Ground state EIT cooling of $^{171}$Yb$^+$ ion
The work propose a scheme of deep laser cooling of $^{171}$Yb$^{+}$. The cooling is based on the effect of electromagnetically induced transparency (EIT) in a polychromatic field with three frequency components are resonant to optical transitions of the $^2S_{1/2} \to \, ^2P_{1/2}$ line. The deep cooling down to the ground motional state in a trap allows for a significant suppression of the second order Doppler shift in frequency standards. Moreover, there is no need to use a magnetic field, which is required for Doppler cooling of $^{171}$Yb$^{+}$ in a field with two-frequency component. The cooling without use of magnetic field is important for deep suppression of quadratic Zeeman shifts of clock transitions from uncontrolled residual magnetic fields.