The thermal fluctuation of mirror surfaces is the fundamental limitation for interferometric gravitational wave (GW) detectors. Here, we experimentally demonstrate for the first time a reduction in a mirrors thermal fluctuation in a GW detector with
sapphire mirrors from the Cryogenic Laser Interferometer Observatory at 17,K and 18,K. The detector sensitivity, which was limited by the mirrors thermal fluctuation at room temperature, was improved in the frequency range of 90,Hz to 240,Hz by cooling the mirrors. The improved sensitivity reached a maximum of $2.2 times 10^{-19},textrm{m}/sqrt{textrm{Hz}}$ at 165,Hz.
