Working in Quantum. Insights and behind-the-scenes with ParityQC’s Leo Stenzel

Here is a new interview from our Working in Quantum series, where we put the spotlight on the people that are part of the flourishing quantum computing industry. This time we talk with Leo Stenzel, a Quantum Software Engineer in the Co-Design department who has been with ParityQC for already 3 years.

Leo comes from Munich and spent his academic years in the Bavarian city, focusing mainly on quantum simulation. He had the chance to work at the prestigious Max-Planck Institute for Quantum Optics, and then decided to focus on mathematical and theoretical physics for his Master’s. His academic career continued further into the field of quantum many-body physics and tensor-network methods. Now at ParityQC, he gets to focus on research of an explorative nature while also giving his unique contribution to the work in our Co-Design department.

What’s your role at ParityQC? 

I’m a “Quantum Software Engineer” which, for me, means working on quantum things and developing “classical” software: for example, software for simulating quantum hardware, or using quantum-inspired algorithms for solving optimization problems. It feels more like doing research than you’d expect from a typical software engineering position, reading lots of papers, occasionally even performing calculations with pen and paper…

Tell us about the path that led you to working in quantum computing.

Like many in our team, I’m a physicist: I studied in Munich with a focus on quantum simulation. During my Bachelor’s I worked on an experiment with ultracold atoms in optical lattices: these systems can be engineered so that the atoms behave just like electrons would in some solids—only on different space and time scales. They may thus serve as “analog” quantum simulators.

I then slightly switched gears, did my Master’s in theoretical and mathematical physics, and settled on numerical quantum simulations for my PhD. Still trying to understand the same “solids” or “atoms in lattices”, but now using tensor-network algorithms, safely behind my computer.

I didn’t think a career in academia was for me, but I wasn’t quite ready to give up on doing quantum physics… Now it’s possible to work on quantum technologies outside of universities, so that’s pretty exciting!

Does your background in quantum simulation and many-body physics make you see quantum computing from a different perspective?

So “QC” and “QSim” are very close in terms of the physical systems; and there is the field of adiabatic (or analog) quantum computing, which is basically a combination of both. Basically, I can’t draw circuit diagrams, but the ideas are all the same.

I think my perspective is rather shaped by my background in numerical methods: I always think about computational complexity. Whether some proposed QC algorithm actually solves a hard problem, or if a classical computer could do the same thing much faster. This might not benefit ParityQC directly, but it’s important for us to be realistic.

You’ve been with ParityQC for 3 years now. What do you think of the development of the company, from the time it was a small spinoff?

Yeah, ParityQC has grown quite a lot, so many people working on so many projects! We used to have all-to-all standups, but now I could never keep track of all the things going on…

Otherwise, not too much has changed, and that’s mostly for the better. There is still a lot of freedom to follow new research directions, try out new ideas. And well, we’re also working on some of the same problems as we did three years ago…

And finally, at least for me, there’s hardly any bureaucratic overhead to worry about (thanks to everyone making this possible!) and I can really focus on my work without spending too much time in meetings.

As the company grew and developed, you are now part of the Co-Design department. What are the overarching mission and activities in the department, and how do you work towards using quantum computers for real-world applications? 

So, in Co-Design, we basically try to make noisy, intermediate-scale quantum devices (NISQ) more useful. Until we have a fault-tolerant, universal quantum computer, we cannot just run our favorite quantum algorithm without worrying about hardware details.

We are currently in a phase where different hardware platforms have their distinct advantages, and it’s not at all clear which one will win in the end. What I like about the Co-Design approach is that we look at the entire stack in order to find some improvements: from low-level quantum dynamics, all the way to mathematical reformulations of the optimization problem which we want to solve, or classical postprocessing. 

Ideally you find that one weird trick to simplify the whole algorithm; but it’s challenging, and you can easily fool yourself. Sometimes your clever reformulation doesn’t generalize to other problem instances, or the classical computer actually does all the heavy lifting…

What are you busy with right now and what is particularly challenging about it?

I usually got a small handful of projects; some with concrete goals and deadlines, others of more explorative nature.

For example, I’ve been involved in our collaboration with NEC for over two years already: they are working on their quantum device based on the ParityQC Architecture, and in this context I perform numerical simulations, trying to find ways for them to improve their hardware. This is challenging, because simulating the “full” physical system is much harder than simulating an ideal quantum system—4 or 8-qubit subsystems can already be fairly difficult, depending on the physical model.

In turn, simulating a quantum system is much harder than solving any “real-world problem” it could be used for. In other projects I look into applying simulation algorithms directly on the optimization problem, avoiding some of this embedding overhead. This leads to things like tensor-networks over semirings. But that isn’t just a mental exercise, such “quantum-inspired” algorithms can be very efficient!

What’s the most rewarding experience you’ve had so far working at ParityQC?

At ParityQC we do company retreats, meeting somewhere in the mountains, and exchanging and developing ideas together for a week, without too much distraction. These are very fun, especially the first one, finally meeting all my colleagues after long Covid restrictions… Quite intense, but also very productive.

How do you “perfect your craft” in your work? What is the mindset, methods, and modes of cooperation you have developed which allowed you to become better and more efficient at what you do?

Well, I don’t know about perfection… But sure, there’s always incremental improvement.

For example in the software-development aspects of my work, I’m trying to learn from open-source projects. About writing more easily maintainable code, better automation, having more structured (remote) discussions, etc. None of that is rocket science, just things you might not have learned in academia.

I think it’s about taking your time and learning new things, even if they don’t seem relevant for your next deadline. And not always hacking things together as fast as possible, but taking a step back, and reflecting on what you want to achieve.

Though, I’m prone to overthinking, so take that with a grain of salt…

What inspires you to come to the ParityQC office (or log in) every day? 

I like the challenges. Most days I get to think about difficult problems, and try to come up with simple and elegant solutions… Of course that doesn’t always work out, and getting stuck is less inspiring. But then it helps to work in a pretty cool team, where everyone will try to help you.

Quantum computing still got a long way to go, so you should enjoy the process.

Finally, could you give three pieces of advice for a student just starting out in this sector? 

You should follow your curiosity: you’ll learn so much more if you’re actually excited about something, if you come up with a question yourself, and not just work out an exercise sheet.

When you’re just starting out, it may be difficult to trust your instincts. But quantum computing is still such a new field that everything will change, anyway; don’t be afraid to miss out on some things.

And be a skeptic; there’s a lot of hype… Not every paper is a breakthrough, and if there’s no paper it’s just PR. 

I think that makes three, depending on how you count?

If you’re interested in joining ParityQC’s journey, don’t forget to check out our open roles on LinkedIn and in our Careers page!

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