Towards optimal, high-efficiency quantum enhancement in the LIGO gravitational wave detectors

The LIGO collaboration has made profound scientific observations over the last 10 years, including our newest astrophysical catalogue update (GWTC-4), which more than doubles our gravitational-wave event count in about 1/3 of the observing time. Key hardware improvements such as the upgrade to frequency-dependent squeezing for broadband quantum noise reduction with quantum-limited sensitivities exceeding the Standard Quantum Limit have made LIGO’s 4th observing run (O4) the most successful to-date. Future observing runs of LIGO and future GW detectors target 10-dB of broadband measured, which would require improved mode-matching and lower losses throughout the many optical cavities that comprise the LIGO detectors. This talk will detail the design, fabrication, and installation of the new output mode cleaner cavities for the LIGO A+ upgrade, and summarize the upcoming upgrade to balanced homodyne detection. We will highlight the collective impact of these changes on expanding the quantum-limited astrophysical reach of the LIGO detectors.