  We searched high resolution spectra of 5600 nearby stars for emission lines
that are both inconsistent with a natural origin and unresolved spatially, as
would be expected from extraterrestrial optical lasers. The spectra were
obtained with the Keck 10-meter telescope, including light coming from within
0.5 arcsec of the star, corresponding typically to within a few to tens of au
of the star, and covering nearly the entire visible wavelength range from 3640
to 7890 angstroms. We establish detection thresholds by injecting synthetic
laser emission lines into our spectra and blindly analyzing them for
detections. We compute flux density detection thresholds for all wavelengths
and spectral types sampled. Our detection thresholds for the power of the
lasers themselves range from 3 kW to 13 MW, independent of distance to the star
but dependent on the competing "glare" of the spectral energy distribution of
the star and on the wavelength of the laser light, launched from a benchmark,
diffraction-limited 10-meter class telescope. We found no such laser emission
coming from the planetary region around any of the 5600 stars. As they contain
roughly 2000 lukewarm, Earth-size planets, we rule out models of the Milky Way
in which over 0.1 percent of warm, Earth-size planets harbor technological
civilizations that, intentionally or not, are beaming optical lasers toward us.
A next generation spectroscopic laser search will be done by the Breakthrough
Listen initiative, targeting more stars, especially stellar types overlooked
here including spectral types O, B, A, early F, late M, and brown dwarfs, and
astrophysical exotica.
