Condensed Matter Physics Seminar: Thomas Agrenius

Abstract: Observation of quantum phenomena often necessitates sufficiently pure states, a requirement that can be challenging to achieve. Recently, we prepared a nonclassical state originating from a mixed state, using dynamics that preserve the initial low purity of the state [1]. We generated quantum superpositions of displaced thermal states within a microwave cavity using only unitary interactions with a transmon qubit, and measured the Wigner functions of these “hot” Schrödinger cat states for an initial purity as low as 0.06. Such highly mixed quantum superposition states could be implemented with other continuous-variable systems, e.g., nanomechanical oscillators, for which ground-state cooling remains challenging. In this talk, I present the work from my perspective as the project's theorist, highlight its lessons about quantum coherence in mixed states, and discuss potential future applications of mixed quantum superposition states.
[1] Ian Yang*, Thomas Agrenius* et al., Hot Schrödinger cat states. Science Advances 11, eadr4492 (2025). DOI: 10.1126/sciadv.adr4492