The A+ Upgrade to Advanced LIGO

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Over the past two years, the NSF-supported LIGO Laboratory has opened the gravitational-wave window on the universe and established the entirely new field of gravitational-wave-enabled multi-messenger astrophysics. These revolutionary achievements were enabled by sub-attometer position measurement technologies made possible over multi-kilometer baselines through the Advanced LIGO detector. This new initiative, the "A+ upgrade", can offer five times the gravitational wave event detection rate over the existing Advanced LIGO design. The enhanced capabilities afforded by A+ will be able to, among other science goals, illuminate the origins and evolution of stellar-mass black holes, allow precision tests of extreme gravity, enable detailed study of the equation of state of neutron stars, and permit new tests of cosmology, including fully independent constraints on the Hubble constant. The A+ upgrade will also foster expanding community participation and enhance opportunities for education and public understanding of science. Both aspects of LIGO technology targeted by A+ are expected to be of keen interest to diverse communities, including quantum optics, quantum information theory, materials science, optical technology, precision metrology and physical standards.The A+ upgrade addresses two technical limitations that have constrained the Advanced LIGO design target sensitivity: quantum mechanical uncertainty and classical Brownian noise in mirror coatings. The improved A+ instruments should reach observational readiness as soon as mid-2024, enabling LIGO to continue its leadership of the worldwide effort to open and exploit this new mode of observation. The A+ plan builds on the expertise and synergy of the joint international team that successfully delivered Advanced LIGO. Approximately 35% of A+ hardware will be contributed by UK and Australian LIGO Scientific Collaboration (LSC) groups, along with significant commitments of effort by UK and Australian personnel. These investments echo the crucial and highly successful UK Science and Technology Facilities Council (STFC) and Australian Research Council (ARC) contributions made to Advanced LIGO.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.