Broadband quantum noise reduction in future long baseline gravitational-wave detectors via EPR entanglement

Broadband quantum noise reduction can be achieved in gravitational-wave detectors by injecting frequency-dependent squeezed light into the dark port of the interferometer. This frequency-dependent squeezing can be generated by combining squeezed light with external filter cavities. However, in future long baseline interferometers (LBIs), the filter cavity required to achieve the broadband squeezing has a low bandwidth-necessitating a very long cavity to mitigate the issue from optical loss. It has been shown recently that by taking advantage of Einstein-Podolsky-Rosen (EPR) entanglement in the squeezed light source, the interferometer can simultaneously act as a detector and a filter cavity. This is an attractive broadband squeezing scheme for LBIs because the length requirement for the filter cavity is naturally satisfied by the length of the interferometer arms. In this paper we present a systematic way of finding the working points for this broadband squeezing scheme in LBIs. We also show that in LBIs, the EPR scheme achieves nearly perfect ellipse rotation as compared to 4-km interferometers which have appreciable error around the intermediate frequency. Finally, we show that an approximation for the optomechanical coupling constant in the 4-km case can break down for longer baselines. These results are applicable to future detectors such as the 10-km Einstein Telescope and the 40-km Cosmic Explorer.