Optical Wiring Enables Larger Quantum Computers

Researchers at ETH Zurich have demonstrated a method for the delivery of multiple laser beams precisely to their intended locations, from within the physical architecture of a single chip. The method is stable enough to allow for delicate quantum operations.

Laser precision has been an obstacle to building larger quantum computers; because the lasers must hit targets that are only a few micrometers in size, even small vibrations disturb operation. To solve this problem, the ETH Zurich researchers integrated tiny waveguides into the chips that contain the electrodes for trapping the ions so that the light can be sent directly to those ions.

The ion-trap chip with integrated waveguides. The laser light is fed into the chip via the optical fibers on the right. Courtesy of K. Metha/ETH Zurich.

“In this way, vibrations of the cryostat or other parts of the apparatus produce far less disturbance,” said Chi Zhang, a Ph.D. student in researcher Jonathan Home’s group.

The researchers commissioned a commercial foundry to produce chips, which contain both gold electrons for the ion traps, and, in a deeper layer, waveguides for laser light. At one end of the chips, optical fibers feed the light into 100-nm-thick waveguides, forming an optical wiring network within the chips. Each waveguide leads to a specific point on the chip, where the light is eventually deflected toward the trapped ions on the surface.

Previous work on architecture, in collaboration with MIT, showed that the approach worked in principle. The present work demonstrated that the process has been developed and refined to a point where it is also possible to use it for implementing low-error quantum logic gates between different atoms.

“With the new chip, we were able to carry out two-qubit logic gates and use them to produce entangled states with a fidelity that, up to now, could only be achieved in the very best conventional experiments,” said Ph.D. student Maciej Malinowski of Home’s group.

The group is currently working on different chips that are intended to control up to 10 qubits at a time, and is pursuing designs for fast and precise quantum operations enabled by the chip technology.

The research was published in Nature (www.doi.org/10.1038/s41586-020-2823-6).