How to replace quantum physics with quantum physics

Julian Leonard is awarded an ERC grant. He is developing quantum simulators that can be used to study intriguing effects from solid-state physics.

The formulas of quantum physics can be used to explain a wide variety of objects: Atoms, for example, but also microchips or electrons in electromagnetic fields. Sometimes, interesting similarities emerge between seemingly very different quantum experiments: One can investigate a quantum object and learn something that also applies to another quantum object - although at first glance one might think that the two objects have nothing to do with each other.

In this case, we speak of "quantum simulators": a physical situation is studied by creating another situation that is similar but easier to investigate. This is precisely the vision of Julian Leonard from the Institute of Atomic and Subatomic Physics at TU Wien (Vienna): He uses electromagnetic fields to manipulate atoms in order to derive insights into the behavior of electrons in solids. For his work, he has now been awarded an ERC Starting Grant from the European Research Council - one of the most prestigious grants in the European research landscape, endowed with around 1.5 million euros.

Mountains and valleys of light

"We work with so-called optical lattices," Julian Leonard explains. "These are, in a sense, crystals of light. With laser beams, you create a periodic intensity distribution in which the atoms can only sit in very specific places, similar to eggs in an egg carton." In some places, the atoms can linger for a long time; in other places, they are pushed away by the light. The atoms can merely hop from one allowed spot to the next allowed spot.

This corresponds to the movement of electrons in a solid material. "These electrons can jump from atom to atom, much like the atoms in our light experiment," Julian Leonard explains. "So we can simulate effects that occur in the solid state with our experiment. But the advantage is that we have much better ways to measure and control these effects."

You can’t directly observe the behavior of an electron inside a solid. The atoms, on the other hand, can be studied in detail with suitable equipment. Moreover, one can adjust the shape and strength of the light field as desired - unlike in solid-state physics, where the parameters of the experiment are fixed by the material used.

Exotic quasiparticles

In this way, one can go in search of exotic effects that are almost impossible to study with other methods. "When several quantum particles interact with each other, it can happen that they behave together as if they were another particle with completely different properties - a so-called quasiparticle."

In a sense, the situation resembles a flock of birds moving collectively through the air on stunningly intricate trajectories: The interaction of the birds creates something new; the flock moves collectively roughly as if it were a different animal, with completely different properties from a bird - perhaps the flock could be called a "quasibird."

Anyons that cannot be swapped

Such quasiparticles can exhibit amazing properties. For example, there are so-called "anyons" that can appear in two-dimensional boundary layers. "If you swap two particles and then swap them back, you usually have the original state restored. But that’s not the case with anyons in two dimensions. You have created a new state that is different from the initial state," says Julian Leonard. "These processes have hardly been studied experimentally yet, and there is still a lot of research to be done here. They are so-called topological effects, which are very promising for new quantum technologies - such as quantum information memories or quantum computers."

Julian Leonard

Julian Leonard studied physics at the Technical University of Munich. While still a student, he made contacts at the Max Planck Institute of Quantum Optics in Garching, and then wrote his diploma thesis at the École Normale Supérieure and Sorbonne Université in Paris. He then began doctoral studies at ETH Zurich, where he graduated in 2017. That same year, he moved to the Physics Institute at Harvard University, USA, as a postdoctoral fellow. In 2021, he won the highly endowed START Award of the Austrian Science Fund FWF, thus returning to Europe and establishing his own research group at TU Wien. With the ERC grant, he will now expand his team at TU Wien and push ahead with new experiments.