Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F36: Superconducting Qubits: Fabrication and CharacterizationRecordings Available
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Sponsoring Units: DQI DMP DCMP Chair: Katarina Cicak, National Institute of Standards and Technology, Boulder Room: McCormick Place W-194A |
Tuesday, March 15, 2022 8:00AM - 8:12AM |
F36.00001: Niobium Hydride Formation in Superconducting Thin Films during Wet Chemical Processing Dominic P Goronzy, Carlos G Torres Castanedo, Cameron Kopas, Jayss Marshall, Matthew J Reagor, Michael J Bedzyk, Mark C Hersam Wet etching procedures are used at different stages in the nanofabrication of superconducting qubits. However, the use of acidic solutions has the potential to introduce hydrogen into the bulk by diffusion via interstitial sites in the Nb lattice. Niobium hydrides have been previously found to play a key role in the quality factor (coherence) of 3D SRF niobium cavities, and more recently similar hydrides have been discovered in niobium films of 2D superconducting qubits. [*] Here we investigate how chemical processes used in Nb film fabrication could lead to different hydrogen loading amounts and precipitation states. The exposure times and pH of various etchant solutions were varied to determine the formation of Nb hydrides as detected by X-ray reflectivity and X-ray diffraction in Nb (110) / Si (001) thin films. Specifically, changes in the Nb film density, thickness, roughness, crystallinity, and d-spacing have been characterized. Additionally, atomic force microscopy was utilized to observe the etching effects on the surface roughness of Nb thin films and the silicon substrate. This study finds that low pH etching solutions have a propensity to convert Nb metal films to NbHx, which is detrimental for superconducting device performance. [*] J. Lee et al, arXiv:2108.10385 |
Tuesday, March 15, 2022 8:12AM - 8:24AM |
F36.00002: Surfaces, Interfaces, and Impurity Elements in Niobium-based Materials for Superconducting Quantum Computing Analyzed by Atom-Probe Tomography Dieter Isheim, Dominic P Goronzy, Carlos G Torres-Castanedo, Michael J Bedzyk, Mark C Hersam, James A Sauls, Jayss Marshall, Cameron Kopas, Mark Field, Gregory Stiehl, Hilal Cansizoglu, Josh Mutus, Matthew J Reagor, Michael Harburn, Alexander Romanenko, Anna Grassellino, David N Seidman Superconductivity with Tc=9.3 K makes niobium the material of choice for thin film resonator structures in transmon qubits, one of the leading candidates in quantum computing. While the superconducting properties of the Nb thin film itself are robust, the qubit coherency times are thought to be limited by dielectric losses originating from defects at surfaces and interfaces. We apply atom-probe tomography (APT) to compositionally and structurally characterize the surfaces, interfaces, and grain boundaries in Nb thin-films fabricated by high-power impulse magnetron sputtering (HIPIMS) on silicon substrate. The results indicate the formation of a 5-8 nm thick surface oxide film with a NbO or Nb2O5 stoichiometry on top of the Nb thin film, and the formation of a 6-8 nm thick silicide interreaction layer between the Nb thin-film and the Si substrate. The single-atom time-of-flight mass spectrometric sensitivity of APT is particularly suitable to track presence, location, and concentrations of interstitial impurity elements, H, C, N, and O. |
Tuesday, March 15, 2022 8:24AM - 8:36AM |
F36.00003: Hydride precipitates in Nb thin film superconducting qubits Jaeyel Lee, ZuHawn Sung, Akshay A Murthy, Matthew J Reagor, Anna Grassellino, Alexander Romanenko We report the observation of hydride precipitates within Nb thin films on Si substrates as a possible decoherence source for superconducting qubits. We performed high-resolution transmission electron microscopy (HR-TEM) and electron diffraction analysis on the Nb thin films and observed hydrides in the Nb thin films of superconducting qubit test devices. The size, morphology, and atomic structures of the hydrides within the device structures are illustrated. Hydride precipitates tend to be seen near the surface of the Nb films and exhibit variation in size and morphology from small (~5 nm) irregular shaped precipitates within the interior Nb grains to large (~10-100 nm) grains partially or fully transformed to niobium hydride. Atomic force microscope (AFM) analysis on the surface of the device film at cryogenic temperatures shows unknown structures, implying possible structural changes on the surface of Nb during cool down, possibly related to hydride precipitation. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis indicates that the hydrogen and other impurity concentration is enriched near the surface, which may play a role in the nucleation of hydrides near the surface regions. Additional investigations of hydrides in various Nb samples imply that hydride formation in Nb is correlated to the grain size, etc. Nb hydrides are non-superconducting and may contribute to the decoherence due to quasiparticles and two-level system (TLS). This study suggests the importance of preventing hydride formation in the Nb films during the Nb thin film growth and fabrication process. Possible pathways to prevent the detrimental hydride precipitates in Nb thin film are also discussed. