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Abstract
Optical quantum communication represents a new generation of information technology, with applications from quantum cryptography for secure communication to shared quantum computing resources for solving novel computational problems. However, quantum memory is required to facilitate synchronized operations across long communication channels. Erbium (Er) is an ideal defect for such quantum memory applications: Er emits in the telecom C-band, exhibits spin coherence times in excess of a millisecond in the literature, and can be integrated into a variety of crystalline materials. In this thesis, we initiate development of erbium-doped cerium dioxide (CeO2) thin films on silicon as a quantum memory platform, finding half-microsecond spin and optical coherence times even in un-optimized CeO2 thin films. We additionally discuss integration of CeO2 onto other substrates, and selection of other material hosts based on additional criteria such as Er optical emission rate.
To benchmark Er-doped CeO2 (Er:CeO2), we grow thin films of Er:CeO2 on (111)-oriented silicon by molecular beam epitaxy (MBE) with a range of Er doping levels. Microstructural characterization confirms the structure of the CeO2 host, while Er spin measurements confirm its embedding into the CeO2 matrix. Characterization of the Er optical spectrum additionally confirms the remainder of the energy level structure relevant to our work. We establish a baseline for this material in the context of key metrics for rare-earth doped defects systems: as-grown CeO2 films on silicon with 2-3 ppm Er doping show spin linewidths as narrow as 245(1) MHz, optical inhomogeneous linewidths down to 9.5(2) GHz, and an optical excited state lifetime as long as 3.5(1) ms. To complete our benchmark of Er:CeO2, we find a homogeneous linewidth of 440 kHz for the Y1 -Z1 optical transition, and Er spin coherence of 0.66 µs in the single-ion limit. These coherence times, currently limited by grown-in defects and high Er doping concentrations, may be increased through optimization of our CeO2 growth conditions and heterostructure design – but are already sufficiently long to warrant further exploration of MBE-grown Er:CeO2 as a quantum memory platform.
We additionally explore CeO2 on (001)-oriented substrates, for increased compatibility with standard nanofabrication platforms such as silicon-on-insulator (SOI). We found that, when grown on Si(001) or (001)-oriented strontium titanate (STO), the inhomogeneous linewidth and spectral diffusion linewidth of Er:CeO2 are both broader than on Si(111). We also find that the multiple domains of CeO2(110) that result upon Si(001) substrates do not appear to cause additional inhomogeneous broadening when compared to single-crystal CeO2(001) on STO(001), corroborating similar results in other materials. We aim to identify a route towards improving optical properties of CeO2 on Si(001) to match or surpass those found on Si(111), again with an eye towards future integration on (001)-oriented SOI.
Finally, through high-throughput computational methods, we compute oscillator strengths for Er embedded in ∼58,000 materials to identify which hosts result in higher optical relaxation rates. We find that careful host selection could lead to order of magnitude greater Er emission rates compared to current hosts of interest such as CeO2. Combined with nanophotonics engineering, we speculate that this could push Er-based quantum memory into the 1 GHz bandwidth regime. The next step for this project is to obtain Er-doped samples of promising hosts to experimentally confirm their brightness, and thus verify our host selection process using this technique.
Over the course of this thesis we have taken three major steps in the advancement of quantum communication, taking a defect-first approach towards development of our chosen platform. We have identified a viable quantum communication qubit platform in Er:CeO2(111) grown on Si(111); we have extended the viability of Er:CeO2 integration to include Si(001) substrates; and we established computational survey techniques to target additional materials that could provide alternative benefits such as faster optical transitions – all in the development of rare-earth based quantum communication technologies.