  An ultra-stable optical clock based on coherent population trapping effect of
alkaline-earth ions, such as Ca$^+$, Sr$^+$, Ba$^+$, is analyzed here. The
proposed transitions use the odd isotopes, so that the frequency shift is
insensitive to the applied magnetic field. To enhance the signal, a large
number of ions are trapped in a linear Paul trap, and laser cooled to
crystallize such that the ions are in the Lamb-Dicke regime to avoid first
order Doppler shift. Other relevant frequency shifts are also analyzed. Further
techniques to improve the stability of the clock signal are discussed.
