 More precisely, it was the ammonia maser operating at 12.5 mm wavelength that was demonstrated by Gordon, Zeiger, and Townes in 1954.One year later the same authors derived theoretically the linewidth of their device by making the reasonable approximations that their ammonia maser

(i) is a true continuous-wave (CW) maser,

(ii) is a true four-level maser, and

(iii) exihibits no intrinsic resonator losses but only outcoupling losses.Notably, their derivation was entirely semi-classical, describing the ammonia molecules as quantum emitters and assuming classical electromagnetic fields (but no quantized fields or quantum fluctuations), resulting in the half-width-at-half-maximum (HWHM) maser linewidth
denoted here by an asterisk and converted to the full-width-at-half-maximum (FWHM) linewidth formula_2.