A different qubit architecture could make it easier to manufacture the building blocks of a quantum computer

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have shown that a type of qubit with an architecture more suited to mass production can perform comparable to the qubits that currently dominate the field. Through a series of mathematical analyses, the scientists have provided a roadmap for simpler qubit manufacturing that enables robust and reliable manufacturing of these building blocks of quantum computers.

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, and builds on years of scientific collaboration focused on improving qubit performance for scalable quantum computers.

Recently, scientists have been working to increase the length of time that qubits retain quantum information, a property known as coherence that is closely related to the quality of a qubit’s junction.

They were particularly interested in superconducting qubits, whose architecture includes two superconducting layers separated by an insulator. This part of the qubit is called a SIS junction, for superconductor-insulator-superconductor. But reliably manufacturing such sandwich junctions is not easy, especially with the precision needed for large-scale production of quantum computers.

“Performing SIS junctions is a real art,” said Charles Black, co-author of the paper published in the Physical examination A and director of the Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility at Brookhaven Lab.

Black and Mingzhao Liu, a senior scientist at CFN and lead author of the paper, were part of C²QA since its inception in 2020. And while they have helped quantum scientists understand the material science of qubits to improve their coherence, they have also become curious about the scalability of this art of qubit construction and its compatibility with the inevitable need to build large-scale quantum computers.

So the scientists looked at qubit architectures with superconducting junctions that consist of two layers connected by a thin superconducting wire, instead of an insulating layer in between. Known as a constriction junction, this architecture is flat rather than stacked like a sandwich. And importantly, the process for making constriction junctions is compatible with standard methods in semiconductor manufacturing facilities.

“As part of our work, we investigated the impact of this architectural change,” Black said. “Our goal was to understand the performance tradeoffs associated with moving to constriction junctions.”

Overcoming increased current flow and linearity

The most common superconducting qubit architecture works best when the junction connecting the two superconductors transmits only a small amount of current. Although the SIS sandwich insulator prevents almost all current transmission, it is thin enough to allow a small amount of current to pass through, through a mechanism known as quantum tunneling.

“The SIS architecture is ideal for today’s superconducting qubits, even though it’s difficult to manufacture,” Black said. “But it’s a bit counterintuitive to replace the SIS with a constriction, which inherently conducts a lot of current.”

With their analysis, the researchers showed that it is possible to reduce the current through a constriction junction to a level suitable for a superconducting qubit. However, the method requires fewer traditional superconducting metals.

“The constriction wire would have to be too thin if we used aluminum, tantalum or niobium,” Liu says. “Other less conductive superconductors would allow us to fabricate the constriction junction to practical dimensions.”

However, constriction junctions behave differently from their SIS counterparts, so scientists also investigated the consequences of this architectural change.

To function, superconducting qubits require some nonlinearity, which limits the qubit to operate between only two energy levels. Superconductors do not naturally exhibit nonlinear behavior: it is the qubit junction that introduces this key property.

Superconducting constriction junctions are inherently more linear than proven SIS junctions, meaning they are less ideal for qubit architectures. However, scientists have found that the nonlinearity of the constriction junction can be tuned by selecting a superconducting material and appropriately designing the junction size and shape.

“We are excited about this work because it directs scientists toward specific goals based on device requirements,” Liu says. For example, the scientists identified that for qubits operating at 5 to 10 gigahertz, which is typical of today’s electronics, there must be specific tradeoffs between the material’s ability to carry electricity, determined by its resistance, and the nonlinearity of the junction.

“There are some combinations of material properties that are simply not feasible for qubits operating at 5 gigahertz,” Black said. But with materials that meet the criteria set by the Brookhaven scientists, qubits with constriction junctions can operate similarly to qubits with SIS junctions.

Liu and Black are currently working with their C²QA colleagues will study materials that can meet the specifications outlined in their new paper. Superconducting transition metal silicides, in particular, have caught their attention because these materials are already used in semiconductor manufacturing.

“In this work, we have shown that it is possible to mitigate the concerning characteristics of constriction junctions,” Liu said. “So now we can start to exploit the benefits of a simpler qubit manufacturing process.”

This work embodies C²A fundamental principle of QA co-design, as Liu and Black explored a qubit architecture that could meet the demands of quantum computing and align with current electronics manufacturing capabilities.

“This type of interdisciplinary collaboration will bring us closer to realizing scalable quantum computers,” Black said. “It’s almost hard to believe that humans have achieved the quantum computers we have today. We’re very excited to play a role in helping C²“Quality assurance achieves its objectives.”