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arXiv:2110.04876 · Physical review. B./Physical review. B

Erbium-implanted materials for quantum communication applications

Paul Stevenson, Christopher M. Phenicie, Isaiah Gray, Sebastian P. Horvath, Sacha Welinski, Austin M. Ferrenti, Alban Ferrier, Philippe Goldner, Sujit Das, R. Ramesh, R. J. Cava, Nathalie P. de Leon, Jeff D. Thompson

Published Jun 10, 2022• 73 citations
arXivHTMLPDF

Abstract

Erbium-doped materials can serve as spin-photon interfaces with optical transitions in the telecom $C$ band, making them an exciting class of materials for long-distance quantum communication. However, the spin and optical coherence times of ${\mathrm{Er}}^{3+}$ ions are limited by currently available host materials, motivating the development of new ${\mathrm{Er}}^{3+}$-containing materials. Here we demonstrate the use of ion implantation to efficiently screen prospective host candidates, and show that disorder introduced by ion implantation can be mitigated through post-implantation thermal processing to achieve inhomogeneous linewidths comparable to bulk linewidths in as-grown samples. We present optical spectroscopy data for each host material, which allows us to determine the level structure of each site, allowing us to compare the environments of ${\mathrm{Er}}^{3+}$ introduced via implantation and via doping during growth. We demonstrate that implantation can generate a range of local environments for ${\mathrm{Er}}^{3+}$, including those observed in bulk-doped materials, and that the populations of these sites can be controlled with thermal processing.

