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Rare-earth (RE) ion doped nano-materials are promising candidates for a rangeof quantum technology applications. Among RE ions, the so-called Kramers' ionspossess spin transitions in the GHz range at low magnetic fields, which allowsfor high-bandwidth multimode quantum storage, fast qubit operations as well asinterfacing with superconducting circuits. They also present relevant opticaltransitions in the infrared. In particular, Er$^{3+}$ has an optical transitionin the telecom band, while Nd$^{3+}$ presents a high-emission-rate transitionclose to 890 nm. In this paper, we measure spectroscopic properties that are ofrelevance to using these materials in quantum technology applications. We findthe inhomogeneous linewidth to be 10.7 GHz for Er$^{3+}$ and 8.2 GHz forNd$^{3+}$, and the excited state lifetime T$_1$ to be 13.68 ms for Er$^{3+}$and 540 $\mu$s for Nd$^{3+}$. We study the dependence of homogeneous linewidthon temperature for both samples, with the narrowest linewidth being 379 kHz(T$_2$ = 839 ns) for Er$^{3+}$ measured at 3 K, and 62 kHz (T$_2$ = 5.14$\mu$s) for Nd$^{3+}$ measured at 1.6 K. Further, we investigate time-dependenthomogeneous linewidth broadening due to spectral diffusion and the dependenceof homogeneous linewidth on magnetic field, in order to get additional clarityof mechanisms that can influence the coherence time. In light of our results,we discuss two applications: single qubit-state readout and a Fourier-limitedsingle photon source.

Optical coherence properties of Kramers' rare-earth ions at the nanoscale for quantum applications