Yongnan Li

High‐dimensional (HD) entanglement of photonic spatial modes endows photon pair with greater shared information, enabling secure quantum communication even in noisy environments. However, adequately characterizing HD spatial mode entanglement remains challenging due to the exponential scaling of measurements and data processing complexity. Here, we develop and experimentally demonstrate a physics‐informed neural network for efficient quantum state tomography (QST) of HD spatial mode entanglement. Our method substantially reduces the number of coincidence counts required for QST and enables physics‐informed reconstruction from only two projective measurements. We achieve experimental fidelities of 0.931 0.005 and 0.975 0.006 for 15‐dimensional entangled states and 5‐dimensional mixed entangled states, respectively. The approach also exhibits enhanced environmental adaptability to atmospheric turbulence and spatial misalignment, which are critical challenges in free‐space quantum communication. By addressing both the scaling bottleneck and photon efficiency, our method offers a practical and scalable solution for characterizing quantum spatial mode systems, paving the way for broader applications in quantum communication and information processing.

Spatially structured light fields governed by global topological invariants provide a robust platform for free-space optical communication. While conventional scalar Möbius topologies are vulnerable to phase and amplitude distortions, vectorial Möbius fields constructed via spatially resolved Stokes parameters overcome this limitation. Because atmospheric turbulence distorts spatial phases but leaves local polarization states largely unperturbed, encoding topological invariants into Stokes fields effectively cancels common-mode phase fluctuations. Advancing beyond conventional two-dimensional vectorial modes, mapping information onto the three-dimensional generalized Möbius topology of these macroscopic fields provides intrinsic structural protection against severe turbulence-induced intensity fragmentation. A free-space optical communication scheme utilizing the global topological twist number as an information carrier, therefore, circumvents the need for active wavefront compensation. By employing a polarimetric detection scheme based on filtered normalized Stokes parameters combined with digital matched filtering, this approach guarantees accurate target field identification across diverse propagation scenarios, including near-field, far-field, single-screen, and multiple-screen turbulence configurations. Evaluations demonstrate that even in strong turbulence regimes with a Rytov variance of up to 3.0, these topological signatures are robustly preserved despite severe intensity fragmentation. Consequently, this scheme achieves negligible error rates for an alphabet of four topological configurations and a transmission capacity approaching 2.8 bits per optical field utilizing a basis of ten orthogonal fields, establishing a robust physical layer framework for high-dimensional optical networks.

The topological properties of vector optical fields are traditionally considered strictly conserved during continuous deformations and linear propagation. However, while structured light has been extended into nonlinear regimes, previous studies have predominantly focused on the intensity modulation of specific orbital angular momentum (OAM) components and the pure frequency conversion of structured light. The critical question of whether macroscopic topological invariants remain robust or experience fundamental breakdown during nonlinear light–matter interactions remains largely unexplored. To address this specific gap, we propose and generate multiple fractional vector optical fields (MF-VOFs), establishing their dynamic topological evolution and inherent conservation laws in free space. It should be noted that our experimental results are limited to free-space propagation. Strikingly, we report a significant departure from this paradigm during light–matter interactions: topological nonconservation anomalies manifest when these optical fields interact with nonlinear materials via second- and third-harmonic generation. Through a comprehensive quantitative analysis of the OAM spectrum, we confirm that the asymmetrical reconstruction and spatial transition of the total OAM along the propagation direction serve as the physical origins driving this topological symmetry breaking. These findings provide a fundamentally novel perspective on topological manipulation in nonlinear optical processes, offering advanced strategies for complex structured light generation and high-dimensional optical information processing.

Scattering imaging techniques are significant tools in biomedicine and underwater detection. Speckle cross correlation imaging of a moving object makes it possible to reconstruct hidden objects through random media. The cross correlation imaging is difficult to apply in dynamic scattering media and requires a moving object. We propose a method based on the far-field transmission of a mutual coherence function to reconstruct the image of object obscured by dynamic scattering media. We characterize the imaging capability through the Euclidean distance and analyze the impact of two system parameters on the Euclidean distance. Subsequently, object images of different sizes were reconstructed. This work provides an innovative applicable method for non-invasive optical imaging.

