Gravitational wave detectors are about to get a major sensitivity upgrade that will let them catch four to seven times more cosmic collisions each year. The Advanced LIGO detectors have already spotted dozens of black hole mergers and one neutron star collision, but their core technology is a decade old. This project, called A+, will install new mirror coatings, larger beam-splitters, improved suspension fibres, and better readout systems to slash the thermal and quantum noise that currently limits sensitivity. The UK team previously built the ultra-low-noise silica suspensions that proved critical for detecting the first signals. If A+ succeeds, the detectors will see fainter, more distant mergers and accumulate more high-quality events. That will let astronomers trace how black hole populations form and use neutron star signals as standard sirens for cosmology. This is fundamental science—there is no immediate practical application. But the same pattern held for the first gravitational wave detection itself, which grew from decades of curiosity-driven engineering and now opens a new window on the universe.
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The discovery of gravitational waves by the Advanced LIGO (aLIGO) detectors in 2015 and the observation of mergers of several pairs of Black Holes (BH) and a pair of Neutron Stars (NS) by the Advanced LIGO and, latterly, Advanced Virgo detectors have revolutionised astronomy. The BH merger signals reveal a previously unknown population of BH's in the 10s of solar mass range. The NS merger and the consequential events including a kilonova were observed throughout the EM spectrum, leading to one of the highest-impact observations in astronomy. Our project - Advanced LIGO plus (A+) - arises in a direct path from the completely successful UK contribution to aLIGO. That project was PPARC/STFC funded from 2002-2011 with operations support from 2012 to 2020. For aLIGO, we designed and delivered a range of equipment, in particular the ultra-low noise fused silica suspension systems that support the main interferometer mirrors. The technology in these suspensions allowed a substantial decrease in the noise in the low-frequency band of the detectors (some 10s of Hz). Of the signals seen thus far, a considerable proportion of the signal to noise ratio (SNR) was accumulated in that band, indicating the primary importance of the UK suspensions in these observations. The technology for the core aLIGO detectors was frozen about a decade ago. Meanwhile, R&D has continued in the applicant groups. The results of this research provide further refinements in key areas of technology including: new materials and improved techniques for mirror coatings to reduce the background "thermal noise" that limits the mid-band in the detector (around 100 Hz); fused silica suspension fibres of enhanced design and strength that allow further reductions in suspension noise; and newly developed interferometric readout systems that optimise the use of non-classical light (squeezed vacuum) to reduce quantum noise. Quantum noise dominates over all other noise sources in the high frequency range above 100 Hz, and - given the low thermal noise associated with the suspensions - has become important also below 100 Hz. The gain offered by squeezing can be optimised by reducing diffraction and clipping loss by increasing the clear aperture of the main beam-splitter. By fully exploiting these enhancements resulting from our R&D, in combination with provision of squeezed light and also "filter cavities" that are required to maximally exploit squeezed light, it becomes possible to almost double the sensitivity of Advanced LIGO. More precisely, we expect to obtain event rates 4 to 7 times higher, depending on the particular type of source (i.e. over the mass range of observable compact binary mergers). This will bring a corresponding increase in high-SNR events that are of particular importance in tracing the origins of the BH population and undertaking cosmology with NS merger signals. The UK contribution to A+ is fully integrated within the US project. We describe our project in terms of seven work packages (WP1-WP7) introduced here: WP1 core optics: main mirrors: to provide replacement interferometer mirrors with upgraded coatings for both detectors; WP2 core optics: beam-splitters: to provide large diameter (450mm) beam-splitters, to reduce diffraction/clipping loss and better permit non-classical detection schemes; WP3 new suspensions: to provide a new suspension to support the WP2 beam-splitters; WP4 enhanced sensing and controls: to upgrade suspension controls for the beam-splitter and other key interferometer systems; WP5: balanced homodyne readout: to provide a novel balanced-homodyne readout scheme compatible with non-classical detection; WP6 suspension enhancement: to upgrade the facility for production of fused silica suspension fibres at the LIGO Hanford Observatory and WP7 project coordination: to support project management and coordination.
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