New Gallium Phosphide-on-Diamond Chip Lets Light Read Out a Single Qubit
University of Washington researchers built more than 600 nanophotonic cavities on diamond with 99.5% yield, a step toward quantum networks that can be manufactured at scale.
Researchers have built a hybrid chip that makes light and a single quantum bit interact strongly and predictably, a long-sought step toward quantum networks that can link devices over long distances. The platform combines gallium phosphide (GaP) with a diamond substrate and is described in the journal Optica Quantum by a team led by Kai-Mei C. Fu of the University of Washington and Pacific Northwest National Laboratory.
Connecting quantum memories over long distances means converting information stored in qubits into optical photons that can travel through fiber-optic networks, and doing so reliably. "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 first author Nicholas S. Yama. GaP also has strong nonlinear optical properties that allow frequency conversion, switching and modulation, all needed for networking.
GaP-on-diamond has been studied for more than a decade as a scalable platform for diamond qubits, but getting the photonic devices and SiV centers to interact strongly enough has been difficult. The target is a regime called high cooperativity, where the interaction is strong enough to produce deterministic behavior. Early devices partly etched the diamond to bring the photonics closer to the qubits, Yama said, but that process is nonstandard and hard to scale with established fabrication methods. In 2023, Fu's group showed it could build GaP-on-diamond devices without modifying the diamond, using a scalable stamp-transfer process, but the optical interactions were too weak for practical use.
For the new work, the team went back to the drawing board. They redesigned the structure so that the physical separation between the SiV centers and the photonics was part of the optimization, which greatly enhanced the interaction without sacrificing other aspects of the device. Through several iterations they raised the stamp-transfer yield to nearly 100%. They fabricated more than 600 GaP nanophotonic cavities on a diamond substrate with a 99.5% yield.
To demonstrate the devices, the researchers grew a GaP membrane on a carrier substrate, patterned the cavities with electron-beam lithography, tested them with optical spectroscopy, and transferred the best ones to the diamond by stamping. Using a laser, they showed above-unity cooperativity, meaning the SiV spin state could be reliably detected through the light scattered from the device. Each spin state produced a distinct spectral signature, letting the team watch the qubit switch states. With spin-relaxation times above 0.4 milliseconds, they achieved single-shot readout of the SiV electron spin.
Next, the researchers want to connect two or more devices on one chip and entangle them to form a larger quantum register, while further optimizing fabrication to improve cooperativity and allow more complex layouts. They say the approach could also work with other emitters, such as silicon carbide, that are difficult to integrate with all-diamond photonics. "If GaP-on-diamond proves to be a viable path forward, it could be among the most straightforward paths to realizing quantum networks at scale for practical applications such as secure communications and cryptography," Yama said. "In this work, we demonstrate that the key fundamental component, the SiV-photon coupling, works."
