Description
Communicating quantum information beyond the order of metres requires minimally-lossy avenues of transmission. Optical fibres - internationally deployed as a backbone of modern, classical telecommunications - present the opportunity for such quantum information to be photonically networked with astonishingly low losses capable of reaching -0.091 dB/km within the telecommunications C-band (1530-1565 nm). Trivalent erbium ions (Er$^{3+}$) in solid-state crystals are promising candidates for scalable quantum networks, owing to their uniquely convenient transition properties. Er$^{3+}$ can exhibit near C-band ground state ($^4$I$_{15/2}$ $\to^4$I $_{13/2}$) transitions that capitalise on the low losses of fibre-based communication, independent of its host material. Indeed, light matter interfaces based on erbium ions are among the leading systems for fibre-based quantum communication networks (Uysal et. al., 2025).
In this paper we present the spectroscopic investigation of CaF$_2$ as a test bed of studying the engineering of implanted Er$^{3+}$ for light matter interfaces. The work was enabled by a home-built cryogenic confocal optical microscope. In CaF$_2$, C$_{3v}$ oxygen-compensated centres were able to be locally and controllably created by co-implantation of Er$^{3+}$ with O$^{2+}$ ions, followed by thermal annealing. These processes rendered sub-GHz inhomogeneous linewidths and 23 ms excited state lifetimes. Along with previous bulk studies (Moull et. al., 2024; Newman, 2025), this shows that centres with favourable properties for quantum networks can be engineered and spatially controlled. Importantly, an implantation depth of 2 $\mu$m is sufficient to enable coupling these engineered centres to high quality factor whispering mode gallery resonators for cavity coupled devices. We will discuss the consequences of this work for rare-earth devices in general and the prospects for transferring these technique to low nuclear-spin hosts.
| I am the presenting author | Yes |
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