Precise control of qubits using new quantum computing method

Instead of designing their own qubits for study, the team used nature-made ones and focused on ways to control them.
Ameya Paleja
Green laser light is the correct energy to manipulate the energy states of barium ions.
Green laser light is the correct energy to manipulate the energy states of barium ions.

University of Waterloo 

Researchers at the University of Waterloo in Canada have developed a novel and robust way to control individual qubits. This ability is a crucial step as humanity attempts to scale up its computational capacities using quantum computing, a press release said.

Much like silicon-based computers use bits as the basic unit of storing information, quantum computers use quantum bits or qubits. A number of elemental particles, such as electrons and photons, have been used to serve this purpose, wherein the charge or polarization of the light is used to denote the 0 or 1 state of the qubit.

Scientists place these qubits in highly controlled environments to protect them from outside influences and are now working on ways to manipulate them. Since even the mere act of observation of a qubit can change the value of a qubit, scientists have been looking for practical approaches for manipulation and measurement. This is where ions are coming in handy.

Controlling Barium with lasers

Recent research in trapped ion quantum computation has shown that Barium is the ideal element to work with. “We use ions because they are identical, nature-made qubits, so we don’t need to fabricate them. Our task is to find ways to control them,” said Crystal Senko, a faculty member at the Institute for Quantum Computing (IQC) at the University of Waterloo, in the press release.

Barium ions have convenient energy states that can be used as zero and one levels for a qubit. More importantly, the element can be manipulated with visible green light against the ultraviolet light needed to manipulate other atom types.

This allowed the research team to use commercially available options for their quantum manipulation, which is impossible when using ultraviolet light. The researchers created a waveguide chip that could divide the laser light into 16 different channels, each of which could be directed into individual modulators. Using fiber optics, these modulators allowed the researchers greater control over the beam's intensity, phase, and frequency, the press release said.

The approach also allowed the researchers to focus the beams to small spacing using a series of lenses, much like a telescope. The focus and the intensity of these beams were confirmed using camera sensors.

Precise control of qubits using new quantum computing method
Artist's illustration of a qubits at work

Research achievements

The waveguide created by the IQC researchers was able to separate and focus the laser beam as little as four microns apart. This is about four-hundredths the width of a human hair and a precision that has never been achieved before.

“Our design limits the amount of crosstalk–the amount of light falling on neighboring ions–to the very small relative intensity of 0.01 percent, which is among the best in the quantum community,” said Dr. K. Rajibul Islam, a professor at IQC in the press release. This precision allows the researchers to manipulate any ion without affecting its neighbors. According to Islam, this is the "most flexible ion qubit control system with high precision" existing in academia or the industry.

This is a simple yet precise method for the manipulation of ions and paves the way for encoding and processing quantum data.

The research findings were published in the journal Quantum Science and Technology.


Trapped ions are one of the leading platforms for quantum information processing, exhibiting the highest gate and measurement fidelities of all contending hardware. In order to realize a universal quantum computer with trapped ions, independent and parallel control over the state of each qubit is necessary. The manipulation of individual qubit states in an ion chain via stimulated Raman transitions generally requires light focused on individual ions. In this manuscript, we present a novel, guided-light individual addressing system for hyperfine Ba+ qubits. The system takes advantage of laser-written waveguide technology, enabled by the atomic structure of Ba+, allowing the use of visible light to drive Raman transitions. Such waveguides define the spatial mode of light, suppressing aberrations that would have otherwise accumulated in a free-space optics set up. As a result, we demonstrate a nearest neighbor relative intensity crosstalk on the order of 10−4, without any active aberration compensation. This is comparable to or better than other previous demonstrations of individual addressing. At the same time, our modular approach provides independent and agile control over the amplitude, frequency, and phase of each channel; combining the strengths of previous implementations.

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