  Quantum uncertainty of laser light limits the sensitivity of
gravitational-wave observatories. In the past 30 years, techniques for
squeezing the quantum uncertainty as well as for enhancing the
gravitational-wave signal with optical resonators were invented. Resonators,
however, have finite linewidths; and the high signal frequencies that are
produced during the scientifically highly interesting ring-down of
astrophysical compact-binary mergers cannot be resolved today. Here, we propose
an optical approach for expanding the detection bandwidth. It uses quantum
uncertainty squeezing inside one of the optical resonators, compensating for
finite resonators' linewidths while maintaining the low-frequency sensitivity
unchanged. Introducing the quantum expander for boosting the sensitivity of
future gravitational-wave detectors, we envision it to become a new tool in
other cavity-enhanced metrological experiments.
