We show that rare-earth (RE-) doped glasses can have homogeneous linewidths as narrow as $287\mathrm{kHz}$ at ${}^4\mathrm{He}$ temperatures. This is far narrower than others reported in glasses in the same temperature range and is suitable for precise spectral hole burning and spatial–spectral holographic applications. It is known that cw spectral hole burning linewidth measurements of RE ions in glasses are hindered by the presence of spectral diffusion but, even in glasses, application of a magnetic field can freeze out RE spin–spin interactions responsible for spectral diffusion and isolate the remaining contribution of two-level systems (TLSs). The ${\mathrm{Er}}^{3+}:2\mathrm{G}2\mathrm{S}$ glasses have unusually low TLS contributions, making it possible to study the real homogeneous linewidth using photon echo measurements. The contribution from TLSs is only $170{\mathrm{T}}^{1.3}\mathrm{kHz}$ when subjected to a field of $5\mathrm{T}$ .