Thousands of papers have been written about quantum metrology, but few have acknowledged the reality that quantum sensors such as gravitational-wave detectors are dynamical systems and the signals are often time-varying. This talk presents fundamental quantum limits to such sensors, as well as measurements to achieve them [Tsang, Wiseman, and Caves, PRL 106, 090401 (2011); Tsang and Nair, PRA 86, 042115 (2012)]. For the task of noise spectroscopy with an optical interferometer, I show the surprising result that spectral photon counting can be far superior to homodyne detection [Ng et al., PRA 93, 042121 (2016); Tsang, PRA 107, 012611 (2023)]. This idea has recently been adopted by Caltech scientists, who will build an experiment based on the idea to detect signatures of quantum gravity under the GQuEST project [McCuller, arXiv:2211.04016; https://gquest.fnal.gov/]