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Advanced Quantum Technologies

Rare‐Earth Doped Thin Films for Optical Quantum Technologies

Philippe Goldner, Alexandre Tallaire, Diana Serrano, Alexey Tiranov, Tian Zhong

Published Oct 8, 2025• 6 citations
Publisher

Abstract

Abstract Rare earth doped crystals show, at low temperatures, extremely narrow optical homogeneous linewidths, as well as long spin coherence lifetimes, a unique combination in the solid state. This makes these materials attractive for optical quantum technologies like quantum communication and processing. Most of the results in this field have so far used bulk crystals because of their exceptional spectroscopic properties. Crystalline thin films can combine these properties with the possibilities offered by integration in photonic circuits in terms of compactness, stability, energy efficiency, and scalability. In this review, recent results on different platforms containing rare earth ions and targeting quantum technologies, including lithium niobate and silicon films, and oxide films deposited on Si, are summarized. Current approaches for obtaining thin films and devices are described, together with RE spectroscopic properties and applications to quantum technologies. The opportunities and challenges offered or faced by the different platforms are also discussed.

DOI
10.1002/qute.202500026

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Abstract Rare earth doped crystals show, at low temperatures, extremely narrow optical homogeneous linewidths, as well as long spin coherence lifetimes, a unique combination in the solid state. This makes these materials attractive for optical quantum technologies like quantum communication and processing. Most of the results in this field have so far used bulk crystals because of their exceptional spectroscopic properties. Crystalline thin films can combine these properties with the possibilities offered by integration in photonic circuits in terms of compactness, stability, energy efficiency, and scalability. In this review, recent results on different platforms containing rare earth ions and targeting quantum technologies, including lithium niobate and silicon films, and oxide films deposited on Si, are summarized. Current approaches for obtaining thin films and devices are described, together with RE spectroscopic properties and applications to quantum technologies. The opportunities and challenges offered or faced by the different platforms are also discussed.
Rare-earth ion ensembles doped in single crystals are a promising materials system with widespread applications in optical signal processing, lasing, and quantum information processing. Incorporating rare-earth ions into integrated photonic devices could enable compact lasers and modulators, as well as on-chip optical quantum memories for classical and quantum optical applications. To this end, a thin film single crystalline wafer structure that is compatible with planar fabrication of integrated photonic devices would be highly desirable. However, incorporating rare-earth ions into a thin film form-factor while preserving their optical properties has proven challenging. We demonstrate an integrated photonic platform for rare-earth ions doped in a single crystalline thin film lithium niobate on insulator. The thin film is composed of lithium niobate doped with Tm 3+ . The ions in the thin film exhibit optical lifetimes identical to those measured in bulk crystals. We show narrow spectral holes in a thin film waveguide that require up to 2 orders of magnitude lower power to generate than previously reported bulk waveguides. Our results pave the way for scalable on-chip lasers, optical signal processing devices, and integrated optical quantum memories.
Since quantum information technologies are expected to offer communication security and high computational capacities, research in the field is currently attracting a lot of attention. Among the materials studied so far, rare earth doped inorganic insulators are one of the most promising. With the different available trivalent rare earth ions, the visible and the IR range including the telecom wavelength at 1.5 μm can be covered. Transitions are usually narrow, and at low temperatures, long optical and spin coherence times can often be observed. Investigations using bulk single crystals have already led to many promising results. Recently, spectroscopic studies have been extended to other forms of inorganic materials, such as transparent ceramics, thin films, and nanoparticles for single rare‐earth qubits. Progress in these areas is expected to offer many new possibilities for the design of quantum light‐matter interfaces and scalable quantum memories and processors.
