Hybrid diamond chip ties light to qubits in the lab

A chip two millimetres wide, glued to the cold finger of a cryostat and cooled to 4 K, is now doing what a diamond qubit needs before it can talk to the outside world: it lets a light signal and a quantum bit act on each other reliably. The team at the University of Washington, led by first author Nicholas S. Yama and working with Kai-Mei C. Fu of Washington University and Pacific Northwest National Laboratory, reports the result in the journal Optica Quantum.

The qubits in question are silicon-vacancy centers, or SiV centers: defects in the diamond crystal where a silicon atom sits next to an empty lattice site. They store quantum information well, but they must send it out as photons, particles of light, to connect with other devices across distance. Fiber-optic networks could carry those photons, and the difficulty lies in the conversion between the two. Gallium phosphide, known as GaP, has long been a candidate for the photonic side, because it can host optical components such as switches and modulators directly on the chip. Until now, however, the GaP layer sat physically apart from the SiV centers, and the two interacted too weakly to be controlled deterministically.

The fix was partly a matter of fabrication and partly of design. Earlier devices etched the diamond to close the gap, a method the team describes as nonstandard and hard to scale. In 2023 Fu's group had shown GaP could be transferred onto unmodified diamond with a scalable stamp process, but the optical interaction was too weak for practical use. This time the researchers redesigned the structure, folding the physical separation into the optimisation of the device. Over several iterations they raised the stamp-transfer yield to nearly 100%, and they produced more than 600 cavities with a 99.5% yield.

The measurements show that the coupling works. The devices reached above-unity cooperativity, a regime in which the light scattered from the device is strong enough to reveal the spin state of the SiV center. Each spin state gave a distinct spectral signature, so the team could follow the qubit switching between states. The devices also kept their spin-relaxation times above 0.4 milliseconds. These are laboratory results, obtained on cooled chips with a laser, and they have not yet been reproduced in a network.

What changes for the field is the route to larger chips. Yama says the platform could eventually connect several SiV qubits on a single chip, with switching and modulation integrated beside them rather than supplied by separate external parts, and that the fundamental coupling is the first piece of that plan. The authors point to secure communications and cryptography as possible uses, and to quantum computing more broadly. The paper does not set a date for any of these, and the team has not yet built the multi-qubit networking chip it describes. For now, the advance is a working interface between a diamond qubit and light, reproducible at scale in fabrication, and it is the first step on that road.