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F36.00004: Bulk Loss Measurements of III-V Semiconductor Materials in a Microwave Cavity at Single Photon Powers and Millikelvin Temperatures Nicholas Materise, Eliot Kapit, David Pappas, Anthony McFadden, Haozhi Wang, William M Strickland, Javad Shabani, Joseph Yuan, SHENG-XIANG LIN, Corey Rae H McRae, John Pitten Superconducting-semiconducting hybrid quantum systems look to integrate the success of superconducting qubits with the electronic tunability and half century of materials development in conventional semiconductor manufacturing. As the materials stack on the semiconductor side grows, there is an increasing need for characterizing the loss in these materials. We propose a method for extracting the bulk loss in semiconducting and dielectric samples loaded in the antinode of the TE101 mode of a superconducting rectangular cavity. Unlike in coplanar waveguide resonator studies where the fields are concentrated on the surfaces, the cavity field strongly probes the bulk of the material. This method lends itself to rapid measurements of materials in a controlled and repeatable microwave environment, opening up new studies that are incompatible with existing planar measurement techniques and measuring materials that are difficult to fabricate. We report bulk losses for Fe-doped, semi-insulating InP substrates loaded in a superconducting rectangular cavity. To validate our approach, we measured intrinsic Si with thermally grown SiO2 whose losses are well-documented [Appl.Phys.Lett. 92, 112903 (2008)]. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F36.00005: Identifying Sources of Decoherence at Defects and Interfaces in Superconducting Qubit Systems Akshay A Murthy, Paul M Das, Cameron J Kopas, Stephanie M Ribet, Matthew J Reagor, Vinayak P Dravid, Roberto dos Reis, Mattia Checchin, Alexander Romanenko, Anna Grassellino Superconducting qubits have emerged as a platform technology for potentially addressing computational problems deemed intractable with classical computing. Despite recent advances enabling coherence lifetimes on the order of hundreds of μs, material quality and interfacial structures continue to curb performance. Two-level system defects in the superconductor and adjacent dielectric regions introduce stochastic noise and dissipate electromagnetic energy at cryogenic operating temperatures. Through a correlative approach combining secondary ion mass spectroscopy and advanced electron microscopy techniques, we systematically investigate interfaces associated with superconducting thin films to build a link between processing parameters, material structure, and resultant properties. We find that during film deposition, oxide and silicide layers form with varying stoichiometries at metal/substrate and metal/air interfaces. Additionally, we observe that lithography and etching procedures lead to the presence of impurity species such as hydrides, carbides, and fluorides, which can impact the superconducting properties, within the large-area contact pads and Josephson junctions. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F36.00006: Understanding and mitigating losses in superconducting radio frequency (SRF) cavities using two-tone spectroscopy Ivan Nekrashevich, Daniil Frolov, Roman Pilipenko, Bianca Giaccone, Alexandr Netepenko, Mattia Checchin, Sam Posen, Alexander Romanenko, Anna Grassellino It was shown that two-level systems (TLSs) are among the major detrimental factors causing energy loss and decoherence in superconducting quantum devices. Understanding and mitigating this loss mechanism is of high importance for practical applications of quantum computing systems. Microscopically, majority of TLSs are defects at dielectric surfaces and interfaces of superconducting quantum hardware. For instance, the major source of TLS in Nb SRF cavities is the layer of natural oxide formed on their inner surface. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F36.00007: Optimization of Nb Superconducting Films for Quantum Devices by H-T Phase Boundary Analysis Kevin M Ryan, Patrick W Krantz, Carlos Torres, Paul Masih Das, KVLV Narayanachari, Dominic P Goronzy, Michael J Bedzyk, Vinayak P Dravid, Mark C Hersam, Antony McFadden, Corey Rae McRae, Mark Field, Cameron J Kopas, Jayss Marshall, Matthew J Reagor, Venkat Chandrasekhar A critical area for the improvement of quantum devices is the materials optimization of superconducting films used in the construction of qubit elements and resonators, where careful considerations of purity and morphology must be made to reduce the effects of TLSs. In materials such as Nb, this involves simultaneously refining both the interfacial properties of the film as well as the superconductivity of the bulk. |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F36.00008: Round Robin: multi-institution characterization of superconducting qubits Josh Y Mutus, Corey Rae H McRae, David Pappas, Roman