Measuring the Quantum State of Dark Matter

Abstract

AbstractIt is shown how the time series obtained from searches for ultralight bosonic dark matter (DM), such as the axion, can be used to determine whether it is in a coherent or incoherent quantum state. The example is essentially trivial, but it is hoped that explicitly addressing it will provoke experimental exploration. In the standard coherent state, oscillations in the number density occur over the coherence time, , where m is the particle mass and v is the galactic virial velocity, leading to a reduction in the constraining power of experiments operating on timescales , due to the unknown global phase. On the other hand, if the DM is incoherent then no such strong number oscillations occur since the ensemble average over particles in different streams gives an effective phase average. If an experiment detects a signal then the coherent or incoherent nature of DM can be determined by time series analysis over the coherence time. This finding is observationally relevant for DM masses, (corresponding to coherence times between a year and 100 s).