The A+ upgrade:Expanding the Advanced LIGO Horizon

Lead Research Organisation: University of Birmingham
Department Name: School of Physics and Astronomy

Abstract

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.

Planned Impact

The consortium involved in this capital proposal has a strong and extensive track record in working with industry, in public outreach and schoolteacher CPD, which will continue throughout and beyond the construction period. Beneficiaries will include the optics industry e.g. companies such as Gooch and Housego - enhancing capability in the area of manufacture of optical components, and such as Helia Photonics via development of low loss optical coatings. Beneficiaries will also include those working in the sectors of energy and security via the application of MEMS gravimeters. The consortium has transferred technical knowledge and will further do so to help company competitiveness and success, all feeding back into the UK economy. The UK economy will further benefit through the spinning off of new companies arising from the research or licensing out of the technology being developed.

We anticipate research developments, spinning off from the gravitational wave work to contribute to the grand challenge areas of health and wellbeing via developments of software algorithms which can help with removal of artifacts in scanning medical imaging devices and in the development of hardware which can lead to the differentiation of a variety of stem cells with major implications for medicine. More globally, as a spin-off from the gravitational waves work at Cardiff a Data Innovation Institute has been established to conduct fundamental research into the aspects of managing, analysing and interpreting massive volumes of textual and numerical information. This will benefit projects in a wide-ranging spectrum of disciplines including social, biological, life and engineering sciences, e.g. in the biological and life sciences by extracting information from data sets without compromising privacy and confidentiality, and interpreting large data sets into reliable and understandable mathematical models.

Public outreach involving television, radio, science festivals, masterclasses and public lectures feature strongly in our present and proposed programmes and the legacy of the effort we have devoted to celebrate the international year of light - such as the development of a laser harp - fit well with the wider public outreach work we undertake in collaboration with the LIGO Scientific Collaboration on the physics of neutron stars, black holes and the Universe as a whole. Working with the Scottish government and Education Scotland members of the consortium will build on previous work contributing strongly to the curriculum for physics in Scotland by extending provision of CPD for schoolteachers in Scotland, producing videos and other material helping them to tackle the challenges introduced by the more interdisciplinary nature of the new school qualifications, and this support is very transportable to be used throughout the UK. The wide range of impact provided by the scale of our programme is excellent for the training of early career researchers and graduate students and we aim to ensure that all our young scientists have experience in these areas, enabling them to have access to a wide range of career opportunities.

Publications

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