Towards Cavity-Enhanced Spectroscopy of Single Europium Ions in Yttria Nanocrystals

A promising approach for realizing scalable quantum registers lies in the efficient optical addressing of rare-earth ion spin qubits in a solid state host. We study Eu³⁺ ions doped into Y₂O₃ nanoparticles (NPs)[1] as a coherent qubit material and work towards efficient single ion detection by coupling their emission to a high-finesse fiber-based Fabry-Pérot microcavity [2,3]. A beneficial ratio of the narrow homogeneous line to the inhomogeneous broadening of the ion ensemble at temperatures below 10K makes it possible to spectrally address and readout single ions. The coherent control of the single ion ⁵D­₀ - ⁷F₀ transition then permits optically driven single qubit operations on the Europium nuclear spin states. A Rydberg-blockade mechanism between ions within the same nanocrystal permits the implementation of a two-qubit CNOT gate to entangle spin qubits and perform quantum logic operations [4]. We observed fluorescence signals from small ensembles of Europium ions at cryogenic temperatures and measured cavity-enhanced optical lifetimes of half the free-space lifetime resulting in effective Purcell-factors of one. Considering the low branching ratio into the desired transition this amounts to a two-level Purcell-factor of 100. We will report on the progress towards single ion readout and control.

[1] Nano Lett. 17 (2017) 778-787

[2] New J. Phys. 12 (2010) 065038

[3] New J. Phys. 20 (2018) 095006

[4] Phys. Rev. A 105 (2022) 032603