"Physics opened up a world of endless possibilities." Tanja Mehlstäubler, professor of quantum optics and metrology, Physikalisch-Technische Bundesanstalt and Leibniz University Hannover
Professor Tanja E. Mehlstäubler is a leading expert in precision laser spectroscopy and ion traps for atomic clocks and quantum technologies. She studied physics at the Julius Maximilians University in Würzburg and at the State University of New York at Stony Brook in the USA, earning her PhD from the University of Hannover for her work on advanced laser cooling techniques for neutral atoms.
After completing a postdoctoral fellowship at the Paris Observatory working on a cold atom gravimeter, she joined the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, where she established PTB’s first junior research group in 2009 to develop a multi-ion clock. She completed her habilitation on the topic ‘Quantum Sensors with Laser-cooled Atoms and Ions’ in 2016, and has held a professorship in quantum optics and metrology at Leibniz University Hannover since 2020. Since 2018, she also holds a visiting professorship at Osaka University in Japan.
Thank you for talking to atom*innen today. What are you currently researching?
Right now, we’re working on benchmarking and advancing the multi-ion clock, which we first proposed over ten years ago. We’ve made significant progress since then. Over the past three years, we have conducted international measurement campaigns using the multi-ion clock. For the first time, we have been able to demonstrate its effectiveness, our proposal has become a reality. Earlier this year, we even achieved a world record by demonstrating an accuracy close to the 18th decimal place and a clock frequency comparison with less than 5x10-18 uncertainty, something no one else had achieved so far. Currently, we’re scaling things up with this new, functional multi-ion clock. We have created a scalable crystal clock that allows us to add more and more qubits.
Another exciting step will be to measure time dilation: We expect that each atom experiences time differently. We intend to measure these time gradients and investigate whether quantum states possess an intrinsic time. It’s a fascinating system where relativity meets quantum mechanics. This is one of the main areas of research in our group.
In another experiment, we are using precision spectroscopy to take a closer look at the atomic nucleus. Here, we are studying how electron shells extend into the nucleus through their quantum wave functions. Currently, we are examining how atomic nuclei change shape when neutrons are added. In the long term, we would like to investigate the thickness of neutron shells. It's an incredibly exciting time. We're collaborating closely with nuclear physicists and learn a lot from them.
That sounds absolutely fascinating, not to mention a lot of work! Have you ever had a moment in your career where you thought, 'This is a turning point; this might be huge'?
I don't think those big 'Eureka!' moments really exist. They belong to the realm of anecdotes. That might sound dull, but I tend to take a strategic approach to things. I usually have plans A, B and C ready to go. For me, a crucial moment is when I decide to pursue an idea properly. But, of course, that only happens after I have carefully studied the theory, analyzed the possibilities and learned from the experiences of others. If I’m convinced that something could work, I go for it, even if everyone else says no. Take the multi-ion clock, for example. I had to fight for it and rely on my intuition. I secured independent funding so that we could develop it step by step. Now it’s up and running!
How did you first become interested in physics? Were you always interested in the subject, or did you consider other career paths as well?
I considered many different options. Coming from a non-academic background, I didn’t really know what careers were out there. I was the first in my family to attend high school and earn a diploma. I considered law or banking because they seemed like respectable careers. But during high school, I narrowed it down. I could see myself becoming a scientist. I considered studying archaeology because I’ve always been interested in understanding the past, and also Latin, my favorite subject.
So, I only studied physics as a minor subject at school, not as a major. In my final year of school, I started visiting different universities and physics won me over. It was concrete and it offered career prospects. Perhaps it was that kind of strategic thinking again. At the same time, it was definitely a gift to be able to study physics in Germany for free, without tuition and with a vast choice of universities all offering an excellent education. Initially, I intended to be an astrophysicist. As a child, I observed sunspots with binoculars and projected them onto paper to track the Sun’s rotation. Later, I realized that physics offered a wide spectrum, including solid state, nuclear physics, optics and lasers. This appealed to me because I never wanted to feel confined. Physics opened up a world of endless possibilities.
