Ultimate Q of optical microsphere resonators

We demonstrate the quality factor Q equals (0.8 plus or minus 0.1) multiplied by 1010 of whispering-gallery modes in fused silica microspheres at 633 nm, close to the limit determined by fundamental material attenuation. The lifetime of ultimate Q is limited by adsorption of atmospheric water. Optical effects of adsorption are investigated and conditions for fabrication of long-lifetime microspheres are clarified.