Electrically conductive metals, semiconductors, superconductivity, and even more exotic quantum properties: In order to facilitate the emergence of key technologies, like semiconductor electronics, physicists continue to expand their knowledge of such crystalline materials.

All these properties are based on the interaction of electrons with ions that are sorted three-dimensionally into a regular “crystal lattice” in these materials. The more complex and exciting a phenomenon is, such as high-temperature superconductivity, the more intense the interaction of electrons moving in the material becomes, and a collective quantum state forms. A multi-particle system like this one poses great challenges to theoretical physics because its overall behavior can no longer be calculated with precision. The reason is that each additional quantum particle in a quantum system doubles its quantum complexity. This exponential explosion in required computing power quickly pushes even supercomputers to their limits. So theory has to make do with approximations.

Until Now, Simpler Models Had to Suffice

This was the case up to now, at least, because now there is a solution. In order to precisely calculate the behavior of a quantum system, scientists take a second quantum system as an “arithmetic unit.” Professor Monika Aidelsburger explains the crucial difference by noting that “this second quantum system must be very controllable.” This is because the electron collective in a material is difficult to access from the outside. So far, this has necessitated indirect methods of investigation, something like broadcasting a soccer match on the radio instead of using pictures on a television.

Researchers hope that quantum simulation will make the leap from radio to TV, so to speak. Aidelsburger, a physics professor at the Ludwig Maximilian Universität (LMU) in Munich, works in this still young but rapidly growing field of basic research. The field holds major promise because these kinds of quantum simulators may one day help in the search for new materials or in understanding chemical reactions. They could also play an important role in the development of quantum computers that are more stable. For Aidelsburger, however, this research revolves around very fundamental questions of physics.

Already Many Awards for Monika Aidelsburger

Aidelsburger has been researching the development of quantum simulators since completing her master’s thesis. The 34-year-old physicist has already received an astounding number of awards for her creative ideas. On November 18, 2021, she will be awarded the Klung Wilhelmy Science Award 2021 at Freie Universität Berlin. This honor is awarded annually to scientists in the fields of chemistry and physics, alternating between the two fields each year. The recipients are chosen by two expert committees at Freie Universität Berlin.

At 50,000 euros, it is one of the most highly endowed, privately financed prizes for Germany’s best young researchers. The jury honored Aidelsburger “for her outstanding research on the experimental realization of synthetic gauge fields in optical lattices and their application to quantum simulation of topological phases of matter.” Aidelsburger’s system juggles individual atoms like balls. The atoms are vaporized in a vacuum chamber and then decelerated with laser beams until they almost stop.

At the micro-level, however, standstill means nothing other than cold. Further tricks are needed to bring these atoms down to a temperature of a few billionths of a degree above absolute zero. This makes it possible to capture them at the intersections of a three-dimensional grid of laser light beams. This rigid structure imitates a crystal. The ultracold atoms in the light grid, however, perform a different task than the atoms in a crystalline material: They are designed to mimic the behavior of the electrons that exhibit interesting properties. The problem is that the electrically neutral atoms do not react to electric and magnetic fields like electrons do. But Aidelsburger solved this issue by developing a sophisticated laser control system that allows the atoms to react like electrons in a magnetic field.

Taking a Break in Asia after Her Doctorate

Aidelsburger wasn’t necessarily predestined to study physics. She could have also gone into art or music, she says. Then friends told her that studying physics was exciting but difficult. “I wanted to try that out,” she says.

She was a successful student at university, and her interest was really ignited in a lecture by Immanuel Bloch, one of the pioneers of quantum simulation in Munich. She applied to write her master's thesis with him and ended up continuing to work with him as a doctoral researcher. After she completed her doctorate, she took six months off and traveled through Asia. Then she went to France as a postdoctoral fellow, but ended up coming back sooner than planned when she received an offer from LMU Munich. She started in Munich as research group leader and is now a professor of physics. Aidelsburger is interested in testing highly abstract yet fundamentally important concepts of physics in the quantum simulator.

Aidelsburger specializes in topology, a field that actually comes from mathematics. Topology is playing an increasingly significant role in physics because it can elegantly describe rare but particularly exciting physical properties. Its growing importance can be seen in the fact that three pioneers from this field received the Nobel Prize in Physics in 2016. What all these complex phenomena have in common is that electrons, as a quantum multi-particle system, produce them. The most famous system with these properties is the quantum Hall effect, which the German physicist Klaus von Klitzing discovered and for which he was awarded the sole Nobel Prize in Physics in 1985. Aidelsburger’s achievement was not simply the replication of this effect in her quantum simulator; her work has gone further and showed that the concepts of topology can be applied beyond normal matter to reveal fascinating properties.

Pretzel, Ring, and Cup in Topology

As is so often the case in physics, this is very abstract for non-experts. In fact, the playing field is an abstract “space” in which the quantum states of the electrons involved in the effect under study have a specific geometry. In this way, it is similar to the topology of objects.

Examples of such objects are a ring, a coffee cup with a handle, and a pretzel. The ring and coffee cup each have a through hole, the latter in the handle, and thus the same topology. They are – from a purely topological point of view – equivalent and can easily be transformed into each other. The pretzel, on the other hand, has a different topology with three holes, so it cannot simply be made into a ring.

This insistence on maintaining a topology can also stabilize electron systems, which leads to exotic phenomena. One example is topological insulators, which conduct electrical currents only on their surface and do so without electrical resistance. Aidelsburger wants to use her quantum simulators to investigate these kinds of quantum phenomena as well as very basic predictions in theoretical physics about other fields, such as particle physics. This could lead to many more exciting discoveries. At any rate, a lack of creativity will not be a problem for Professor Aidelsburger.

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This text originally appeared in German on October 2, 2021, in the Tagesspiegel newspaper supplement published by Freie Universität Berlin.