  Compared to the quantum noise in the measurement of the translational motion
of a suspended mirror using laser light, the quantum noise in the measurement
of the angular motion of a suspended mirror has not been investigated
intensively despite its potential importance. In this article, an expression
for the quantum noise in the angular motion measurement is explicitly derived.
The expression indicates that one quadrature of the vacuum field of the
first-order Hermite-Gaussian mode of light causes quantum sensing noise and the
other causes quantum backaction noise, or in other words the first-order vacuum
field is ponderomotively squeezed. It is also shown that the Gouy phase shift
the light acquires between the mirror and the position of detection of the
light corresponds to the homodyne angle. Therefore, the quantum backaction
noise can be cancelled and the standard quantum limit can be surpassed by
choosing the appropriate position of detection analogously to the cancellation
of quantum radiation pressure noise by choosing an appropriate homodyne angle.
