Entanglement purification is an essential component of quantum repeaters, as it can improve the fidelity of the distributed entangled states and mitigate the effects of the noisy channel. Successful purification yields entangled states with increased fidelity, whereas failed events can still retain residual entanglement that remains usable for further purification when the error rates of the two degrees of freedom (DOFs) are unbalanced. In this paper, we demonstrate a single-copy entanglement purification scheme based on hyperentanglement using silicon chips and experimentally observe the presence of residual entanglement. Leveraging the reconfigurability of integrated photonics, our scheme ensures that, under bit-flip noise acting on the two DOFs, residual entanglement suitable for further purification can always be obtained, regardless of which DOF has the higher error rate. Our results demonstrate the advantages of integrated photonics for quantum information processing and provide guidance for the optimized utilization of entanglement resources in on-chip entanglement purification and future quantum repeater systems.

Raman lasing in microresonators has been observed in various material platforms, such as silicon, silica, lithium niobate, diamond, and silicon carbide. Frequency matching between cavity resonances and a Raman gain profile enables a lasing threshold as low as a milliwatt level with a continuous-wave pump. However, the distinct thermal responses of the cavity resonances and the Raman phonon frequency can induce frequency mismatch and lead to unstable Raman lasing with varying temperatures. This temperature-dependent Raman lasing has not been sufficiently investigated to date. Here, we characterize two prominent Raman phonons at 265 and 776 cm(-1) in 4H-silicon carbide (SiC) microresonators and analyze their thermal evolution. We observe that the Raman signal near 776 cm(-1)exhibits a thermal shift rate closely matching that of the cavity resonance, enabling stable Raman lasing for a temperature range exceeding 88 K. In contrast, Raman lasing near 265 cm(-1 )behaves in an unstable manner, due to the narrow Raman gain bandwidth and the large mismatch between the thermal response of the Raman gain and the cavity resonance. These findings demonstrate the stable lasing behavior of the dominant Raman signal at similar to 776 cm(-1) of 4H-SiC and establish crucial guidelines for achieving thermally robust Raman lasing in microresonators.

Quantum repeaters are employed in quantum communication to overcome the long-distance transmission loss of quantum states. The quantum repeater is based on various key technologies, including quantum entanglement swapping, quantum memory, and entanglement purification. In particular, quantum purification can distil high-quality entanglement from the degraded entangled states which is propagating through noisy quantum communication channels. Although previous reports have demonstrated on-chip entanglement swapping and teleportation through the less-noisy channel, current entanglement purification experiments still rely on off-chip discrete devices, leading to limitations on scalability, stability, and controllability. In this paper, for the first time, we demonstrated chip-to-chip hyperentanglement distribution and quantum entanglement purification based on integrated silicon chips. Path-encoded high-dimensional entangled photon pairs are produced on the chip, converted to fibre-based polarization-spatial hyperentanglement by grating couplers, distributed to the receiver silicon chip, and finally purified by consuming the spatial degree of freedom. Our purification scheme by integrated photonics finished the last puzzle of on-chip quantum repeater, which will promote the realization of the quantum repeater based on integrated photonics.

We demonstrate a Brillouin OTDR sensing range of 250 km on a telecommunication fiber. Including a normal dispersion fiber at a selected position helps to reduce the nonlinear noise arising from modulation instability at the remote end.

We demonstrate Raman lasing in 4H-SiC microring resonators, analyze and characterize the temperature-dependence of the lasing. We evaluate the thermo-optic coefficient to be $4.50\times 10^{-5}\mathrm{K}^{-1}$. The result shows the robustness of the thermal tuning of the microring for Raman lasing.

Using intermodal four-wave mixing in an optical step-index fiber, we demonstrate tunable low-noise frequency conversion of single photons emitted from a InAs/GaAs quantum dot with peak conversion efficiency of -0.6 dB.

The reliable distribution of high-dimensional entangled quantum states, an important resource in quantum technologies, through optical fibre networks is challenging due to the need to maintain coherence across multiple modes. Here we demonstrate the distribution of four-dimensional path-encoded entangled quantum states between photonic chips, enabled by a novel multimode phase stabilisation algorithm. The algorithm utilises the reconfigurability of the integrated photonic circuits to complete one iteration of phase stabilisation in just two measurement rounds for an arbitrary number of modes, and requires no additional hardware to the quantum measurements it enables. As a result, we are able to perform complete quantum state tomography across two chips using the minimum number of local projective measurements to verify the fidelity of the distributed entangled state to be 86% (compared to 8.1% without the phase stabilisation) with an entanglement entropy of 0.995±0.002.

