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physica status solidi (a)

Rare‐Earth‐Doped Y2SiO5 Crystal Directly Bonded on Glass for Efficient Optical Quantum Technologies Platform

A. Talneau, Sacha Welinski, Alban Ferrier

Published Jan 11, 2024• 3 citations
Publisher

Abstract

For high‐efficiency optical quantum technology platforms, bonding rare‐earth doped crystals on glass are of great interest, enabling the enhancement of the optical guided mode interaction with the active rare‐earth ions. Optical coherence lifetimes and stable operation are operated at very low temperature (3–4 K). Bonding should be preferably performed without adding any intermediate sticking layer in order not to hamper the optical mode distribution and propagation. Directly bonded Y2SiO5 crystal on a borosilicate glass is shown and the bonding mechanisms are discussed.

DOI
10.1002/pssa.202300718

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For high‐efficiency optical quantum technology platforms, bonding rare‐earth doped crystals on glass are of great interest, enabling the enhancement of the optical guided mode interaction with the active rare‐earth ions. Optical coherence lifetimes and stable operation are operated at very low temperature (3–4 K). Bonding should be preferably performed without adding any intermediate sticking layer in order not to hamper the optical mode distribution and propagation. Directly bonded Y2SiO5 crystal on a borosilicate glass is shown and the bonding mechanisms are discussed.
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.
Abstract Eu³⁺-doped borate and phosphate glasses were synthesized by the conventional melt-quenching technique and investigated as potential materials for quantum photonic and solid-state quantum computing applications. X-ray diffraction analysis confirmed the highly amorphous and isotropic nature of the prepared glasses, providing favorable conditions for coherent photon propagation with minimal scattering losses. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) revealed homogeneous morphology and uniform distribution of Eu³⁺ ions, which are essential for stable light–matter interactions and optical qubit confinement. Physical and optical characterization demonstrated distinct advantages for both glass systems. Borate glasses exhibited higher density, refractive index, field strength, and optical confinement, indicating enhanced photon localization and stronger light–matter interactions suitable for integrated quantum photonic circuits. In contrast, phosphate glasses showed superior rare-earth ion solubility, improved ion dispersion, reduced concentration quenching, and greater spectral stability, making them promising candidates for optical quantum memory and photon-buffering applications. Optical absorption and photoluminescence studies revealed intense Eu³⁺ radiative transitions, particularly the hypersensitive ⁵D₀ → ⁷F₂ transition near 612 nm, confirming strong electric-dipole interactions and efficient quantum optical emission behavior. Judd–Ofelt analysis indicated enhanced asymmetry and covalent bonding around Eu³⁺ ions, resulting in increased radiative transition probabilities, optical gain, and stimulated emission characteristics. These properties are highly beneficial for coherent optical amplification, photon manipulation, and photonic qubit operations. The combined structural, physical, and spectroscopic results demonstrate that Eu³⁺-activated borate and phosphate glasses possess the characteristics required for rare-earth-based quantum emitters, optical quantum memory devices, integrated photonic circuits, quantum sensors, and solid-state quantum computing architectures. The study establishes both glass systems as promising multifunctional platforms for emerging quantum photonic technologies and advanced quantum information processing applications.
With an assortment of narrow line-width transitions spanning the visible and IR spectrum and long spin coherence times, rare-earth doped crystals are the leading material system for solid-state quantum memories. Integrating these materials in an on-chip optical platform would create opportunities for highly integrated light-matter interfaces for quantum communication and quantum computing. Nano-photonic resonators with high quality factors and small mode volumes are required for efficient on-chip coupling to the small dipole moment of rare-earth ion transitions. However, direct fabrication of optical cavities in these crystals with current nanofabrication techniques is difficult and unparallelized, as either exotic etch chemistries or physical milling processes are required. We fabricated hybrid devices by mechanically transferring a nanoscale membrane of gallium arsenide (GaAs) onto a neodymium-doped yttrium silicon oxide (Y2SiO5) crystal and then using electron beam lithography and standard III-V dry etching to pattern nanobeam photonic crystal cavities and ring resonator cavities, a technique that is easily adapted to other frequency ranges for arbitrary dopants in any rare earth host system. Single crystalline GaAs was chosen for its low loss and high refractive index at the transition wavelength. We demonstrated the potential to evanescently couple between the cavity field and the 883 nm 4I9/2- 4F3/2 transition of nearby neodymium impurities in the host crystal by examining transmission spectra through a waveguide coupled to the resonator with a custom-built confocal microscope. The prospects and requirements for using this system for scalable quantum networks are discussed.
Rare earth doped crystals are a promising platform for long-distance quantum communications, where they can perform quantum light storage. Among different systems, 171{}^{171}171Yb3+{}^{3+}3+-doped Y2{}_{2}2​SiO5{}_{5}5​ (YSO) crystals stand out because of long optical and spin coherence times, and large hyperfine splitting, enabling long-time storage over large bandwidths. Here, the authors study the mechanisms affecting the optical decoherence in a broad range of temperatures (40 mK to 18 K) using high-resolution spectroscopic techniques, and identify ways to further improve the crystal's properties.