PHOTONICS INSIGHTS
quantum correlations
Figure 4. Observation of frequency-dependent squeezing in the LIGO Livingston detector. Noise spectra without squeezing( black) are compared to measurements with frequency-independent( green) and frequency-dependent( purple) squeezing. Frequency-independent squeezing reduces shot noise by 5.8 dB near 1 kHz but increases radiation pressure noise at low frequencies. While, frequency-dependent squeezing yields a 1 – 2 dB broadband noise reduction from ~ 60 Hz to 1 kHz, enhancing sensitivity across the whole band. Figure from [ 8 ].
disturbances are far less significant. The world is much quieter in the MHz region, with respect to the forest of disturbances in the Virgo-LIGO operation bandwidth. It took, then, decades to learn how to produce a high level of squeezing in the frequency band relevant for gravitational-wave detectors. Finally in April 2019, after months of prototyping and commissioning, squeezed vacuum is injected into Virgo and LIGO. Quantum noise drops by 3 dB: the same effect as doubling laser power. This increases detection rate by up to 50 % for the LIGO – Virgo network. It is a great experimental success [ 4,5 ]. However, a closer look at the sensitivity below 50 Hz reveals a slight increase in noise when squeezing is injected. The reason is subtle: the reduced-noise quadrature( the minor axis of the squeezing ellipse), which is aligned with the phase quadrature to match the gravitational-wave signal, undergoes a 90-degree rotation at low frequencies due to the opto-mechanical interaction with the suspended test masses. This means that, at low frequencies, the fluctuations( i. e., the noise) are actually amplified compared to the case where standard vacuum is injected. In practice, the effect is similar to that of a power increase, though without the drawbacks of mirror heating and other issues discussed above. Although the sensitivity degradation induced at low frequency because of this effect was barely observable during Observation Run 3 due mainly to the presence of technical noises, it is expected to become more and more detrimental as the detectors approach their design sensitivity. Fortunately, a solution was found to enable broadband quantum noise
reduction: the injection of frequency-dependent squeezed( FDS) vacuum, in which the squeezing ellipse rotates as a function of frequency to compensate for the rotation induced by the interferometer( see Fig. 3, right panel). Such rotation of the squeezing ellipse can be obtained reflecting the frequency independent squeezed states by an optical cavity( known as filter cavity) operated slightly out of its resonant condition. In practice this rotation is produced by the cavity’ s asymmetric reflection of the upper and lower sidebands of the vacuum field. A detailed explanation and quantitative discussion can be found in [ 6 ]. Squeezing angle rotation in the region of interest for gravitational wave detectors has been demonstrated with a full-scale filter cavity prototype in 2020 [ 7 ]. For the next observation run( O4), started in May 2023, these frequency-dependent squeezed states have been produced combining the frequency independent squeezed state previously used in Virgo and LIGO with 300 m long suspended filter cavities. LIGO is currently injecting frequency dependent squeezing, obtaining an impressive broadband quantum noise reduction( See Fig. 4) reaching up to 5.8 dB at high frequency and producing an increase in the detection rate of 65 %. [ 8 ]. As for Virgo, despite the successful construction of the frequency dependent squeezing source [ 9 ], it is not yet
Figure 5. View inside the Virgo tunnel showing the filter cavity vacuum pipe running alongside the north arm of the interferometer.
58 www. photoniques. com I Photoniques 134