Characterizing The Quantum State Of Dark Matter: An Open Quantum Systems Approach

Obtaining insight into the constituents of DM and their interactions with normal, i.e., Standard Model (SM) matter, has inspired a wide range of large and small-scale experimental efforts that harness current technology to look for the feeble interactions between SM matter and DM with unprecedented precision. Several novel experimental approaches involve quantum systems or measurements performed at the limits imposed by quantum mechanics. This is particularly relevant for the case of ultralight bosonic dark matter (UBDM), where dark matter is assumed to be a bosonic field/particle present in high occupation numbers around the earth. While a classical treatment of UBDM and its detectors is well-motivated, future detection strategies would benefit from a quantum treatment of both the field and its interaction with a, perhaps, quantum detector. The field of quantum optics already provides a rigorous formalism for characterizing bosonic fields. Here we apply the quantum theory of optical coherence to characterize the statistical properties of the UBDM field and an open quantum system approach to the interaction between the UBDM field and a detector. Our theoretical treatment has implications in uncovering the astrophysical history of the UBDM field, as well as informing quantum metrology-based strategies for its detection.