As an active material with favorable linear and nonlinear optical properties, thin-film lithium niobate has demonstrated its potential in integrated photonics. Integration with rare-earth ions, which are promising candidates for quantum memories and transducers, will enrich the system with new applications in quantum information processing. Here, we investigate the optical properties at 1.5 micron wavelengths of rare-earth ions (Er 3+ ) implanted in thin-film lithium niobate waveguides and micro-ring resonators. Optical quality factors near a million after post annealing show that ion implantation damage can be successfully repaired. The transition linewidth and fluorescence lifetime of erbium ions are characterized, revealing values comparable to bulk-doped crystals. The ion-cavity coupling is observed through a Purcell enhanced fluorescence, from which a Purcell factor of 3.8 is extracted. This platform is compatible with top-down lithography processes and leads to a scalable path for controlling spin-photon interfaces in photonic circuits.As an important material in modern photonics, lithium niobate (LN) displays favorable piezoelectric, electro-optic, optical, photoelastic and photorefractive properties 1 . It is widely used for electro-optic modulators, frequency doublers, optical parametric oscillators and Q-switches for lasers. However, LN had been limited to bulk crystal components in all of these applications until a recent breakthrough in LN thin-film nanofabrication technology 2 made compact and dense photonic integrated circuits possible. Subsequent works on high-performance electro-optical modulators 3,4 , ultra-efficient second harmonic generation 5,6 and microwaveoptical transduction 7 have stimulated intense interest and promise for integrated photonics. This advance in LN thinfilm nanofabrication technology also raises the interest in incorporating rare-earth ions (REIs) into patterned LN waveguides and micro-cavities for scalable photonic integrated circuits with added functionalities enabled by the REIs.REIs are well known for their applications in nonlinear optics such as lasers and amplifiers 8 due to their stable optical transitions, high fluorescence quantum efficiencies, and long population lifetimes. Their narrow homogeneous linewidths 9,10 , which allow the burning of ultra-narrow spectral holes, also find themselves useful in photonic signal processing 11 as well as frequency stabilization 8,12,13 , medical imaging 14 and optical filtering 8,15 . Over the past decades, REIs have emerged as a promising candidate for quantum information processing 16-18 thanks to the weak coupling of their 4f electrons to the environment 19 and long coherent spin states 20 . REIs have been used to demonstrate quantum memory protocols for quantum networks 21 , light-matter interactions 22 and quantum-state teleportation 23 . Among the several well-studied REIs, erbium (Er) has received much attention due to its optical transitions in the telecommunications band, avoiding the need for frequency conversion a...

Superconducting cavity electro-optics presents a promising route to coherently convert microwave and optical photons and distribute quantum entanglement between superconducting circuits over long-distance. Strong Pockels nonlinearity and high-performance optical cavity are the prerequisites for high conversion efficiency. Thin-film lithium niobate (TFLN) offers these desired characteristics. Despite significant recent progresses, only unidirectional conversion with efficiencies on the order of 10−5 has been realized. In this article, we demonstrate the bidirectional electro-optic conversion in TFLN-superconductor hybrid system, with conversion efficiency improved by more than three orders of magnitude. Our air-clad device architecture boosts the sustainable intracavity pump power at cryogenic temperatures by suppressing the prominent photorefractive effect that limits cryogenic performance of TFLN, and reaches an efficiency of 1.02% (internal efficiency of 15.2%). This work firmly establishes the TFLN-superconductor hybrid EO system as a highly competitive transduction platform for future quantum network applications.

Rare earth emitters enable critical quantum resources including spin qubits, single photon sources, and quantum memories. Yet, probing of single ions remains challenging due to low emission rate of their intra-4f optical transitions. One feasible approach is through Purcell-enhanced emission in optical cavities. The ability to modulate cavity-ion coupling in real-time will further elevate the capacity of such systems. Here, we demonstrate direct control of single ion emission by embedding erbium dopants in an electro-optically active photonic crystal cavity patterned from thin-film lithium niobate. Purcell factor over 170 enables single ion detection, which is verified by second-order autocorrelation measurement. Dynamic control of emission rate is realized by leveraging electro-optic tuning of resonance frequency. Using this feature, storage, and retrieval of single ion excitation is further demonstrated, without perturbing the emission characteristics. These results promise new opportunities for controllable single-photon sources and efficient spin-photon interfaces.

Rare earth ions are known as promising candidates for building quantum light-matter interface. However, tunable photonic cavity access to rare earth ions in their desired host crystal remains challenging. Here, we demonstrate the integration of erbium doped yttrium orthosilicate (Er3+:Y2SiO5) with thin-film lithium niobate photonic circuit by plasma-activated direct flip chip bonding. Resonant coupling to erbium ions is realized by on-chip electro-optically tuned high Q lithium niobate micro-ring resonators. Fluorescence and absorption of erbium ions at 1536.48 nm are measured in the waveguides, while the collective ion-cavity cooperativity with micro-ring resonators is assessed to be 0.36. This work presents a versatile scheme for future rare earth ion integrated quantum devices.

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