Bulletin of the American Physical Society
2024 APS March Meeting
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session N47: Superconducting Qubits: Metal Films II |
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Sponsoring Units: DQI DCMP DMP Chair: Katarina Cicak, National Institute of Standards and Technology, Boulder Room: 200CD |
Wednesday, March 6, 2024 11:30AM - 11:42AM |
N47.00001: Near-field THz spectroscopy of tantalum films for superconducting quantum devices Peter A Jacobson, Xiao Guo, Zachary Degnan, Julian Steele, Eduardo Solano, Bogdan C Donose, Karl Bertling, Arkady Fedorov, Aleksandar D Rakić Superconducting quantum circuits are one of the leading quantum computing platforms. To advance superconducting quantum computing to a point of practical importance, it is critical to identify and address material imperfections that lead to decoherence. In this talk, I will show how terahertz Scanning Near-field Optical Microscopy (SNOM) can be used to probe functional devices such as coplanar microwave resonators and inform the processing of new materials for quantum technology. I will discuss the recent observation of a localized vibrational excess on the surface of tantalum films and how this excess is connected to the boson peak, a universal signature of amorphous materials. The nanoscale identification and localization of amorphous oxides provides critical insight as amorphous materials host tunneling two level systems (TLS) – the major performance limiting factor in superconducting quantum devices. |
Wednesday, March 6, 2024 11:42AM - 11:54AM |
N47.00002: Characterization of encapsulated superconducting microwave resonators Aranya Goswami, Sameia Zaman, Andres E Lombo, Pablo M Perez, Kevin Grossklaus, Terry P Orlando, Simon Gustavsson, Kyle Serniak, Joel I Wang, Jeffrey A Grover, Kevin P O'Brien, William D Oliver Surface amorphous oxides in superconducting films, commonly used to fabricate superconducting qubits, have been demonstrated to host two-level systems that can limit their coherence. Encapsulation with other materials can potentially reduce the formation of these lossy oxides and improve qubit performance. In this work, we study the effects of encapsulation of superconducting films with non-superconducting materials. For encapsulation, we explore both in-situ capping techniques, after the growth of superconducting films, as well as alternate methods, involving ex-situ etching and subsequent passivation. We first comprehensively study the surface morphology and crystallinity of these superconducting heterostructures using atomic force microscopy, electron microscopy, X-ray photoemission spectroscopy, and X-ray diffraction. Next, we characterize the critical temperature and residual resistivity ratio of these encapsulated films. Finally, we measure microwave resonator devices fabricated from encapsulated and non-encapsulated films in a dilution refrigerator operating at milliKelvin temperatures. Through this study, we aim to correlate the observed material properties with the DC and microwave behavior of superconducting microwave resonators encapsulated with normal metals. |
Wednesday, March 6, 2024 11:54AM - 12:06PM |
N47.00003: Surface Passivation of Niobium and Aluminum Superconducting Thin-Films using Self-Assembled Monolayers to Suppress the Growth of Oxides Mohammed Alghadeer, Omar Abdulsahib Saleh, Khan Alam, Saleem Rao In the development and optimization of superconducting quantum circuits, post-fabrication surface passivation of superconducting thin films is pivotal [1,2]. This research directs a spotlight on harnessing self-assembled monolayers (SAMs) as a means to achieve enhanced surface passivation, countering the deleterious effects of two-level-system (TLS) defects from metal oxides, which are notorious for significantly precipitating decoherence and impeding the optimal functioning of quantum devices. Traditional methods of mitigating these defects, such as employing an inorganic passivating layer, are discounted due to their propensity to further deteriorate interface conditions. The deployment of SAMs in this context has been explored to effectively forestall defect regrowth post-etching processes, resulting a significant improvement in the quality factor of the superconducting co-planar waveguide (CPW) resonators. The main focus is placed on materials analysis, with techniques such as Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS) being employed to validate the efficacy of SAMs in both inhibiting oxide regrowth and steadily enhancing superconducting quantum circuit performance. This detailed analysis underscores the pivotal role of SAMs in realizing and maintaining improved surface passivation for Niobium and Aluminum superconducting thin films in cQED applications. |
