Between crystals, cats and quantum

What she does every day is difficult to convey to laypeople, Yiwen Chu says. Because in everyday life we don’t see anything that behaves quantum mechanically.

As Professor of Solid State Physics at ETH Zurich, Chu experiments with hybrid quantum systems. She combines different types of quantum objects and attempts to transfer quantum information between microwave circuits, sound, light and the intrinsic angular momentum of electrons, known as spins. When these different quantum systems interact with each other, interesting physical phenomena arise – and one day, so will more powerful technologies, Chu hopes. “We’re using our current understanding of quantum mechanics to develop new technologies that could be useful in everyday life.”

In different places at once

Many technologies that are already in daily use are based on quantum science: for example, computer chips, lasers and magnetic resonance imaging (MRI). In the tiny world of quantum mechanics, the rules that apply differ from those in the world that is visible to the human eye. For example, particles can be in several places at once, or they can be in a quantum superposition – meaning in several states at the same time – until they are measured.

While researchers have long been able to visualise and work with the various quantum states at the microscopic scale, so far this hasn’t been possible at the scale of everyday objects. “We’re working to develop new systems that will let us detect quantum mechanical phenomena, such as superpositions, in larger objects,” Chu says.

Specific research for fundamental questions

Quantum research celebrates its 100th birthday next year. In recent years especially, it has experienced a surge of growth all around the world – not least because of the race to build powerful supercomputers based on the laws of quantum mechanics.

Nevertheless, the Hybrid Quantum Systems Group at ETH Zurich, which Chu founded in 2019, continues to conduct research in a highly specialised field. “We use extraordinary materials, special physical objects and unusual techniques for the experiments in our laboratory,” she says. “There are very few groups in the world working on anything similar.”

Much is still unknown in quantum research, Chu says. For example, whether or not quantum mechanics can also be applied to everyday objects. We assume it isn’t possible to conduct Schrödinger’s famous thought experiment in the real world, she says, because of course no one has ever seen a cat dead and alive at the same time. “But nobody knows for sure whether or not a cat can in fact be in a superposition,” the quantum physicist says with a laugh.

Another mystery is how quantum mechanics interacts with other forces, such as gravity. “These are two of the biggest questions in physics to which we currently have no conclusive answers. Our research is trying to find answers to these questions.”

Producing Schrödinger’s cat in the lab

Last year, Chu and her team managed for the first time to produce a particularly heavy Schrödinger cat: an oscillating crystal to represent the cat, a superconducting circuit that takes on the role of the original atom and a layer of piezoelectric material that generates an electric field when the crystal changes shape while oscillating. The researchers succeeded in making the crystal oscillate in two directions simultaneously – up/down and down/up, for instance. These two directions represent the “alive” and “dead” states of the cat.

These findings could be used to make quantum technologies more efficient. For instance, quantum information stored in qubits could be made more robust by using cat states made up of a huge number of atoms in a crystal rather than relying on single atoms or ions, as is currently done.

Vibrating crystals

Chu’s group is now working on further increasing the mass of these crystal cats. The experimental quantum physicist says that this will enable her and her colleagues to better understand the reasons behind the disappearance of quantum effects in the macroscopic world of real cats.

To date, the largest possible objects that behave quantum mechanically are crystals measuring half a millimetre across. Never before have quantum superpositions been detected in larger objects.

Observing quantum phenomena is fundamentally difficult because quantum states are very fragile. The larger an object, the more complex it becomes to ensure that all components retain their quantum properties. If even the tiniest amount of energy escapes from the system, this can destroy the quantum state.

What makes crystals so suitable for these experiments is the fact that they can store energy for a very long time. That’s why watches have always used quartz crystals. “We use these oscillations in the crystal because they can hold the energy and the quantum states – in other words, the quantum information – for a very long period, which gives us time to actually do something with them,” Chu says.

Her passion for hybrid quantum mechanics is both evident and inspiring. The 38-year-old is unstoppable when it comes to her area of expertise. Her enthusiasm for quantum physics dates back to her studies: “I liked the practical work in the lab. But I was also fascinated by the concepts of quantum physics.”

The moment it clicked

Chu spent the first years of her life in China’s capital, Beijing. At the age of eight, she moved with her parents to the United States and grew up in Pittsburgh. During her doctorate at Harvard in 2014, she researched how diamonds behave in quantum optical systems. As a postdoctoral researcher at Yale, Chu began working on mechanics in crystals and hybrid quantum systems – but more by chance than anything else. “During a break, I was chatting to researchers from another group whose lab happened to be in the same corridor as our quantum optics lab. I was fascinated and came up with the idea of combining what I was working on at the time with their research fields of nonlinear optics and optomechanics.”

This took Chu into uncharted scientific territory. “For this new type of system, we had to put objects together that I wasn’t familiar with, and I didn’t know whether it would work in the end,” she says. “I remember sitting in the laboratory and carrying out measurements, and then realising the data confirmed that the device worked just as I had designed it on paper.” She remembers how excited she was, and she still has the lab records with the note saying “Yes, it works!”.

At the time, Chu thought the topic was something new and interesting that she could probably work on for a few years. “I had no idea how far I would get with it. And now it’s given birth to this whole research programme,” she says, still looking amazed.

“Expectations exceeded”

Coming to ETH Zurich was an easy decision, Chu says. “There are many people here working in similar areas, and we can all benefit from that.” However, the founding of the Quantum Centre at ETH in 2021 was also important. “This research centre brings the community closer together,” she says.

Chu’s colleagues really enjoy working with her. “Yiwen already had a stellar career before joining us in 2019,” says Atac Imamoglu, Professor at the Institute of Quantum Electronics. Their expectations of her were correspondingly high. “However, her achievements at ETH, working in a very competitive field and under the limitations posed by the pandemic during the build-up time of her laboratory, simply amazed me and my colleagues. We are really lucky to have her in our department.” It was Imamoglu who nominated his colleague for the Latsis Prize.

“Recognition for the whole team”

Chu was awarded the Sackler Prize in Physics in 2021. On 16 November, the young professor will be adding the ETH Zurich Latsis Prize to her collection. Endowed with 25,000 Swiss francs, the prize acknowledges “her outstanding achievements in the field of hybrid quantum systems,” to quote the presentation speech.

Chu says that she feels very honoured and that it’s nice to know her research area is attracting interest, perhaps even from a wider audience. However, she adds that the entire team deserves the award. “I may be the one who’s won the prize, but the work was done by so many people in my group. This award is testament to the effort that everyone put in.”

Stepping outside the comfort zone

In the meantime, the physicist has set herself a new challenge: Chu became a mother in summer. “It’s lovely to be trying something completely different,” she says. In her day-to-day research into quantum physics, lots of things run in an orderly fashion. The birth of her son has taken her and her partner, who is also a physicist, back out of their comfort zone, says the professor. “Raising a child is something we can’t control. That energises me and encourages me to try new things and take risks.”

It also has an impact on her day-to-day work, Chu says, having recently returned to ETH following her maternity leave. “I feel revitalised and fulfilled in my professional work.”