  We characterized a double torsion pendulum system, including measurements of
the photon-spin-induced torque. Our experimental strategy was similar to that
used in Beth's experiment, which was performed in 1936 to measure
photon-spin-induced torque using forced oscillation caused by polarization
modulation of light incident on a suspended object. Through simple passive
isolation of the suspended object from external vibration noise, the achieved
torque sensitivity was 2 x 10^{-17} N m in a measurement time of 10^4 s, which
is close to the thermal noise limit and one order smaller than the minimum
torque measured in Beth's experiment. The observed spin-induced torque exerted
on the light-absorbing optics is consistent with the angular momentum transfer
of hbar per photon.
