  We give a systematic theoretical treatment of linear quantum detectors used
in modern high energy physics experiments, including dark matter cavity
haloscopes, gravitational wave detectors, and impulsive mechanical sensors. We
show how to derive the coupling of signals of interest to these devices, and
how to calculate noise spectra, signal-to-noise ratios, and detection
sensitivities. We emphasize the role of quantum vacuum and thermal noise in
these systems. Finally, we review ways in which advanced quantum techniques --
squeezing, non-demolition measurements, and entanglement -- can be or currently
are used to enhance these searches.
