Observation of a Sub-10-Hz Linewidth $^{88}\hbox{Sr}^{+ \ \ 2}\hbox{S}_{1/2}$–$^{2}\hbox{D}_{5/2}$ Clock Transition at 674 nm

In this paper, a pair of diode lasers at 674 nm, locked to two independent ultralow-expansion high-finesse cavities, have been characterized by monitoring the beat frequency between them. The individual laser linewidth of 1.4 Hz for a 3-s averaging time broadened to 4 Hz at 30 s. The relative frequency stability of the beat at 1s is 2.5 10<sup>-15</sup>. One of the lasers has been used to interrogate the <sup>88</sup>Sr<sup>+</sup> <sup>2</sup>Si<sub>1/2</sub>-<sup>2</sup>D <sub>5/2</sub> transition at 674 nm, and a transition linewidth of 9 Hz has been observed

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