Researchers at the University of Washington have created a platform integrating gallium phosphide (GaP) with a diamond substrate to enable deterministic interactions between photons and diamond-based qubits. The chip-based system addresses a critical bottleneck in quantum networking: reliably converting qubit information into optical photons for long-distance transmission.
“We show that our hybrid-integrated GaP-on-diamond nanophotonic platform can address this challenge by realizing reliably controlled interactions between photons and a type of qubit called a silicon-vacancy (SiV) center in diamond,” said Nicholas S. Yama, first author of the study. The fabrication process yielded over 600 GaP nanophotonic cavities on diamond with 99.5% efficiency, suggesting a scalable path toward practical quantum networks.
GaP-on-Diamond Achieves High Cooperativity for SiV-Photon Coupling
Achieving above-unity cooperativity, a measure of strong light-matter interaction, the new platform enables reliable detection of a silicon-vacancy (SiV) qubit’s state via scattered light, a critical step toward scalable quantum networks. Until recently, realizing this strong interaction proved difficult due to the physical separation between the GaP photonic layer and the SiV centers within the diamond substrate, according to Yama.
The team overcame this limitation by refining a stamp-transfer process initially demonstrated in 2023, allowing for GaP devices to be directly integrated onto the unetched diamond without compromising the interaction between the SiV centers and photons.
This fabrication process yielded devices with spin-relaxation times exceeding 0.4 milliseconds, a key metric for maintaining qubit coherence and enabling single-shot readout of the SiV electron spin. The combination of high cooperativity and extended spin-relaxation times represents an advancement in the field of diamond-based quantum technologies. The design integrates the strengths of both materials. Unetched diamond provides an ideal environment for strong SiV-photon interaction, essential for transferring quantum information, while GaP facilitates the on-chip integration of optical components like switches and modulators.
“Our GaP-on-diamond platform could eventually be used to connect multiple SiV qubits on a single chip to integrate switching, modulation and waveguides alongside SiV qubits rather than relying on separate external components,” explained Yama. The researchers validated the platform’s functionality by fabricating GaP membranes, patterning nanophotonic cavities using electron-beam lithography, and then transferring these devices onto the diamond substrate.
Optical spectroscopy confirmed the devices possessed the necessary properties for strong coupling, and laser testing demonstrated the achievement of above-unity cooperativity. “In this work, we demonstrate that the key fundamental component — the SiV-photon coupling — works, which could enable these larger networking chips in the future,” said Yama, highlighting the potential for scaling up the technology. Future work will focus on connecting multiple SiV devices on a single chip to create larger quantum registers and further optimizing the fabrication process to enhance cooperativity and enable more complex photonic chip layouts.
Scalable Stamp-Transfer Fabrication of Hybrid Nanophotonic Devices
The fabrication process achieved a 99.5% yield of over 600 gallium phosphide (GaP) nanophotonic cavities on a diamond substrate, a figure critical for translating laboratory demonstrations into scalable quantum technologies. Researchers addressed this by redesigning the device structure and optimizing the fabrication process itself, focusing on the physical separation between silicon-vacancy (SiV) centers and the photonic elements. The team’s approach bypasses the need for partial diamond etching, a technique previously used to improve performance but difficult to scale using standard manufacturing methods.
The current method relies on transferring pre-fabricated GaP structures onto the diamond, streamlining production and opening the way to larger-scale integration. GaP’s inherent properties also contribute to the platform’s versatility; its strong nonlinear characteristics enable optical frequency conversion, switching and modulation, all essential functions for quantum networking applications. Beyond SiV centers in diamond, the hybrid integration approach holds promise for incorporating other qubit technologies that present challenges for traditional all-diamond photonics. The platform could potentially accommodate qubits difficult to integrate with existing systems, expanding the range of compatible quantum emitters.
This adaptability extends to other materials as well, including silicon carbide and single emitters in 2D materials, offering a versatile toolkit for advancing quantum technologies. “If GaP-on-diamond proves to be a viable path forward, it could be among the most straightforward to realizing quantum networks at scale for practical applications such as secure communications and cryptography,” the researchers suggest, highlighting the potential for real-world impact.
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