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Title of the thesis: Microwave optomechanics for milligram-scale gravitational force experiments

Thesis defender: Joseph Depellette 
Opponent: Associate Professor Pierre-François Cohadon, École normale supérieure, France   
Custos: Professor Mika Sillanpää, Aalto University School of Science

Measuring gravity between very small objects is one of the major challenges in modern experimental physics. This doctoral thesis studies how sensitive mechanical devices and microwave measurement techniques can be developed for future experiments that aim to detect the gravitational force produced by milligram-scale masses. 

The work is based on cavity optomechanics, where a microwave cavity is coupled to a vibrating mechanical element. In this thesis, the mechanical elements are silicon nitride membranes. These membranes are excellent for acceleration sensing, and by adding mass to them they can also be used to produce a stronger, measurable gravitational signal. This makes them promising tools for studying weak forces and, in the future, possible links between gravity and quantum physics. 

A central challenge is that heavier membranes vibrate at low frequencies. At these frequencies, cooling the device in a refrigerator is not enough to prepare it close to its quantum ground state. The thesis shows that feedback cooling offers an effective alternative. The motion of a membrane was measured with a nearly quantum-limited amplifier and used to apply a corrective force. This significantly reduced the membrane’s motion, an important step toward controlling massive mechanical objects at the quantum level. 

The thesis also presents a method for reducing microwave amplitude noise from signal generators. This noise can heat mechanical resonators and limit measurement sensitivity. By cancelling the noise, the lowest achievable temperature in a cooling experiment was improved by a factor of two. The method is simple, adjustable and potentially useful in many precision microwave experiments. 

Finally, the work demonstrates key components for an experiment designed to measure the gravitational force from a 1 mg mass. A mass-loaded membrane was driven electrically at cryogenic temperatures and successfully combined with a microwave cavity. 

The results provide new methods for cooling, controlling and measuring massive mechanical resonators. They support the development of experiments that can probe extremely weak gravitational forces and help bring fundamental physics tests closer to reality.

Thesis available for public display 7 days prior to the defence at .