Wednesday, March 6, 2024 12:06PM - 12:18PM |
N47.00004: Nb-based Superconducting Qubits with Capping Layers: Optimizing the Proximity Effect John F Zasadzinski, Seth J Rice, Maria Iavarone Recent studies within the SQMS center of transmon qubits, utilizing thin capping layers on the Nb capacitor to eliminate the RF lossy Nb oxide, have shown significant increases in the T1 relaxation time. Such superconducting/normal metal (S/N) bilayers necessarily involve the proximity effect and we discuss how this can be optimized. First, a review is presented of the extensive tunneling research that shows that thin capping layers of Ta, Al and Mg on Nb can be fabricated in the specular limit where the Arnold theory of the proximity effect is realized. In this case, the continuum of quasiparticle states above the induced gap ΔN << ΔS are replaced with a discrete set of Andreev bound states near the top of the potenial well ΔS - ΔN. This allows the capping layer to transmit the superconducting properties of the underlying Nb including an effective gap parameter close to the bulk value of 1.55 meV for Nb. The inclusion of a quasiparticle scattering term in the Arnold model shows how low-lying quasiparticle states near ΔN re-emerge which might be detrimental to qubit performance. This gives guidance to the materials processing, suggesting that the capping layer should be clean and the N/S interface should not introduce significant scattering. The model, as well as experiment with Nb/Mg, also shows that the N layer need not be superconducing at all in bulk. This opens the door to using capping layers of normal metals such as Au or other metals that have a minimal native oxide layer that might cause RF losses. |
Wednesday, March 6, 2024 12:18PM - 12:30PM |
N47.00005: Encapsulating superconducting circuits to avoid oxide-related dielectric loss Nana Shumiya, Ray Chang, Matthew Bland, Faranak Bahrami, Russell A McLellan, Alexander Pakpour-Tabrizi, Kevin D Crowley, Chenyu Zhou, Kim Kisslinger, Junsik Mun, Rebecca Cummings, Aswin kumar Anbalagan, Conan Weiland, Andrew L Walter, Mingzhao Liu, Robert J Cava, Andrew A Houck, Nathalie P de Leon Single qubit coherence remains a major limiting factor in building scalable processors based on superconducting qubits. Tantalum-based superconducting qubits have been recently discovered to enable long lifetimes and coherence times because of their chemical robustness and well-behaved surface oxide [1,2,3,4]. One of the remaining major sources of dielectric loss has been measured to be two-level systems (TLSs) residing in the amorphous native oxide layer [5,6]. One strategy for avoiding such loss is to eliminate the oxide layer entirely. In this presentation I will describe our approach to eliminate the oxide layer by encapsulating tantalum with a noble metal thin film. We are able to deposit thin continuous films that prevent oxide formation while being thin enough to be fully proximitized, and we see no additional quasiparticle poisoning. We systematically study the impact of the encapsulation layer on TLS loss by correlating resonator loss measurements with direct materials characterization. |
Wednesday, March 6, 2024 12:30PM - 12:42PM |
N47.00006: Encapsulating Ta films with Pt for novel Josephson junctions and quantum information applications Param J Patel, John W Lyons, Junwon Choi, Jacob J Repicky, Chung Wa Shum, Maria F Nowicki, Israa G Yusuf, Benjamin M Hunt, Michael Hatridge The coherence time of superconducting qubits limits the scale and complexity of quantum circuits. Tantalum-based transmon qubits have demonstrated that coherence times can exceed 0.3 ms[1]. However, the amorphous oxide surface of the Ta hosts TLS losses and introduces dielectric losses that could limit qubit lifetimes. In addition, this insulating oxide layer poses a challenge in creating ohmic contact to the Ta layer limiting the potential kinds of junctions made. We demonstrate a technique to encapsulate the Ta films with a Pt layer to eliminate the Ta oxide growth. By growing a thin uniform layer of Pt in situ on top of the Ta, we will show heavily diminished oxide peaks seen through XPS. This enables us to create unique novel Josephson junctions using the Ta films and other insulating barriers like van der Waals heterostructures such as hexagonal Boron Nitride. We will demonstrate some results using these films in superconducting quantum circuits, such as resonators and JJ-based circuits. |
