Feedback cooling of a mechanical resonator from room temperature close to its ground state

In the last two decades, cavity optomechanics has gained a central spot in the research on quantum physics, thanks to its premise to integrate macroscopic oscillators of different nature – optical and mechanical – into a single platform. Here lies the challenge though, as mechanical resonators are typically not in their ground state at room temperature, unlike light. In the sideband unresolved regime, where the optical damping rate is much larger than the mechanical angular frequency, feedback cooling has proven to be a valid approach to overcome this problem. Its working principle consists of a continuous measurement of the mechanical displacement and application of a proportional damping force. Unfortunately, so far its success has relied on operation in a cryogenic environment, which represents a major obstacle when scaling up experiments. In this work, we set to explore the possibility to use feedback cooling to steer a millimeter-sized mechanical resonator into a state of phonon occupancy as low as possible starting from room temperature. Although the minimum occupancy we achieve is 1800, far from the onset of quantum behavior, we pave the way for future success by offering a discussion of which parameters need further optimization in order to obtain a truly macroscopic quantum state. Our experimental scheme operates at 1550 nm, ensuring the viability of integration with other computation and telecommunication protocols. We use phase-sensitive detection of light reflected off high-finesse optical cavities to monitor the mechanical motion and pay great attention to minimizing optical losses, thus making our platform suitable for interface with sources of non classical light such as squeezers. Finally, the low frequency and high quality factor of our mechanical resonators produce long coherence times, particularly appealing for implementing quantum protocols such as state transfer.