"I want to solve the puzzle"

Understand and marvel at the world

As a postdoc at the Walther Meißner Institute, Christian Schneider is responsible for advancing the work on fluxonium qubits. Creating something that has never existed before is what the physicist finds fascinating about his job. Not only is he passionate about the development of superconducting qubits, but he also sees exciting puzzles in many areas that have yet to be solved.

By Maria Poxleitner

"My parents used to swear a lot because I took almost everything apart as a child," remembers Christian Schneider. Alarm clock, telephone, computer. He didn't always put everything back together again afterwards, the 34-year-old physicist adds with a smile. He was always very curious, always wanted to get to the bottom of things and understand them. After graduating from high school, however, he didn't really know what he wanted to do, Christian says thoughtfully. "I was always drawn to computers". When he was seven years old, his father brought home the first devices, and Christian and his brother played with them. He therefore thought about studying computer science. But his good friend Julian then convinced him to study physics – for which he is still very grateful: "He said it would teach you about the world."

Today, as a postdoc at the Walther Meißner Institute (WMI), he is responsible for advancing the work on fluxonium qubits at WMI and bringing together all the individual projects to ultimately develop a fluxonium processor. The fluxonium qubit, or 'fluxonium' for short, is a specific design of a superconducting circuit – the term ‘architecture’ is also often used. Superconducting circuits are one way to realize qubits. And for the actual design, there are again many possibilities. You actually have a very small Lego set, you only need two or three elements, Christian explains. Capacitors, coils and Josephson junctions can be built into the circuit. The latter is a special, nanometer-sized component that is essential for any superconducting qubit architecture. "You can now combine these elements wildly. And people are doing that and design circuits that look completely crazy," the postdoc explains, his face reflecting the fascination that something useful can come out of it at all.

Transmon versus Fluxonium

Further research into this "qubit zoo", as Christian calls it, is always aimed at discovering qubit designs that have better properties, such as being less error-prone or more scalable. "But you always pay with complexity," the physicist points out. This is also the success story of the transmon qubit, the circuit architecture that is probably the most established today, with companies such as Google and IBM relying on it. What’s nice about the transmon, as Christian sees it: "It's stripped down to the bare essentials and it works. You take out everything that could cause interference and make the system as simple as possible.” But now, after 20 years of research and fine-tuning, there are limits, he adds.  The underlying architecture makes it difficult to improve certain properties. And that, in turn, motivates research into other architectures, such as the fluxonium.

The postdoc enters the "Fluxonium Booster Lab". He starts to laugh: That's what the doctoral students had called the lab. The cryostat is open. Christian puts on his gloves, picks up a small wrench and starts to check that the golden nuts are tight. Fluxonium qubits are very difficult to manufacture, he explains: "You have to make a chain of 80 to 100 Josephson junctions in a reproducible way. Not many facilities in the world can do that." But even with them, things can go wrong, which is why the newly manufactured chips are first tested before more complex measurements are made: Open the cryostat, place the new chip, connect and check cables, tighten screws and nuts. "Working on the cryostat always involves a lot of fiddling," says Christian. The cryostat here is rather small, so that it can be cooled down faster and the test chips can be changed more quickly. In the neighboring room, a larger cryostat is being used for a measurement. You can hear the cooling system working. A periodic high-pitched wheezing. "I already have a slight hearing loss at this frequency," jokes the physicist with a smile.

Christian Schneider, 34




Walther Meißner Institute (BAdW)



Christian studies the fluxonium qubit which is a special type of superconducting circuit. Although the development of the fluxonium qubit is not as advanced as that of the well-established transmon qubit, the research community is very interested in this alternative qubit architecture because of its promising properties. Christian's task is to bring together all the work on the fluxonium qubit at WMI and to advance the development of a fluxonium processor.

Christian at the open cryostat in the "Fluxonium Booster Lab". The many cables, screws and nuts have to be checked and tightened regularly.

Getting to the bottom of things

Intense hours in the lab or at his desk he often experiences as meditative, Christian says. When he is faced with a problem and an idea for a solution comes to him, he is "completely immersed" and blocks out everything else around him. Getting to the bottom of things and really understanding them is what drives him: "I want to solve the puzzle”.

However, superficiality seems to be foreign to the postdoc in other areas of life as well. The guitarist and bassist played in a band with friends from his school days in Munich until the end of his master's program. They played mainly blues and rock, performing at birthdays and weddings: "You play there, but somehow you also become part of the family. You get into other people’s lives for an evening, which is kind of weird because you never see them again." It was an exciting experience and he misses it a bit, admits Christian, his expression becoming somewhat pensive.

After completing his master's degree in Munich, the physicist moved to Innsbruck for his doctorate. "Innsbruck blew me away. The people there all had such an intuitive understanding and could explain everything very well." Christian was particularly drawn to Innsbruck by Gerhard Kirchmair, who had moved from Yale to Innsbruck as a young professor to establish what was then Austria's first research group to study superconducting quantum systems. "The papers from Yale were super exciting. Gerhard did some really cool physics there and I was fascinated," says Christian. He wanted to join the team in Innsbruck. They researched macroscopic systems put into a quantum mechanical state, i.e. exploring the "limits" of quantum physics. "What happens when quantum systems get really big, and how does gravity affect quantum mechanics, for example? That's where our expertise ends." Christian explains that he himself studied a kind of small "springboard" the size of a hair. He once thought about what this system would look like after a certain excitation: "It’s deflected up and down at the same time, but does not oscillate." His voice resonates with an almost disbelieving, but mostly fascinated laugh at this observation. "That's really strange. Very, very difficult to imagine." But that's what Christian loves about his job: "We're poking around at the edge of our knowledge and trying to create something that has never existed on Earth before."

"Munich was the most exciting spot on the quantum map"

For his postdoc, Christian decided to return to Munich, to the WMI, where he had already written his master's thesis. However, he feels that a lot has changed there since he left Munich: "What I appreciated so much in Innsbruck has now also developed in Munich." Through projects like Munich Quantum Valley, many exciting people are coming together in one place to discuss physics and advance quantum technology, he adds. He knew he wanted to stay in Europe: "And Munich was the most exciting spot on the quantum map.”

As part of MQV, the WMI is conducting parallel research on the transmon, the "classic," and the somewhat newer fluxonium. "The quality of our transmon qubits is at record levels," says Christian. Great progress has already been made with the fluxonium, but they are not yet at the forefront, the physicist states . He recalls a paper from the Massachusetts Institute of Technology (MIT) that caught his and his colleagues' attention last year. Researchers there had developed a special coupling element between two fluxonium qubits that made it possible to perform gates with particularly high accuracy. "That was a real breakthrough," emphasizes Christian. However, the physicist does not seem to feel any envy or disappointment that it was not his own paper. "I was just impressed. The error rates. You can only congratulate them.”

The physicist is not driven by his own career, but by new insights and the desire to ‘understand more’. "For me, understanding actually means gaining intuition, getting a feeling for why something behaves the way it does," Christian says. When it comes to quantum mechanics, however, you do reach your limits quite often. Unlike taking apart an alarm clock or a telephone, you have to rely on experiments that you can't see with your own eyes or feel with your own hands, he adds. Getting to the bottom of things and understand everything down to the smallest detail – the quantum systems that Christian studies don't always make it easy for him. "But that's also the beauty of quantum mechanics: You're always surprised." He definitely wants to stay in research for the next few years. That way, he can continue to push the boundaries of understanding a little further and learn more about the world.


Published 26 April 2024; Interview 13 February 2024