Waterloo researchers are making a significant step toward the reliable processing of quantum information

Using laser light, researchers have developed the most powerful method currently known to control qubits made of the chemical element barium. The ability to reliably control a qubit is an important achievement for realizing the functional computers of the future.

This new method, developed at the University of Waterloo’s Institute for Quantum Computing (IQC), uses a small glass waveguide to split the laser beams and focus them four microns apart, about four hundredths of the width of one human hair. The precision and extent to which a laser beam focused on its target qubit can be controlled coherently is unmatched by previous research.

“Our design reduces the amount of crosstalk – the amount of light falling on neighboring ions – to a minimum of 0.01 percent, which is among the best in the community,” said Dr. K. Rajibul Islam, professor. at IQC and the Department of Physics and Astronomy at Waterloo. “Unlike previous methods of fine-tuning each ion, fiber-based modules are unaffected.

“This means we can talk to any ion without affecting its neighbors while also maintaining the ability to control each ion to the extent possible. This is the most flexible and high-precision ion qubit system we know of anywhere, both in academia and industry. “

The researchers focused on barium ions, which have become very popular in the field of ion quantum computation. Barium ion has simple energy states that can be used as a zero and one qubit level and is driven by visible green light, unlike the high energy ultraviolet light required for other types of atoms for the same transformation. This allows researchers to use commercially available optical technology that is not available at ultraviolet wavelengths.

The researchers created a waveguide chip that splits a single laser into 16 different modes of light. Each channel is then routed to independent fiber-based modules that provide instant control over each laser beam’s intensity, frequency, and phase. The laser beams are then focused down into a small space using a series of telescope-like optical lenses. The researchers verified each laser’s focus and control by measuring them with precision camera sensors.

“This work is part of our effort at the University of Waterloo to build barium ion quantum processors using atomic processes,” said Dr. Crystal Senko, principal investigator of Islam and faculty member at IQC and Waterloo’s Department of Physics and Astronomy. “We use ions because they are the same, qubits that are made by nature, we don’t have to make them. Our job is to find ways to control them.”

The new waveguide method presents a simple and precise control method, which shows the promise of ion control for embedding and processing of quantum data and implementation in quantum simulation and computing.

Paper, A guided light system for fast interfacing of Ba+ qubits with 10^-4 Level crosstalk, published by Ali Binai-Motlagh, Dr. Matt Day, Nikolai Videnov, Noah Greenberg, Senko and Islam Quantum Science and Technology.