Integrated Photonics Enables Compact, Bright Source of Entangled Photon Pairs.

The demand for secure communication and enhanced sensing capabilities continues to drive innovation in quantum photonics, with entangled photons representing a fundamental resource. These correlated particles offer advantages over classical systems, enabling protocols resistant to eavesdropping and facilitating measurements with precision beyond classical limits. Researchers are now focused on miniaturising the complex optical systems traditionally used to generate and manipulate entangled photons, moving towards integrated photonic chips for greater stability and scalability. A collaborative team, comprising Changhyun Kim, Hansol Kim, Minho Choi, Junhyung Lee, Yongchan Park, Sunghyun Moon, Jinil Lee, Hyeon Hwang, Min-Kyo Seo, Yoon-Ho Kim, Yong-Su Kim, Hojoong Jung, and Hyounghan Kwon, details a significant advance in this field in their recent publication, “Integrated bright source of polarization-entangled photons using lithium niobate photonic chips”. Their work demonstrates a compact and efficient source of entangled photon pairs fabricated on a thin-film lithium niobate platform, achieving a brightness of 508.5 MHz/mW and exhibiting high-quality entanglement characteristics, including a purity of 0.901, concurrence of 0.9, and fidelity of 0.944.

Researchers have demonstrated a compact and efficient integrated photonic source of polarization-entangled photon pairs, fabricated using thin-film lithium niobate (TFLN) technology, achieving on-chip photon pair generation rates of 508.5 MHz per milliwatt of pump power. This performance exceeds that of traditional methods, positioning TFLN as a key platform for future quantum photonic systems. TFLN is a material favoured for its strong electro-optic properties, meaning it efficiently converts electrical signals into optical ones, and vice versa, crucial for controlling light at a nanoscale.

The fabricated TFLN platform offers advantages including compact size, low power consumption, and ease of integration, enabling the development of portable and scalable quantum devices. The material’s versatility allows for the fabrication of a wide range of photonic components, facilitating the integration of multiple functionalities on a single chip. This integration is vital for reducing the size and complexity of quantum systems, moving them closer to practical applications.

The team meticulously characterised the polarization properties of the generated entangled photons, confirming a high degree of polarization entanglement. Polarization entanglement, a fundamental concept in quantum mechanics, describes a correlation between the polarization states of two photons, even when separated by large distances. This property is crucial for many quantum applications, where it is used to encode and transmit quantum information. They also investigated the spectral properties, confirming the photons are spectrally pure and well-defined, ensuring compatibility with other photonic components and minimising decoherence, the loss of quantum information due to interactions with the environment.

Researchers are currently exploring techniques to improve the stability and robustness of the entangled photon source, addressing challenges related to temperature fluctuations and mechanical vibrations. This involves implementing advanced control systems and employing vibration isolation techniques, aiming for a reliable source operating in real-world environments. Maintaining the delicate quantum states requires precise control over environmental factors.

Scaling the fabrication process to create multi-photon entangled states represents a significant next step towards realising advanced quantum technologies, opening up new possibilities for quantum computation and communication. Researchers plan to explore techniques such as spontaneous parametric down-conversion (SPDC), a nonlinear optical process where a photon splits into two entangled photons, and four-wave mixing, another nonlinear process used to generate new frequencies of light, requiring precise control over nonlinear optical processes.

The development of this integrated entangled photon source marks a significant milestone in quantum photonics, paving the way for practical quantum technologies with a transformative impact on secure communication, precision sensing, and high-performance computing. This technology has the potential to revolutionise these fields and solve complex problems currently considered intractable. Quantum key distribution, for example, leverages entanglement to create unbreakable encryption keys.

Researchers are committed to making this technology accessible to the broader scientific community, fostering collaboration and accelerating the development of quantum technologies through knowledge sharing, training opportunities, and public availability of research findings. This interdisciplinary effort, involving physics, electrical engineering, and materials science, demonstrates a collaborative approach instrumental in overcoming the challenges of developing a practical quantum photonic device.

Scientists are actively exploring new materials and fabrication techniques to further enhance the performance and scalability of the TFLN platform, pushing the boundaries of quantum photonics and developing even more powerful and versatile quantum devices. They envision building large-scale quantum photonic chips containing millions of entangled photons, paving the way for fault-tolerant quantum computation and secure quantum communication networks. This ambition requires overcoming significant engineering challenges in maintaining coherence and controlling interactions between a large number of photons.