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Abstract

Precision measurements of space and time, like those made by the detectors of
the Laser Interferometer Gravitational-wave Observatory (LIGO), are often
confronted with fundamental limitations imposed by quantum mechanics. The
Heisenberg uncertainty principle dictates that the position and momentum of an
object cannot both be precisely measured, giving rise to an apparent limitation
called the Standard Quantum Limit (SQL). Reducing quantum noise below the SQL
in gravitational-wave detectors, where photons are used to continuously measure
the positions of freely falling mirrors, has been an active area of research
for decades. Here we show how the LIGO A+ upgrade reduced the detectors'
quantum noise below the SQL by up to 3 dB while achieving a broadband
sensitivity improvement, more than two decades after this possibility was first
presented.