13 research outputs found

    Self-Optimizing Photoelectrochemical Growth of Nanopatterned Se–Te Films in Response to the Spectral Distribution of Incident Illumination

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    Photoelectrochemical growth of Se–Te films spontaneously produces highly ordered, nanoscale lamellar morphologies with periodicities that can be tuned by varying the illumination wavelength during deposition. This phenomenon has been characterized further herein by determining the morphologies of photoelectrodeposited Se–Te films in response to tailored spectral illumination profiles. Se–Te films grown under illumination from four different sources, having similar average wavelengths but having spectral bandwidths that spanned several orders of magnitude, all nevertheless produced similar structures which had a single, common periodicity as quantitatively identified via Fourier analysis. Film deposition using simultaneous illumination from two narrowband sources, which differed in average wavelength by several hundred nanometers, resulted in a structure with only a single periodicity intermediate between the periods observed when either source alone was used. This single periodicity could be varied by manipulating the relative intensity of the two sources. An iterative model that combined full-wave electromagnetic effects with Monte Carlo growth simulations, and that considered only the fundamental light-material interactions during deposition, was in accord with the morphologies observed experimentally. Simulations of light absorption and concentration in idealized lamellar arrays, in conjunction with all of the available data, additionally indicated that a self-optimization of the periodicity of the nanoscale pattern, resulting in the maximization of the anisotropy of interfacial light absorption in the three-dimensional structure, is consistent with the observed growth process of such films

    Morphological Expression of the Coherence and Relative Phase of Optical Inputs to the Photoelectrodeposition of Nanopatterned Se-Te Films

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    Highly anisotropic and ordered nanoscale lamellar morphologies can be spontaneously generated over macroscopic areas, without the use of a photomask or any templating agents, via the photoelectrodeposition of Se–Te alloy films. To form such structures, the light source can be a single, linearly polarized light source that need not necessarily be highly coherent. In this work, the variation in the morphologies produced by this deposition process was evaluated in response to differences in the coherence and relative phase between multiple simultaneous linearly polarized illumination inputs. Specifically, the morphologies of photoelectrodeposits were evaluated when two tandem same-wavelength sources with discrete linear polarizations, both either mutually incoherent or mutually coherent (with defined phase differences), were used. Additionally, morphologies were simulated via computer modeling of the interfacial light scattering and absorption during the photoelectrochemical growth process. The morphologies that were generated using two coherent, in-phase sources were equivalent to those generated using only a single source. In contrast, the use of two coherent, out-of-phase sources produced a range of morphological patterns. For small out-of-phase addition of orthogonal polarization components, lamellar-type patterns were observed. When fully out-of-phase orthogonal sources (circular polarization) were used, an isotropic, mesh-type pattern was instead generated, similar to that observed when unpolarized illumination was utilized. In intermediate cases, anisotropic lamellar-type patterns were superimposed on the isotropic mesh-type patterns, and the relative height between the two structures scaled with the amount of out-of-phase addition of the orthogonal polarization components. Similar results were obtained when two incoherent sources were utilized. In every case, the long axis of the lamellar-type morphology component aligned parallel to the intensity-weighted average polarization orientation. The observations consistently agreed with computer simulations, indicating that the observed morphologies were fully determined by the nature of the illumination utilized during the growth process. The collective data thus indicated that the photoelectrodeposition process exhibits sensitivity toward the coherency, relative phase, and polarization orientations of all optical inputs and that this sensitivity is physically expressed in the morphology of the deposit

    Microscopic Relaxation Channels in Materials for Superconducting Qubits

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    Despite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties and qubit coherence are poorly understood. Here, we perform measurements of transmon qubit relaxation times T1T_1 in parallel with spectroscopy and microscopy of the thin polycrystalline niobium films used in qubit fabrication. By comparing results for films deposited using three techniques, we reveal correlations between T1T_1 and grain size, enhanced oxygen diffusion along grain boundaries, and the concentration of suboxides near the surface. Physical mechanisms connect these microscopic properties to residual surface resistance and T1T_1 through losses arising from the grain boundaries and from defects in the suboxides. Further, experiments show that the residual resistance ratio can be used as a figure of merit for qubit lifetime. This comprehensive approach to understanding qubit decoherence charts a pathway for materials-driven improvements of superconducting qubit performance

