"My heart beats for physics"

Getting one step closer to quantum communication with quantum dots

As an ambitious swimmer, Michelle Lienhart learned early on how to work towards a long-term goal. Today, her perseverance and ambition are helping her in her Ph.D. in the field of quantum dots - constantly driven by her curiosity and enthusiasm for physics. 

By Veronika Früh

"Hang on, I need to wash my rubber glove hands!" shouts Michelle Lienhart as she comes running out of her lab at the Walter Schottky Institute (WSI), slippers on her feet. She had to quickly switch on the laser cooling system so that she could get straight back to work later. Here at the institute in Garching, the 27-year-old MQV fellow is doing her Ph.D. in experimental physics in the field of quantum dots.

Switching on the cooling of the lasers in the lab – that's how most of Michelle's working days start: "I press two buttons, then I have about half an hour to check my emails in the office while the lasers are running cold." This is important so that the powerful lasers don't overheat later during Michelle's experiments. It then takes another few minutes for the lasers to start up after being switched on. And then comes the tedious work. "I usually spend about two hours setting everything up," says Michelle. The laser beam has to be meticulously aligned in order to hit the sample perfectly and not lose intensity on its way through the numerous optics and mirrors. Also, as few photons as possible should be lost on the way from the sample to the detector. It’s all about micrometers. "It's always a long, rather annoying procedure, I find," admits Michelle, "because it's always the same every day and there are a lot of steps involved." But that is simply the price you pay for working so precisely. In the meantime, the doctoral student has established a certain routine, but even if she is faster than at the beginning, going through the entire system from the laser to the sample and then to the detector takes time. "By the time everything is in place, it's usually just before lunchtime," Michelle says with a laugh. Then she starts with some characterization measurements to check her system. And then her experiments can finally start.

Research on artificial atoms

Quantum dots, the subject of the doctoral student's research, are also known as artificial atoms. They consist of tens of thousands of atoms. Due to their size in the nanometer range, however, quantum mechanical effects can occur that are similar to those in a single atom. These artificial atoms have the advantage that their physical properties can be very well defined by the material and geometry of the quantum dot. Quantum dots can also be used well as a single photon source, which is why they are of great importance for research in the field of quantum computing and quantum information. Michelle works with quantum dot molecules, two quantum dots grown on top of each other that are coupled by their small distance. "My aim is to entangle spins in the quantum dot molecules with photons," explains the doctoral student. "In the broadest sense, of course, the aim is that these entangled photons will bring us closer to realizing a quantum computer," she says, casting a glance at the big picture. "And quantum dot molecules are a very promising candidate here."

The quantum dot molecules themselves are produced in Bochum and further processed in Berlin before being finished in the clean room at WSI. "A few more things are etched down here and gold contacts are attached," explains Michelle. So far, she is still working with her predecessor's sample, but when the next sample arrives at her lab, the fabrication of the quantum dot molecule will also be one of her tasks. 

Michelle Lienhart, 27


Position

MQV doctoral fellow


Institute

Walter Schottky Institute – Chair of Semiconductor Nanostructures and Quantum Systems (SNQS)


Degree

Physics and Computer Science


Michelle is researching quantum dot molecules, which consist of two coupled quantum dots, also known as artificial atoms. Her goal is to entangle spins in the quantum dot molecules with photons and thus come one step closer to the realization of quantum computing and quantum communication.

All the components in Michelle's set-up have to be precisely adjusted before she can start her experiments.

Michelle is fascinated by the fact that the fabrication goes through so many steps at different locations and that so many different areas of work are involved in producing this potential candidate for quantum computing technology. She particularly likes the sense of community that this creates: "When you're in the lab, you sometimes feel like you're doing it alone. But you're actually researching this big goal together and everyone is working on their part. And even if the goal is still a long way in the future, in ten years' time you might be able to say that you have contributed a little to the progress in quantum technologies."

The fact that Michelle ended up in the field of quantum dots was also a bit of a coincidence – "it turned out great," she says. The 27-year-old wrote her Bachelor's thesis, with which she completed her physics degree at the University of Augsburg, in biophysics. And even though she really enjoyed working on cell recognition with machine learning, when she got talking to colleagues in the same department about quantum dots, she found this topic even more exciting. "At the beginning of your Bachelor's degree, you just start somewhere, you don't even know what you want to do yet," she recalls. For her Master's degree in physics, Michelle then knew all the more precisely which direction she wanted to take and focused on semiconductor physics and optics, dealing with quantum dots in her Master's thesis.

Driven by curiosity

"I'm generally enthusiastic about a lot of things because I'm simply interested in physics," says the doctoral student. It was therefore clear to her early on that she wanted to study physics when she was still at school – and she "100% enjoyed" her studies. When she realized during her Bachelor's thesis that computer science would help her reach her goals faster, she quickly took up a double degree and also completed her Bachelor's degree in this subject. "You always have to evaluate data and program things," she says, explaining her decision. She really enjoyed programming and thinking about algorithms – "but my heart beats for physics," says Michelle, beaming.

