Ten years after the plan of the Large-scale Cryogenic Gravitational wave Telescope (LCGT) project, its construction started in June, 2010. The scientific objective of the project is to detect gravitational waves that are produced by astrophysical events in the Universe. Since the events are so rare that the detection capability of the detector should be enhanced to the extent where its sensitivity is limited not by technical reasons but by ultimate physics of instrument such as thermal noise, quantum noise, and so on. LCGT adopts cryogenic mirror to reduce thermal noise, which is meaningful only after ultimate sensitivity is technically attained. Cryogenic mirror is still adventurous technique after ten years of development. I present here the overview of LCGT project.
The thermal noise of mirrors in laser interferometric gravitational wave (GW) detectors is inevitable, fundamental noise and is crucial to whether a sufficiently high sensitivity to detect GWs is reached. The cryogenic mirror technique is one method to reduce the thermal noise. Japanese GW collaboration started in a study of the cryogenic mirror technique in 1997 and continues now through the Cryogenic Laser Interferometer Observatory (CLIO) project, which started in 2002. The CLIO interferometer was completed in an underground site of the Kamioka mine in 2006 as the first cryogenic