Diamond and Lithium Niobate Combine to Build Efficient Quantum Light Channels

Integrated quantum photonic platforms combining dissimilar materials advance scalable quantum networks. Sophie W. Ding and Chang Jin, from the John A. Paulson School of Engineering and Applied Sciences at Harvard University, led a collaborative effort with researchers from The University of Tokyo, Pritzker School of Molecular Engineering and the Department of Physics at the University of Chicago, Q-NEXT at Argonne National Laboratory, and further contributions from F. Joseph Heremans and colleagues at the University of Chicago.

The successful heterogeneous integration of thin-film lithium niobate with thin diamond films creates a platform using diamond’s efficient quantum memory interfaces alongside lithium niobate’s strong nonlinear and electro-optic properties. This integration enables the creation of low-loss photonic circuits capable of efficiently transferring light between the two materials and collecting photons from quantum emitters. This represents a key step towards practical, scalable quantum networking technologies. Zixi Li and Nicholas Achuthan also contributed to this development.

High-quality factor cavities enable efficient diamond-lithium niobate photon transfer

Q factors exceeding 5×10^4 at 735nm represent a key advancement, surpassing previous diamond photonic crystal cavity performance by an order of magnitude. This high quality—a measure of how efficiently a cavity traps light—was previously unattainable due to limitations in integrating diamond with other photonic platforms. Existing methods introduced substantial signal loss through bonding agents or inefficient light coupling, hindering the creation of functional quantum circuits.

A new integration technique achieves low-loss “escalators” for light transfer, with losses of approximately 1 dB per coupler, enabling efficient photon transfer between diamond and lithium niobate. The platform supports the collection of photons emitted from silicon vacancies within the diamond structure, paving the way for scalable quantum networks and integrated photonic circuits.

A 91% efficiency for transferring light between diamond and lithium niobate “escalators” at 775nm was measured using a cutback method to exclude grating coupler losses. Five fabricated diamond photonic crystal cavities consistently achieved on-target resonances with an average error of only 0.33%, alongside high quality factors consistent with leading diamond platforms.

Error rates dropped. Critically coupled cavities at 735nm reached a Q factor of 5.3×10^4, with a scattering Q factor of approximately 1.1×10^5, confirming the platform’s ability to trap light effectively. At cryogenic temperatures of 5K, photons emitted from silicon vacancies within the diamond were collected, observing both SiV emission and high-Q cavity mode reflection. These devices utilise 200nm thick lithium niobate layers on silicon, with waveguide widths of 330nm, ensuring single-mode operation and polarization control.

Low-loss diamond lithium niobate integration at cryogenic temperatures enables photonic circuit

Thin-film diamond has been successfully integrated with a lithium niobate (TFLN) platform, achieving light transfer losses of approximately one decibel per coupler. Efficient photon transfer is enabled by low-loss coupling between diamond and TFLN, but a fully functional quantum network remains elusive. The fabrication complexity and associated costs of these integrated circuits remain undisclosed.

High-Q diamond photonic crystal cavities, exceeding a Q factor of 50,000 at 735 nanometres, were lithographically aligned with the TFLN platform. Attempts at integration previously employed pick-and-place, transfer printing, or flip-chip bonding, often introducing losses due to bonding agents or inefficient coupling. This new approach avoids such issues by achieving direct coupling and minimising the need for intermediate materials.

For practical quantum technologies, operation at cryogenic temperatures—specifically 5K—is necessary. This requirement for cooling severely limits potential applications and adds to system complexity. Further analysis of fabrication costs and scalability is needed to determine the platform’s suitability for large-scale quantum networks. Diamond photonic crystal cavities with Q factors exceeding 5×10 4 at 735nm are lithographically aligned with a lithium niobate photonic backbone.

Direct coupling underpins this integration, avoiding the signal degradation common in previous methods. The strong nonlinear properties of TFLN enable the creation of essential photonic components, such as modulators and frequency converters, which are absent in diamond itself. These properties enhance the platform’s potential for building complex quantum circuits.

Low-loss diamond-lithium niobate integration advances quantum photonic systems

Thin-film diamond has been successfully integrated with a thin-film lithium niobate (TFLN) platform, achieving light transfer losses of approximately 1 dB per coupler. This represents a substantial improvement over previous diamond integration methods, which often suffered from signal degradation due to inefficient coupling or bonding agents. Several groups are currently pursuing similar integrations of diamond with other photonic materials, including silicon nitride (SiN) at the University of Bristol.

Companies like Quantum Motion and Riverlane are focused on developing diamond-based quantum processors, requiring efficient photonic interfaces for control and readout. Prior approaches typically relied on adhesives or direct bonding, introducing significant optical loss and hindering scalability. The ability to create low-loss “escalators” for light transfer between diamond and TFLN unlocks new possibilities for scalable quantum networks.

No prior method matched this. If this integration can be scaled, it could enable the construction of complex quantum circuits with increased functionality and performance. Currently, the demonstrated integration requires cryogenic temperatures of 5K for operation. Achieving room-temperature operation remains a significant hurdle before widespread deployment becomes feasible.

Also, the fabrication complexity and associated costs of these integrated circuits are not detailed, potentially limiting practical scalability. A scattering Q factor of approximately 1.1×10^5 was measured alongside a critically coupled Q factor of 5.3×10^4 at 735nm, confirming the platform’s ability to trap light. This integration of diamond and lithium niobate establishes a new platform for scalable quantum technologies, overcoming a key limitation of using either material alone.

Diamond provides efficient quantum memories, while lithium niobate—a special type of glass—enables precise control of light through electrical signals. Successfully collecting light emitted from silicon vacancies—defects within the diamond—via the lithium niobate circuit confirms the platform’s functionality. Investigation into achieving similar performance without the need for extremely low, cryogenic temperatures is now prompted, essential for practical applications.

The platform’s potential is further enhanced by TFLN’s strong nonlinear properties, allowing for the creation of modulators and frequency converters absent in diamond. This combination of materials offers a promising route towards advanced quantum photonic systems. Speed doubled.