Fundamental sensitivity limit of lossy cavity-enhanced interferometers with external and internal squeezing

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 the available energy and the 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 the mitigation of readout loss. We demonstrate the application of internal squeezing to various scenarios and confirm that it indeed allows one 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.