Three 2nd generation gravitational-wave detectors (the two LIGO and Virgo) are about to come online again, in an upgraded version, for a new data-taking period. The increased sensitivity of the network is expected to result in a higher rate of detections with louder signals so as to enter a mature phase of gravitational wave astronomy.
Meanwhile a fourth detector, KAGRA, is being completed in Japan, and it is expected to join the network within the end of 2019. KAGRA is a 2.5 generation GW detector, more advanced with respect to the others, as it includes some new technologies that have never been applied in large scale detectors.
First of all, it is an underground detector, built inside the Kamioka mine. This feature allows to reduce the seismic excitation, which induces a motion of the suspend test masses, disturbing the detector operation and limiting its sensitivity at low frequency. Newtonian noise, originated by the changes in the terrestrial gravity field, will also benefit from this configuration.
Another crucial improvement is represented by the cryogenic operation. The test masses are cooled down to 20 K in order to reduce the thermal excitation of their atoms which is currently the limiting noise in the mid-frequency region.
Finally, the implementation of squeezing techniques to mitigate the quantum noise in the whole bandwidth is being prepared.
The goal of this lecture is first to present the expected scientific payoffs, in term of the cosmology and astrophysics, due to the presence of this additional detector and then to review and discuss the new technologies applied in KAGRA, as they play a pioneering role with respect to the future 3rd generation detectors.