Suffolk Reporter

UPTON, N.Y. — Scientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have demonstrated that a type of qubit with an architecture more suitable for mass production can perform comparably to those currently dominating the field. Through a series of mathematical analyses, they have provided a roadmap for simpler qubit fabrication, enabling robust and reliable manufacturing of these quantum computer building blocks.

This research was conducted as part of the Co-design Center for Quantum Advantage (C2QA), a DOE National Quantum Information Science Research Center led by Brookhaven Lab. It builds upon years of scientific collaboration focused on improving qubit performance for scalable quantum computers. Recently, scientists have been working to increase the amount of time qubits retain quantum information, known as coherence, which is closely linked to the quality of a qubit’s junction.

The focus has been on superconducting qubits whose architecture includes two superconducting layers separated by an insulator, known as an SIS junction (superconductor-insulator-superconductor). However, reliable manufacturing of such sandwich-like junctions is challenging, especially at the precision needed for large-scale production.

“Making SIS junctions is truly an art,” said Charles Black, co-author of the paper recently published in Physical Review A and director of the Center for Functional Nanomaterials (CFN), a DOE Office of Science user facility at Brookhaven Lab.

Black and Mingzhao Liu, senior scientist at CFN and lead author on the paper, have been part of C2QA since its inception in 2020. While helping quantum scientists understand the materials science of qubits to improve their coherence, they also explored the scalability and compatibility with large-scale quantum computer manufacturing.

The scientists turned their attention to qubit architectures with superconducting junctions comprised of two layers connected by a thin superconducting wire instead of a middle insulating layer. Known as a constriction junction, this architecture lays flat rather than stacking like a sandwich and is compatible with standard semiconductor manufacturing methods.

“In our work, we investigated the impact of this architectural change,” said Black. “Our goal was to understand the performance tradeoffs of making the switch to constriction junctions.”

The prevalent superconducting qubit architecture works best when the junction transmits only a small amount of current. Though the insulator in SIS sandwiches prevents nearly all current transmission, it allows some via quantum tunneling.

“The SIS architecture is ideal for today’s superconducting qubits even though it’s tricky to manufacture,” said Black. “But it’s counterintuitive to replace SIS with a constriction that conducts more current.”

Their analysis showed that reducing current across a constriction junction to appropriate levels for superconducting qubits requires less traditional superconducting metals.

“The constriction wire would have to be impractically thin if we used aluminum, tantalum or niobium,” explained Liu. “Other superconductors that do not conduct as well would let us fabricate practical dimensions.”

However, constriction junctions behave differently from SIS counterparts. The scientists investigated these differences and found that nonlinearity—essential for limiting operation between two energy levels—can be tuned through material selection and design adjustments.

“We’re excited about this work because it points materials scientists towards specific targets based on device requirements,” explained Liu. For example, they identified tradeoffs between material resistance and nonlinearity necessary for qubits operating between 5 and 10 gigahertz.

“Certain combinations just aren’t workable for 5 gigahertz qubits,” said Black. But with materials meeting outlined criteria, constriction-junction-based qubits can operate similarly to SIS-junction-based ones.

Liu and Black are collaborating with C2QA colleagues to explore suitable materials like superconducting transition metal silicides already used in semiconductor manufacturing.

“In this work, we showed it is possible to mitigate concerning characteristics of constriction junctions,” said Liu. “So now we can begin exploiting simpler fabrication processes.”

This research embodies C2QA’s foundational co-design principle by exploring architectures satisfying both quantum computing demands and electronics manufacturing capabilities.

“These interdisciplinary collaborations will bring us closer to scalable quantum computers,” said Black. “It’s almost hard to believe we’ve attained today’s quantum computers; we’re excited to help C2QA achieve its goals.”