The ability to convert the spatial mode of single photons opens up a promising path to enhancing quantum communication protocols by enabling high-dimensional encoding and efficient multiplexing. In this work, we demonstrate spatial mode conversion of single photons at 1550.6 nm using a fiber long-period grating (LPG). The fundamental $$\hbox {LP}_{01}$$ mode was converted to higher-order modes $$\hbox {LP}_{11}$$ and $$\hbox {LP}_{02}$$ , with quantum mode conversion efficiencies of 87.5 ± 1.4% and 96.1 ± 1.6%, respectively. The characterization of the converted single photons was carried out using a time-of-flight technique and coincidence measurements, by taking advantage of the differences in group velocity between the modes. We also performed loss measurements at the single-photon level and demonstrated mode re-conversion by using a second LPG to restore the photons back to the fundamental mode. These results highlight the potential of LPGs as a versatile tool for spatial mode manipulation at the single-photon level, with applications in high-dimensional quantum communication and nonlinear optical interactions.

We demonstrate efficient Bragg-scattering four-wave mixing frequency conversion in a few-mode fiber with two pumps spectrally separated from the signal and idler by 600 nm. The wideband frequency conversion is made possible by propagating the frequency components in two different spatial modes, the pumps are excited in the LP11 mode, while the signal is excited in the LP01 mode, and the Bragg scattering (BS) idler is generated in the LP01 mode. For these processes, we experimentally characterize their conversion efficiency and bandwidth. The dependency of conversion efficiency on peak pump power and the separation between BS components are measured, demonstrating a peak conversion efficiency of up to 79%.

A deterministic entanglement purification integrated photonics circuit is proposed. The purification protocol is simulated based on characterized data of on-chip devices and the state fidelity rises from 86% to 97.9%.

Erbium-doped fiber designs are currently constrained by the general requirement to guide only one mode at the pump and signal wavelengths. Parameters such as the core diameter and numerical aperture must be carefully controlled to maintain single-mode operation. This limits the flexibility in the doped fiber design, and the small mode field diameter reduces the power handling limit and increases the splice losses to standard single-mode fibers. Relaxing this constraint to permit higher-order mode propagation at the pump wavelength would enable the development of new doped fiber designs with enhanced properties. However, the consequences of permitting higher-order pump modes to propagate within the amplifier fiber remain largely unexplored. Here, we present gain and noise figure measurements for a pure LPII pump mode that is usually not excited and compare the results to standard $\text{LP}_{01}$ pumping. The setup shown in Fig. 1 is used to measure the gain and noise figure on 8 m of an OFS LRXL erbium-doped fiber. Simulations and cut-off wavelength measurements show that the $\text{LP}_{01}$ and LPII modes are supported at the pump wavelength. The fiber is pumped by a 976 nm laser diode that is combined with the signal using a wavelength division multiplexer (WDM). A long-period fiber grating (LPG) is thermally inscribed on a fiber [1] and converts the $\text{LP}_{01}$ mode into an LPII mode at the pump wavelength with a conversion efficiency of 99 %. The signal wavelengths pass through the LPG and remain in the $\text{LP}_{01}$ mode, which is the only mode supported by the fiber at those wavelengths. By replacing the LPG fiber with an identical fiber without any inscriptions, the pump mode can be changed to an $\text{LP}_{01}$ mode.

4H‐silicon carbide (SiC) has recently emerged as a promising material for nonlinear photonic integrated circuits, thanks to its low loss, wide bandgap, as well as strong second‐order () and third‐order () nonlinearities. Though its unique crystal structure allows versatile processes, spontaneous parametric down‐conversion (SPDC) still remains unrealized. In this work, photon‐pair generation in a 4H‐SiC‐on‐insulator integrated platform is demonstrated through modal‐phase‐matched type‐I SPDC. Furthermore, type‐0 and type‐II nonlinear interactions are shown in 4H‐SiC waveguides, thus highlighting the potential to exploit diverse phase‐matching mechanisms on this platform. These results underscore the potential of 4H‐SiC for advancing the development of integrated quantum photonics in the realms of quantum information processing and quantum communication.

