Researchers have developed the most robust method for controlling individual qubits made of the chemical element barium using laser light. The ability to control a qubit reliably is a significant step toward realizing future functional quantum computers.
The Institute for Quantum Computing (IQC) at the University of Waterloo developed this new method, which uses a small glass waveguide to separate and focus laser beams four microns apart, or about four hundredths of the width of a single human hair. Previous research has not been able to match the precision and extent to which each focused laser beam on its target qubit can be controlled in parallel.
“Our design limits the amount of crosstalk-the amount of light falling on neighbouring ions-to the very small relative intensity of 0.01 per cent, which is among the best in the quantum community,” said Dr. K. Rajibul Islam, a professor at IQC and Waterloo’s Department of Physics and Astronomy. “Unlike previous methods to create agile controls over individual ions, the fibre-based modulators do not affect each other.
Our design limits the amount of crosstalk-the amount of light falling on neighbouring ions-to the very small relative intensity of 0.01 per cent, which is among the best in the quantum community. Unlike previous methods to create agile controls over individual ions, the fibre-based modulators do not affect each other.
Dr. K. Rajibul Islam
“This means we can communicate with any ion without affecting its neighbors while also retaining the ability to control each individual ion to the greatest extent possible.” This is the most versatile ion qubit control system with this level of precision that we are aware of in academia or industry.”
The researchers focused on barium ions, which are gaining popularity in the field of trapped ion quantum computation. Barium ions have convenient energy states that can be used as the zero and one levels of a qubit and manipulated with visible green light, as opposed to the higher energy ultraviolet-light required for the same manipulation with other atom types. This enables the researchers to employ commercially available optical technologies that were previously unavailable for ultraviolet wavelengths.
The researchers created a waveguide chip that divides a single laser beam into 16 different channels of light. Each channel is then directed into individual optical fibre-based modulators which independently provide agile control over each laser beam’s intensity, frequency, and phase. The laser beams are then focused down to their small spacing using a series of optical lenses similar to a telescope. The researchers confirmed each laser beam’s focus and control by measuring them with precise camera sensors.
“This work is part of our effort at the University of Waterloo to build barium ion quantum processors using atomic systems,” said Dr. Crystal Senko, Islam’s co-principal investigator and a faculty member at IQC and Waterloo’s Department of Physics and Astronomy. “We use ions because they are identical, naturally occurring qubits, and we don’t need to fabricate them.” Our task is to find ways to control them.”
The new waveguide method demonstrates a simple and precise method of control, showing promise for manipulating ions to encode and process quantum data, as well as for implementation in quantum simulation and computing.
