Tailoring Quantum Walks in Integrated Photonic Lattices Enables Continuous Interference across Entire Structures

Photonic circuits offer exciting possibilities for manipulating light, but achieving complex behaviours typically requires carefully designed structures, and researchers are now exploring how to create more versatile systems. A. Raymond, P. Cathala, and M. Morassi, alongside colleagues at various institutions, investigate the potential of integrated photonic lattices to tailor the behaviour of quantum walks, where photons explore multiple paths simultaneously. Their work demonstrates a systematic comparison between standard photonic circuits and nonlinear lattices that generate photons internally, revealing how to tune these systems to produce increasingly complex, non-classical states of light. Crucially, the team also pioneers an inverse-design approach to engineer unconventional, aperiodic lattices capable of generating highly entangled states, such as the biphoton W-state, paving the way for compact and powerful quantum technologies. Step-by-step using a series of optical elements, arrays of coupled waveguides enable photons to interfere continuously across the entire structure. When composed of a nonlinear material, such arrays can also directly generate quantum states of light within the circuit. On-Chip Entangled Photon Generation and Control This research explores the creation and manipulation of entangled photons within integrated photonic circuits, aiming to build complex quantum systems on a chip for advanced information processing, simulation, and communication. The work focuses on miniaturizing quantum photonics, moving away from bulky optical setups to compact, integrated circuits, which is crucial for scalability and practical applications. Scientists utilize nonlinear materials, such as aluminum gallium arsenide and lithium niobate, to generate entangled photon pairs through a process called Spontaneous Parametric Down-Conversion. These photons are then guided and manipulated using precisely designed waveguide structures, forming the fundamental building blocks for quantum circuits.

The team also investigates topological photonics, exploring how to create robust and protected quantum states and circuits, and dynamically controls photon behaviour by modulating the waveguide structures. Researchers develop techniques to fully characterize the generated quantum states, including methods for phase retrieval and state reconstruction. Experiments utilize materials like aluminum gallium arsenide for fabricating the waveguide circuits and lithium niobate on insulator for its strong nonlinear properties. Key characterization techniques include Hong-Ou-Mandel interference to verify photon indistinguishability, coincidence counting to detect entangled photon pairs, and quantum state tomography to reconstruct the density matrix of the generated quantum states. Specific research areas include investigating Anderson localization, where disorder affects photon propagation, and exploring dynamic localization through modulated waveguides. Scientists implement quantum walks of correlated photons on the chip and explore quantum state transfer across waveguide networks. They also utilize Floquet engineering to create novel photonic band structures and functionalities, and generate and characterize high-dimensional entangled states. The research focuses on improving the efficiency and brightness of on-chip entangled photon sources and developing advanced techniques for phase retrieval and state reconstruction. The research highlights challenges such as finding materials with strong nonlinearities and low losses, creating high-quality waveguide structures with precise control, minimizing losses and decoherence, and scaling up the number of components on the chip. Future directions include integrating multiple quantum components, developing new materials, advancing control techniques, demonstrating quantum algorithms, and developing quantum communication and networking protocols. In summary, this work presents a comprehensive overview of cutting-edge research in integrated quantum photonics, paving the way for practical applications in quantum information processing, simulation, and communication.

Nonlinear Waveguides Control Quantum Walks and Entanglement Scientists have demonstrated a new approach to manipulating photons using continuously-coupled waveguide arrays, achieving precise control over quantum walks and entanglement. The work validates predictions regarding both linear arrays, where photons are injected externally, and nonlinear arrays, where photon pairs are generated directly within the circuit via parametric down-conversion. Experiments utilized III-V semiconductor nonlinear waveguide lattices, enabling researchers to tune the depth of quantum walks over an order of magnitude, revealing the gradual emergence of non-classical behaviour in the output state. Analysis of the system reveals that the output state of a nonlinear array can be understood as a coherent superposition of quantum walks in linear arrays of continuously varying lengths, providing valuable insight into the underlying dynamics. Researchers observed that in a linear array, with two indistinguishable photons injected into the central waveguide, the probability of bunching or anti-bunching depends on the waveguide length and coupling constant. Specifically, perfect bunching is achieved at lengths of m + 1/2 Lc, while perfect anti-bunching occurs at mLc, where Lc represents the half-beat length and m is an integer. Furthermore, the team successfully implemented an inverse-design approach to engineer aperiodic waveguide arrays with optimized coupling profiles, generating maximally entangled states such as the biphoton W-state. These experiments demonstrate the ability to create and control high-dimensional entanglement within compact photonic architectures. The research establishes a pathway toward hybrid schemes combining linear and nonlinear arrays on a single chip, potentially expanding possibilities for quantum state engineering and characterization.

Engineered Waveguides Generate Maximally Entangled Photon States This research presents a systematic comparison of quantum walks in both linear and nonlinear waveguide arrays, validated through experiments using semiconductor structures with tunable properties.

The team demonstrated that nonlinear arrays, which generate photon pairs directly within the circuit, offer a particularly effective means of creating spatially entangled quantum states. By carefully controlling the geometry of these arrays, researchers were able to tune the depth of the quantum walks and observe the gradual emergence of non-classical behaviour in the output light. Furthermore, the scientists introduced a novel inverse-design approach to engineer aperiodic waveguide arrays, successfully generating maximally entangled states, such as the biphoton W-state, with relatively modest experimental requirements. These findings underscore the potential of continuously-coupled photonic systems to harness high-dimensional entanglement within compact devices. Looking forward, researchers propose integrating nonlinear and linear sections onto a single chip, allowing for dynamic control of photon-pair generation and enabling more versatile quantum state engineering and characterization, potentially including on-chip quantum state tomography. This hybrid approach promises to significantly expand the range of accessible quantum states while maintaining a simplified experimental setup. 👉 More information 🗞 Tailoring quantum walks in integrated photonic lattices 🧠 ArXiv: https://arxiv.org/abs/2512.11608 Tags: