Coherent feedback cooling of a nanomechanical membrane with atomic spins

This thesis focuses on the engineering of light-mediated interaction between distinct quantum system. Specifically, we present experiments on the light-mediated interaction between a nanomechanical membrane oscillator and a collective atomic spin. The mechanical oscillator, a silicon nitride membrane mounted in a single-sided cavity, couples to the light via radiation pressure in a room temperature environment. The spin oscillator, consisting of an optically pumped ensemble of cold Rubidium atoms held in a dipole trap, couples to light via the off-resonant Faraday interaction. By engineering a light mediated interaction between the mechanical oscillator and the atomic spin in a loop geometry, we experimentally demonstrate strong bidirectional Hamiltonian coupling between membrane and spin. We observe normal-mode splitting and coherent energy exchange oscillations as signatures of strong coupling. Combining this strong coherent coupling with the versatile quantum control on the atomic spin we demonstrate for the first time, coherent feedback cooling of a mechanical oscillator using the atomic spins as a coherent controller. We explore different coupling regimes, i.e. from incoherent overdamped cooling to strong stroboscopic coherent feedback. Spin-membrane state swaps along with stroboscopic spin pumping allows us to cool our mechanical oscillator from room-temperature to T = 216 mK in 200 μs. Moreover, we study the effect of delays on the cooling performance. In a further experiment, we exploit the strong correlation between the atomic spin and light to achieve ponderomotive squeezing of light, which is a hallmark of reaching the backaction dominated regime. The squeezing of light allows one to perform measurements with a precision beyond the standard quantum limit, which has many strong implications in quantum metrology.