Hu and Colleagues Demonstrate 0.943 Efficiency Photonic Platform for Quantum Information Processing

Integrated photonic circuits create scalable and robust quantum technologies by combining photon sources and linear optical networks on compact chips. Yi-Han Luo and colleagues at International Quantum Academy and Shenzhen Futian SUSTech Institute for Quantum Technology and Engineering and University of Science and Technology of China and Southern University of Science and Technology have addressed a key limitation in these systems, optical loss, by demonstrating a monolithic silicon nitride platform for discrete-variable quantum information processing. Their architecture achieves high-fidelity Einstein-Podolsky-Rosen (EPR) states (0.9875 ±0.0003) and near-unity photon indistinguishability, enabling the synthesis and characterisation of four-photon Greenberger-Horne-Zeilinger states with a record fidelity of 0.943 ±0.008 and a fourfold count rate of 27Hz. This represents a substantial advance over previous silicon-photonic implementations. These results, combined with CMOS-compatible fabrication, position ultralow-loss silicon nitride integrated photonics as a viable platform for large-scale, deployable quantum processors. High-rate multi-photon entanglement achieved via integrated silicon nitride photonics A fourfold count rate of 27Hz for four-photon Greenberger-Horne-Zeilinger states represents a new level of performance, exceeding previous silicon-photonic implementations by more than two orders of magnitude. This significant threshold overcomes the rate-loss barrier that previously prevented the creation of complex multi-photon states essential for scalable quantum computing. Prior quantum photonic systems often struggled to reliably generate even single entangled photons, limiting the complexity of quantum algorithms that could be implemented. The fundamental challenge lies in maintaining the fragile quantum state of photons as they propagate through optical circuits, with losses rapidly degrading the signal. The silicon nitride platform integrates essential components, narrowband photon sources, qubit-fusion circuits, and state-analysis interferometers, onto a single chip, streamlining the process and critically minimising optical loss. This integration reduces the number of optical interfaces, which are primary sources of photon loss and decoherence. The Greenberger-Horne-Zeilinger state, a specific entangled state of four qubits, is particularly sensitive to loss, making its high-fidelity generation a strong indicator of the platform’s capabilities. The ability to generate and measure these states is crucial for demonstrating quantum non-locality and validating the performance of quantum algorithms. The fabrication process utilises 150-mm-diameter wafers, aligning with standard manufacturing techniques and strongly suggesting scalability for mass production. Achieving a fidelity of 0.9875 ±0.0003 for Einstein-Podolsky-Rosen states, complex entangled pairs of photons used as fundamental building blocks for quantum computation and communication, was accomplished at International Quantum Academy and the University of Science and Technology of China. These sources also exhibit near-unity photon indistinguishability, confirmed by a heralded Hong-Ou-Mandel interference visibility reaching 0.990 ±0.006, which is vital for successful quantum operations. Photon indistinguishability is a critical requirement for two-photon interference, enabling the creation of more complex entangled states. The Hong-Ou-Mandel effect demonstrates this indistinguishability by showing that two identical photons will bunch together rather than scatter when they meet at a beam splitter. Fusing two EPR states on a single chip synthesised and characterised four-photon Greenberger-Horne-Zeilinger states. This achieved a record fidelity of 0.943 ±0.008. Further optimisation of off-chip connections and detection methods, such as employing superconducting nanowire single-photon detectors with higher efficiency and lower dark count rates, may enhance photon generation rates and overall system performance. Improving the coupling efficiency between the chip and optical fibres is also a key area for future development. Monolithic silicon nitride integrates photon sources and qubit-fusion circuits for high-fidelity Silicon nitride proved key, enabling the fabrication of a platform where narrowband photon-pair sources seamlessly integrate with low-loss qubit-fusion circuits. These circuits combine quantum bits for computation, functioning much like logic gates in conventional computers, but operating on quantum information. The qubit-fusion process is essential for scaling up quantum computations, allowing for the entanglement of multiple qubits and the implementation of complex quantum algorithms. Carefully controlling light travel through the chip’s intricate network minimises optical loss, a major obstacle in quantum computing where photons can be lost during processing, leading to errors and decoherence.

The team employed microresonators, tiny circular structures with high quality factors, to generate entangled photon pairs via spontaneous four-wave mixing, enhancing light interaction within a small space. This process efficiently converts pump photons into signal and idler photons, creating the entangled pairs. The use of silicon nitride, with its exceptionally low optical absorption, is crucial for maintaining the coherence of the photons over long distances within the chip. Compared to silicon, silicon nitride exhibits significantly lower material loss at the wavelengths commonly used for quantum photonics, typically around 1550nm. Ultralow-loss silicon nitride fabrication paves the way for scalable quantum processors Scientists at International Quantum Academy and the University of Science and Technology of China have engineered a silicon nitride platform to address optical loss, a persistent challenge in scaling quantum computers. The new architecture successfully generates complex multi-photon states with record fidelity, but the demonstration highlights a reliance on off-chip coupling and detection, which currently limit the overall system performance. The current system still relies on linking the chip to external components for sending and receiving light, introducing losses that hinder peak performance. These off-chip connections require careful alignment and can be susceptible to environmental noise. Addressing this limitation will involve developing integrated light sources and detectors directly on the silicon nitride chip, eliminating the need for external components. This would require further advancements in materials science and nanofabrication techniques. Nevertheless, this demonstration of ultralow-loss silicon nitride remains significant as it establishes a viable manufacturing process for building larger, more complex quantum processors. The ability to fabricate these devices on standard wafers promises a pathway towards scalable quantum computing, even with ongoing improvements needed in off-chip integration. This demonstration of a silicon nitride platform integrating all necessary components for discrete-variable quantum information processing establishes a new benchmark in scalability. It moves beyond simply improving existing silicon approaches and demonstrating a monolithic circuit capable of generating and manipulating complex multi-photon states with record fidelity. The potential applications of such a platform extend beyond quantum computing to include secure quantum communication networks and advanced quantum sensing technologies. The development of a scalable and robust quantum photonic platform is a crucial step towards realising the full potential of quantum information science. Researchers have demonstrated a new silicon nitride integrated photonic platform capable of generating complex four-photon states with a fidelity of 0.943. This achievement addresses a key limitation in scaling quantum computers, optical loss, by creating a monolithic circuit with ultralow loss. The platform integrates photon sources, qubit-fusion circuits and state-analysis interferometers onto a single chip, representing a significant step towards building larger and more complex quantum processors. Although the current system relies on external connections, the authors note that future work will focus on integrating light sources and detectors directly onto the chip to further improve performance. 👉 More information 🗞 An ultralow-loss integrated photonic platform for discrete-variable quantum information processing 🧠 ArXiv: https://arxiv.org/abs/2606.26910 Stay currentSee today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals. Tags: