10.7.2025
Julian Schmitt recently joined the Kirchhoff Institute for Physics – bringing fresh momentum to the field of quantum optics. With his new research group Optical Quantum Systems, he aims to explore how light and matter behave in the quantum world. At the heart of his work lies a particularly fascinating question: How can quantum states of light be deliberately created, controlled, and understood?
Before taking up his professorship in Heidelberg, Julian Schmitt spent several years as a junior research group leader at the University of Bonn – where he also completed his doctorate. In between, he spent time as a postdoc at the University of Cambridge. Now he’s back in Germany and looking forward to new challenges – in the lab, the lecture hall, and with his team.
Johanna Schwarz, administrative director of the faculty, spoke with him about his research and plans in Heidelberg:
Johanna:Julian, you've been at Heidelberg University for about six months now – have you settled in well?
Julian:Absolutely! Heidelberg is one of those places you’ve probably heard of or even visited before – either privately or through science. For example, I once attended a DPG conference here as a doctoral student – that was a great time and left a good impression. Living here now is, of course, something entirely different. The city has so much to offer: the surrounding nature, the mountains right on your doorstep, the river – there's so much to explore and do outdoors. It's vibrant, and in my opinion, just beautiful.
Johanna:You were previously at the University of Bonn – what does this move to Heidelberg mean for you?
Julian:It was the logical next step – but of course also a big one. In Bonn, as a junior group leader, I worked on developing ideas from my postdoc time in Cambridge, where I studied ultracold atomic gases – in other words, quantum matter. The goal was to transfer these concepts to quantum gases of light. The exciting part is: it works at room temperature – so you don’t need extreme cooling like with atoms. With the help of an ERC Starting Grant, we were able to deepen this line of research, especially in relation to topological phenomena.
Now, as a professor, I have completely new possibilities: more freedom, more room for creativity, but also more responsibility. That means I can continue with established research directions like quantum gases of photons and topology – but also venture into new directions. One area we’re launching here in Heidelberg is physical computing. The professorship was actually advertised for this very topic – it’s about the idea of building optical systems that can solve computationally demanding problems very efficiently. So essentially, computers made of light and matter – and I find that incredibly exciting!
Johanna:What exactly will you be researching here – and what can students expect from your lectures?
Julian:In research, we want to tackle two big questions: First, we’re interested in the phenomenon of superfluidity – that is, frictionless flow. It's well-known in quantum fluids at very low temperatures, but we want to see if something similar can be realized with light at room temperature and what’s the role of topology in it. That would be truly special. And second, as mentioned, the computing topic: Can we build systems that solve complex optimization problems – like the famous Traveling Salesperson Problem – very quickly?
In teaching, my “home zone” is quantum optics – everything related to light and its interaction with matter in a quantum mechanical description. That means I’ll offer lectures on conventional topics in quantum optics, but also more specialized subjects like quantum gases of light and matter or quantum simulation. And of course, we offer bachelor’s and master’s theses in our group – from small optical setups to more complex experimental projects. There's something for a wide range of interests.
Johanna:How does your research connect to the existing work in the faculty?
Julian:I’d say our research complements what’s already being done here – and that opens up many possibilities for fruitful exchange, without being in competition. There are many exciting groups here, for example colleagues working with ultracold gases – like the groups of Chomaz, Oberthaler, Jochim, and Weidemüller. They’re doing very similar physics to us, but with atoms, whereas we work with photons. It’s a different platform, but the fundamental questions – about thermodynamics or collective quantum states – are very much the same.
Then there are groups like Wolfram Pernice’s, who are focused on photonics – figuring out how to guide light efficiently and how to implement computing processes with it. That’s technologically fascinating and methodologically very helpful for us – we benefit a lot there.
And since we're also working on computing, there are points of connection to theoretical physics and information science – such as: What are the actual classes of problems we can solve with our physical systems? Ultimately, we're sitting at the intersection between basic research and application-oriented ideas. I also see myself as a kind of bridge between the various areas at our institutes.
Johanna:Could you put it in really simple words – what exactly happens in your lab?
Julian:It’s kind of like what happens when water vapor cools down and condenses into a droplet – we do that with light: we trap light particles in a small box, cool them down – and then a kind of droplet forms, but made out of photons.
Johanna:You mainly do fundamental research – but what societal impact could your work have in the long run?
Julian:If physical computing works out, it definitely has huge potential. It’s about building systems with light that can solve logistic problems very efficiently – in a short time and with very low energy consumption. Our optical systems also constitute light sources – with properties that classical lasers don't offer, for example, controllable coherence properties. Such optical sources are potentially interesting for imaging applications or as lighting technologies for interferometry applications.
If we truly come to understand if and how to realize superfluids at room temperature, this would be a big step. Conceptually, this can help us to drive our understanding of superconductivity at room temperature. Current flowing without resistivity at room temperature – that would be revolutionary.
Johanna:You’re based in the new EINC building; it’s a great space, but in some areas it still feels like a construction site, and the hallways are pretty quiet – how does it feel to be one of the first to start here?
Julian:I agree – it’s a really beautiful building – modern, bright, with a lot of space. And that gives us the opportunity to fully develop our work here. But of course, it’s also a challenge at the beginning! Right now, it’s about filling the building with life and the labs with experiments – my group is currently growing, and hopefully more colleagues will join soon.
Ideally, the building will soon have a spirit similar to the other physics buildings. Sure, those have lecture halls, so it’s not quite the same. But I’d love to see this space lively – with students around, bulletin boards, events happening. For example, we have a big event space here in the building, and we’d like to make more use of it – for workshops, or formats that attract larger groups.
Please spread the word: the EINC is open, it’s a place where research from the Kirchhoff Institute is happening! Students should feel like this is a building they can enter – maybe even get involved in the research someday. And by the way, theorists are very welcome too – for scientific exchange, work, and conversations.
Johanna:That sounds very inspiring! By the way – how do you actually pronounce “EINC”?
Julian:(laughs) Best to say “KIP-EINC”– like “eyens” the German word for number “one.”
Johanna:Speaking of inspiration – what was your most recent "wow moment" in research?
Julian:I recently read something that I found totally crazy: researchers managed to make tissue transparent in living mice by rubbing their fur with a special fluorescent dye material. That changed the refractive index, and suddenly they could see through the skin – revealing the organs underneath. I thought that was pretty cool – especially since you can just wash it off, and everything goes back to normal. But also because it's not that far off from what we do – we also work with dye molecules.
Johanna:What do you do when you’re not in the lab?
Julian:I love cycling – and Heidelberg is great for that. You can choose: do I want to ride up into the hills and gain some elevation, or stay on flat terrain? Both are possible. I also enjoy hiking, and something I really love is gardening. In Bonn, we had a small allotment garden, which we sadly had to give up. But luckily, we’ve found a little garden plot here again. It’s just great for switching off – being outside, doing something with your hands.
Johanna:Is there anything that would make your start at the faculty even easier? Any tips or tricks from your colleagues?
Julian:The start has been very open and collegial – that’s been fantastic. What’s especially helpful now are contacts for questions about teaching practices and formats that have proven successful here at the faculty.
And what I’d find interesting: more stories about the university itself! Heidelberg has these very old buildings – like those on the Philosophenweg or in the Old Town, places where Bunsen or Kirchhoff once worked… There’s not just history in that, but also many little stories and anecdotes. Learning more about those would be fascinating!
Johanna:Thank you, Julian, for the conversation – and once again, welcome to the faculty! We wish you a great start, lots of joy in research, teaching, and networking – and of course, in collecting the little and big stories that make Heidelberg and our faculty so special.
