  Quantum backaction disturbs the measurement of the position of a mechanical
oscillator by introducing additional fluctuations. In a quantum backaction
measurement technique, the backaction can be evaded, although at the cost of
losing part of the information. In this work, we carry out such a quantum
backaction measurement using a large 0.5 mm diameter silicon nitride membrane
oscillator with 707 kHz frequency, via a microwave cavity readout. The
measurement shows that quantum backaction noise can be evaded in the quadrature
measurement of the motion of a large object.
