Lithium Niobate Chip Converts Time-bin to Path Qubits with over 97% Fidelity for On-chip Networks

The increasing demands of modern internet infrastructure require efficient on-chip processors and reliable connections between them, yet current quantum communication methods present challenges in integrating these systems. Path-encoded photonic qubits excel at on-chip information processing, while time-bin encoded qubits are better suited for long-distance communication; a crucial missing link is a device that seamlessly converts between these two encoding methods. Now, Xiaosong Ren, Zhanping Jin, Xiaotong Zou, and colleagues at Tsinghua University and the Shanghai Institute of Microsystem and Information Technology have demonstrated a photonic circuit that efficiently converts time-bin encoded qubits into path-encoded ones. The device utilizes a thin-film lithium niobate high-speed switch and carefully designed delay lines to achieve this conversion on a single chip. Experiments on a fabricated sample demonstrate that the converted path qubits achieve an average fidelity exceeding 97%, a significant step towards practical quantum networks. Furthermore, the team successfully demonstrated entanglement and key distribution, highlighting the potential of this on-chip converter to underpin future internet technologies by bridging the gap between information transmission and on-chip photonic processing.

Thin-film lithium niobate for quantum networks

Researchers are developing a quantum communication network using integrated photonics built on thin-film lithium niobate (TFLN). This work aims to create a practical and scalable quantum key distribution (QKD) system, potentially forming the basis of a future quantum internet. The research addresses limitations in current QKD systems, such as signal loss and system complexity, by harnessing the unique advantages of TFLN technology. TFLN serves as the core material due to its exceptional properties, exhibiting a strong electro-optic effect and low signal loss. It is also compatible with standard CMOS fabrication processes, reducing manufacturing costs and increasing scalability.

The research focuses on creating all necessary components for a QKD system directly on a chip, including waveguides, modulators, beam splitters, interferometers, and single-photon detectors. The team utilizes energy-time entanglement, a robust form of entanglement suitable for long-distance communication, and employs periodically poled lithium niobate to efficiently generate entangled photon pairs. They have successfully developed high-performance TFLN-based components with low loss and high bandwidth, demonstrating QKD over significant distances, reaching 242 kilometers in one instance. This research is geared towards building a network where multiple users can securely communicate, distributing entangled photons over long distances and exploring higher-dimensional entanglement to increase key rates and enhance security. Notably, the team achieved entanglement without the need for post-selection, simplifying the system and improving efficiency. This work has implications for quantum key distribution and the long-term vision of a quantum internet capable of transmitting quantum information securely and efficiently over vast distances, promising secure communication networks for sensitive applications in government, finance, and other sectors.

Time-Bin to Path Qubit Conversion on Chip

Scientists engineered a photonic circuit to convert time-bin-encoded qubits into path-encoded qubits, a crucial step towards integrating quantum communication with on-chip photonics. This innovative approach enables seamless compatibility between long-distance quantum communication, which favors time-bin encoding, and on-chip processing, which benefits from path encoding. The experimental setup began with a pulsed laser to pump a quantum light source based on spontaneous four-wave mixing. This source, carefully filtered and amplified, generated entangled photon pairs, stabilised by a feedback system.

Following amplification, further filtering removed unwanted noise. Scientists then separated the entangled photons, detecting one as a heralding signal and inputting the other into the time-bin qubit preparation section before directing it through the time-bin-to-path encoding converter. An optical delay line and polarization controller precisely aligned the photons before conversion. An arbitrary waveform generator, triggered by the pulsed laser, modulated the electro-optic switch on the lithium niobate chip, controlling the qubit conversion process. Synchronization between the modulation signal and photon arrival time was achieved by adjusting the optical delay line, ensuring accurate qubit manipulation. To characterize the conversion fidelity, scientists performed tomography on the converted path-encoded qubits using a Mach-Zehnder interferometer, meticulously calibrating each electro-optic switch and aligning the photon arrival time. Results demonstrate that the converted path qubits consistently achieve an average fidelity exceeding 97%, confirming the effectiveness of the encoding converter and its potential for building advanced quantum networks.

High-Fidelity Time-to-Path Qubit Conversion Demonstrated

Scientists have developed a novel on-chip converter capable of transforming time-bin encoded photonic qubits into path-encoded qubits, a crucial advancement for future quantum networks. The research demonstrates a functional bridge between long-distance quantum communication and on-chip information processing using photons, leveraging a thin-film lithium niobate (TFLN) platform with high-speed optical switches and low-loss matched optical delay lines. Experiments confirm the converter achieves an average fidelity exceeding 97% in converting time-bin encoded qubits to their path-encoded counterparts, demonstrating the converter’s ability to accurately preserve quantum information during the encoding process. Researchers prepared time-bin entangled states on the chip and transmitted the photons through two converters connected by 12.

4 kilometers of optical fiber, successfully demonstrating path-entanglement via two-photon interference and confirming the converter’s compatibility with long-distance quantum communication protocols. As a practical application, scientists performed an entanglement-based quantum key distribution (QKD) experiment, showcasing the converter’s potential as a foundational component in future quantum internet systems. The results demonstrate the ability to seamlessly integrate long-distance transmission with on-chip quantum information processing, paving the way for scalable and efficient quantum networks and establishing a robust infrastructure for secure quantum communication and distributed quantum computing.

This work demonstrates a functioning on-chip converter capable of transforming time-bin encoded photonic qubits into path-encoded qubits with high fidelity, exceeding 97% on average. This achievement addresses a critical need for integrating different qubit encoding schemes within future quantum networks and information processors, bridging the gap between long-distance communication and on-chip processing. The successful demonstration of entanglement and key distribution experiments utilising this converter highlights its potential for practical applications in quantum communication protocols. Future work may focus on integrating this encoding converter with other photonic components to create more complex on-chip quantum circuits, representing a significant step towards realising a practical quantum internet and paving the way for more sophisticated quantum technologies.