Michelle's curiosity continues to drive her in her current work. When her system is perfectly adjusted and everything is running smoothly in the lab, she sometimes forgets about the time. "When I see that everything is working and I get exciting results, I can't stop," says the doctoral student. "This curiosity drives me on and on," she pauses briefly, "and then the hours just fly by." As a result, Michelle often sits in the lab until late in the evening carrying out her measurements. "Our lab is dark, so I don't even notice what's going on outside," she laughs. The fact that she can perhaps do without adjusting her experimental set-up for two hours on another day is a welcome side effect.

The optical head: all paths from the experiment converge here and are passed downwards to the sample.

The mounts of the optical components, densely arranged in Michelle's experimental setup, sparkle in a bright purple with golden screws. "I thought to myself that science doesn't always have to be so serious," says the doctoral student, explaining her decision to use colorful components instead of the usual black and silver combination. All the paths that the laser beam can take across the optical table converge at one point: the optical head. It is not only called this because of its functional similarity to the human brain – all impressions converge here before they are collectively passed on to the sample. The cube-shaped structure, which is connected to the bulbous cryostat in which the sample sits via a narrow rod, is also reminiscent of a robot-like head. Michelle built the optical head at the beginning of her doctorate. "I'm quite proud of it because it's used every day, and not just by me," she says. And here, too, it is clear how diverse the doctoral student's interests are: Michelle was responsible for everything from the design with a modeling program to the consultation with the workshop for the precise production of components. "I really enjoyed building it and I still do, also because it was once again very interdisciplinary work," enthuses the doctoral student. "And because it simply works and is also very useful for other people," she adds.

What frequently causes her problems, however, is the "body" on which the optical head sits: the cryostat occasionally heats up unintentionally, which could be dangerous for the sensitive sample. "That's not so nice and it was a big challenge for me to accept that it's not my fault and that I can't change anything at the moment," says the 27-year-old. She is generally very good at hiding away in her office to think about a problem. However, she has since learned when it's time to draw a line and clear her head: "I then try to make the cut and tell myself that I'm going to have a nice weekend and then think about it again on Monday." And then things usually work out much better again.

From Augsburg to Munich

Being patient with yourself is something Michelle would also recommend to other early career researchers, especially at the beginning of their doctorate. "Patience and talking to other people," she summarizes. The best way to learn from your colleagues at the beginning is to simply ask. It is also important that you feel comfortable in your group, says the doctoral student: "It helped me a lot that I looked at the group beforehand. I would recommend everyone to visit the group for at least one day, to see what the people are like, what the labs are like." As an MQV fellow, Michelle was able to choose which research group at a Bavarian university she would like to join. "I thought it was a very good idea that I could apply first, but didn't have to commit myself completely yet," she says. So she looked at various groups in Bavaria that work with quantum dots and eventually ended up with Prof. Finley at the WSI. "At least I made it from Augsburg to Munich," says the doctoral student and laughs.

One reason Michelle chose the Semiconductor Nanostructures and Quantum Systems group was that it is a very international group: "The people all have different backgrounds. I thought that was great." She had already really enjoyed the international working environment during her time in California. The doctoral student has already been to the sunny state on the west coast of the USA twice, once for a three-month research internship at Berkeley Lab and once for a one-month research stay at UC Santa Barbara. The doctoral student can summarize her experience there concisely: "Very refreshing!" She really liked the way of working there; everything was much faster and more spontaneous. "It's also just nice, of course. You go to work there in the morning, the sun is shining. You work, you go home, the sun is still shining," she recalls.

Also ambitious outside the lab

Michelle also likes to set herself ambitious goals away from the lab. A few months ago, she started surfing on the Eisbach wave. "I'm not that good at it yet, but that's my leisure goal for this year," she says. She used to admire the Eisbach surfers every time she visited Munich. And now that she lives here, she really wants to learn. By the end of the summer, she wants to be able to stand on the wave. "On the small one. The big one – we'll see." She has already bought a beginner's board and wetsuit. "I really want to be able to do it," Michelle says with emphasis and laughs.

The water is her element anyway, Michelle has been in the swimming club almost her whole life. She currently swims "just for fun", as she says, and takes part in only one or two competitions a year, "just to see people from the past". She no longer wants to call herself an active swimmer. No wonder, because her own standards are high: as a student, Michelle wanted to pursue a professional swimming career and trained 14 times a week at the elite sports school in Erfurt. "I had a school routine there where swimming was almost above school," she recalls of her time at Pierre-de-Coubertin-Gymnasium. School, boarding school, swimming hall. Swim training twice a day and athletics training in the gym once a day. Even though her swimming career didn't come to fruition, Michelle was able to learn a lot in terms of perseverance and ambition. "I would say I learned a lot back then for my doctorate now," she says. Including how to deal with setbacks: "I know that something can break in the lab, that something doesn't work the way you thought it would. And then you just keep going." Always in mind: her long-term goals.

Published 26 July 2024; Interview  26 March 2024