Professor Yiwen Chu (@MPL, Susanne Viezens)

Yiwen Chu's research group "Hybrid Quantum Systems" deals with the study of massive mechanical objects in the quantum regime. In physics classes, a spring with a mass attached to it is often used as a model to teach both classical as well as quantum mechanics. While the “classical” periodic elongation and compression of a spring forms part of our everyday experience, quantum mechanics predicts a wealth of counter-intuitive phenomena. Most strikingly, the mass can mimic the iconic "Schroedinger cat," meaning it could be in a superposition of two classically incompatible states. A Schroedinger cat is alive and dead at the same time, similarly a mass-spring system in a Schrodinger-cat state vibrates simultaneously with two opposite oscillation phases. Perhaps unsurprisingly, such a bizarre behaviour has not yet been observed. Yiwen Chu explained: "This is because a block of a certain material attached to a spring is highly complex. It has many degrees of freedom, many components and, most importantly, many ways of interacting with the environment, which all leads to, for example, decoherence. And that makes it very difficult to then observe their quantum mechanical behaviour in a coherent way."

In modern physics laboratories, systems with no or little mass, such as light or molecules made up of thousands of atoms, are routinely used to realize quantum-mechanical experiments including Schroedinger-cat states. Yiwen Chu's research group aims to enlarge the realm for these phenomena to include the collective dynamics of significantly more massive objects. In her talk, Yiwen Chu presented her latest experiments with bulk acoustic resonators which have masses in the microgram regime and consist of up to 10¹⁷ constituent atoms. Her devices also comprise a superconducting circuit mounted on the resonator. The bulk acoustic waves in the resonator interact strongly with the microwave radiation which circulates in a superconducting circuit and is itself in a quantum state. The interaction with the microwave radiation can be tuned using a sequence of microwave pulses. This setup makes it possible to manipulate the quantum state of the resonator, e. g adding phonons (quantized vibrational excitation) one by one or steering it into a Schroedinger cat state.

During the lecture, Chu introduced the audience to the applications of these systems in quantum information, quantum sensing and fundamental physics research.

You can find this and other lectures in the DLS series on our YouTube channel: https://www.youtube.com/playlist?list=PL6yOOrXfatatYcdjK_N90smVlWbVLrfj6