Rare earth ions doped into solid-state crystals provide exceptional optical and hyperfine coherence, which renders them promising candidates for quantum optical applications, ranging from quantum memories, quantum-nonlinear optics and quantum processing. However, the dipole-forbidden nature of the narrow transitions results in very weak interaction with light, hence limiting most experiments to macroscopic ensembles. Efficient access to a single ion allows it to act as a qubit, thereby allowing us to exploit the full potential of these systems. For example, this could enable us to demonstrate deterministic narrowband single photon sources as well as single-ion spin-photon interfaces. The existence of permanent electric dipole moment in rare earth ions allows for interaction between nearby ions, which can be utilized for quantum computing purposes.
In this project, we gain efficient access to individual ions and small ensembles by coupling solid-state nano-structures to a high finesse optical fibre micro-cavity [1]. Purcell enhancement can enhance the emission rate by several orders of magnitude, thereby making the weak transitions bright. As this emission is in the mode of the cavity, it also allows for a very efficient interaction.
The system we work with is erbium ions doped into a yttrium oxide nano-crystal (Er:Y2O3). This is because erbium has a transition at telecom wavelengths where losses in commercial fibers is low. This means that the emitted photons can be transmitted with low loss, hence making them very promising for use in long-distance networks. The nano-crystals are synthesized in collaboration with the group of Dr. Philippe Goldner at Chimie ParisTech (Paris, France). As for the cavity, we use a micro-cavity based on laser-machined optical fibers as mirror substrates, which combine high finesse with small mode cross sections and full tunability. The fibers are fabricated in collaboration with the group of Prof. David Hunger at KIT (Karlsruhe, Germany).
References:
- [1] B. Casabone et al. “Cavity-enhanced spectroscopy of a few-ion ensemble in Eu3+:Y2O3” . New Journal of Physics 20 (2018).
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Funding: