  Quantum optical sensors are ubiquitous in various fields of research, from
biological or medical sensors to large-scale experiments searching for dark
matter or gravitational waves. Gravitational-wave detectors have been very
successful in implementing cavities and quantum squeezed light for enhancing
sensitivity to signals from black hole or neutron star mergers. However, the
sensitivity to weak forces is limited by available energy and optical
decoherence in the system. Here, we derive the fundamental sensitivity limit of
cavity and squeezed-light enhanced interferometers with optical loss.This limit
is attained by the optimal use of an additional internal squeeze operation,
which allows to mitigate readout loss. We demonstrate the application of
internal squeezing to various scenarios and confirm that it indeed allows to
reach the best sensitivity in cavity and squeezed-light enhanced linear force
sensors. Our work establishes the groundwork for the future development of
optimal sensors in real-world scenarios where, up until now, the application of
squeezed light was curtailed by various sources of decoherence.