High‐dimensional (HD) entanglement of photonic orbital angular momentum (OAM) offers significant potential for enhancing channel capacity and improving noise resistance in quantum information processing. However, the challenge of achieving simple and rapid measurement has limited its practical applications. In this work, a quantum state tomography (QST) framework is demonstrated that utilizes convolutional neural networks to rapidly reconstruct the density matrix of OAM entanglement from only two coincidence measurements. The experimental results for a 5D OAM entangled state yield a fidelity of 0.973 ± 0.005. This method is also applicable to mixed OAM entangled states and scenarios with incomplete tomographic measurements. These findings represent a significant step toward implementing high‐speed QST for applications involving HD spatial mode quantum state, whether in free space or integrated systems.

Twisted photons can in principle carry a discrete unbounded amount of orbital angular momentum (OAM), which are of great significance for quantum communication and fundamental tests of quantum theory. However, the methods for characterization of the OAM quantum states present a fundamental limit for miniaturization. Metasurfaces can exploit new degrees of freedom to manipulate optical fields beyond the capabilities of bulk optics, opening a broad range of novel and superior applications in quantum photonics. Here we present a scheme to reconstruct the density matrix of the OAM quantum states of single photons with all-dielectric metasurfaces composed of birefringent meta-atoms. We have also measured the Schmidt number of the OAM entanglement by the multiplexing of multiple degrees of freedom. Our work represents a step toward the practical application of quantum metadevices for the measurement of OAM quantum states in free-space quantum imaging and communications.

High-dimensional (HD) entanglement enables an encoding of more bits than in the two-dimensional case and promises to increase communication capacity over quantum channels and to improve robustness to noise. In practice, however, one of the central challenges is to devise efficient methods to quantify the HD entanglement explicitly. Full quantum state tomography is a standard technology to obtain all the information about the quantum state, but it becomes impractical because the required measurements increase exponentially with the dimension in HD systems. Hence, it is highly anticipated that a new method will be found for characterizing the HD entanglement with as few measurements as possible and without introducing unwarranted assumptions. Here, we present and demonstrate a scan-free tomography method independent of dimension, which only requires two measurements for the characterization of two-photon HD orbital angular momentum (OAM) entanglement. Taking Laguerre-Gaussian modes of photons as an example, the density matrices of OAM entangled states are experimentally reconstructed with very high fidelity. Our method is also generalized to the mixed HD OAM entanglement. Our results provide realistic approaches for quantifying more complex OAM entanglement in many scientific and engineering fields such as multiphoton HD quantum systems and quantum process tomography.

The generation of a quadratic soliton in a pulse-pumped microresonator has attracted significant interest in recent years. The strong second-order nonlinearity and high peak power of pumps offer a straightforward way to increase efficiency. In this case, the influence of the third-order nonlinearity effect becomes significant and cannot be ignored. In this paper, we study the quadratic soliton in a degenerate optical parametric oscillator driven synchronously by the pulse pump with third-order nonlinearity. Our simulations verify that the robustness of quadratic soliton generation is enhanced when the system experiences a perturbation from pump power, cavity detuning, and pump pulse width. These results represent a new way of manipulating frequency comb in resonant microphotonic structures.

Reliable generation of single photons is of key importance for fundamental physical experiments and quantum protocols. The periodically poled lithium niobate (LN) waveguide has shown promise for an integrated quantum source due to its large spectral tunability and high efficiency, benefiting from the quasi-phase-matching. Here we demonstrate photon-pair sources based on an LN waveguide periodically poled by a tightly focused femtosecond laser beam. The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW (peak power is 2.9 kW). Our results prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source. This method can be extended to three-dimensional domain structures, which provide a potential platform for steering the spatial degree of freedom of the entangled two-photon states.