Taking qubits to the next level

Tuesday, January 26, 2021

Researchers have implemented a gate used in important quantum algorithms in one step on a three-level quantum system—a qutrit—for the first time.

The new work led by Ali Yurtalan, Research Associate at IQC and in the University of Waterloo Department of Physics and Astronomy, identifies and addresses key challenges to controlling a qutrit, bringing researchers one step closer to developing quantum computers based on these multi-level quantum processors.

In computing, a gate is a logical operation that processes inputs to generate an output. In a classical computer, a gate returns a 0 or a 1. A gate implemented on a qubit—a quantum bit—returns a 0, 1, or some superposition of those possibilities. A gate implemented on a qutrit has many more possible outputs, including a 0, 1, 2, or some superposition between any or all these states.

An extra level in each processing unit of a computer means a lot more options for implementing existing quantum algorithms and developing new ones. It also opens the possibility of quantum processors that process information with two levels like a qubit but use the third level of a qutrit to help with things like error correction.

But adding another level to a system doesn’t just add capability. It also adds complexity.

“There are challenges when you’re addressing a qutrit,” said Yurtalan. “When you try to control all three levels of the system at the same time, there are going to be shifts to the energy levels of the qutrit.”

The researchers, working with faculty member Sahel Ashhab of Hamad Bin Khalifa University, were able to identify and characterize the shifts of the energy levels in their superconducting qutrit. They then dynamically compensated for these shifts while they implemented their gate across all three levels of the qutrit with one microwave pulse.

“We’ve identified the natural challenges that will arise when trying to control these qutrits, and we identified how to address them,” said Yurtalan. “This understanding will hopefully help us generalize our gate implementation to systems with more than three levels.”

The gate that the team implemented is common to many critical quantum algorithms that will have powerful capabilities once there are quantum computers powerful and stable enough to run them. And while the gate is just one small part of any given algorithm, Yurtalan is hopeful that this work is a step towards control of powerful quantum computers made up of qudits, the general term for quantum systems with three or more levels.

“Implementing this gate is a step towards implementing critical quantum algorithms, so this development is another small step towards useful quantum computation.”

The superconducting qutrit the researchers implemented their gate on was also the object of research for the latest IQC Achievement Award winner, Michal Kononenko, also an author on this paper.

Implementation of a Walsh-Hadamard Gate in a Superconducting Qutrit was published in Physical Review Letters October 27, 2020.

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