  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 $1~\rm{cm^{-3}}$.
