Cryogenic Mirrors.The State-of-the-art in Interferometric Gravitational Wave Detectors

The "cryogenic mirror" is one of the most important key issues for the Large-scale Cryogenic Gravitational wave Telescope (LCGT), which is the Japanese interferometric gravitational wave detector project. In this article, three main advantages of cryogenic mirrors are introduced: reduction in thermal noise, decrease of the thermal lens effect, and suppression of the parametric instability. A future project planned in Europe, the Einstein Telescope (ET), which includes the use of cryogenic interferometers, is also introduced.