A Telecom-Compatible Quantum Memory in the Solid-State: Single Erbium Ions Coupled to Silicon Nanophotonic Circuits

Abstract

Single atoms and atom-like defects in solids are promising platforms for realizing single photon sources and long-lived quantum memories, which are essential ingredients for the development of long-distance quantum networks. However, most atomic transitions are in the ultraviolet-NIR regions with wavelengths shorter than 1 µm, where propagation losses in optical fibers are prohibitively large. A notable exception is erbium ion, whose optical transition at 1.5 µm is in the “telecom band”, allowing minimal fiber transmission losses. Isolating and addressing individual erbium ions using an optical interface have been elusive so far because of the poor emission rate of erbium due to the electric dipole-forbidden nature of its intra-4f optical transition. We report the observation of fluorescence from single erbium ions for the first time. We achieve this by integrating erbium ions in a low loss, small mode-volume silicon nanophotonic cavity and enhancing their emission rate by over two orders of magnitude.

A crucial component of optically interfaced solid-state defects-based platforms is high-fidelity, projective measurement of the spin state, which is generally accomplished using fluorescence on an optical cycling transition. We demonstrate that the cavity modifies the local electromagnetic environment of an erbium ion (which otherwise lacks strong cycling transitions) and improves its cyclicity by greater than 100-fold, thus enabling high-fidelity single-shot quantum nondemolition readout of the ion’s spin. We also identify dozens of spectrally distinct ions coupled to the same cavity. Combining an optical frequency-domain multiplexing technique and microwave rotations, we individually initialize, manipulate, and perform single-shot spin measurement of six such ions. Our approach is not limited by the spatial separation between individual ions and is readily scalable to tens or hundreds of ions.

Finally, we demonstrate coherent coupling of an erbium electronic spin to a nearby nuclear spin and implement single-qubit and two-qubit gates on them, thus extending our platform’s prowess as a quantum memory by making a long-lived nuclear spin register available for storage and retrieval of information. These results are a significant step towards realizing long-distance quantum networks by utilizing multiplexed quantum repeater protocols and deterministic quantum logic for photons based on a scalable silicon nanophotonics architecture.