Integrated Photonic Quantum Computing Advances Millimeter-Scale Devices with Thousands of Components

Researchers are rapidly advancing integrated photonic quantum computing, seeking to build scalable and functional quantum processors. Hui Zhang, Yiming Ma, and Di Zhu, from Tongji University, alongside Yuancheng Zhan from the National University of Singapore, and et al, present a comprehensive review charting progress from silicon-based circuits to the emerging potential of lithium niobate platforms. This work is significant because it details how lessons learned from the mature silicon technology can accelerate the development of lithium niobate , a material uniquely suited for efficient quantum state control and complex optical processing. By examining the functional integration mechanisms of lithium niobate, the team highlights its transformative impact and speculates on its future role in revolutionising fields like quantum communication and computation. By examining the functional integration mechanisms of Lithium niobate, the team highlights its transformative impact and speculates on its future role in revolutionising fields like Quantum communication and computation.,. Silicon-lithium niobate integration for quantum scalability Devices fabricated from this material exhibit remarkably high efficiency in generating, manipulating, converting, storing, and detecting photon states, establishing a solid foundation for deterministic multi-photon generation and precise single-photon quantum interactions. Experiments show that lithium niobate’s unique properties facilitate comprehensive frequency-state control, a critical element for advanced quantum protocols. The research delves into the functional integration mechanisms of lithium niobate, specifically focusing on its role in electro-optic tuning and nonlinear energy conversion, highlighting its transformative impact on the entire photonic quantum computing process. This detailed analysis reveals how these mechanisms enhance the performance and versatility of quantum devices, moving beyond the limitations of traditional approaches. This work establishes a clear roadmap for future research, suggesting that continued innovation in lithium niobate photonics will unlock unprecedented capabilities in quantum information processing and usher in a new era of quantum technologies, potentially impacting fields ranging from secure communication networks to advanced materials discovery.

Lithium Niobate Integration for Quantum State Control offers This research leverages insights from silicon-based systems to further advance lithium niobate technology. Researchers engineered a six-mode triangularly arranged Mach-Zehnder interferometer (MZI) network to demonstrate single- and two-qubit gates, including an integrated heralded CNOT gate and six-dimensional complex Hadamard operations. Quantum state teleportation, a crucial element of photonic quantum computing, was initially achieved using silica slab waveguides, but later advanced to chip-to-chip teleportation utilising four microring resonators to produce high-quality entangled states. Photons embodying the quantum states were transmitted between chips via a quantum photonic interconnect, converting path to polarization and reconstructing states through tomography. The study pioneered a resource-efficient transmission of high-dimensional quantum states by employing a trained quantum autoencoder to compress input states, subsequently decompressing them at the receiver chip before state reconstruction. To address the probabilistic nature of photonic quantum gates, the team developed re-programmable photonic chips capable of processing multiple quantum tasks and algorithms on a single platform, particularly for variational learning. A variational eigenvalue solver was implemented on a photonic quantum processor featuring a CNOT gate and reprogrammable single-qubit gates, efficiently computing Hamiltonian expectation values and achieving a significant speed advantage over traditional methods, experimentally determining the ground-state molecular energy of He, H+. Furthermore, scientists interfaced a silicon-photonics quantum simulator with a diamond nitrogen-vacancy center’s electron spin, employing Bayesian inference to accurately deduce the Hamiltonian, pinpointing the Rabi frequency with an uncertainty of approximately 10-5. High-dimensional encoding, specifically path encoding, was implemented on integrated chips using beam splitters and MZIs to route and manipulate photons across various paths.

The team coherently pumped d identical spontaneous four-wave mixing (SFWM) sources, generating a photon pair in superposition across the source array, creating a maximally entangled d-level two-qudit state with a fidelity of 95.5% for d=3 to d=15, and achieving quantum interference with visibilities exceeding 96.5%. Silicon-Lithium Niobate Quantum Chip Demonstrations show promising results This establishes a foundation for deterministic multi- generation and single- quantum interactions, alongside comprehensive frequency-state control. Experiments revealed that the purity of a heralded single photon is quantified by g) (!)(∆t), described by the equation B((%(∆2) B(‘%(∆2)B(#%(∆2) P( (4), where P&((∆t) and P<((∆t) represent coincidence count probabilities at a specific delay time, and P( is the signal photon probability. Measurements of g) (!)(0) showed values below 0.5, indicating non-classical behaviour, with the team striving for values closer to 0 for high-purity single-photon sources. The 14 coincidence to accidental ratio (CAR), defined as CAR = F(%”F) F) (5), was also measured to assess the noise in photon counts, demonstrating a strong dependence on pump power. Researchers found that at low pump power, the CAR value is high due to minimal multi-pair generation, but decreases with increasing pump power as noise sources proliferate. Silicon-based spontaneous four-wave mixing (SFWM) sources were widely demonstrated in Si, SiO2, and Si3N4 waveguides, utilising both spiral and microring resonator structures. A dual-mode pump-delayed excitation scheme, detailed in Fig0.3e, achieved a near-ideal spontaneous photon source by engineering spectrally pure photon pairs through inter-modal SFWM in low-loss multi-mode waveguides. To mitigate two-photon absorption (TPA) and free carrier absorption (FCA) in silicon, a reverse-biased p-i-n structure was incorporated into a silicon microring resonator, doubling the generation rate without compromising photon pair purity. Tests prove that an advanced resonator source embedded in asymmetric MZIs, shown in Fig0.3g, achieves a preparation heralding efficiency of 52.4% and spectral purity of 0.95 at relatively low pump power. Employing a phase shift between temporally delayed Gaussian pulses resulted in a spectral purity of 0.98, demonstrating alternative approaches to generating spectrally unentangled photon pairs. An array of 18 SFWM photon sources in SiO2 waveguides and 4 SFWM photon sources in Si microring resonators, depicted in Fig0.3h and Fig0.3i, respectively, were fabricated, showcasing potential for large-scale integration. 👉 More information 🗞 Integrated Photonic Quantum Computing: From Silicon to Lithium Niobate 🧠 ArXiv: https://arxiv.org/abs/2601.