  In this work the inhomogeneous (zero-phonon line) and homogeneous line
widths, and one projection of the permanent electric dipole moment change for
the Ce 4f-5d transition in Y_2SiO_5 were measured in order to investigate the
possibility for using Ce as a sensor to detect the hyperfine state of a
spatially close-lying Pr or Eu ion. The experiments were carried out on Ce
doped or Ce-Pr co-doped single Y_2SiO_5 crystals. The homogeneous line width
was measured to be about 3 MHz, which is essentially limited by the excited
state lifetime. Based on the line width measurements, the oscillator strength,
absorption cross section and saturation intensity were calculated to be about
9*10^-7, 5*10^-19 m^2 and 1*10^7 W/m^2, respectively. One projection of the
difference in permanent dipole moment, Delt_miu_Ce, between the ground and
excited states of the Ce ion was measured as 6.3 * 10^-30 C*m, which is about
26 times as large as that of Pr ions. The measurements done on Ce ions indicate
that the Ce ion is a promising candidate to be used as a probe to read out a
single qubit ion state for the quantum computing using rare-earth ions.
