Improving cooling performance in an optomechanical system using a nonlinear cavity – Part 1*

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 objects near the fundamental quantum limits. However even in cryogenic systems, massive (low frequency) mechanical resonators are in highly excited thermal states, cooling is thus required to observe quantum effects. Here we describe an experiment, where we couple a magnetic cantilever to a microwave cavity, where an embedded SQUID makes the cavity sensitive to magnetic fields and also nonlinear. The tunable flux sensitivity of the SQUID provides in-situ control of the single-photon single-phonon coupling strength, where we demonstrate coupling strengths of up to 10 kHz. Higher coupling strengths towards 100 kHz are possible, however in the current setup flux noise prevents us from harnessing these high couplings.

Remarkably, the nonlinearity of the cavity crucially influences the backaction, and opens doors towards ground state cooling in the unresolved sideband regime.