minimal signal degradation for both classical and quantum channels. The multimode waveguide design exhibits excellent robustness: a width variation of \u0026plusmn;\u0026thinsp;20 nm induces only\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 nm wavelength drift and negligible additional \u003cem\u003eIL\u003c/em\u003e, defining a practical process window for large-scale manufacturing and addressing a key commercialization barrier for integrated photonic devices. Importantly, high channel isolation prevents high-power classical signals from overwhelming quantum channels, preserving QKD transmission quality even with adjacent classical traffic. Together, these features make the TFLN chip an ideal choice for real-world networks requiring simultaneous reliability, spectral efficiency, and security.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePerformance comparison of quantum-classical coexistence systems.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eYear\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCore Platform\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOn-Chip Integration\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNumber of Channels (Quantum/Classical)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eChannel Spacing\u003c/p\u003e \u003cp\u003e(nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eIL\u003c/em\u003e (Classical)\u003c/p\u003e \u003cp\u003e(dB)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eIsolation (Classical)\u003c/p\u003e \u003cp\u003e(dB)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eChip Size\u003c/p\u003e \u003cp\u003e(mm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003cp\u003e(ref. 8)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSingle-mode fiber TDM system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2(1/1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e15.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e~\u0026thinsp;6.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e10.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2020\u003c/p\u003e \u003cp\u003e(ref. 9)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eWeakly-coupled few-mode fiber\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2(1/1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.60 (LP\u003csub\u003e01\u003c/sub\u003e)\u003c/p\u003e \u003cp\u003e3.70 (LP\u003csub\u003e02\u003c/sub\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e23.58 (LP\u003csub\u003e01\u003c/sub\u003e)\u003c/p\u003e \u003cp\u003e23.20 (LP\u003csub\u003e02\u003c/sub\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2020\u003c/p\u003e \u003cp\u003e(ref. 10)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7-core multicore fiber\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8 (1/7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e5.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e44.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2024\u003c/p\u003e \u003cp\u003e(ref. 11)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSingle-mode fiber DWDM system\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNo\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e61(1/60)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2019\u003c/p\u003e \u003cp\u003e(ref. 19)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSilicon photonic chip\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eYes\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2(1/1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e35.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eThis Work\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTFLN-based integrated photonics\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eYes\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8\u003c/p\u003e \u003cp\u003e(Dynamically Reconfigurable)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e12.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.81 (TM)\u003c/p\u003e \u003cp\u003e1.86 (TE)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e56.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e2.7 \u0026times; 0.68\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"8\"\u003eNote: \u003cem\u003eIL\u003c/em\u003e: Insertion Loss; TDM: Time Division Multiplexing; DWDM: Dense Wavelength Division Multiplexing; LP: Linear Polarization; TFLN: Thin-Film Lithium Niobate; NA: Not Available.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe unique material properties of TFLN underpin system performance by addressing key limitations of alternative platforms. TFLN combines ultra-low propagation loss, strong electro-optic modulation, and a broad transparency window, making it inherently suitable for quantum-classical coexistence. Silicon nitride cladding enhances mode confinement and reduces bending loss, enabling compact footprints without sacrificing optical performance. These material advantages, combined with hybrid wavelength-polarization multiplexing, achieve a balanced combination of channel density, isolation, and loss. Supporting both classical and quantum signals on-chip with realistic spacing and power levels bridges the gap between laboratory demonstrations and real-world quantum-secure networks. Beyond device-level performance, the system-level architecture offers transformative capabilities for optical network design: by leveraging wavelength and polarization degrees of freedom, the chip provides 8 dynamically allocable channels between classical and quantum signals, enabling flexible resource management for time-varying network demands. This flexibility suits diverse scenarios: in metropolitan networks, the chip can function as a compact coexistence node that adds or drops QKD channels from existing high-capacity fiber links without dedicated dark fibers; in data centers and short-reach scenarios, its small size and low power consumption make it ideal for high-bandwidth-density links requiring both throughput and security.