Pilipenko, Daniil Frolov, Matthew J Reagor, Anna Grassellino, Alexander Romanenko In order to advance state-of-the-art superconducting qubit device performance as well as the cryogenic microwave measurement systems used to operate them, an understanding of the decoherence mechanisms is critical. Because decoherence can be a result of elements intrinsic to the device and factors external to the chip such as the microwave chain and magnetic environment, disentangling decoherence mechanisms and sources has historically not been possible. This is compounded by a lack of systematic study of qubit performance over time, between cooldowns, and between laboratories. There is also a lack of data about environmental factors, for example the susceptibility to cosmic rays of a given location. In this study we are characterizing the same physical qubits in multiple environments in a first-of-its-kind study to provide new insight into these factors toward advancing state of the art performance. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F36.00009: Scanning Tunneling and Electron Spectroscopies of Nb Surface Oxides for Qubits and SRF Cavities John F Zasadzinski, Jasmine Panthee, Thomas Proslier, Lin Zhou, Xiaotian Fang Scanning point contact tunneling (SPCT) spectroscopy has been performed on high purity Nb pieces, including cut outs of superconducting RF cavities as well as on patterned Nb films coated with Al. Hot spot regions of conventionally processed SRF cavities reveal localized magnetic moments in the Nb oxide as evidenced by a zero bias conductance peak that exhibits Zeeman splitting in a field, and a logarithmic T dependence, consistent with the Appelbaum-Anderson model of spin flip tunneling. Similar magnetic moment features are observed from the Nb oxide layer at the edges of lithographically patterned Nb films where the Al layer has peeled off. Electropolished Nb crystals exhibit significant broadening of the BCS density of states attributed to magnetic pairbreaking. These results are consistent with scanning TEM/EELS studies that show a continuous change of the Nb oxidation state, from the surface Nb2O5 to the presence of an amorphous, likely metallic NbOx layer at the interface with Nb. Combined, these experiments indicate that oxygen vacancies are the probable cause of local moments in the oxide layer. Magnetic moments and pairbreaking may be playing a role in qubit decoherence. |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F36.00010: Atomic-scale Investigation of the Nb/air Interface for Niobium Radio Frequency Cavity Xiaotian Fang, Matthew J Kramer, Alexander Romanenko, Anna Grassellino, Lin Zhou Niobium radiofrequency cavities hold great promise to realize quantum computation, while the coherence time loss of quantum bit (qubit) by surface resistance, in particular oxides, impairs the system's lifetime. We present a comprehensive structural and chemistry study of the Nb/air interface from a niobium cavity using aberration-corrected transmission electron microscopy (TEM) and spectroscopy techniques. The results directly prove that the oxidation process is oxygen diffusion-controlled, and the NbOx/Nb interface is related to the crystallographic orientation of the Nb metal. Moreover, we will discuss different surface protection methods during focus ion beam TEM sample preparation on the Nb oxide layer structure. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F36.00011: Investigation of the π-junction characteristics in Nb/Fe3O4/Nb Josephson junctions Derya Farisogullari, Yasin Ozturk, Tuna Alp, Ege Aygit, Ara Rahimpour, Ozhan Ozatay, Yilmaz Simsek Josephson junctions with a ferromagnetic barrier (F-JJ) so-called π-junctions have attracted a great deal of interests because their rich physics concept orientating from the co-existence of magnetism and superconductivity allows to develop novel electronics which are potentially used in cryogenic memory and superconducting circuits. More recently, some theoretical studies on ferromagnetic insulator-based Josephson junctions (FI-JJ) have been proposed to develop new type π-junctions which are promising candidates to avoid strong dissipation associated with low-energy quasiparticle excitations in ferromagnetic metals and to increase the voltage jump across the junction as well. To prove the viability of the theoretical approaches, we have established an experimental program aiming at developing FI-JJ thin-film platforms consisting of two Niobium superconducting electrodes separated by a magnetite (Fe3O4) tunneling barrier. For this purpose, we have sequentially grown a series of Nb/Fe3O4/Nb epitaxial thin films with gradually varying thickness of the Fe3O4 barrier from 2 nm to 15 nm and fabricated small junction arrays on the thin films. Then a series of out-of-plane transport measurements across the junctions have been carried out at various sample temperatures, and magnetic fields to investigate the characteristic features of π-junctions such as superconducting phase shift and super-spin current. We will discuss the prerequisite signatures of the π-junctions in their transport