Internal squeezing in coupled cavity systems

Gravitational waves from merging neutron stars which are typically in the kHz regime carry information about nuclear matter at extreme conditions. Kilo-Hertz gravitational waves are a direct way of probing the remnants of these mergers and these are interesting objects because they are the only place in the Universe that provide us with access to these beautiful astrophysical laboratories as they look at the extremely high density regime which cannot be produced in terrestrial particle colliders [1].

By combining the techniques of using optically coupled long baseline cavities and internal squeezing, the sensitivity of a gravitational wave detector can be increased in the kHz regime without increasing the total circulating power in the interferometer [2]. Other applications of such a coupled cavity system are in experiments involving optomechanics like optomechanically induced transparency, negative mass systems and precision metrology experiments.

A simplified layout of a gravitational wave detector with internal squeezing and a long signal recycling cavity is shown in figure a. The power recycling mirror (PRM), signal recycling mirror (SRM), input test masses (IX and IY) together with the beam splitter (BS) and the end test masses (EX and EY) are shown in a. The entire interferometer however can be reduced to a three mirror coupled cavity system where the origin of the GW signal and quantum noise is shown in figure b.

Comparison of the quantum noise limited sensitivity of aLIGO-HF with internal and external squeezing (solid red) to third generation GWDs such as Einstein telescope (magenta) and cosmic explorer (blue). Also shown is the sensitivity of aLIGO-HF with only internal squeezing (dashed red line) which shows the enhancement of the coupled cavity pole at high frequencies [2].

 

[1] K. Ackley, V. B. Adya, P. Agrawal et.al., Neutron Star Extreme Matter Observatory: A kilohertz-band gravitational-wave detector in the global network, arXiv:2007.03128.

[2] V B Adya, M J Yap, D Töyrä, et.al., Quantum enhanced kHz gravitational wave detector with internal squeezing, Classical and Quantum Gravity, Volume 37, Number 7, 2020.