Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y30: Materials and Fabrication in Superconducting Qubits II - MaterialsFocus Live
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Sponsoring Units: DQI Chair: Corey Rae McRae, University of Colorado, Boulder |
Friday, March 19, 2021 11:30AM - 11:42AM Live |
Y30.00001: New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds Alexander Place, Lila Rodgers, Aveek Dutta, Pranav Mundada, Basil Smitham, Mattias V Fitzpatrick, Zhaoqi Leng, Anjali Premkumar, Jacob Elvin Bryon, Sara Sussman, Guangming Cheng, Trisha N Madhavan, Harshvardhan Babla, Berthold Jaeck, Andras Gyenis, Nan Yao, Robert Cava, Nathalie De Leon, Andrew Houck We employ tantalum transmon qubits with coherence times above 0.3 ms to demonstrate the importance of materials engineering in realizing a superconducting quantum processor. In this talk we characterize the regions and mechanisms of loss in state-of-the-art two-dimensional qubits. To do so, we efficiently iterate our fabrication procedure using materials spectroscopy. We correlate the spectroscopic results with time domain measurements to enable rapid screening of new materials and processing techniques. We further elucidate the dominant loss sources by characterizing time, frequency, geometry, and temperature fluctuations of coherence. Our fabrication techniques can be easily employed in standard industry and academic cleanrooms, and integrated into existing quantum processor architectures. |
Friday, March 19, 2021 11:42AM - 11:54AM Live |
Y30.00002: A study of intrinsic dielectric loss of tantalum based superconducting quantum circuits Xiaohang Zhang, Wenlong Yu, Feng Bao, Hao Deng, Ran Gao, Xun Jiang, Hsiang-Sheng Ku, Zhisheng Li, Xiaotong Ni, Jin Qin, Zhijun Song, Hantao Sun, Chengchun Tang, Tenghui Wang, Feng Wu, Tian Xia, Gengyan Zhang, Jingwei Zhou, Xing Zhu, Hui-Hai Zhao, Chunqing Deng Over the past decade, scientists have devoted tremendous efforts into improving design and fabrication of superconducting qubits, especially materials and interface for higher coherence times [1]. To realize large-scale fault-tolerant quantum computing, the quality of materials and interface of superconducting quantum devices need to be further improved to enhance the coherence times. In this talk, we present our work on thin film deposition and etching of tantalum together with film quality analysis. We studied superconducting resonators and qubits with engineered participation ratios which reveals the intrinsic dielectric loss of the tantalum films. In the end, we will present our work towards superconducting qubits based on Ta/TaOx/Ta Josephson junctions. |
Friday, March 19, 2021 11:54AM - 12:30PM Live |
Y30.00003: Discovering new platforms for high coherence qubits using direct materials characterization Invited Speaker: Nathalie De Leon Many platforms for quantum technologies are limited by noise and loss arising from uncontrolled defects at surfaces and interfaces, including superconducting qubits, color centers in diamond, trapped ions, and semiconductor quantum dots. I will describe our recent efforts to tackle noise and microwave losses in superconducting qubits. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. We have fabricated two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device [1]. We have observed increased lifetimes for many devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors. |
Friday, March 19, 2021 12:30PM - 12:42PM Live |
Y30.00004: Dielectric Loss in Titanium Nitride and Aluminum Superconducting Resonators Alexander Melville, Wayne Woods, Greg Calusine, Kyle Serniak, Evan Golden, Bethany Niedzielski, David K Kim, Arjan Sevi, Jonilyn Yoder, William Oliver Uniquely characterizing loss from two-level systems (TLS) in dielectric materials in coplanar waveguide resonators is challenging due to the nearly proportional scaling of the electric field participation in response to changes in geometry and anisotropic trench depth. With isotropic trenching, however, we can design a set of device geometries where each geometry amplifies the participation of a specific dielectric region. We fabricate this set of devices for both titanium nitride (TiN) and aluminum (Al) resonators and characterize the specific loss tangent of each dielectric for both materials [1]. In this talk, we show that the metal-air interface is the dominant loss in Al devices, whereas each dielectric region contributes significantly in TiN. Lastly, a post-process hydrofluoric (HF) etch reduces losses from the substrate-air interface in TiN devices [2]. |
Friday, March 19, 2021 12:42PM - 12:54PM Live |
