Ultrastable laser system for spectroscopy of the 1S0 – 3P0 clock transition in Sr atoms

A laser system with a spectral linewidth less than 1 Hz for spectroscopy of the ¹S₀ – ³P₀ clock transition in strontium atoms has been demonstrated. A semiconductor laser emitting at a wavelength of 698 nm was stabilised to an external high-finesse Fabry–Perot cavity with vibration and temperature compensation near the zero expansion point. After laser cooling to a temperature below 3 μK, ⁸⁸Sr atoms were loaded into an optical lattice at a magic wavelength of 813 nm. The laser system was used to characterise the ⁸⁸Sr clock transition by magnetically induced spectroscopy. The resonance spectral width was determined to be 130 ± 17 Hz, which corresponds to a quality factor of 3 × 10¹².