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Quantum Photonic and Rare-Earth Qubit Behavior of Eu³⁺-Activated Borate and Phosphate Glasses for Solid-State Quantum Computing Applications

Vijaya Kumar Chavan, M. Sreenivasulu, V. Ravi Teja

Published Jun 10, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.21203/rs.3.rs-9898408/v1

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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.
This thesis describes investigations of the use of cryogenically cooled rare-earth-ion-doped crystals for quantum information processing and quantum optics. Several aspects of the coherent interaction between light and rare-earth ions in solids are addressed. Quantum information science has given physicists new views of quantum mechanics. The transmission of quantum states has already found practical use and full scale quantum computers may one day perform computations and simulations that would be impossible on a conventional computer. The work presented in this thesis can be seen as a part of a broad effort to learn how to control and manipulate quantum mechanical systems, which will become necessary as science and technology continue to push ever deeper into the nanoscopic world. Coherent radiation, such as laser light, provides us with an ideal tool for these investigations and, along the way, we may also learn more about the quantum nature of light. Rare-earth ions in inorganic crystals have several unusual properties that are interesting for applications within quantum information science, including long coherence times and long-lived ground state sublevels that can be used for storage of quantum and classical information. As part of the work presented in this thesis, new materials have been investigated with respect to these properties, and ways to enhance the useful properties of the materials were explored. In one investigation, the lifetime of information stored in the ground state population distribution of Tm3 ions in YAG was shown to increase by several orders of magnitude with the application of a magnetic field. It is demonstrated how the optical inhomogeneous absorption profile can be prepared, so that the light only interacts with a selected group of ions, absorbing on a specific transition. Narrow absorbing structures, with widths approaching the optical homogeneous linewidth, have been prepared with no absorption in the surrounding spectral interval. This thesis addresses the use of such structures as hardware for quantum bits. Tailored pulses, capable of inducing controlled changes in the quantum states of the ions (qubits), even in the presence of unknown variations of coupling strengths and frequencies, have been realised experimentally and used for multiple transfer of ions between energy levels. Ion-ion interactions, which can be used for performing quantum logic operations, have been investigated in some detail. Techniques for selecting strongly interacting ions, by transferring weakly interacting ions to auxiliary states, have been demonstrated. A scheme for storing the quantum state of light in a solid, using photon-echo-like techniques, is proposed and analysed. In the proposed scheme, an optical wave packet is absorbed and subsequently re-emitted by an inhomogeneous absorption profile, which is tailored and externally controlled by the application of an electric field Additionally, an accumulated photon echo experiment has been performed using faint optical pulses. The experiment can be viewed as a demonstration of delayed self-interference of a single photon and as a demonstration of how a single photon can act as two of the fields in a photon echo process.
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.
Rare earth ions (REI) in solid materials are among the leading systems for quantum technology applications. However, developing practical REI quantum devices with long-lived coherent states remains challenging due to great difficulties in growing high-quality REI materials and incomplete understanding of their decoherence mechanisms. In this work, we report a long optical coherence time of 422 ± 11 μs for the 7F0 → 5D0 transition, as well as a lifetime exceeding 30 hours for the 7F0 hyperfine spin states in Eu3+:Y2O3 optical ceramics. We identify the absence of two-level-system induced optical decoherence in short-range-ordered crystals and a decoherence mechanism caused by perturbing magnetic centers that were not detected previously below 1.5 K. Furthermore, we demonstrate coherent light storage over 5 μs by using the atomic frequency comb protocol. These results provide a promising proof-of-principle demonstration of quantum memory using Eu3+:Y2O3 optical ceramic system, highlighting its significant potential for practical quantum applications. Rare earth ions (REI) in solids are promising for quantum technologies. Here, the authors demonstrate a record optical coherence lifetime and the absence of two-level-system induced decoherence in Eu3+:Y2O3 optical ceramics. These findings advance the optimization of REI-based quantum systems and enable tailored material designs.
