Speaker:   Eleonora Polini, Massachusetts Institute of Technology

When:        Friday, January 13, 2023,
                   starting at 10:30

Where:      Christian Doppler lecture hall (3rd floor, Boltzmanngasse 5)

Hosted by: Philip Walther

Abstract:

Gravitational astronomy began following the first direct detection of gravitational waves by the LIGO detectors in the US. Afterwards, the European Virgo detector joined the observation network, which enabled a much more precise localization of the gravitational wave sources. The detection of a coalescence of neutron stars initiated the so-called multi-messenger astronomy. In total up to the end of the last observing run O3, 90 coalescence events were revealed, reaching a rate of one event per week. The detectors were further improved in view of the next O4 run, to allow an even larger portion of the Universe to be seen. Three of the major upgrades made in Virgo are addressed in this presentation. The first is the implementation of an innovative quantum technique, known as frequency-dependent squeezing, to reduce the quantum noise over the entire detection band,. I contributed to the successful implementation of this technique up to the first measurement of squeezed states with rotation at a frequency of 25 Hz, which is what is needed to optimize the noise reduction in Virgo. The second contribution concerns the study and mitigation of scattered light from the detector's suspended benches. The scattered light that re-couples with the interferometer's main beam worsens its low-frequency sensitivity and risks hiding the benefit introduced by frequency-dependent squeezing. Projections of the scattered light noise arising from the suspended benches has been made after measuring the scattering property of some of the optics with a scatterometer developed at LAPP. Spurious beams that can create extra stray light have also been mitigated. The third contribution concerns the replacement of the two output mode cleaner cavities with a new high finesse low loss cavity. The main motivation was to reduce optical losses that degrade squeezing performance. The new cavity has successfully been characterized, installed and commissioned. The upgrades described in this presentation for O4 will allow for greater sensitivity and thus reveal more distant events, allowing us to uncover some of the secrets that lurk in our universe.