Axion search with quantum nondemolition detection of magnons
  The axion provides a solution for the strong CP problem and is one of the
leading candidates for dark matter. This paper proposes an axion detection
scheme based on quantum nondemolition detection of magnon, i.e., quanta of
collective spin excitations in solid, which is expected to be excited by the
axion-electron interaction predicted by the Dine-Fischer-Srednicki-Zhitnitsky
(DFSZ) model. The prototype detector is composed of a ferromagnetic sphere as
an electronic spin target and a superconducting qubit. Both of these are
embedded inside a microwave cavity, which leads to a coherent effective
interaction between the uniform magnetostatic mode in the ferromagnetic crystal
and the qubit. An upper limit for the coupling constant between an axion and an
electron is obtained as $g_{aee}<1.6\times10^{-7}$ at the 95\% confidence level
for the axion mass of $33.117$$\mu$eV$<m_{a}<33.130$$\mu$eV.
