Gravitational-wave detections have opened up a new window to the universe. We have detected signals from many compact-binary events, including the famous binary neutron star event GW170817, the first joint detection with electromagnetic and gravitational waves. GW170817 has revolutionized our understanding of neutron star physics and the origin of heavy elements. However, due to limitations from the detector sensitivity, we have not observed the gravitational signals from the most energetic part of this event---the catastrophic merger of the two neutron stars. The 4km Advanced LIGO and 3km Virgo are most sensitive around a few hundred Hz, while the merger signals happen at several kHz. In this talk, I will introduce how to improve the detector sensitivity at kHz by using quantum measurement techniques and show the design of a next-generation 20km detector targeting the merger signals.