DOI
10.1103/physrevb.105.224106

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Erbium-doped materials can serve as spin-photon interfaces with optical transitions in the telecom CCC band, making them an exciting class of materials for long-distance quantum communication. However, the spin and optical coherence times of Er3+{\mathrm{Er}}^{3+}Er3+ ions are limited by currently available host materials, motivating the development of new Er3+{\mathrm{Er}}^{3+}Er3+-containing materials. Here we demonstrate the use of ion implantation to efficiently screen prospective host candidates, and show that disorder introduced by ion implantation can be mitigated through post-implantation thermal processing to achieve inhomogeneous linewidths comparable to bulk linewidths in as-grown samples. We present optical spectroscopy data for each host material, which allows us to determine the level structure of each site, allowing us to compare the environments of Er3+{\mathrm{Er}}^{3+}Er3+ introduced via implantation and via doping during growth. We demonstrate that implantation can generate a range of local environments for Er3+{\mathrm{Er}}^{3+}Er3+, including those observed in bulk-doped materials, and that the populations of these sites can be controlled with thermal processing.
Atomic and atomlike defects in the solid state are widely explored for quantum computers, networks, and sensors. Rare earth ions are an attractive class of atomic defects that feature narrow spin and optical transitions that are isolated from the host crystal, allowing incorporation into a wide range of materials. However, the realization of long electronic spin coherence times is hampered by magnetic noise from abundant nuclear spins in the most widely studied host crystals. Here, we demonstrate that Er 3+ ions can be introduced via ion implantation into TiO 2, a host crystal that has not been studied extensively for rare earth ions and has a low natural abundance of nuclear spins. We observe efficient incorporation of the implanted Er 3+ into the Ti 4+ site (>50% yield) and measure narrow inhomogeneous spin and optical line widths (20 and 460 MHz, respectively) that are comparable to bulk-doped crystalline hosts for Er 3+ . This work demonstrates that ion implantation is a viable path to studying rare earth ions in new hosts and is a significant step toward realizing individually addressed rare earth ions with long spin coherence times for quantum technologies.
Rare-earth ion dopants in solid-state hosts are ideal candidates for quantum communication technologies, such as quantum memories, due to the intrinsic spin–photon interface of the rare-earth ion combined with the integration methods available in the solid state. Erbium-doped cerium oxide (Er:CeO2) is a particularly promising host material platform for such a quantum memory, as it combines the telecom-wavelength (∼1.5μm) 4f–4f transition of erbium, a predicted long electron spin coherence time when embedded in CeO2, and a small lattice mismatch with silicon. In this work, we report on the epitaxial growth of Er:CeO2 thin films on silicon using molecular beam epitaxy, with controlled erbium concentration between 2 and 130 parts per million (ppm). We carry out a detailed microstructural study to verify the CeO2 host structure and characterize the spin and optical properties of the embedded Er3+ ions as a function of doping density. In as-grown Er:CeO2 in the 2–3 ppm regime, we identify an EPR linewidth of 245(1) MHz, an optical inhomogeneous linewidth of 9.5(2) GHz, an optical excited state lifetime of 3.5(1) ms, and a spectral diffusion-limited homogeneous linewidth as narrow as 4.8(3) MHz. We test the annealing of Er:CeO2 films up to 900 °C, which yields narrowing of the inhomogeneous linewidth by 20% and extension of the excited state lifetime by 40%.
Erbium-doped solid-state crystals present an attractive platform for quantum communication, combining the telecom C-band spin–photon interface and long coherence times of erbium (Er3+) with the versatility of solid-state hosts. However, across commonly investigated materials, Er3+ is a “dim” ion, with low optical emission rates of kilohertz at most. This makes implementing quantum information transfer protocols (e.g., quantum teleportation) at usable rates a major challenge. One route to increasing Er3+ radiative rates is by careful selection of crystal hosts: the crystal field around an Er3+ ion may hybridize states of different parity, potentially increasing the radiative rate of its telecom transition. Here, using high-throughput computational methods, the radiative rates of Er3+ embedded in 9215 materials are estimated. The computation shows that careful host selection could increase Er3+ radiative rates by an order of magnitude compared to commonly studied hosts. Calculated results are compared with experimental radiative rates in a subset of materials, finding notable clustering but weak correlation. Nevertheless, the “brightest” candidates combined with suitable nanophotonics engineering could push Er3+-based quantum memories into the megahertz bandwidth regime.
Rare-earth-doped materials have garnered significant attention as material platforms in emerging quantum information and integrated photonic technologies. Concurrently, advances in its nanofabrication processes have unleashed thin film lithium niobate (LN) as a leading force of research in these technologies, encompassing many outstanding properties in a single material. Leveraging the scalability of ion implantation to integrate rare-earth erbium (Er3+), which emits at 1532 nm, into LN can enable a plethora of exciting photonic and quantum technologies operating in the telecom C-band. Many of these technologies rely on coupling via polarization-sensitive photonic structures such as waveguides and optical nanocavities, necessitating fundamental material studies. Toward this goal, we have conducted an extensive study on the role of implantation and post-implantation processing in minimizing implantation-induced defectivity in x-cut thin film LN on an insulator. By leveraging this, we have demonstrated a cutting-edge ensemble optical linewidth of 140 GHz for Er emission in x-cut thin-film LN at 77 K. This finding highlights the significant progress in minimizing implantation-induced defects through careful ion implantation engineering and post-implantation processing. To the best of our knowledge, this linewidth measured at a higher temperature (77 K) is the narrowest when compared to the values reported for bulk-doped and implanted LN crystals at liquid helium temperatures (∼3 K), showcasing the potential of our approach for higher-temperature operation devices. Furthermore, we show that the Er photoluminescence (PL) is highly polarized perpendicular to the x-cut LN c-axis through systematic and combinational PL and high-resolution scanning transmission electron microscopy (HRSTEM) studies. These results indicate that using Er emitters in thin film LN, along with their polarization characteristics and related ion implantation engineering, presents an opportunity to produce highly luminescent Er-doped LN photonic integrated devices and circuits for quantum and nanophotonic applications at telecom wavelengths.
Abstract Doping erbium ions into europium-based hosts offers a promising platform for quantum repeater nodes, combining europium’s exceptional coherence properties for long-term quantum storage with erbium’s microwave compatibility and direct telecom-band emission for efficient optical interfacing. In this work, we investigated erbium-doped EuCl3 · 6H2O stoichiometric crystals as a candidate for such nodes. We demonstrate that erbium doping shifts the optical transition frequencies of nearby Eu3+ ions, producing well resolved satellite lines in the inhomogeneous absorption profile. We experimentally probe the coupling between Er3+ and Eu3+ ions under varying temperature and magnetic field conditions, quantifying the interaction strength, which ranges from tens to hundreds of kilohertz, depending on field orientation, magnitude, and the lattice position of the Eu3+ ions. At 60 mK and a moderate magnetic field of 0.1 T, we observed a strong frozen core effect from Er3+ spins, substantially extending the Eu3+ optical coherence time from 62 us to 162 us, approaching the lifetime limit, and enabling hour-long hyperfine state lifetimes. These results underscore the potential of dual-species rare-earth systems for photonic quantum technologies and highlight their promise for precise quantum control.
Atomic layer deposited (ALD) Y 2 O 3 thin films have been thoroughly investigated for optical or electronic applications. The coherent spectroscopy of lanthanide ions doped into this material has also recently attracted increasing interest in the field of quantum technologies for which they are considered promising candidates in quantum memories or as spin–photon interfaces. However, these most demanding applications require a deep control over the local positioning of the ions and their close environment in the crystalline matrix. This study focuses on the structural as well as optical properties of Eu 3+ and Er 3+ dopants in Y 2 O 3 using photoluminescence (PL), luminescence decay times, and inhomogeneous line width (Γ inh ) measurements within this particular context. While as-grown ALD films do not provide an ideal host for the emitters, we demonstrate that by optimizing the deposition conditions and using appropriate annealing post treatments narrow inhomogeneous lines can be obtained for the 7 F 0 ↔ 5 D 0 transition of Eu 3+ even for nanoscale films. Furthermore, about 1.5 ms lifetime has been measured for the infrared telecom transition of Er in ultrathin films (<10 nm), which is an order of magnitude higher than in nanoparticles of the same size. These results validate optimized rare-earth-doped ALD Y 2 O 3 films as a suitable platform for photonics applications where few-nanometer-thick films with well-localized emitters are mandatory. This approach provides the first building blocks toward the development of more complex devices for quantum sensing or hybrid structures coupled with other systems such as two-dimensional materials.