  Obtaining insight into the constituents of dark matter and their interactions
with normal matter has inspired a wide range of experimental efforts. Several
approaches, particularly those involving searches for ultralight bosonic dark
matter (UBDM) fields, involve the use of quantum systems or measurements
performed at the limits imposed by quantum mechanics. While a classical
treatment of UBDM and its detectors is satisfactory, a fully quantum
description would assist in developing future detection strategies. Here, we
present an open quantum systems approach that accomplishes this while providing
intuition into the quantum nature of the detection process itself. Furthermore,
we apply the quantum theory of optical coherence to characterize the
statistical properties of the UBDM field. Using representative examples, we
show that this theoretical treatment has implications in uncovering signatures
of the cosmological production mechanism of the UBDM field and its galactic
merger history. By adapting tools from quantum optics, this work will help
facilitate the creation of novel methods to extract astrophysically relevant
information from correlation measurements.