\u003c/p\u003e \u003cp\u003eDespite these significant advances, key challenges persist for TFLN-based quantum-classical coexistence. The fixed wavelength-polarization mapping limits adaptability to dynamic networks. Future iterations could integrate thermo-optic and electro-optic tuning to enable real-time channel reconfiguration, thereby suppressing FWM noise and enhancing overall efficiency. Long-haul transmission requires hybrid amplification schemes: erbium-doped fiber amplifiers (EDFAs) for classical signals, and quantum repeaters or heralded single-photon sources for QKD. The chip\u0026rsquo;s low \u003cem\u003eIL\u003c/em\u003e reduces reliance on quantum amplification, lowering security vulnerability risks. Finally, system-level integration with on-chip light sources and detectors will eliminate fiber pigtailing losses and enable fully integrated transceivers with smaller form factors and improved reliability.\u003c/p\u003e \u003cp\u003eLooking forward, this study redefines the paradigm of optical networks with intrinsic quantum security. By demonstrating on-chip coexistence of high-intensity classical signals and quantum channels with realistic spacing, low loss, and high isolation, it advances the vision of a quantum-enhanced internet, where classical infrastructure simultaneously transmits high-speed data and unbreakable encryption based on quantum mechanics. With continued advances in dynamic resource allocation and long-haul optimization, TFLN-based integrated QKD will provide high-security encryption for scenarios ranging from metropolitan networks to global quantum backbones. These chip-scale results validate that quantum channels can share a platform with high-intensity classical signals while maintaining strict security guarantees, paving the way for the seamless transition of existing networks to quantum-secure communications via deliberate photonic hardware design.\u003c/p\u003e"},{"header":"4. Methods","content":"\u003cp\u003e \u003cb\u003eDevice simulation\u003c/b\u003e \u003c/p\u003e \u003cp\u003eTE-polarized photonic band structures are simulated using periodic boundary conditions on photonic crystal unit cells with the finite element method. The dependence of the effective refractive index on waveguide width is calculated using a finite-difference eigenmode (FDE) solver. For all other simulation tasks, a 3D-FDTD method is employed. Lithium niobate and silicon nitride are retrieved from the default material database. At 1550 nm, LN has refractive indices of ~\u0026thinsp;2.14 (extraordinary, \u003cem\u003en\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e) and ~\u0026thinsp;2.22 (ordinary, \u003cem\u003en\u003c/em\u003e\u003csub\u003eo\u003c/sub\u003e), while silicon nitride has a refractive index of ~\u0026thinsp;1.99. To ensure reliable results, the FDTD simulation time is set to 10,000 fs, and an auto-shutoff level of 1 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e is used. An automatic mesh with an accuracy level of 4 is adopted to balance computational cost and numerical precision. A mode expansion monitor is introduced to enhance the accuracy of the 3D-FDTD simulations, which significantly improves the evaluation of \u003cem\u003eIL\u003c/em\u003e, \u003cem\u003eER\u003c/em\u003e, and crosstalk. All light propagation profiles are presented as continuous distributions to facilitate physical interpretation and analysis.\u003c/p\u003e \u003cp\u003e \u003cb\u003eDevice fabrication\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe system is fabricated on a commercial \u003cem\u003ex\u003c/em\u003e-cut TFLN wafer incorporating a 300 nm-thick lithium niobate film supplied by NANOLN. First, a 300 nm-thick silicon nitride layer is deposited on top of the TFLN wafer by reactive sputtering. Device structures are then defined in the silicon nitride layer using electron-beam lithography (EBL), followed by inductively coupled plasma (ICP) etching.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (W2411059, 62405125, T2325022, U23A2074), the Gansu Provincial Science and Technology Major Special Project (25ZDWA001, 25ZDGA005), the Key Research and Development Program of Gansu Province (24YFGA007), the Joint Research Fund of Gansu Province (25JRRA1126), the CAS Project for Young Scientists in Basic Research (YSBR-049), the Australian Research Council (CE230100006), the Fundamental Research Funds for the Central Universities, and the Talent Scientific Fund of Lanzhou University. We thank the Micro Nano Research Facility (MNRF) and the Australian Microscopy \u0026amp; Microanalysis Research Facility at RMIT University for access to facilities and for scientific and technical assistance. Parts of this work were carried out at the Melbourne Centre for Nanofabrication (MCN), the Victorian node of the Australian National Fabrication Facility (ANFF), and at the OptoFab node of ANFF, with support from NCRIS and the Victorian state government.