Public defence in Engineering Physics, M.Sc. Cheng Wang

Title of the thesis: Measurement and Control of Micromechanical Oscillators in the Quantum Regime

Doctoral student: Cheng Wang
Opponent: Associate Professor Aurelien Dantan, Aarhusin University, Denmark
Custos: Professor Mika Sillanpää, Aalto University School of Science, Department of Applied Physics

Micro- and nanomechanical devices are key components ubiquitous in many real-world applications, and have become indispensable tools for fundamental studies in physics, in particular, macroscopic quantum phenomena of the motion of mechanical objects. However, a significant challenge in exploring the quantum nature of these devices arises from the thermal disturbances of the environment, which induce decoherence and obscure their quantum properties. Advancements in microfabrication, as well as cooling techniques such as direct refrigeration and cooling methods developed based on quantum control protocols, have propelled the study of macroscopic mechanical systems into the quantum regime.

The micromechanical devices studied in this thesis are superconducting microwave circuits in which aluminum drumhead mechanical oscillators are strongly coupled to circuit electromagnetic resonances. This circuit configuration, sometimes termed circuit electromechanics, can be viewed as a circuit realization of cavity optomechanics, which explores the interaction between the micromechancial motion and electromagnetic radiation.

In this thesis, several endeavors aimed at achieving quantum control of micromechanical motion are experimentally investigated for the first time. We first implemented feedback control in circuit electromechanics and achieved feedback cooling of a 8 MHz oscillator near to its motional ground state. We also explored using feedback to stabilize an otherwise unstable regime in optomechanics. Next, we considered a noise-driven approach in optomechanics, in which we inject strong band-limited electromagnetic noise into the motional sideband of a circuit optomechanical device. Ground-state cooling and optomechanical heating phenomena are investigated with different noise driving conditions. Efficient quantum control over these systems and the ability to prepare them into target quantum states may open up the possibility of using these mechanical devices as valuable resources for future quantum applications.

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

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Doctoral theses at the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52