We introduce a versatile, easy-to-implement Gaussian process model that provides a flexible framework for modeling core-radius fluctuations of an optical fiber. The flexibility of the model allows for easy modification of the fiber profile to fit known characteristics, such as forcing the profile through carefully measured radii at specific points. The model is then used to show the drastic impact that fluctuations have on the efficiency of wide-spanning four-wave mixing systems. Furthermore, by optimizing the fiber design of a simple step-index fiber, we show that the efficiency of the four-wave mixing can be drastically improved even in the presence of large core-radius fluctuations.

We present a characterisation technique for a photonic-lantern-based spatial-division multiplexed link using temporally-resolved coincidence measurements of SPDC photon pairs. Differential modal delay and input path imbalance allow unambiguous identification of spatial mode propagation and coupling errors, providing information that can be used for link optimization. (c) 2025 The Author(s)

We demonstrate the first on-chip deterministic entanglement purification based on silicon photonics. To evaluate the purification performance, we simulate the bit-flip and phase-flip errors by reconfigurable circuits on chip. The state fidelity improves from 0.71 to 0.82 under a 20% bit-flip error. rate

Point sensors, where a short piece of few-mode fiber (FMF) is spliced between single-mode fibers (SMF), work by monitoring the phase shift in the interference between two guided modes of the FMF, induced by e.g. changes in the temperature or the strain applied to the FMF. So far, such sensors have only been demonstrated as single-point sensors. Here, the method of [3], introduced to distinguish the interference between the fundamental mode and two higher-order modes, is applied to separate the interferences generated in FMF sections of different lengths.

Space-division multiplexing (SDM) has emerged as a key solution to meet the growing demands for high-capacity optical communication systems. In this context, ring-core fibers (RCFs) have gained attention due to their unique potential for supporting a large number of orbital angular momentum (OAM) modes [1]. OAM modes exhibit a helical phase front $(\exp(il\phi))$, where $l$ represents the topological charge and $\phi$ the azimuthal angle [2]. With OAM's theoretically infinite number of orthogonal eigenstates, it offers substantial potential for increasing the capacity of mode-division multiplexing (MDM) systems [3]. This work presents an air-gap fiber with two ring cores with reduced inter-core crosstalk (IC-XT) [4] and improved fiber fuse resistance [5]. It supports 52 OAM modes with IC-XT below -100 dB/100 km, ideal for high-capacity, long-distance space-division multiplexing. The cross-section of the proposed fiber design and its modal analysis, conducted using COMSOL Multiphysics, are shown in Fig. 1. The results confirm the confinement of OAM modes from $\ell=1$ to $\ell=7$ with $\Delta \mathrm{n}_{\text{eff}} > 10^{-4}$ ensuring minimal intermodal interference. The simulated IC-XT was below -100dB/100 km for $\ell=5$ to $\ell=7$ at a 140 mm bending radius. Based on the designs, we fabricated a two-core RCF. The preform was formed and drawn into a 145 $\mu \mathrm{m}$ fiber as shown in Fig 2, though some deformation in the air gap and core attachment occurred. These initial results indicate areas for further optimization in the drawing process to meet our design targets.

Using a theoretical model, we investigate the impact due to Raman scattering on frequency conversion obtained via Bragg scattering four-wave mixing in optical fibers. We predict the second order correlation functions and show that by using a specific polarization configuration of the pumps and signal it is possible to significantly mitigate the impact of Raman scattering by reducing the second order correlation function by at least a factor of two.

We demonstrate an experimental method to characterize the transfer function for quantum frequency conversion via Bragg scattering four-wave mixing by performing measurements in the classical regime.

Silicon carbide (SiC) photonic integrated platform has attracted significant research interest for on-chip optical applications, owing to its exceptional optical properties such as a broad transparency window, high refractive index, and strong nonlinearity. Among the various types of SiC, amorphous SiC (a-SiC) has particularly emerged as an accessible choice for forming thin-film SiC-on-insulator (SiCOI) stacks, demonstrating promising capabilities for wafer-scale photonic applications. In this work, we prepare three a-SiCOI samples using the plasma-enhanced chemical vapor deposition, with different refractive indices. We fabricate optical waveguides, conduct four-wave mixing measurements, and characterize the nonlinear refractive index in these samples. Our findings reveal that an increase in the refractive index of a-SiC leads to a corresponding increase in the nonlinear refractive index, which is comparable to that of silicon. Hence, a-SiC offers an approach to develop a SiC platform with a wider bandgap than that of silicon, minimizing two-photon absorption while also providing a higher refractive index and stronger nonlinearity compared to crystalline SiC.