It sounds like physics also sparked some bigger, more fundamental questions for you.
Yes. Since we explain everything through math and theories, which we try to verify by measurements, physicists are often asked if they are religious. I think many physicists are also philosophers or even religious because the universe leaves us with so many unanswered questions. We still don't understand everything — not even close! There are so many deeply philosophical questions about physics: How was matter and anti-matter created? What is time? What is causality? What happened before the Big Bang? How will the universe evolve?These questions influence our perception of ourselves as humans. Science and religion have always interacted, sometimes collaboratively, sometimes oppositionally. The give-and-take between philosophical ideas and scientific measurement continues to this day.
Back to research: I imagine that being a scientist can be tough, with its deadlines, grant writing, teaching and public engagement. What motivates you to do science?
This job offers many sources of satisfaction. I agree: Bureaucratic work is not one of them. However, what motivates us is the immense joy of doing something we find fascinating. In academia, we have the freedom to follow our curiosity. As a professor, you decide what research to pursue. I started out researching atomic clocks, then moved on to solid-state physics, and now I’m exploring nuclear physics. It hasn't been a straightforward journey.
Having the freedom to be creative and explore is incredibly fulfilling. That’s why I’m happy to spend a weekend reading papers. It brings me joy. Of course, on the other hand, we must avoid exploitation. Being a physicist is a profession, not just a hobby. However, it is a deeply meaningful profession that benefits society. Science teaches us to think analytically and distinguish between right and wrong. It encourages us to ask: what am I really seeing? Is it just noise, or is it real? These critical thinking skills are important for society as a whole.
I also find teaching incredibly satisfying. You're constantly learning something new. Sharing my knowledge with young people, engaging in dialogue and mentoring them brings me joy. So, for me, it really is the perfect job!
Let's talk about gender. Have you ever felt that being female has affected your career?
That’s a tricky question! At school, it didn’t matter. Not during my studies, not during my PhD and not even during my postdoctoral research. Up until the age of 30, I would have said there was no difference between men and women in academia. Looking back, there were maybe a few odd moments (things that probably wouldn’t have happened to a male student) but I handled them well at the time.
Things changed when I became independent. As the first junior group leader at PTB, I had to forge a path where there was none. I had to fight for resources, lab space, funding and visibility just to be recognized. It was then that I began to notice a shift: I wasn’t being seen in the same way as others. Why hadn’t I received a single award when others with similar publications had? I realized that I hadn’t promoted myself enough. I hadn’t fought hard enough. Others had been more assertive, and it had paid off.
Another social factor that we don't talk about enough, in my opinion, is class. I come from a working-class background. This shapes not only how you see yourself, but also how you conduct yourself in elite spaces. You don't automatically walk into those spaces thinking that you belong. You wonder: is this really my place? Is this just a lucky accident?
Could mentoring be helpful here?
Absolutely. We need mentors who truly listen and relate to people’s needs. This is not just important for women, but for everyone. For many people in society, access to science is difficult. This is also reflected at top management level: as far as I can tell, I have very rarely met a single non-academic in a top management position. Those in positions of power need to build bridges across social backgrounds and be open to learning from others. What I’d like to see is more awareness and genuine engagement.
Last question: What advice would you give a female student just starting out?
Firstly, study something you enjoy. Complete a solid bachelor's degree. Towards the end, reflect on what you have learned. What did I enjoy most? Optics? Atomic and particle physics? Solid state?... Theoretical or experimental physics? And then, also consider changing locations. Experiencing new environments is incredibly valuable. I highly recommend gaining international experience. You will gain so much culturally, personally and academically. Apply for scholarships. Be mindful of building your profile. And, most importantly, find a mentor. Ideally, you should do this by the end of your bachelor's degree. Talk to professors, they're usually very willing to help.
Learn more about Tanja Mehlstäubler and her research group Quantum Clocks and Complex Systems.
Author: Karoline Irschara
published on 2025-09-11