Quantum memory is a key device for quantum repeaters that are a prerequisite for building global quantum networks. In particular for on-chip quantum memory operating at the telecommunication wavelength, Er-doped $\mathrm{CeO}_{2}\left(\mathrm{Er}: \mathrm{CeO}_{2}\right)$ is a promising material platform, in which a long coherence time is expected. This is because the Er ions are dilutely embedded in the nuclear-spin-free CeO2matrix (crystal). Although we have prepared high-quality Er: CeO2crystals on Si substrates by molecular beam epitaxy, further improvement of their optical properties is desired. Here, we introduced a post-growth annealing process and explored the optimum annealing conditions using Bayesian optimization with a view to increasing the intensity and narrowing the optical absorption linewidth. The post-growth annealing process turned out to be effective, and the resultant improvement in the optical properties can be attributed to improved crystallinity of the host material CeO2that contributes to providing a uniform environment around the Er ions.
Quantum memory is a key device for quantum repeaters that are a prerequisite for building global quantum networks. In particular for on-chip quantum memory operating at the telecommunication wavelength, Er-doped CeO2(Er:CeO2) is a promising material platform, in which a long coherence time is expected. This is because the Er ions are dilutely embedded in the nuclear-spin-free CeO2matrix (crystal). Although we have prepared high-quality Er: CeO2crystals on Si substrates by molecular beam epitaxy, further improvement of their optical properties is desired. Here, we introduced a post-growth annealing process and explored the optimum annealing conditions using Bayesian optimization with a view to increasing the intensity and narrowing the optical absorption linewidth. The post-growth annealing process turned out to be effective, and the resultant improvement in the optical properties can be attributed to improved crystallinity of the host material CeO2that contributes to providing a uniform environment around the Er ions.
Epitaxial growth of erbium-doped cerium oxide (Er:CeO2) is achieved on Si (111) substrates by the cooperative integration of Si and oxide molecular beam epitaxy (MBE) technologies. Lattice matching between CeO2 and Si provides an attractive opportunity to build dilutely doped Er-based optical devices on a Si chip. The CeO2 host crystal is optically transparent for the telecom C-band wavelength and has quite a small magnetic moment, which serves as a disturbance-free environment for the two-level system formed in the doped Er. After the systematic optimization of the growth conditions for stoichiometric Er:CeO2; i.e., (Er + Ce)/O = 1/2, we varied the Er concentration in a range of 1 ~4%. The doped Er showed well-defined optical transitions at the wavelength of 1.533 μm irrespective of the Er concentration. With decreasing Er concentration, enhancement of the luminescence intensity, narrowing of the spectral width, and an increase in the radiative lifetime were observed. The results suggest that CeO2 on Si is promising as a platform for the doped-Er-based optical devices and their quantum optics applications.
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 (Er) is a rare-earth element that is expected to be a platform for manipulation of quantum information by utilizing telecommunication-wavelength photons.However, high-quality epitaxial growth of Er-doped host crystals is difficult, and the memory time of quantum information, which determines the performance of quantum manipulation, is much shorter than theoretically predicted.We investigated a good-compatibility rare-earth oxide as a host crystal for Er.The host oxide crystal, cerium oxide (CeO 2 ), had its nuclear spin removed (i.e., magnetically purified).The nuclear spin is the main factor that shortens the lifetime of quantum-information retention time.In this article, we describe high-quality epitaxial growth of CeO 2 and its optical properties as a thin film grown on a silicon substrate.
The obtention of quantum-grade rare-earth-doped oxide thin films that can be integrated with optical cavities and microwave resonators is of great interest for the development of scalable quantum devices. Among the different growth methods, chemical vapor deposition (CVD) offers high flexibility and has demonstrated the ability to produce oxide films hosting rare-earth ions with narrow linewidths. However, growing epitaxial films directly on silicon is challenging by CVD due to a native amorphous oxide layer formation at the interface. In this manuscript, we investigate the CVD growth of erbium-doped yttrium oxide (Er:Y2O3) thin films on different substrates, including silicon, sapphire, quartz, or yttria stabilized zirconia (YSZ). Alternatively, growth was also attempted on an epitaxial Y2O3 template layer on Si (111) prepared by molecular beam epitaxy (MBE) in order to circumvent the issue of the amorphous interlayer. We found that the substrate impacts the film morphology and the crystalline orientations, with different textures observed for the CVD film on the MBE-oxide/Si template (111) and epitaxial growth on YSZ (001). In terms of optical properties, Er3+ ions exhibit visible and IR emission features that are comparable for all samples, indicating a high-quality local crystalline environment regardless of the substrate. Our approach opens interesting prospects to integrate such films into scalable devices for optical quantum technologies.
The obtention of quantum-grade rare-earth doped oxide thin films that can be integrated with optical cavities and microwave resonators is of great interest for the development of scalable quantum devices. Among the different growth methods, Chemical Vapour Deposition (CVD) offers high flexibility and has demonstrated the ability to produce oxide films hosting rare-earth ions with narrow linewidths. However, growing epitaxial films directly on silicon is challenging by CVD due to a native amorphous oxide layer formation at the interface. In this manuscript, we investigate the CVD growth of erbium-doped yttrium oxide (Er:Y2O3) thin films on different substrates, including silicon, sapphire, quartz or yttria stabilized zirconia (YSZ). Alternatively, growth was also attempted on an epitaxial Y2O3 template layer on Si (111) prepared by molecular beam epitaxy (MBE) in order to circumvent the issue of the amorphous interlayer. We found that the substrate impacts the film morphology and the crystalline orientations, with different textures observed for the CVD film on the MBE-oxide/Si template (111) and epitaxial growth on YSZ (001). In terms of optical properties, Er3+ ions exhibit visible and IR emission features that are comparable for all samples, indicating a high-quality local crystalline environment regardless of the substrate. Our approach opens interesting prospects to integrate such films into scalable devices for optical quantum technologies.
Clock-transition-enabled long electron spin coherence of telecom emitters in CeO2
Rare-earth-ion-doped crystals has emerged as a promising platform for quantum technologies, owing to their narrow telecom-band optical emission, long spin memory, and compatibility with silicon integrated photonic architectures. However, the realization of scalable quantum devices requires host materials with intrinsically dilute spin environments to suppress decoherence. In this context, erbium (Er3+) doped cerium oxide (CeO2) is an attractive candidate due to the ultra-low concentration of nuclear spins in the host matrix and its compatibility with silicon-based technologies. Here we perform a comprehensive investigation of the coherence properties of Er3+ electron spin qubit in CeO2 via semiclassical as well as detailed cluster correlation expansion simulations. By systematically exploring magnetic field strength, pulse sequences, erbium concentration, and spin temperature, we identify regimes where decoherence from the spin bath is strongly suppressed. Our investigations illustrate that at dilute doping concentration (of the order of 10 ppb) and sub-Kelvin temperatures, operation near clock transitions enables Hahn-echo coherence times to approach the second timescale even at natural isotopic abundance. Importantly, from a practical standpoint, coherence times on the order of ∼10 ms are expected even at liquid helium temperature (about 2 K) for similar concentrations. Moreover, we demonstrate that an additional enhancement can be obtained with conventional multi-π-pulse dynamical decoupling protocols. Thus, our findings establish Er3+ doped CeO2 as a front-runner for realizing spin qubits, quantum memories, and integrated quantum networks.