Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X39: Superconducting circuits: Fabrication and materials I |
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Sponsoring Units: DQI Chair: Jonilyn Yonder, Massachusetts Institute of Technology Room: LACC 501B |
Friday, March 9, 2018 8:00AM - 8:12AM |
X39.00001: Characterization of low loss microstrip resonators as a building block for circuit QED in a 3D waveguide David Zoepfl, Phani Raja Muppalla, Christian Schneider, Stephan Kasemann, Stefan Partel, Gerhard Kirchmair We perfomed microwave characterization on a new kind of microstrip resonator made from niobium and aluminum, by placing it in a rectangular waveguide. Our novel approach combines the low loss advantages of three dimensional structures with a compact planar design. The setup allows flexible coupling to the resonator, depending on its position within the waveguide. The waveguide represents a well defined microwave environment, which makes our setup appealing for testing materials. In the low temperature, low power regime, we measure a single photon internal quality factor of up to one million. The demonstrated quality factor is comparable to state of the art coplanar waveguide resonators, but does not require complex fabrication procedures. We measured the internal quality factor with increasing temperature, where our data shows good agreement with theoretical models predicting temperature dependent behavior. |
Friday, March 9, 2018 8:12AM - 8:24AM |
X39.00002: Cryogenic thermal stresses induced in superconducting TiN Margaret Samuels, Christopher Richardson Thin film superconductors used for quantum computing applications require cryogenic operating temperatures. Researchers growing superconducting TiN often use room temperature stress as an optimization metric for film quality. During cryogenic use, the mismatch in coefficients of thermal expansion between the TiN film and Si substrate can lead to very large stresses in the film and perhaps plastic deformation. |
Friday, March 9, 2018 8:24AM - 8:36AM |
X39.00003: High internal Quality factor MBE grown TiN superconducting resonators Christopher Richardson, Ashish Alexander, Christopher Weddle
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Friday, March 9, 2018 8:36AM - 8:48AM |
X39.00004: Decontamination and Passivation of Superconducting Al Resonators with Supercritical NF3 Chris Barrett, Rodrigo Guerrero, Bruce Arey, Shutthanandan Vaithiyalingam, Marvin Warner, Ashish Alexander, Christopher Weddle, Christopher Richardson Surface contamination encountered during microfabrication and packaging of superconducting devices is known to limit qubit performance. Here, a method and system of abating participation of surface contamination on coplanar waveguide resonators using supercritical NF3 is introduced. Application of NF3 in its supercritical form, as opposed to a plasma, is explored as a less-invasive means of device processing, mitigating any further artifact formation and extending greater control over the chemical treatment. Previous efforts with supercritical CO2 have been shown to reduce the spread of measured internal quality factor (Qi) in Al resonators, with Qi greater than 106 at the single photon level. However, subsequent analysis of processed devices has indicated that carbon is a persistent surface adsorbate that would appear to contribute to losses. With this in mind, the oxidizing behavior of supercritical NF3 is being explored to both scavenge residual carbon, while also fluorinating the underlying native oxide surfaces. Using observations from x-ray photoelectron spectroscopy and helium ion microscopy, ongoing efforts to evaluate and exploit the cleaning potential of NF3, and subsequent passivation of Al surfaces will be discussed. |
Friday, March 9, 2018 8:48AM - 9:00AM |
X39.00005: Self-aligned coplanar waveguides with crossovers, for resonators, couplers, and amplifiers Russell Lake, Xian Wu, Mustafa Bal, Junling Long, Hsiang-Sheng Ku, David Pappas We implement a self-aligned fabrication technology for low-loss, superconducting coplanar waveguides (CPWs). In this talk I describe our fabrication process and present loss measurements of resonators fabricated with this technique. We form the CPW center lines and ground planes using a common hard mask. Using this approach, we can also connect separated ground planes by forming crossover bridges within the ground plane layer. This obviates the need to make crossovers in separate lithography steps or use flip-chip geometries. The narrow gaps (down to 200 nm) yielded by this process give characteristic impedances approaching 50 ohms even when using high-kinetic inductance superconductors such as NbTiN. More importantly, our process is compatible with modern planarization techniques to enable large-scale integration of 2D quantum circuits. Initial measurements for 20-nm-thick NbTiN on Si(100) yielded Qi = 4.63e5 at 7.9756 GHz with a 1 micrometer gap. |
Friday, March 9, 2018 9:00AM - 9:12AM |