Coherent quantum devices play a central role in quantum science and engineering. Rare-earth ions such as erbium in solids feature numerous 4f- intra-shell transitions that are effectively shielded from their crystalline surroundings by closed outer shells, allowing for long spin coherence times and narrow optical transitions. Recent ensemble experiments have established the rare-earth doped crystals as the leading materials for optical quantum memories. However, the development of large-scale quantum devices based on rare-earth doped materials has remained a challenge. Here we describe a scalable quantum photonic platform based on epitaxially grown oxide thin films doped with trivalent ions on silicon substrates. The platform builds upon the recent developments of high quality epitaxial rare-earth doped oxides on silicon, and highlights the integration of quantum coherent materials with existing silicon technologies for ultimate device scalability. The proposed device platform builds upon our capability to epitaxially grow high quality, near-lattice-matched Y2O3 thin films on silicon. The epitaxial thin film is of high structural quality and offers several distinct advantages: (1) significantly less optical loss and better quantum emitter coherence; (2) atomic precision placement of rare-earth emitters (via delta doping) with respect to the photonic mode for controllable ion-photon coupling; (3) scalable top-down device fabrication using standard lithography and etching techniques; (4) heterogeneous integration with silicon takes full advantage of existing silicon photonics technologies for routing, tuning, and modulation of the quantum emission from rare-earth ions.
Abstract Diamond containing nitrogen‐vacancy centers (NV−) is one of the most investigated materials for quantum technologies, because of this system's exceptional spin properties. Although the NV− optical transition is very bright, it suffers from spectral diffusion and weak zero‐phonon line, and is in the visible range. This limits integration into quantum photonic structures and interfacing with optical fiber networks. In contrast, rare earth (RE) ions exhibit extremely narrow and stable optical transitions, including erbium's 1.5 µm telecom wavelength. Combining RE with NV− properties through short‐range interactions is however challenging as RE do not readily enter the diamond lattice. In this work, a thin‐film‐based architecture in which RE and NV− centers can interact while preserving their unique properties is introduced. Thin films of Er3+:Y2O3 are grown by chemical vapor deposition on diamond substrates with well‐crystallized and highly textured structures. An extensive spectroscopic study of Er3+ transitions at room and low temperatures further reveals that photoluminescence spectra and decays are close to bulk materials. It is also shown that Y2O3 thin film deposition has no detrimental effects on NV− optical and spin properties. RE thin films deposited on diamond can thus be suitable for building hybrid materials for new functionalities in quantum sensing, communication, and processing.
Rare-earth ion doped crystals possess stable solid state physicochemical properties and long optical coherence time and spin coherence time, thus showing important development prospect in quantum information science and technology area. Investigations on macroscopic bulk rare-earth single crystals have obtained many promising results, especially in the field of optical quantum memory. With the rapid development of quantum information science, a variety of new functions or multifunctional integrations are found in rare earth crystal systems, such as on chip quantum storage, microwave to optical frequency conversion, scalable quantum single photon sources, and quantum logic gates. As a result, beyond the macroscopic bulk rare-earth single crystals, micro/nano-scale rare-earth crystals have received much attention in recent years and they are regarded as promising candidates in highly integrated hybrid quantum systems and miniaturized quantum devices. Moreover, wet chemical method synthesized micro/nano-scale rare-earth crystals have lower growth difficulty and more flexible manipulation in volume, shape and composition. Therefore, exploring high-performance micro/nano-scale rare-earth crystals and precisely manipulating their quantum states have become one of the important directions in today’s quantum information science and technology research. In this review, we first briefly introduce the basic concepts and high resolution spectroscopic techniques that are commonly used in rare earth ion doped crystals for quantum information science and technologies, such as hole burning technique and photon echo technique. Then we summarize comprehensively recent research status and development trends of rare earth ion doped polycrystalline nanoparticles, thin films, single crystal based micro systems, and some other micro/nano-scale rare earth platforms in terms of material fabrication, quantum coherence property, dephasing mechanisms, and also quantum device explorations. The latest research advances in quantum information applications such as quantum storage, quantum frequency conversion, quantum single photon sources and quantum logic gates are given. Finally, we discuss the possible optimization directions and strategies to improve the component design, material synthesis and quantum performance of micro/nano-scale rare earth crystals and their related quantum devices. This review highlights that the micro/nano-scale rare earth crystals may offer many new possibilities for designing quantum light-matter interfaces, thus are promising quantum systems to develop scalable and integrated quantum devices in the future.