Quantum science: The magic of the collective

At this moment in his life, Milan Allan often deals in scales with which we are all familiar – with dimensions measured in meters and centimeters. The laboratory space in LMU’s main building which he will be using is between 2.83 and 2.87 meters tall. Although this poses a challenge for some of his instruments, it fascinates him nonetheless to work in this historic place. “We’ll be doing research in the old laboratories of Arnold Sommerfeld,” says Allan. More than a century ago, this legendary scientist trained a whole generation of theoretical physicists at LMU.

Scientific heroes are part of the pull that brought Allan from Leiden University to Munich, where there is “an unbelievable density of wonderful researchers in quantum physics.” As Chair of Experimental Physics – Quantum Metrology and Sensing, he will contribute his expertise to the university and within the MCQST Cluster of Excellence in Munich. And here he will have to grapple with much smaller scales.

The most magical states on Earth

“We’re trying to understand novel quantum materials,” says Allan. He is referring to materials where the quantum mechanical nature of individual particles – say, the electrons – is exhibited in their macroscopic properties. High-temperature superconductors are a good example. “All electrons can occupy the same state, and as a consequence, the materials do not have any electrical resistance,” explains Allan. “We’re trying to observe these collective microscopic electron states, which are important for macroscopic properties like superconductivity and magnetism. For me, these collective states are the most magical states that exist on Earth. Thus far, we’ve not really managed to understand them.” Ultimately, they connect quantum mechanics with macroscopic – that is, perceptible – properties.

It is uncharted territory, then, that Allan and his staff are exploring. Generally speaking, there are no instruments on the market for really understanding a physical problem or behavior at the microscopic level in this area. “So we build them ourselves,” laughs Allan. “We often need very special microscopes, and if we find an issue important enough, we’ll spend years if needs be to invent an instrument that’s better than anything else on the planet.”

Understanding novel materials

The goal is to use these instruments to investigate things like new superconductors. “These materials are often very heterogeneous, so you really have to know what’s happening in them locally at the microscopic level. Macroscopically, you get a completely different picture.” Allan and his team attempt to precisely measure things like electrical currents in novel materials.

To this end, he plans to build a so-called scanning SQUID microscope in Munich over the coming years and develop new components for it. Just like he did in Leiden for another instrument known as a scanning tunneling noise microscope (STNM), which measures tunneling currents in materials. While previously it was only possible to measure the averaged current, a newly developed amplifier allowed the researchers to record the tiniest fluctuations. As a result, they could investigate certain phenomena in novel materials that were formerly inaccessible.

Milan Allan recalls how he became interested in quantum mechanics and collective quantum states while studying at ETH Zurich: “I found these collective condensates fascinating. I wanted to do something new and I like difficult problems.” Back in “gymnasium”, his outlook was very different. “I was very unmotivated for school in those days, and only at college did I discover my passion for mathematics and quantum physics – and it never let me go since then.”

“You need to see something to understand it”

After stints at the University of St Andrews in Scotland, Cornell University in the United States, and a postdoc under renowned quantum physicist Andreas Wallraff at ETH Zurich, Allan was appointed professor at Leiden University. For the first time, he was able to plan and build his instruments with his own research group. “My early days as a professor were like a honeymoon – it’s wonderful to get up every morning and be able to implement your ideas,” he recounts. In Leiden, he built a series of microscopes that make novel observations possible, including one with the daunting name: spectroscopic imaging-scanning tunneling microscope (SI-STM).

This instrument is capable of imaging the main quantum mechanical degrees of freedom – such as the entanglement of electrons – in a quantum material at the atomic level. Using finite element calculations, Allan developed a novel, stable STM head made of sapphire, which can scan samples with greater precision. Used to scan material samples from top to bottom, and thus precisely measure them, these microscopes are large, often ceiling-high apparatuses that require elaborate cooling.

Allan loves this special type of imaging. “I like pictures,” he says. “I think that in physics you often need to see something to understand it.” Building new instruments can be extremely useful in this regard. “It’s often the only way to make progress in a field. I have the feeling I can make a difference here.” Overall, he is a “happy experimental physicist” and explains that you get a lot back “when you build an instrument that allows you to see a quantum mechanical degree of freedom that nobody has ever seen before.”

Good environment with Cluster of Excellence

Allan is now bringing his experimental expertise and engineering skills to Munich. He had already established initial contacts while in Leiden and collaborated with the research group led by LMU physicist Dmitri Efetov. “I like how broadly based the research is here in the field of quantum physics with MCQST, with regards to applications as well as basic research and theory,” says Allan. “There’s also the whole range in between, with groups investigating quantum simulations and novel materials. The students are good, and the environment is great. It’s fun to be here in the midst of it all.”

Allan is getting to know his new colleagues in Munich, such as the physicist Fabian Grusdt, who, like himself, is working on high-temperature superconductors, but at the theoretical level, or well-known experts in theoretical physics like Jan van Delft and Ulrich Schollwöck. “I’m still discovering brilliant new researchers here.”

He also hopes to talk physics soon with researchers such as quantum physicists Immanuel Bloch und Monika Aidelsburger and forge new collaborations with them. “Immanuel Bloch is a real role model. Like me, he’s interested in doped Mott insulators, for example.” Mott insulators such as copper oxide are exciting materials for physicists because, according to the models of physics, they should be electrical conductors, but in experiments, they prove to be insulators. For physicists, they’re still a puzzle. “I’m always amazed at Immanuel Bloch’s perspective on them, so different to our group’s, and what he thinks important to measure – entanglement, for instance.” For Allan, Munich is a place where he feels: “My God, I still have so much work to do; I’m really not where I need to be.” But his work in the former basement of Arnold Sommerfeld will provide an excellent platform to get there.