One of the most comprehensive research efforts to study the question of why quantum bits can seemingly be identical in every way—yet perform very differently—is now offering the clearest look yet at nanoscale features that could advance the development of more reliable quantum computers.
The research, led by a team of physicists at Fermilab’s Superconducting Quantum Materials and Systems Center (SQMS), involved a first-of-its-kind blinded study, focused on 22 superconducting transmon qubits, a kind of superconducting charge qubit possessing reduced sensitivity to “noise” resulting from its charge.
The study employed seven different materials characterization techniques spanning six different institutions, as reported in a new study that appeared in Applied Physics Reviews, which links variations observed in quantum performance with subtle differences that occur in various device materials, as well as fabrication processes.
The Coherence Equation
The issue of maintaining coherence—the fragile quantum state in which qubits can store and process information—represents one of the most significant obstacles in the development of superconducting quantum computers.
Even when qubits are manufactured together, and from the same materials, and following the same designs, they can exhibit vastly different performance characteristics, an issue that has left researchers probing for the underlying microscopic causes.
In the Fermilab team’s recent research, the team investigated high- and low-performing devices while deliberately withholding information about their performance from the teams conducting the materials analyses. Only after this blinded portion of the study was completed were researchers permitted to compare their findings with measurements of the qubits’ energy-relaxation lifetimes, which helped them to minimize bias in spotting any potential correlations that might exist.
The team’s work revealed several significant features during their analysis. One involved observation of surface oxide thickness, where, to the team’s surprise, variations of so little as a single nanometer in oxide thickness could significantly impact performance.
Another feature the team identified involved the angle of etched sidewalls in materials, where simulations showed that sharper sidewall edges were the clear winner in terms of improving performance, with models indicating an improvement by as much as 20% to 30% compared with broader angles.
Finally, the depth of trenches surrounding device structures appeared as a prominent third component, revealing that small differences in shallow trench depths were also linked to substantial performance changes.
The Path to Better-Performing Qubits
Overall, the team’s research reveals that differences in these three nanoscale features could account for as much as a twofold variation in performance in qubits that were made under almost identical manufacturing conditions.
By contrast, the research showed that much larger imperfections, such as damage from scratches or the accumulation of particulates, showed almost no relationship to overall performance in the devices the team studied.
These findings are significant, since they could have important implications as manufacturers attempt to scale quantum processors to increasingly large numbers of qubits.
As a next step, the researchers now aim to extend their work to other measures of coherence, as well as different devices and materials. In the future, the findings could ultimately help reshape our current development of quantum hardware, moving it from its current process involving experimentation to a process that relies on predictive processes, offering manufacturers much clearer guidance for engineering more reliable quantum processors.
Bindu Nair, associate director of Basic Energy Science at the U.S. Department of Energy, said that our current efforts to advance quantum information science are inherently connected to our current capabilities with the control of matter at the atomic level.
“The coordinated effort led by SQMS provides a scientifically grounded guide to the material defects behind variability in superconducting device performance, moving us from observation to correlation,” Nair said of the recent work in a statement.
“This foundational knowledge is essential to building the next generation of reliable, reproducible and powerful quantum technologies for the nation,” Nair added.
The team’s recent study, “Identifying materials-level sources of performance variation in superconducting transmon qubits,” was published in the journal Applied Physics Reviews.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.