Quantum Noise reduction via frequency dependent squeezing for next generation gravitational-wave detectors – Quantum-FRESCO

Gravitational-wave (GW) astronomy, which began in 2015, has already achieved a number of very important results in various fields, such as general relativity, astrophysics and cosmology. The "second generation" (2G) detector network, which includes LIGO, Virgo and KAGRA, has a rich scientific program for the next decade, including the investigation of fundamental open questions such as the nature of gravitation and dark energy, the properties of nuclear matter, and the formation of black-holes. Meanwhile, "third generation" gravitational-wave detectors are being studied, including the European Einstein Telescope (ET), with the goal of increasing sensitivity tenfold over LIGO and Virgo. One of the major limitations to the sensitivity of current and future detectors is the quantum noise, imposed by the vacuum fluctuations of the electromagnetic field entering the detector from its output port. A key technology for reducing quantum noise is the replacement of the "ordinary" vacuum with the so-called "squeezed vacuum." In a squeezed vacuum state, the amplitude or phase fluctuations (the same in the "ordinary" vacuum) are reduced below the level of an ordinary vacuum state. Squeezed states are usually represented as an ellipse in the phase-amplitude plane, while the ordinary vacuum is a circle. If the GW signal is aligned in quadrature with reduced uncertainty, the signal-to-noise ratio improves. Squeezed states have already been used in Virgo and LIGO, providing a significant increase in sensitivity in part of the detector band (above ~100 Hz). In the near future, a more sophisticated "frequency-dependent squeezing", achieved by reflecting states through a 300-m slightly out-of-resonance optical cavity, will be used to mitigate quantum noise in the entire detector band of Virgo and LIGO. This so-called “quantum filter cavity” allows the squeezing ellipse to rotate as a function of the Fourier frequency, to compensate for an optomechanical-driven rotation of the ellipse inside the interferometer. In this way the gravitational-wave signal is always aligned with the quadrature with less quantum fluctuations. For current Virgo and LIGO, a single optical cavity is sufficient to optimize frequency-dependent squeezing, but future detectors, such as ET and perhaps LIGO Virgo and KAGRA upgrades, will use an optical configuration called, "detuned signal recycling." In fact, the signal recycling technique, currently used in 2G detectors, in which a mirror is placed between the detector output and the photodiodes, allows to modify the detector's frequency response and optimize the signal-to-noise ratio for some GW sources. For "flat" responses of the signal recycling, just one filter cavity is enough to optimize injected squeezing. For detuned signal recycling, the interferometer will rotate the squeezing ellipse in a non-trivial way, and 2 cavities in series or a 3-mirror cavity (a coupled cavity) will be required to optimize the ellipse rotation and then the squeezing injection. This project aims to study frequency-dependent squeezing production with non-trivial rotation of the squeezing ellipse. We will first study the two possible configurations in simulation and then experimentally test this more complex frequency-dependent squeezing source, using the most promising configuration. Our objective will be the experimental demonstration, never obtained so far, of a frequency-dependent squeezing source with non-trivial rotation of the squeezing ellipse, optimized for a detuned interferometer, with a great impact for future GW detectors and their science.