Optical wiring in chip

‘Optical wiring’ allows manipulation of many qubits

Image credit: K. Metha / ETH Zurich

Researchers at the Institute for Quantum Electronics, ETH Zurich, have demonstrated a method for delivering multiple laser beams to targets with the level of stability and precision necessary to perform delicate operations on qubits.

Scientists and engineers scaling up quantum computers face many serious technical hurdles. For quantum computers based on individual ions trapped in electric fields, it is necessary to aim hundreds or thousands of laser beams precisely from a distance of several metres at regions a few micrometres in size which contain the cold atoms.

Any unwanted vibrations can disturb the system, knocking the atoms out of coherence.

“If you want to build quantum computers with several thousand qubits, which will probably be necessary for practically relevant applications, current implementations present some major hurdles,” said Dr Karan Mehta, first author of a paper describing the work published in Nature. “Already in current small-scale systems, conventional optics are a significant source of noise and errors – and that gets much harder to manage when trying to scale up.”

In order to direct light precisely, the researchers integrated tiny waveguides in the chips containing the electrodes which trap the atoms, allowing light to be directly transmitted to the ions. This resulted in the vibrations of the cryostat and other components producing far less disturbance.

They commissioned a foundry to produce chips which contain both gold electrodes for the ion traps and – in a deeper layer – waveguides. At one end of the chip, optical fibres feed the light into the 1nm thick waveguides, effectively forming “optical wiring” within the chips which transmit the light to the trapped ions on the surface.

Previous work – involving some of the authors of the Nature study – had demonstrated that this approach would work in principle. Now the ETH Zurich group has refined the technique, demonstrating that it is even possible to use to implement low-error quantum logic gates between different atoms: an important prerequisite for building quantum computers. Logic gates acting on atoms in an entangled state are particularly sensitive to disturbance.

“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 PhD candidate Maciej Malinowski.”

The researchers are currently experimenting with different chips intended to control up to ten qubits at a time. They are also pursuing new designs for fast, precise quantum operations which could be enabled by the optical wiring.

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