I will describe our studies on hybrid quantum systems that demonstrate the interplay between an ultracold spin ensemble and a mesoscopic optomechanical system. Optomechanical systems, i.e. devices that exhibit a parametric coupling between light and a mesoscopic mechanical resonator, have emerged as a promising arena not only for ultrasensitive sensor technologies but also for fundamental studies of precision measurement, macroscopic quantum effects and decoherence.

However, in contrast to ultracold atomic gases, quantum state preparation of such mesoscopic mechanical systems has proved challenging. I will describe our experimental realization of a hybrid quantum system in which an ultracold atomic gas is optically interfaced to an optomechanical system. Through this interface, the atomic gas mediates a 'spin-photon-phonon' interaction that allows for new forms of nonlinear optomechanical interactions and sympathetic cooling of a mechanical resonator using the ultracold gas. 

I will show that the use of such hybrid strategies circumvents many of the limitations prevalent in conventional optomechanical systems and offers a promising route to quantum sensors that surpass the standard quantum limit. Lastly, I will describe recent results on novel phase transitions and emergent critical behavior that arise in these systems due to the interplay between coherent dynamics and dissipation.