Millicharge Dark Matter Detection with Mach-Zehnder Interferometer

If the dark sector exists and communicates with Standard Model through the $U(1)$ mixing, it is possible that electromagnetism would have influence on matter fields in dark sector, so-called millicharge particles (mCPs). Furthermore, the highest mCPs could be dark matter particles. Recently it has been shown that the mCPs would be slowed down and captured by the earth. As a result, the number density of accumulated mCPs underground is enhanced by several orders of magnitude as compared to that of dark matter in our solar system. In this study, we propose to use the Mach-Zehnder (MZ) laser interferometer to detect earth bound mCPs through the detection of phase shifts of photons. We show that, for mass of mCPs lager than $1$ GeV, the sensitivity of probing the mixing parameter $\epsilon$ could reach as low as $10^{-11}$ if number density is larger than $1~\rm{cm^{-3}}$.