The quantum state of light changes its nature when being reflected off a mechanical oscillator due to the latters susceptibility to radiation pressure. As a result, a coherent state can transform into a squeezed state and can get entangled with the m
otion of the oscillator. The complete tomographic reconstruction of the state of light requires the ability to readout arbitrary quadratures. Here we demonstrate such a readout by applying a balanced homodyne detector to an interferometric position measurement of a thermally excited high-Q silicon nitride membrane in a Michelson-Sagnac interferometer. A readout noise of $unit{1.9 cdot 10^{-16}}{metre/sqrt{hertz}}$ around the membranes fundamental oscillation mode at $unit{133}{kilohertz}$ has been achieved, going below the peak value of the standard quantum limit by a factor of 8.2 (9 dB). The readout noise was entirely dominated by shot noise in a rather broad frequency range around the mechanical resonance.
