PhD Defense by William H. P. Nielsen

Quantum Cavity Optomechanics with Phononic Bandgap Shielded Silicon Nitride Membranes 

Almost a hundred years after the advent of Quantum Mechanics, the fundamental problem of identifying and explaining the mechanisms by which macroscopic objects behave classically, i.e. distinctly non-quantum mechanically, remains perplexing and, in some fundamental respects, unresolved. A possible route to investigate the problem experimentally is offered by the preparation and detection of quantum behaviour in ever-larger objects. To this end, the field of cavity optomechanics, in which an electromagnetic field interacts with the mechanical motion of solid objects (often even visible to the naked eye), has shown a great potential in recent years, with several such systems reaching the quantum regime. 

In this work, we present the results of our own endeavours in the direction of establishing a "quantum enabled" cavity optomechanical system. More specifically, our system consists of a highly stressed silicon nitride membrane, acting as a square-drum mechanical resonator, coupled via the radiation pressure force to the light field of a high-finesse optical cavity. 

One of the main hindrances to observing quantum effects is decoherence from the environment, a particularly outspoken challenge for the mechanical part of the system. We overcome this obstacle by embedding the membrane in a phononic crystal structure providing a significant suppression of background modes and allowing for a very compact cavity design. In this setting, we manage to bring the optomechanical system well into the quantum regime. As evidence hereof, we observe strong continuous squeezing of the quantum noise of the light at a level hitherto unprecedented. As a secondary result, a mechanical mode of the membrane is cooled very close to its quantum ground state.