X39.00006: An argon ion beam milling process for native AlOx layers enabling coherent superconducting contacts Uwe Von Luepke, Lukas Gruenhaupt, Daria Gusenkova, Sebastian Skacel, Nataliya Maleeva, Steffen Schloer, Alexander Bilmes, Hannes Rotzinger, Alexey Ustinov, Martin Weides, Ioan-Mihai Pop We present an argon ion beam milling process [1] to remove the native oxide layer forming on aluminum thin films due to their exposure to atmosphere in between lithographic steps. Our cleaning process is readily integrable with conventional fabrication of Josephson junction quantum circuits. From measurements of the internal quality factors of superconducting microwave resonators with and without contacts, we place an upper bound on the residual resistance of an ion beam milled contact of $50$ $m\Omega \cdot \mu m^{2}$ at a frequency of 4.5 GHz. Resonators for which only 6\% of the total foot-print was exposed to the ion beam milling, in areas of low electric and high magnetic field, showed quality factors above $10^{6}$ in the single photon regime, and no degradation compared to single layer samples. We believe these results will enable the development of increasingly complex superconducting circuits for quantum information processing. |
Friday, March 9, 2018 9:12AM - 9:24AM |
X39.00007: Possible quasiparticle localization induced by substrate treatment in superconducting TiN film resonators Timothy Kohler, Peng Xu, Evgeniya Lock, Serena Eley, Yaniv Rosen, Leonardo Civale, Kevin Osborn
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Friday, March 9, 2018 9:24AM - 9:36AM |
X39.00008: Probing Micro-eV States at the Material Interfaces of Superconducting Devices Yaniv Rosen, Dongxia Qu, Bahman Sarabi, Kevin Osborn, Jonathan DuBois Micro-eV states in amorphous materials and at material interfaces have long plagued planar superconducting circuits. These states interfere with qubit lifetimes and often set the limit on resonator quality factors in the single photon regime. We propose a superconducting resonant circuit that enables characterization of these states for planar devices. Using a biased-bridge interdigitated capacitor circuit we apply a slow-varying (megahertz) electric field on the defect states and sweep their energies. We will discuss different experiments, including probes of energy distributions, dipole moments, and reaching the strong photon-µeV state coupling limit in these devices. |
Friday, March 9, 2018 9:36AM - 9:48AM |
X39.00009: Quantitative Analysis of Surface Losses in Coplanar Waveguide Resonators, Part 1: Materials and Fabrication Alexander Melville, Greg Calusine, Wayne Woods, Rabindra Das, Evan Golden, Corey Stull, Vlad Bolkhovsky, Danielle Braje, David Hover, David Kim, Xhovalin Miloshi, Danna Rosenberg, Arjan Sevi, Jonilyn Yoder, Eric Dauler, William Oliver Two-level systems (TLS) at metal-substrate, metal-air, and substrate-air interfaces are a significant contributor to loss in superconducting resonators probed at the single-photon limit. The choice of fabrication methods for different combinations of superconducting material and substrate affects the TLS density at these interfaces. In this talk, we discuss the process development of high quality factor resonators (Qi >1M) for aluminum, titanium nitride, and niobium, and how both anisotropic and isotropic etching facilitates device geometries designed to focus TLS participation on specific interfaces. Material-specific fabrication challenges are addressed to enable a comparison of surface losses between materials or for a specific fabrication process. Lastly, we report progress on applying this understanding to superconducting qubit devices. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X39.00010: Quantitative Analysis of Surface Losses in Coplanar Waveguide Resonators Part 2: Anisotropic Trenching Greg Calusine, Alexander Melville, Wayne Woods, Rabindra Das, Evan Golden, Corey Stull, Vlad Bolkhovsky, Danielle Braje, David Hover, David Kim, Xhovalin Miloshi, Danna Rosenberg, Arjan Sevi, Jonilyn Yoder, Eric Dauler, William Oliver Deep anisotropic etching into the substrate drastically alters the surface participation ratios of coplanar waveguide resonators. This technique provides one method to mitigate losses by reducing overall participation or to shift losses between interfaces for comparison to modeling. Here we demonstrate titanium nitride coplanar waveguide resonators with mean quality factors exceeding two million and controlled trenching reaching 2.2 µm into the silicon substrate. We measure sets of resonators with a range of sizes and trench depths and compare these results with finite-element simulations to demonstrate quantitative agreement with a model of interface dielectric loss. We then apply this analysis to determine the extent to which trenching can improve resonator performance. Furthermore, we report progress on understanding other loss contributions in these systems and the application of trenching to superconducting qubit devices. