“I'm fascinated by the fact that you can harness quantum physics for your benefit”
In search of alternative materials for qubits
When Vera Bader is in the lab, she is, in a sense, conducting experiments on the “open heart” of superconducting quantum computers. As a postdoctoral researcher at the Walther Meissner Institute, she is investigating alternative materials for Josephson junctions, the core component of superconducting qubits. Her degree in materials science has perfectly prepared her for this work at the intersection of chemistry, physics, and engineering.
By Veronika Früh
Vera Bader keeps zooming in on her presentation slides showing superconducting quantum chips. First, a golden, sparkling cryostat comes into view. Then the chip itself, cut from a silicon wafer, a few millimeters long and wide. Finally, the fine geometric structures on the chip become visible: conductive paths to carry signals and the qubits on their capacitor surfaces, which are intended to perform the computational operations of quantum computers – all just large enough to be seen with the naked eye. Finally, a high-resolution electron microscope image: two fine arms, crossed, almost like microscopic swords. These so-called Josephson junctions are the heart of superconducting qubits.
About six months after her presentation at the “World of Quantum” trade fair, Vera is sitting in the library of the Walther Meissner Institute, where she works as a postdoctoral researcher. “The Josephson junction is the smallest element in our circuit; we’re talking nanometers here,” she explains. At first glance, it seems very unremarkable, but it is absolutely essential for every single qubit, as she goes on to explain: “The Josephson junction ensures non-linearity. That means you can truly represent two states – zero and one. So you need the Josephson contact for every circuit.”
Traditionally, the two electrodes of a Josephson junction – the two “arms” – are made of aluminum, a superconducting metal. What matters is the name-giving “junction,” a non-superconducting barrier just a few nanometers thick between the two superconductors. Between the electrodes is an insulator. With aluminum, this is an oxide layer that grows very well and is self-limiting,” Vera explains. That is why aluminum was a good starting point for the fabrication of qubits. To improve the lifetimes of qubits for future high-performance quantum computers, research is being conducted into alternative materials that have better properties than aluminum. That is what Vera is working on as well.
Instead of aluminum: Niobium and tantalum as promising alternative materials
“Niobium or tantalum are currently the materials of choice in materials research for superconducting qubits,” explains the scientist. Both metals have higher transition temperatures than aluminum, meaning that quantum mechanical phenomena such as the required superconductivity occur even at higher temperatures. “If you imagine packing a large number of qubits into a cryostat in the future, it can get warm, which reduces the coherence times of the qubits,” Vera illustrates the problem. However, “warm” must always be viewed in the context of low-temperature research – the transition temperatures of all three materials are below ten Kelvin, around -263 °C.
Vera Bader, 32
Position
Postdoc
Institute
Walther Meißner Institute (BAdW)
SQQC
Degree
Materials Science
In her research, Vera is investigating alternative materials and fabrication processes for superconducting qubits. More favorable material properties are also aimed at advancing the scalability of quantum computers. Tantalum and niobium are promising candidates to replace the aluminum traditionally used in the fabrication of Josephson junctions.
In addition to the transition temperature, other properties such as oxidation behavior are also crucial in the search for alternative qubit materials. “Of course, testing new materials is always risky because they aren’t necessarily better,” Vera explains. “But I think that’s just the nature of research.” It is also still too early to say whether and to what extent the lifetimes of qubits will improve with new materials.
New materials require new processes
New materials also mean new manufacturing processes: “My research also involves the search for new fabrication methods to build the qubits,” explains Vera, “the processes would differ significantly.” The aluminum electrode arms of the Josephson junction, for example, are manufactured using a so-called “lift-off process.” In this process, the aluminum layer is first deposited onto the entire wafer. The material is then removed from predefined areas, leaving only the desired structures on the wafer. For niobium or tantalum, however, Vera is investigating an alternative manufacturing process in which all material layers are first applied in a stacked configuration, and the structures – lines for controlling and reading out the qubits, the qubits themselves, or the Josephson junction – are etched in using a “top-down” approach.
Ultimately, the goal here is also to be able to scale these new methods so they can be integrated into the entire quantum chip production process on an industrially relevant scale. One of the challenges involved is creating reproducibly identical qubits. “Our cleanroom here at the WMI is very well suited for experimental work,” explains the researcher, “but when it comes to matters of scaling, you really need cleanrooms like the ones at the HLL.” The Max Planck Society’s Semiconductor Laboratory (HLL) is located right next door. Currently separated by a fence, which will soon be fitted with a door to further capitalize on the proximity of the two institutes. Vera is already very much looking forward to the even closer collaboration with the HLL planned for the future.
For now, however, her WMI lab is getting an upgrade: a new sputtering machine is currently being set up there. In sputtering, a material is vaporized at the atomic level and then settled on a substrate, where it forms a layer. “Maybe the machine will even be finished today, then new materials can be tested there soon,” the scientist says happily. “When it’s ready, I definitely want to start with tantalum.” She played a key role in setting up the system. However, before it can actually be used to test new materials, there is still quite a bit of work to be done: “Gas lines still need to be installed, and then we have to test the system first with materials we’re familiar with to verify the parameter settings,” she explains.
Always new perspectives
Once everything is fully set up, Vera also hopes that many exciting topics for theses will emerge. She has been active in teaching for a few semesters now. At first, she helped out with various courses on occasional dates; last semester, she taught about half of the lectures in a module. “The preparation takes a lot of time, of course. But I learn the topics in a completely different way because I really want to understand every detail so I can teach it to others,” she says. What she enjoys most are the lectures themselves. Especially in the master’s lectures with fewer participants, discussions and interesting questions often arise that she had never thought about before.
Constantly exploring new questions is also the common thread running through Vera’s research career so far. Vera describes her decision to attend the University of Stuttgart after high school as “still rather arbitrary.” She had to move away from the small town near Lake Constance where she’s from anyway, because there weren’t any good study options for her nearby. For both her Ph.D. and her postdoc, the scientist shifted her research focus, always following the topics she found most exciting at the time. “It’s very labor-intensive because you’re basically starting from the bottom again. But you learn a lot; it’s always exciting,” she says. During her Ph.D., the scientist studied quantum spin liquids, a state of matter in which no aligned quantum spins occur even at the lowest temperatures – contrary to the usual behavior of magnetic materials, in which the spins of all electrons align identically at low temperatures. “I was working in the field of solid-state physics and investigating candidates for quantum spin liquids,” Vera explains. “Well, I didn’t find a new spin liquid,” she says with a laugh, “this state of matter has been theoretically predicted so far, but it hasn’t yet been indisputably demonstrated in a material.” However, during the step-by-step characterization of the materials, she was always able to discover very interesting properties, which she also considers a valuable gain.
And what interests Vera most about her current field of research is that she is looking for things that are new and learning a lot about the materials in the process. Compared to her Ph.D. in basic research, however, her current work is significantly more application-oriented.
“I’m fascinated by the fact that you can essentially harness quantum physics for your own use,” she says, “that you can work with quantum physics and then build a quantum computer from it.” And the heart of the quantum computer – that’s what she’s building.
Published 27 March 2026; Interview 11 December 2025