Abstract
Relaxation of the off-diagonal density-matrix elements between the metastable $3\text{ }{^{2}D}_{3/2}$ and $3\text{ }{^{2}D}_{5/2}$ states in a single ${^{40}\text{C}\text{a}}^{+}$, which are separated from each other by 1.82 THz, is studied experimentally by using the Ramsey method of separated oscillating fields. The two metastable states are connected with a stimulated Raman transition by using two infrared lasers, which are phase locked with their frequency difference bridged by an optical frequency comb. By performing Ramsey measurements for two Zeeman components of the stimulated Raman transition with different sensitivities to magnetic fields, the coherence is investigated quantitatively and the effect of magnetic-field fluctuations on that is evaluated. By using a spin-echo $\ensuremath{\pi}$ pulse in a Ramsey sequence which is effective for inhomogeneous broadening processes, a visibility decay time of 5.1 ms is obtained. This is more than twice as long as that for normal sequences. This work is an essential step for realization of a qubit using the two metastable states and demonstrates coherent manipulation of the internal states separated by around a terahertz using phase-locked light sources.
Published Version
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