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X39.00011: Quantitative Analysis of Surface Losses in Coplanar Waveguide Resonators Part 3: Surface Loss Extraction Wayne Woods, Alexander Melville, Greg Calusine, Rabindra Das, Evan Golden, Corey Stull, Vlad Bolkhovsky, Danielle Braje, David Hover, David Kim, Xhovalin Miloshi, Danna Rosenberg, Arjan Sevi, Jonilyn Yoder, Eric Dauler, William Oliver Uniquely characterizing TLS defect layers around superconductors is challenging due to the nearly proportional scaling multiple of the defect layer participations in response to changes in geometry and anisotropic trench depth. We design a set of superconducting coplanar waveguide resonators utilizing deep isotropic etching into the silicon substrate to enable the extraction of independent surface losses. We then combine finite element electromagnetic simulations with statistical characterization of these isotropically etched resonators to determine the independent loss contribution from different interfaces for a high-Q TiN superconductor fabrication process. This characterization technique can be used to quantify the impact of process changes on individual defect layer losses and also as a general process monitor of multiple defect layer losses in co-fabricated superconducting qubit circuits. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X39.00012: Effect of surface treatment on superconducting qubit coherence Bradley Christensen, Pradeep Kumar, JJ Nelson, Yebin Liu, Andrew Ballard, Britton Plourde, Robert McDermott A promising approach toward solid-state universal quantum computer relies on the use of frequency-tunable qubits to apply the necessary entangling gates. However, while the qubits are detuned from a flux insensitive point, they are susceptible to flux noise, causing rapid dephasing. Furthermore, weakly-coupled defects can resonantly interact with the qubit, leading to energy relaxation. Thus, these points of defect interaction must be avoided during operation, thereby exasperating the frequency crowding issue present in large qubit arrays. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X39.00013: Exploration of Alternate Fabrication and Processing Techniques for Superconducting Qubit Junctions Yebin Liu, JJ Nelson, Jaseung Ku, Britton Plourde The performance of superconducting qubits can be significantly impacted by the fabrication process for forming the qubit junctions and shunt capacitance. Contamination and processing residues on the surfaces and interfaces can lead to substantial reductions in the coherence of superconducting qubits and quality factor of superconducting resonators. Therefore, it is of great importance to develop fabrication techniques that minimize such residues for producing high-quality qubits. In addition, we are exploring different lithographic approaches for achieving these goals. We present our ongoing efforts on alternate fabrication and processing techniques for superconducting qubit junctions. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X39.00014: Circuit quantum electrodynamics of granular Aluminum resonators Nataliya Maleeva, Lukas Gruenhaupt, Florence Levy-Bertrand, Martino Calvo, Alexey Ustinov, Hannes Rotzinger, Alessandro Monfardini, Mikhail Fistul, Ioan-Mihai Pop Granular Aluminum (GrAl) thin film structures can be used as superinductors in superconducting qubits, or as high kinetic inductance detectors for mm wavelength radiation. Their microstructure consists of pure Aluminum grains of 2-5 nm diameter, separated by thin aluminum oxide barriers, forming a self-assembled network of Josephson junctions. We model microwave resonators made of GrAl as a 1D array of effective Josephson junctions, directly relating their dispersion relation and nonlinearity to GrAl microstructure. Below 20 GHz, we can measure the dispersion relation of GrAl stripline resonators using a circuit quantum electrodynamics (cQED) setup. We observe self-Kerr coefficients in the range of 1-100 Hz/photon, depending on the resonator geometry and the sheet resistance of the film. Using an optical setup designed for mm wave spectroscopy we measure the plasma frequency of the film around 75 GHz. These results are in agreement with values calculated using our effective Josephson junction chain model, and they are encouraging for the design of cQED elements such as qubits or parametric amplifiers using GrAl. |
Friday, March 9, 2018 10:48AM - 11:00AM |
X39.00015: Quasiparticle dynamics in microwave resonators from granular aluminum close to the superconductor to insulator transition Lukas Gruenhaupt, Nataliya Maleeva, Sebastian Skacel, Florence Levy-Bertrand, Alexey Ustinov, Hannes Rotzinger, Alessandro Monfardini, Ioan-Mihai Pop Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (GrAl) is a particularly interesting material since it has already shown a kinetic inductance in the range of nH/square and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microstrip resonators in the GHz range fabricated from GrAl thin films with resistivity and geometry dependent kinetic inductances between 0.3 and 1.7 nH/square. Our results suggest that non-equilibrium quasiparticles limit their internal qualitiy factors at a level of 105. We extract quasiparticle relaxation times on the order of 1 s and we observe quasiparticle producing events approximately every 30 s. The current level of coherence of GrAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of non-equilibrium quasiparticles. |
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