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 conducive to mass production can perform comparably to those currently leading 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 duration qubits retain quantum information, known as coherence, which is closely linked to the quality of a qubit’s junction.
They have particularly focused 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 they’ve been helping quantum scientists understand the materials science of qubits to improve their coherence, they’ve also grown curious about scalability and compatibility with large-scale manufacturing needs.
The scientists turned their attention to qubit architectures with superconducting junctions comprised of two layers connected by a thin superconducting wire instead of an 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.”
Overcoming increased current flow and linearity
The prevalent superconducting qubit architecture works best when the junction transmits minimal current. The insulator in SIS sandwiches prevents nearly all current transmission but allows a small amount 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 the SIS with a constriction that conducts more current.”
Through analysis, researchers showed it is possible to reduce current across a constriction junction to appropriate levels using less traditional superconducting metals.
“The constriction wire would need impractically thin dimensions 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’ consequences on architectural changes.
Superconducting qubits require nonlinearity limiting operation between two energy levels. Superconductors don’t naturally exhibit nonlinear behavior; it’s introduced by the qubit junction.
Superconducting constriction junctions are inherently more linear than SIS junctions but can be tuned through material selection and design adjustments.
“We’re excited because it points materials scientists towards specific targets based on device requirements,” explained Liu. For example, operating between 5-10 gigahertz requires specific tradeoffs between material resistance and nonlinearity.
“Certain combinations just aren’t workable at 5 gigahertz,” said Black. But materials meeting outlined criteria allow similar operations as SIS junctions.
Liu and Black are exploring materials meeting specifications outlined in their new paper with C2QA colleagues. Transition metal silicides are promising due to existing semiconductor use.
“We showed it’s possible to mitigate concerning characteristics,” said Liu. “Now we can exploit simpler fabrication benefits.”
This work embodies C2QA's co-design principle exploring architectures aligning with electronics manufacturing capabilities while satisfying quantum computing demands.
“These interdisciplinary collaborations bring us closer to scalable quantum computers,” said Black. “It’s hard to believe we’ve attained today’s quantum computers; we’re excited about contributing towards C2QA goals.”