Quantum mechanics defines the minimum uncertainty in a continuous position detection of a harmonic oscillator. In a cavity detector of mechanical motion, this fundamental uncertainty originates from the back-action force generated by the quantum fluctuations of light. However, it has been also expected that other observables, a quadrature for instance, can be measured with arbitrary precision by manipulating the quantum back-action. I describe three recent experimental results regarding the quantum back-action in a microwave cavity detection of micro-mechanical motion: existence of the quantum back-action force from microwave photons, reduction of the quantum back-action noise by 8.5dB in a single quadrature measurement, and the quadrature imprecision 2.4dB below the mechanical zero-point fluctuation level. In addition to advancing precision force measurements, our results could provide a route toward quantum squeezing of a mechanical oscillator.