characteristics. |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F36.00012: Exploring high-coherence overlap junction qubits fabricated with a subtractive etch process Jeroen Verjauw, Rohith Acharya, Tsvetan Ivanov, Daniel Perez Lozano, Fahd A. Mohiyaddin, Danny Wan, A. M. Vadiraj, Jacques Van Dame, Julien Jussot, Yann Canvel, Xiaoyu Piao, Marc Heyns, Massimo Mongillo, Anton Potočnik, Iuliana P Radu, Bogdan Govoreanu State-of-the-art superconducting qubits are fabricated using shadow evaporation and lift-off techniques. However, these are generally not compatible with CMOS manufacturing capabilities, which could greatly help advance the upscaling of qubit architectures. Overlap junction qubits are a promising alternative [1]. During fabrication, the vacuum is broken and Ar milling is needed to remove native oxide before junction oxidation. While this potentially compromises the junction’s integrity, its effect on the qubit performance is still unclear. In our work, we explore fully fab-compatible subtractive etch processes to fabricate overlap junctions, which are studied in different qubit designs. We find that average qubit lifetimes of our best devices exceed 60 μs, with values up to 100 μs. Furthermore, the quality factors of different qubit designs scale in accordance with surface oxide participation ratios, indicating that overlap junctions are not limiting the qubit’s T1 coherence time up to 60 μs. This work paves the way toward large-scale superconducting qubit integration with industrially standardized process techniques. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F36.00013: Aluminum air bridges for superconducting quantum devices realized using a single step e-beam process Noah Janzen, Michal Kononenko, Shaun Ren, Adrian Lupascu Superconducting quantum devices typically rely on co-planar waveguides (CPWs) for qubit readout and information storage. CPWs break up the ground plane and introduce decoherence through mode mixing. These issues can be mitigated through the use of air bridges. These bridges act as an interconnect to improve uniformity of the ground plane, reduce slot line modes in resonators, and enable a greater design space for planar devices by allowing crossing of signal lines. We present our results on the development of a process for fabricating superconducting aluminum air bridges that rely on the use of a single electron-beam lithography (EBL) grayscale exposure step followed by aluminum deposition and liftoff. Besides the convenience of implementation in a single lithography step, this method offers flexibility with regards to design changes and positioning of the bridges. This process produces high-yield air bridges of various lengths, ranging from 10 micrometers to 100 micrometers. The air bridges are tested on superconducting resonators and are found to have little effect on the internal quality factor. We also discuss the integration of these bridges in flux-type superconducting devices for improved design capabilities, device density, and coherence. |
Tuesday, March 15, 2022 10:36AM - 10:48AM |
F36.00014: Improving the frequency reproducibility of transmon qubits to lower the crosstalk in quantum processors Amr Osman, Jorge Fernández-Pendás, Sandoko Kosen, Marcus Rommel, Giovanna Tancredi, Anton Frisk Kockum, Jonas Bylander, Anita Fadavi Roudsari This work targets frequency crowding in superconducting quantum computing devices that is a profound challenge towards scalability of architectures with fixed-frequency qubits. Fixed-frequency qubits show superior performance in terms of coherence compared to tunable qubits. However, they suffer from deviations in their frequency from the designed value, due to imperfections in the fabrication process of Josephson junctions that cause variation in junctions' normal state resistance. This leads to crosstalk between neighboring qubits and to scale beyond hundreds of qubits, a significant reduction in the frequency variation is required. We discuss and demonstrate possible means of improving the spread in Josephson junction resistance, to values close to the required limit for scalability. |
Tuesday, March 15, 2022 10:48AM - 11:00AM |
F36.00015: Probing the Role of Low Temperature Vacuum Baking on Photon Lifetimes in Superconducting Niobium 3-D Resonators Daniel Bafia, Anna Grassellino, Alexander Romanenko We discuss a potentially dramatic source of quantum decoherence in three-dimensional niobium superconducting resonators and in two-dimensional transmon qubits that utilize oxidized niobium: an aggravation of two-level system (TLS) induced losses driven by vacuum baking at temperatures and durations typically used in transmon qubit fabrication. By coupling RF measurements on cavities with time-of-flight secondary ion mass spectrometry studies on an SRF cavity cutout, we find that modest vacuum baking (150-200 C for 5 min-11 hrs) produces a partially depleted native niobium oxide which likely contains a large concentration of oxygen vacancies that drive TLS losses. Continued baking is found to eliminate this depleted layer and mediate these additional losses. |
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