Optical and hyperfine spectroscopic investigations on europium ions doped in yttrium orthosilicate waveguides fabricated by focused ion beam milling

Quantum memory is a crucial element in large-scale quantum networks. Integrated quantum memories based on micro-/-nano structures, such as waveguides, can significantly enhance the scalability and reduce the consumption of optical and electrical power. Eu-151(3+):Y2SiO5 stands out as an exceptional candidate material for quantum memory, because it possesses a spin coherence lifetime of 6 h and an optical storage lifetime of 1 h. Here we employ focused ion beam technology to fabricate a triangular nanobeam on the surface of a Y2SiO5 crystal. The width and length of the nanobeam are 2 mu m and 20 mu m, respectively. The optical lifetime and inhomogeneous broadening of Eu-151(3+) in the triangular nanobeam are measured by fluorescence spectroscopy. The optical lifetime is (1.9 +/- 0.1) ms and the optical inhomogeneous broadening is (1.58 +/- 0.05) GHz at a doping level of 0.07% for Eu-151(3+). The hyperfine transition spectra are measured by using optically detected magnetic resonance and spin inhomogeneous broadening of (19 +/- 3) kHz is obtained. Furthermore, we analyze the coherence properties of optical and hyperfine transitions, respectively, via transient spectral hole burning and spin echo measurement. We obtain an optical homogeneous linewidth down to (22 +/- 3) kHz, which is still limited by the instantaneous spectral diffusion. The spin coherence lifetime under the geomagnetic field is (5.1 +/- 0.6) ms. The results demonstrate that Eu-151(3+) ions embedded within the 2 mu m triangular nanobeam essentially retain the same optical and hyperfine transition properties as those observed in bulk crystals. Consequently, this research lays a foundation for studying the integrated quantum memories based on Eu-151(3+) ensembles and the detection of the single Eu-151(3+) ion based on the focused ion beam technique.