Ground-state cooling of a nanomechanical oscillator with N spins.

Typical of modern quantum technologies employing nanomechanical oscillators is to demand few mechanical quantum excitations, for instance, to prolong coherence times of a particular task or, to engineer a specific non-classical state. For this reason, we devoted the present work to exhibit how to bring an initial thermalized nanomechanical oscillator near to its ground state. Particularly, we focus on extending the novel results of D. D. B. Rao \textit{et al.}, Phys. Rev. Lett. \textbf{117}, 077203 (2016), where a mechanical object can be heated up, squeezed, or cooled down near to its ground state through conditioned single-spin measurements. In our work, we study a similar iterative spin-mechanical system when $N$ spins interact with the mechanical oscillator. Here, we have also found that the postselection procedure acts as a discarding process, i.e., we steer the mechanics to the ground state by dynamically filtering its vibrational modes. We show that when considering symmetric collective spin postselection, the inclusion of $N$ spins into the quantum dynamics results highly beneficial. In particular, decreasing the total number of iterations to achieve the ground-state, with a success rate of probability comparable with the one obtained from the single-spin case.