Harvesting quantum coherence from axion dark matter
  Quantum coherence is one of the most striking features of quantum mechanics
rooted in the superposition principle. Recently it has been demonstrated that
it is possible to harvest the quantum coherence from a coherent scalar field.
In order to explore a new method of detecting axion dark matter, we consider a
point-like Unruh-DeWitt detector coupled to the axion field and quantify a
coherent measure of the detector. We show that the detector can harvest the
quantum coherence from the axion dark matter. To be more precise, we consider a
two-level of electron system in an atom as the detector. In this case, we
obtain the coherence measure $C=2.2\times 10^{-6}\gamma\, \left(T/1{\rm
s}\right)$ where $T$ and $\gamma$ are an observation time and the Lorentz
factor. At the same time, the axion mass $m_a$ we can probe is determined by
the energy gap of the detector.
