  Lasers with high spectral purity can enable a diverse application space,
including precision spectroscopy, coherent high-speed communications, physical
sensing, and manipulation of quantum systems. Already, meticulous design and
construction of bench Fabry-Perot cavities has made possible dramatic
achievements in active laser-linewidth reduction, predominantly for
optical-atomic clocks. Yet there is increasing demand for miniaturized laser
systems operating with high performance in ambient environments. Here, we
report a compact and robust photonic-atomic laser comprising a 2.5 cm long,
20,000 finesse, monolithic Fabry-Perot cavity integrated with a micromachined
rubidium vapor cell. By leveraging the short-time frequency stability of the
cavity and the long-time frequency stability of atoms, we realize an
ultranarrow-linewidth laser that enables integration for extended measurements.
Specifically, our laser supports a fractional-frequency stability of $1\times
10^{-13}$ at an averaging time of 20 ms, $7 \times 10^{-13}$ at 300 s, an
integrated linewidth of 25 Hz that results from thermal noise, a Lorentzian
linewidth as low as 0.06 Hz$^2$/Hz, and a passive vibration immunity as low as
$10^{-10}$/g. Our work explores hybrid laser systems with monolithic photonic
and atomic packages based on physical design.

Optics