It's 23 October 2019, and Leon Sander is sitting in Prof. Michael J. Hartmann's office for a preliminary interview for his bachelor's thesis. Suddenly, the phone won't stop ringing. Journalists are calling to get a statement from Hartmann on the just-published Google paper on quantum supremacy, in which the Bavarian quantum researcher was involved. "That's when I had the feeling that I was in the right place at exactly the right time," Leon recalls. He still has that feeling today – by now he is working on his doctorate at Hartmann's Chair of Theoretical Physics at the Friedrich Alexander University of Erlangen-Nuremberg.

On a table in the corner of his office, which Leon shares with two other doctoral students, a small blue espresso machine catches your eye, looking a bit like a robot of days gone by. Insiders know immediately what model it is and that it suggests Leon is a coffee nerd. The coffee scale and hand grinder next to it reinforce this suspicion, which the physicist can laughingly confirm. When making coffee, too, you can experiment with the variables until the result comes out right. Much like in Leon's research.

Leon has enjoyed physics since his school days. This was also due to a good teacher who came up with exciting topics for his only very small physics course in high school. That's also how he first came into contact with quantum physics at school – although it was this area, of all things, that was his biggest weakness back then in high school, he recounts with a laugh. "I think I was a little resistant to it at first. I thought there must be a better explanation for these effects," says the now 24-year-old looking back. But that's exactly what fascinated him in the end and drew him to the subject, he says.

Coding as self-realization

In his own research, Leon deals with machine learning models and neural networks. "More specifically, I'm researching Quantum Convolutional Neural Networks (QCNNs). This is a relatively new concept," the Ph.D. student explains. So far, QCNNs have been applied primarily to one-dimensional spin chains; Leon is working on applying them to two-dimensional systems.

He sits down in front of his computer and briefly types a few lines of code into the command line. "I'm trying to find a sequence of procedures by which properties can be efficiently read out of a complex quantum system. A QCNN helps by recognizing the property in question, so to speak, and developing it in such a way that the information can be read out with far fewer measurements than traditionally. To do this, I plot two-dimensional qubit lattices and apply operations to them," Leon says, summarizing his research project. He jokes with his girlfriend that he's been playing four-in-a-row ever since she looked over his shoulder in his home office. And indeed: Leon presses Enter again and his grids appear on his screen, squares of red and blue circles that clearly remind of the game.