Improving cooling performance in an optomechanical system using a non-linear cavity

The possibility to operate massive mechanical resonators in the quantum regime has become central in fundamental sciences. Optomechanics, where photons are coupled to mechanical motion, provides the tools to control mechanical motion near the fundamental quantum limits. However even in cryogenic systems, massive (low frequency) mechanical resonators are in highly excited thermal states. Cooling them to, or close to, the motional ground state usually requires that the system is in the good cavity limit, i.e. the mechanical frequency greater than the photonic loss rate. Despite being in the bad cavity limit, we show a possible way to overcome this limitation by using a non-linear cavity. More specifically, we couple a magnetic cantilever to a microwave cavity, where an embedded SQUID makes the cavity magnetic field sensitive and also non-linear. We show, that the non-linearity has to be included in describing the back action and demonstrate a one order of magnitude improvement in the cooling. With our system it seems to be possible to overcome the back action limit, which limits the cooling performance in linear cavities. It could even be a way reach the ground state with a system in the bad cavity limit.