12 May 2020, 12:45 EDT: The original print version of this article implied that the 1986 paper by H.O. Everitt et al. (Appl. Phys. Lett. 49, 995) was the first to suggest that a terahertz molecular gas laser might be built. In fact, such lasers had been demonstrated in the 1970s; the paper by Everitt et al. showed how they might be made far more compact.
In addition, we should clarify that the QCL-pumped THz molecular-gas laser reported in 2019 (P. Chevalier et al., Science 366, 856) was not the first THz laser pumped by a QCL; A. Paiges et al. (APL Photon. 1, 031302) published a report of a QCL-pumped THz molecular-gas laser in 2016. One distinction of the laser reported in 2019, according to that paper, was the more recent laser design’s greater potential THz tunability.
Two paragraphs on page 33 have been rewritten to reflect these details. We thank Romain Peretti, IEMN–CNRS, France, for making us aware of problems in the original description.