Quantum Enhancement of LIGO beyond the Standard Quantum Limit

LIGO, a gravitational-wave detector, continuously measures the relative positions of four suspended 40-kg mirrors to observe spacetime modulations from cataclysmic astrophysical events. However, an optical measurement of position introduces momentum backaction which increases the uncertainty of subsequent position measurements. This momentum backaction associated with the position measurement gives rise to a limit the ultimate sensitivity with which individual photons can measure the position of a freely moving object, known as the standard quantum limit (SQL). The recently upgraded LIGO A+ detector injects backaction-evading squeezing light to leverage the quantum correlations between photons and massive 40 kilogram mirrors that breaks the SQL. Here, we present the first analysis of quantum noise in kilometer-scale gravitational wave detectors operating with frequency-dependent squeezing. We find that the LIGO interferometers surpass the SQL by up to 3 dB at 50 Hz while reducing quantum noise at nearly all astrophysical frequencies.