This thesis reports the construction of a hybrid setup consisting of an optomechanicalsystem in and an atomic spin ensemble capable of quantum back-action evadingmeasurements of mechanical motion by use of an itinerant optical field.The optomechanical system consists of a highly stressed, 500 μm × 500 μm × 60nm, SiN membrane placed in the middle of an unresolved, high finesse, one-sidedoptical cavity. This cavity is placed in a cryostat operated at 4.4K. The motionof the dielectric membrane modulates the cavity resonance frequency, and thus thelight field populating the cavity. This couples the light degrees of freedom withthe motion of the mechanics allowing a sensitive read out of mechanical motionby interrogation of the light quadratures output from the cavity. The limit of thesensitivity with which this motion can be read out, without disturbing the system, isset by the quantum back-action introduced by the probing light onto the mechanicalmotion.The significant influence of the quantum back-action on the motion of the mechanicsis initially demonstrated by a (−3.18 ± 0.18) dB (equivalent to (53 ± 2)%below shot noise) observed ponderomotive squeezing. Correcting for detection efficiencyand additional classical laser noise gives an ideal squeezing of −8.6 dB (equivalentto 86% below shot noise). The squeezed light noise occurs as a result of a projectionof the optically transduced mechanical motion onto the optical quadrature,whose quantum correlations were the drive for the motion in the first place.Using an atomic spin ensemble pumped to its most energetic state, we realize anoscillator with an effective negative mass. Using the motion of this oscillator as areference allows for the evasion of back-action on the mechanical oscillator. The spinensemble consists of a vapour cell with 109 spin polarized cesium atoms confined toa micro-channel of length 300 μm × 300 μm × 10mm coated with a spin preservinglayer. The cell is placed in a magnetically controlled environment allowing for precisecontrol of the oscillator frequency and spin direction. The spins are coupled to anoptical probe via the Faraday effect and read out in transmission before being filteredand directed towards the optomechanical setup.Measuring the output optical phase quadrature from this cascaded hybrid systemallows for measurements of mechanical displacement with enhanced sensitivity inthe relevant regime where the mechanical displacement sensitivity is overwhelmedby the added noise caused by the quantum back-action. This quantum back-actionis demonstrated to be evaded by −1.8 dB (equivalent to a (−34 ± 5)% reduction)and is well understood by a detailed theoretical hybrid model. The model providesfurther insight into how to significantly boost back-action evasion in such a hybridsystem.Demonstrating an optically linked hybrid system consisting of two quantum enabledsubsystems, capable of significant back-action evasion, lays the foundation forgenerating Einstein-Podolsky-Rosen entanglement between the two