Wednesday, March 6, 2024 12:42PM - 12:54PM |
N47.00007: Characterization of loss mechanisms in high kinetic inductance materials: Part 1 Neel Thakur, Vishakha Gupta, Patrick Winkel, Peter van Vlaanderen, Yanhao Wang, Suhas S Ganjam, Luigi Frunzio, Robert J Schoelkopf High kinetic inductance superconducting thin films are an attractive platform to build compact quantum circuits with high characteristic impedances well beyond the vacuum impedance. To fully exploit the properties of such materials, we must better understand their loss mechanisms, and identify ways to integrate them with established superconductors like aluminum or tantalum for quasiparticle mitigation. In part 1 of this talk, we discuss fabrication techniques for building microwave resonators with these materials, and outline our approach to identifying different loss mechanisms. |
Wednesday, March 6, 2024 12:54PM - 1:06PM |
N47.00008: Characterization of loss mechanisms in high kinetic inductance materials: Part 2 Vishakha Gupta, Neel Thakur, Patrick Winkel, Peter van Vlaanderen, Yanhao Wang, Suhas S Ganjam, Luigi Frunzio, Robert J Schoelkopf High kinetic inductance superconducting thin films are an attractive platform to build compact quantum circuits with high characteristic impedances well beyond the vacuum impedance. To fully exploit the properties of such materials, we must better understand their loss mechanisms, and identify ways to integrate them with established superconductors like aluminum or tantalum for quasiparticle mitigation. In part 2 of this talk, we present results on devices constructed of such high-kinetic inductance materials. By employing different circuit geometries, we study and discuss the relative interplay between kinetic inductance, non-linearity and conductor loss in such devices. |
Wednesday, March 6, 2024 1:06PM - 1:18PM |
N47.00009: Numerical Analysis of the Kinetic Inductance Fraction of Superconducting Thin-film Resonators Matthew Snyder, Yen-An Shih, Gabriel Spahn, Shravan Patel, David C Harrison, Robert McDermott Superconducting transmission line resonators play a crucial role in the measurement of solid-state quantum bits and the detection of weak astrophysical signals. Robust design of resonator arrays requires a thorough understanding of the modification of propagation velocity and mode impedance by the kinetic inductance of the superconducting thin film. Here, we describe a numerical approach to the calculation of kinetic inductance fraction in superconducting transmission lines that can be applied to structures with arbitrary 2D cross section and arbitrary penetration depth. We use fluxoid conservation and matrix inversion to access the current distribution in the structure; from this, we obtain the kinetic inductance contribution of the signal and ground traces along with the modification of the geometric inductance due to finite penetration depth. We compare our numerical results against experimental data from aluminum and niobium coplanar waveguide resonators. |
Wednesday, March 6, 2024 1:18PM - 1:30PM |
N47.00010: Quantum Circuits Using the Disordered Superconductor WSi (Part I) Trevyn Larson, Sarah Garcia Jones, Tamas Kalmar, Sai Pavan Chitta, Pablo Aramburu Sanchez, Stephen T Gill, Akash V Dixit, Varun Verma, Jens Koch, Sae Woo Nam, Raymond W Simmonds, Andras Gyenis Kinetic inductance materials have been widely used in the superconducting detector community and are now applied in circuit QED, allowing for the implementation of distributed high inductance and highly nonlinear elements. However, devices based on such materials have not yet reached the simultaneous levels of high nonlinearity and low loss commonly achieved with Josephson junction-based devices. In this project, we aim to address this problem by optimizing devices based on WSi, an amorphous superconductor commonly used in single-photon detectors for its lack of grain boundaries and high-quality thin films. |