    Hamiltonian and materials engineering for superconducting qubit lifetime enhancement

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    The potential for quantum computing to expand the number of solvable problems has driven researchers across academia and industry, in multiple disciplines, to develop a variety of different qubit platforms, algorithms, and scaling strategies. At its core, quantum computation relies on the robustness, or coherence, of its building blocks (``qubits"). In current small-scale superconducting qubit processors, the fidelity of operations is often limited by qubit coherence. The coherence time of a single qubit depends on its lifetime T1T_1 and pure dephasing time TϕT_{\phi}. In this thesis, we focus on the problem of improving T1T_1. Strategies for improving lifetimes are informed by models for relaxation - specifically Fermi's Golden Rule. Relaxation rates depend on noise properties of the environment and on properties of the qubit states. This dependence suggests two strategies for engineering longer lifetimes: environment engineering involves mitigating or filtering the noise that the qubit sees, and Hamiltonian engineering refers to optimizing the qubit circuit and its resulting eigenstates to optimize T1T_1. Significant enhancements of qubit lifetimes will require paradigm shifts in our approaches to both environment and Hamiltonian engineering. First, I present a side-by-side study of transmon coherence and materials measurements of the constituent Nb films, including synchrotron x-ray spectroscopy and electron microscopy. We found correlations between qubit lifetimes and materials properties such as grain size, grain boundary quality, and surface suboxides. This study expands the scope of superconducting qubit research by presenting a broad set of materials analyses alongside device measurements. Second, I will give an overview of Hamiltonian engineering, including the concepts behind intrinsic protection against relaxation and dephasing processes. I'll describe the soft 0−π\mathrm{0-\pi} qubit, which is the first experimentally realized superconducting qubit to show signatures of simultaneous T1T_1 and T2T_2 protection. We improved coherence in the soft 0−π\mathrm{0-\pi} through optimized fabrication processes. We have also characterized the effects of non-computational levels on gate fidelity, specifically AC Stark shifts and leakage. From the results in this thesis, we have gained a deeper understanding of what limits qubit coherence, informing future directions on both the materials and Hamiltonian engineering fronts

    Polarization Control of Morphological Pattern Orientation During Light-Mediated Synthesis of Nanostructured Se–Te Films

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    The template-free growth of well ordered, highly anisotropic lamellar structures has been demonstrated during the photoelectrodeposition of Se–Te films, wherein the orientation of the pattern can be directed by orienting the linear polarization of the incident light. This control mechanism was investigated further herein by examining the morphologies of films grown photoelectrochemically using light from two simultaneous sources that had mutually different linear polarizations. Photoelectrochemical growth with light from two nonorthogonally polarized same-wavelength sources generated lamellar morphologies in which the long axes of the lamellae were oriented parallel to the intensity-weighted average polarization orientation. Simulations of light scattering at the solution–film interface were consistent with this observation. Computer modeling of these growths using combined full-wave electromagnetic and Monte Carlo growth simulations successfully reproduced the experimental morphologies and quantitatively agreed with the pattern orientations observed experimentally by considering only the fundamental light-material interactions during growth. Deposition with light from two orthogonally polarized same-wavelength as well as different-wavelength sources produced structures that consisted of two intersecting sets of orthogonally oriented lamellae in which the relative heights of the two sets could be varied by adjusting the relative source intensities. Simulations of light absorption were performed in analogous, idealized intersecting lamellar structures and revealed that the lamellae preferentially absorbed light polarized with the electric field vector along their long axes. These data sets cumulatively indicate that anisotropic light scattering and light absorption generated by the light polarization produces the anisotropic morphology and that the resultant morphology is a function of all illumination inputs despite differing polarizations
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