Frequency-Dependent Squeezed Vacuum Source for Broadband Quantum Noise Reduction in Advanced Gravitational-Wave Detectors

The astrophysical reach of current and future ground-based gravitational-wave detectors is mostly limited by quantum noise, induced by vacuum fluctuations entering the detector output port. The replacement of this ordinary vacuum field with a squeezed vacuum field has proven to be an effective strategy to mitigate such quantum noise and it is currently used in advanced detectors. However, current squeezing cannot improve the noise across the whole spectrum because of the Heisenberg uncertainty principle: when shot noise at high frequencies is reduced, radiation pressure at low frequencies is increased. A broadband quantum noise reduction is possible by using a more complex squeezing source, obtained by reflecting the squeezed vacuum off a Fabry-Perot cavity, known as filter cavity. Here we report the first demonstration of a frequency-dependent squeezed vacuum source able to reduce quantum noise of advanced gravitational-wave detectors in their whole observation bandwidth. The experiment uses a suspended 300-m-long filter cavity, similar to the one planned for KAGRA, Advanced Virgo, and Advanced LIGO, and capable of inducing a rotation of the squeezing ellipse below 100 Hz.

Figure 1

Schematic diagram of the experimental setup. The squeezed vacuum beam is produced by the OPO and injected into the filter cavity through the in-vacuum injection telescope. The reflected frequency-dependent squeezing is rejected by the in-vacuum Faraday and characterized by the homodyne detector. The green beam produced by the SHG is used to pump the OPO and as auxiliary beam for the control of the filter cavity.

Figure 2

Noise spectra of the frequency-dependent squeezing, measured for different homodyne angles. Each curve has been fitted, assuming the degradation parameters in Table 2, to extract homodyne angle and detuning frequency. The degradation parameters were used to infer the expected quantum noise reduction achievable by injecting such frequency-dependent squeezed state into a detector with the proper shot-noise–radiation-pressure crossover frequency. Each spectrum has a 0.5-Hz resolution and is averaged 100 times, resulting in an acquisition time of 200 s.

Figure 3

Estimated degradation budget for the frequency-dependent squeezing source. The black curve shows the expected improvement of the quantum noise for a gravitational-wave detector such as KAGRA. The squeezing degradation parameters used are those reported in Table 2. The dashed line shows the change in the quantum noise when injecting a frequency-independent squeezing state (with the same squeezing magnitude of the frequency-dependent case). As expected, quantum noise is reduced at high frequencies, while it highly increased at low frequencies.