Without using any trapping lasers, rare-earth (RE) ions can be doped into some nanofabricated materials in form of nanocrystals in a solid state matrix, thus rendering a natural trap for potential application as a quantum computing device. In particular, passive nuclear spin states of RE ions can be utilized as (i) optical quantum memory (with huge density as yielded from nanocrystals), (ii) qubits by encoding in ground hyperfine states and operated as (iii) two-qubit gate when transferring them to active magnetized electronic states. A feasible implementation of qubits is realised by using those doped RE ions whose open-shell (for inner highly shielded 4f4 f4f shell electrons) energy levels and statevectors are well described by the crystal field (CF) interaction and hyperfine coupling in which the Hamiltonian is composed of various freeion interactions, the CF potential, electronic and nuclear Zeeman effects as well as the hyperfine interaction. For those qubit encoded in the two ground hyperfine states of RE ions, they can have fairly long lifetimes of days and coherence times (from 1.3s for Er3+\mathrm{Er}^{3+}Er3+ to 6 hours for Eu3+\mathrm{Eu}^{3+}Eu3+) due to the unique spectroscopic properties of RE ions. Because of the recent progress in optical single-ion detection for addressing individual qubits with a specific frequency channel, RE ions could be packed in a solid state 3D matrix at nanometer spacing, hence rendering very high qubit densities for quantum memories. Meanwhile, this configure also leads to strong dipole-dipole interactions (or high connectivity) between qubits which are not nearest neighbors. In this study, Kramers ions like 167Er{ }^{167} \mathbf{E r}167Er and 171Yb{ }^{171} \mathbf{Y b}171Yb doped in yttrium orthosilicate Y2SiO5{Y}_{2} {SiO}_{5}Y2​SiO5​ (YSO) nanocrystals will be investigated as a promising system for quantum computing because of their weak spin bath and possibility for being directly milled in single nanocrystals as nano-resonators. By using the famous Wien2k package to carry out the first-principles calculations, we can obtain the CF parameters and hyperfine constants as well as various electronic properties like energy gap and band structure for those Kramers RE ions doped in YSO. We shall present some essential findings from first-principles calculations to obtain various properties of the ground and excited CF and hyperfine states for those RE ions with an aim to optimize RE ions doping in YSO nanocrystals for quantum memories and qubits, contributing to scalable quantum computing architectures.
Abstract An optical quantum memory is a device that can store photonic quantum information and release it after a controlled time. It is an essential component for overcoming channel losses in large‐scale quantum networks. Optical quantum memories have been demonstrated with various physical systems including atomic gases, single atoms in optical cavities, and rare‐earth‐ion doped solids. Now, quantum memories are marching toward miniaturization and integration for large‐scale practical applications. Solid state systems stand as a natural choice due to the physical stability and ease of micro or nano fabrication using well‐established techniques. In the past decade, considerable efforts have been devoted to developing photonic integrated quantum memories, that is, quantum memories based on micro/nano‐photonic structures manufactured in solids. Remarkable performances have been achieved with integrated quantum memories, with the advantages of lower laser/electric power requirements, small volumes, large storage densities, and easy implementations. In this article, the basic concepts of optical quantum memories, the state‐of‐the‐art technologies for fabricating integrated quantum memories in rare‐earth ions doped crystals, and recent advances are introduced, and the roadmap for developing practically useful devices for applications in quantum networks is discussed.
Quantum light-matter interfaces that reversibly map the quantum state of photons onto the quantum states of atoms, are essential components in the quantum engineering toolbox with applications in quantum communication, computing, and quantum-enabled sensing. In this talk I present our progress towards developing on-chip quantum light-matter interfaces based on nanophotonic resonators fabricated in rare-earth-doped crystals known to exhibit the longest optical and spin coherence times in the solid state. We recently demonstrated coherent control of neodymium (Nd3+) ions coupled to yttrium orthosilicate Y2SiO5 (YSO) photonic crystal nano-beam resonator. The coupling of the Nd3+ 883 nm 4I9/2-4F3/2 transition to the nano-resonator results in a 40 fold enhancement of the transition rate (Purcell effect), and increased optical absorption (~80%) - adequate for realizing efficient optical quantum memories via cavity impedance matching. Optical coherence times T2 up to 100 μs with low spectral diffusion were measured for ions embedded in photonic crystals, which are comparable to those observed in unprocessed bulk samples. This indicates that the remarkable coherence properties of REIs are preserved during nanofabrication process. Multi-temporal mode photon storage using stimulated photon echo and atomic frequency comb (AFC) protocols were implemented in these nano-resonators. Our current technology can be readily transferred to Erbium (Er) doped YSO devices, therefore opening the possibility of efficient on-chip optical quantum memory at 1.5 μm telecom wavelength. Integration with superconducting qubits can lead to devices for reversible quantum conversion of optical photons to microwave photons.