WASHINGTON — Researchers have developed and demonstrated a new hybrid photonic platform that integrates quantum emitters with photonic devices, providing a way to reliably control the interaction between photons and quantum information carriers. The chip-based platform, which combines gallium phosphide (GaP) with a diamond substrate, could help make quantum networks more practical by providing a way to connect quantum devices over long distances.
Scaling up quantum technology to create communication networks requires connecting quantum memories, or qubits, over long distances. Although fiber-optic networks offer a promising way to do this using optical signals, a key challenge is reliably converting information stored in qubits into optical photons that can travel through the network.
"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 the paper's first author Nicholas S. Yama from the University of Washington . "GaP also has strong nonlinear properties that enable optical frequency conversion, switching and modulation, all of which are needed for networking applications."
In the Optica Publishing Group journal Optica Quantum, researchers led by Kai-Mei C. Fu from Washington University and Pacific Northwest National Laboratory describe their new hybrid platform, including a scalable fabrication process they developed to address a longstanding challenge in producing diamond-based photonic devices. They used this process to fabricate more than 600 GaP nanophotonic cavities on a diamond substrate with a 99.5% yield.
"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," said Yama. "The same technology could also support broader quantum computing applications, while the ability to create quantum systems spanning large distances could open up possibilities well beyond networking."
Overcoming the separation challenge
GaP-on-diamond has been studied for more than a decade as a potential scalable platform for diamond-based qubits. However, getting the photonic devices and SiV centers to interact strongly enough to reliably control the qubits has proven difficult. This requires a regime known as high cooperativity, in which the interaction is strong enough to produce deterministic behavior.
"Until now, it has been difficult to achieve high cooperativity with GaP-on-diamond because the GaP photonic layer was spatially separated from the SiV centers in the diamond," said Yama. "Early devices used partial etching of the diamond to mitigate this, but because this process is non-standard, it is difficult to scale up using established fabrication methods."
In 2023, Fu's group showed that they could fabricate GaP-on-diamond devices without modifying the diamond by using a scalable stamp-transfer process. However, the optical interactions were not strong enough for practical use. In the new work, they went back to the drawing board and redesigned the structure to improve the performance while also perfecting the fabrication process.
Through several iterations, they improved the stamp transfer process yield to nearly 100%. They also integrated the physical separation of the SiV and photonics into the design optimization process, which made it possible to greatly enhance the interaction without sacrificing other aspects of the device.
The new platform combines the strengths of two materials. While unetched diamond provides a pristine host for the strong SiV-photon interaction needed to transfer quantum information, GaP enables optical components such as switches and modulators to be integrated directly into the chip.
"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," said Yama. "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."
Testing the new platform
To demonstrate the devices, the researchers started with a GaP membrane grown on a carrier substrate. They then patterned the nanophotonic cavity devices into the membrane using an electron-beam lithography process and tested their performance with optical spectroscopy to ensure that they have the right properties. They selected the best sets of devices and transferred those to the diamond substrate via the stamp transfer process they developed.
Using a laser, they were able to show that the devices achieved above-unity cooperativity, meaning the interaction was strong enough for the SiV spin state to be reliably detected through the light scattered from the device. Each spin state produced a distinct spectral signature, allowing the researchers to "watch" the SiV qubit switch between states. The devices also had spin-relaxation times exceeding 0.4 milliseconds, which, together with the high cooperativity, enabled single-shot readout of the SiV electron spin.
Next, the researchers would like to connect two or more GaP-on-diamond SiV devices on the same chip and show that they can be entangled with each other to form a larger quantum register. They also plan to further optimize the fabrication process, which would improve cooperativity values while also enabling the creation of more complex integrated photonic chip layouts.
The hybrid integration approach could likely be used for applications beyond SiV centers and diamond. For example, qubits that are difficult to integrate with traditional all-diamond photonics could potentially be combined with GaP-on-diamond. Similar approaches could also be adapted to other quantum emitter platforms, such as silicon carbide and single emitters in 2D materials, providing new tools for scaling quantum technologies.
Paper: N. S. Yama, C.-C. Wu, F. Hatami, K.-M. C. Fu, "Efficient spin-photon interactions in a scalable nanophotonic platform," 4 (2026).
