Next-level squeezed light for Advanced Virgo
AEI researchers help set up and characterize the frequency-dependent squeezed-light source at the gravitational-wave detector near Pisa
Current gravitational-wave detectors are limited by quantum noise over a wide part of their observing frequency range. This noise is caused by quantum vacuum fluctuations entering the detector from the detector output. It appears in two different forms: shot noise at frequencies above approximately 100 Hz, and radiation pressure noise at frequencies below that. In 2010, the first implementation of injecting squeezed vacuum states into the GEO600 gravitational-wave detector showed how this method can reduce the high-frequency quantum mechanical noise. In 2018, researchers from the Max Planck Institute for Gravitational Physics in Hannover and Leibniz University Hannover developed and installed a squeezed-light source for the Advanced Virgo detector. Now, together with Virgo researchers the team has helped to push squeezing to the next level: By installing a 285 meter long optical resonator, the entire team implemented a frequency-dependent upgrade of the squeezed-light source that will be used to suppress radiation pressure noise and shot noise simultaneously. This frequency-dependent squeezed-light source at the Advanced Virgo detector was now characterised. The device fulfills the requirements for the observation run O4. A reduction of high / low frequency quantum noise by 4.5 dB / 2 dB, respectively, is expected after integration into the interferometer, which will increase the sensitivity to gravitational-wave signals.
In this paper we present the design and performance of the frequency dependent squeezed vacuum source which will be used for the broadband quantum noise reduction of the gravitational-wave detector Advanced Virgo Plus in the upcoming O4 observation run. The frequency dependent squeezed field is generated by a phase rotation of a frequency independent squeezed state through a 285 m long, high-finesse, near-detuned optical resonator. With about 8.5 dB of generated squeezing, up to 5.6 dB of quantum noise suppression has been measured at high frequency while close to the filter cavity resonance frequency, the intra-cavity losses limit this value to about 2 dB. Frequency dependent squeezing is produced with a rotation frequency stability of about 6 Hz RMS, which is maintained on the long term. The achieved results fulfill the frequency dependent squeezed source requirements for the upcoming observation run (O4) for Advanced Virgo Plus. With the current squeezing source, considering also the estimated squeezing degradation induced by the interferometer, we expect a reduction of the quantum shot noise and radiation pressure noise up to 4.5 dB and 2 dB, respectively.