Optical Ramsey spectroscopy of a single trapped ⁸⁸Sr⁺ ion

Coherent optical spectroscopy has been performed on the narrow 5s ²S_1∕2–4d ²D_5∕2 quadrupole transition in a single Doppler-cooled and trapped ⁸⁸Sr⁺ ion. High-contrast optical Ramsey spectra have been observed with fringe visibilities up to ∼90%. The visibility decreased as the free precession period was increased, and was limited by the interrogation laser’s coherence. The effect of varying the relative phase of the second Ramsey pulse was investigated. By measuring the difference in excitation probability on reversing a 90° relative phase shift between the two Ramsey pulses, we have demonstrated Ramsey’s anti-symmetric line shape in the optical domain. A significant advantage of this line shape is the zero crossing at line center and the independence of this center frequency on drifts in signal amplitude. This optical Ramsey line shape is suitable for stabilizing a local oscillator to an atomic reference transition in an optical frequency standard. All observed optical Ramsey signals are well described by a model using the optical Bloch equations.