Towards generation of millihertz-linewidth laser light with 10−18 frequency instability via four-wave mixing.

Laser light with spectral purity and frequency stability is pursued in precision spectroscopy and precision measurements. We propose a scheme to generate millihertz-linewidth laser light with a frequency instability of 10−18 via optical four-wave mixing in alkaline-earth atoms. We show that the linewidth of the mixing laser light is ultimately limited by the natural linewidth of the atomic transition rather than by the linewidth of the input lasers. The frequency stability of the mixing laser light depends largely on the intensity stability of the input lasers. It is possible to generate a millihertz-linewidth laser light with a frequency instability of 10−18 and a power of 10−12 W when the input lasers with a relative intensity instability of 10−4 and a spectral width of 1 Hz interact with strontium (Sr) atoms with a density of 1 × 1011 cm−3. [ABSTRACT FROM AUTHOR]