Wednesday, March 6, 2024 1:30PM - 1:42PM |
N47.00011: Quantum Circuits Using the Disordered Superconductor WSi (Part II) Sarah Garcia Jones, Trevyn Larson, Sai Pavan Chitta, Tamas Kalmar, Pablo Aramburu Sanchez, Stephen T Gill, Akash V Dixit, Varun Verma, Jens Koch, Sae Woo Nam, Raymond W Simmonds, András Gyenis Kinetic inductance materials have been widely used in the superconducting detector community and are now applied in circuit QED, allowing for the implementation of distributed high inductance and highly nonlinear elements. However, devices based on such materials have not yet reached the simultaneous levels of high nonlinearity and low loss commonly achieved with Josephson junction-based devices. In this project, we aim to address this problem by optimizing devices based on WSi, an amorphous superconductor commonly used in single-photon detectors for its lack of grain boundaries and high-quality thin films. |
Wednesday, March 6, 2024 1:42PM - 1:54PM |
N47.00012: Quantum Circuits Using the Disordered Superconductor WSi (Part III) Sai Pavan Chitta, Trevyn Larson, Sarah Jones, Heli Vora, Sae Woo Nam, Raymond W Simmonds, Andras Gyenis, Jens Koch In part three of this three-part talk, we focus on circuit quantization of a RF-squid circuit which using a weak-link WSi junction instead of a conventional AlOx tunnel junction. We demonstrate that experimental data cannot be modeled based on a simple sinusoidal current-phase relation, but is consistent with a sawtooth-like current-phase relationship. Results from both semi-classical and quantum calculations exhibit detailed agreement with experimental one-tone spectroscopic data. |
Wednesday, March 6, 2024 1:54PM - 2:06PM |
N47.00013: Vortex viscosity in superconducting granular aluminum resonators Jadrien T Paustian, Clayton Larson, Kenneth R Dodge, B.L.T. Plourde The high kinetic inductance of granular aluminum resonators presents a novel regime for studying vortex dynamics in superconductors. Vortices trapped in regions of large microwave currents typically contribute excess loss for superconducting resonators and qubits. However, in highly disordered films, such as granular aluminum, we observe an anomalously low microwave loss due to vortices. We measure granular aluminum resonators of different sheet resistances cooled in a range of magnetic fields. By analyzing the dependence of the loss and reactance as a function of field and resonator frequency, we extract the vortex viscosity. For the lowest sheet resistance samples, we find the vortex viscosity to be consistent with the conventional Bardeen-Stephen model. However, for more disordered films with higher sheet resistance, we observe a vortex viscosity that is anomalously low. We discuss possible theoretical explanations for this behavior. |
Wednesday, March 6, 2024 2:06PM - 2:18PM |
N47.00014: Probing Kinetic Inductance in Thin Niobium Diselenide (NbSe2) through Microwave Measurements Sameia Zaman, Joel I Wang, Miuko Tanaka, Thomas Werkmeister, Max Hays, Daniel Rodan Legrain, Aranya Goswami, Thao H Dinh, Michael A Gingras, Bethany M Niedzielski, Hannah M Stickler, Mollie E Schwartz, Jonilyn L Yoder, Kenji Watanabe, Takashi Taniguchi, Terry P Orlando, Jeffrey A Grover, Simon Gustavsson, Kyle Serniak, Pablo Jarillo-Herrero, Philip Kim, William D Oliver We developed hybrid superconducting microwave resonators incorporating van der Waals (vdW) superconductors to explore the microwave (MW) response of superconducting 2D materials in the GHz regime. We first established a reliable technique to contact thin NbSe2, entirely encapsulated with hexagonal Boron Nitride (hBN), to a coplanar Al resonator. Then, we fabricated a hybrid Al-NbSe2 resonator and measured the kinetic inductance of thin NbSe2 at low temperature in the low-photon number limit. In this talk, we share our findings from the microwave characterizations of Al-NbSe2 resonators, where the thickness of NbSe2 will vary. Furthermore, we discuss the observed relation between the kinetic inductance and the thickness of the thin NbSe2. Our approach contributes to understanding the MW properties of superconducting 2D materials with potential implications for their utilization in emerging technologies. |
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