Gravitational waves (GW) are ripples in spacetime caused by dramatic astrophysical
events such as the merger of binary neutron stars or black holes. The groundbreaking
first detection of GW occurred in 2015 by the Laser Interferometer Gravitational-Wave
Observatory (LIGO) in the United States. Subsequently, an international network of GW
detectors has identified dozens of GW events, opening a new window into the field of
astronomy. As of May 2023, LIGO has commenced its fourth observation run (O4), during
which it has detected over ten GW events in just a few months. Virgo in Europe, and
KAGRA in Japan, are set to join this observation run within the next half a year,
promising enhanced accuracy in sky localization of the sources of GW, which
multi-messenger astronomy is heavily reliant on. LIGO continually enhances its
sensitivity with each observation run. A significant advancement leading up to O4 is the
introduction of frequency-dependent squeezing, designed to mitigate quantum noise
resulting from light's quantum fluctuations. This innovative technique has boosted
LIGO's sensitivity by 5.8 dB at high frequencies, expanding its observable volume
compared to the previous observation run. In this presentation, I will provide an
overview of the ongoing observation run and delve into the details of LIGO's upgrades,
particularly the advancements in frequency-dependent squeezing.