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eM.Y. and Y.T. conceived the project and optimized the research plan. M.Y. designed and performed numerical simulations of the hybrid (de)multiplexer, with contributions from H.L. and X.Z. M.X.L., T.G.N., and G.R. fabricated and characterized the devices. M.Y. performed the transmission measurements and analyzed the data, with additional analysis support from B.X., X.H., Z.C., and L.F., who characterized the classical-quantum coexistence system. M.Y. and Y.T. wrote the manuscript with input from all authors. Y.T. supervised the overall project. G.R., A.M., X.R., and Y.T. provided guidance and feedback. All authors discussed the results and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the data supporting the findings in this study are available in the paper and Supplementary Information. Source data are provided with this paper. Additional data related to this paper are available from the corresponding authors upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eO\u0026rsquo;Brien, J. L., Furusawa, A., \u0026amp; Vučković, J. Photonic quantum technologies. \u003cem\u003eNature Photonics\u003c/em\u003e\u003cstrong\u003e3\u003c/strong\u003e, 687\u0026ndash;695 (2009).\u003c/li\u003e\n\u003cli\u003ePittaluga, M., et al. Long-distance coherent quantum communications in deployed telecom networks. \u003cem\u003eNature\u003c/em\u003e\u003cstrong\u003e640\u003c/strong\u003e, 911\u0026ndash;917 (2025).\u003c/li\u003e\n\u003cli\u003eBernstein, D. J., \u0026amp; Lange, T. Post-quantum cryptography. \u003cem\u003eNature\u003c/em\u003e\u003cstrong\u003e549\u003c/strong\u003e, 188\u0026ndash;194 (2017).\u003c/li\u003e\n\u003cli\u003eDing, X., et al. 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Quantum-classical coexistence in multi-band optical networks: a noise analysis of QKD. \u003cem\u003eIEEE Communications Letters\u003c/em\u003e\u003cstrong\u003e28\u003c/strong\u003e, 488\u0026ndash;492 (2024).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"quantum key distribution (QKD), classical optical communication, coexistence system, integrated photonic circuits, thin-film lithium niobate (TFLN)","lastPublishedDoi":"10.21203/rs.3.rs-8525819/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8525819/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIntegrated photonics-based coexistence systems enable classical data channels and quantum channels to share the same optical waveguide by integrating quantum key distribution (QKD) into existing telecommunications infrastructure, reducing the costs and complexities of deploying quantum-secure links. Achieving this requires photonic components that efficiently reject intense classical light while preserving the integrity of ultra-weak single-photon quantum signals. Here, we demonstrate the first on-chip quantum-classical coexistence platform based on thin-film lithium niobate (TFLN). The chip integrates a polarization splitter-rotator (PSR) with two four-channel wavelength-division multiplexers, creating eight distinct channels encoded by wavelength and polarization in a highly compact millimeter-scale footprint. The device features ultra-low on-chip insertion loss (\u003cem\u003eIL\u003c/em\u003e), high polarization isolation, and robust suppression of nonlinear noise, minimizing crosstalk from classical to quantum channels. Single-photon experiments verify its compatibility with QKD operations under realistic scenarios of simultaneous classical and quantum transmission. Its fabrication-robust design eases lithography constraints, enabling scalable and cost-effective manufacturing. Overall, this chip offers a viable path to integrate QKD into dense optical networks and advances the development of scalable quantum-secure communications.\u003c/p\u003e","manuscriptTitle":"On-chip coexistence of classical optical communication and quantum key distribution on etchless thin-film lithium niobate","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-29 00:37:39","doi":"10.21203/rs.3.rs-8525819/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f1d8b3d1-f13b-42a6-bf47-89f853381039","owner":[],"postedDate":"January 29th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":61641103,"name":"Physical sciences/Optics and photonics/Applied optics/Integrated optics"},{"id":61641104,"name":"Physical sciences/Optics and photonics/Applied optics/Optoelectronic devices and components"}],"tags":[],"updatedAt":"2026-02-25T12:16:59+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-29 00:37:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8525819","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8525819","identity":"rs-8525819","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}