Y30.00005: Process Optimization for Superconducting Resonators via Identification and Mitigation of Interface Loss Mechanisms Archan Banerjee, AHMED HAJR, Cassidy Berk, John Mark Kreikebaum, Virginia Altoe, David Ivan Santiago, D. Frank Ogletree, Irfan Siddiqi CPW resonators plays a vital role for many quantum circuits. They also serve as a platform to characterize loss mechanisms in superconducting materials in the microwave frequency range. We report on the characterization and iterative process development of niobium-based CPW resonators. Using a set of post-fabrication chemical etching techniques, we have systematically reduced the metal-air and substrate-air oxide thickness, improving the device quality factor beyond 5 million at single-photon-excitation power (measured at 100 mK). Following these low temperature electrical measurements, resonator samples have been probed using a suite of structural characterization tools (XPS, TEM and AFM) in order to determine the thickness, chemical composition and location of parasitic interface layers and their contribution towards quality factor. |
Friday, March 19, 2021 12:54PM - 1:06PM Live |
Y30.00006: Fabrication of Transmon Qubits with Molecular-Beam Epitaxy Aluminum Yizhou Huang, Frederick C Wellstood, Christopher J K Richardson, Benjamin Palmer I will discuss the design, fabrication, and measurement of a superconducting transmon qubit, made mostly from molecular-beam epitaxy (MBE) aluminum. To reduce loss at superconductor-substrate interface, the qubit incorporates a thin film of aluminum grown by MBE on a high resistivity silicon substrate.[1] The initial MBE aluminum layer is patterned using photolithography and wet etching. An Al/AlOx/Al Josephson junction is then added using electron-beam lithography, an ion milling of the initial layer, and double angle evaporation of aluminum by electron-beam evaporation with an oxidation step in between. I will discuss the process and present some transmon and resonator coherence data. |
Friday, March 19, 2021 1:06PM - 1:18PM Live |
Y30.00007: Fabricating low loss, lumped element niobium resonators Debadri Das, Kevin Karan Singh Multani, Hubert Stokowski, Amir Safavi-Naeini, Paul B. Welander, Emilio A Nanni Over the past decade there has been a significant improvement in the quality of on-chip superconducting microwave components for various applications including quantum sensing and computation. However, scaling these sensitive quantum devices for potential applications like Stage-IV CMB experiments which require more than 500,000 superconducting TES bolometers for its proposed telescope or a high-efficiency microwave to millimeter-wave quantum transducer, will require these components to be made more compact while maintaining their high quality. Current on-chip microwave resonators with large interdigitated capacitors (IDCs) are incompatible with such future devices due to larger footprints and degraded performance with increasing capacitances. As opposed to some of the previous attempts to make compact high quality components from Al or TiN, our approach is to use niobium for our resonators. We have modelled our device using the planar 3D EM simulator SONNET, developed a fabrication procedure, fabricated initial prototype circuits, and evaluated device performance in a cryogenic environment. We will report our most recent results. |
Friday, March 19, 2021 1:18PM - 1:30PM Live |
Y30.00008: Thickness-Dependent Superconductor-Insulator Transition of TaN Thin Film Grown with Atomic Layer Deposition Wonho Song, Sungchul Jung, Junhyung Kim, Gahyun Choi, Joonyoung Lee, Yonuk Chong, Dongwoo Shin, Jeehoon Kim, Kibog Park Atomic layer deposition (ALD) is a well-known method to grow a thin film which can ensure the uniformity and conformality of the grown film. In this work, TaN thin films with thicknesses ranging 8.9 nm to 32.6 nm were grown by using plasma enhanced ALD with Tris(diethylamido)(tert-butylimido)tantalum(TBTDET) precursor and H2 reactant. The electrical properties of grown films including carrier density, mobility, and Hall coefficient obtained from Hall effect measurements are presented. From the temperature-dependency of sheet resistance and Hall coefficient above superconducting critical temperature ~4.3 K, the thickest TaN film appeared strongly disordered with kFl ~ 0.4 and showed the unusual metallic behavior (dρ/dT<0). The slope of Hall coefficient vs. sheet resistance plot was found to be more toward the strong localization limit, which would be a valid interpretation under the weak scattering assumption. Most relevantly, the critical temperature was extracted to keep decreasing as the film became thinner and thinner. From the thickness dependence of critical temperature, superconductor-insulator transition is expected to occur as the film thickness goes below ~18.5 nm. |
Friday, March 19, 2021 1:30PM - 1:42PM Live |
Y30.00009: Low-loss superconducting coplanar waveguide resonators fabricated with TiN Ran Gao, Wenlong Yu, Feng Bao, Hao Deng, Xun Jiang, Hsiang-Sheng Ku, Zhisheng Li, Xiaotong Ni, Jin Qin, Zhijun Song, Hantao Sun, Chengchun Tang, Tenghui Wang, Feng Wu, Tian Xia, Gengyan Zhang, Xiaohang Zhang, Jingwei Zhou, Xing Zhu, Hui-Hai Zhao, Chunqing Deng Titanium nitride (TiN) is an attractive superconducting material for superconducting circuits due to its high kinetic inductance. Here, we report low-loss superconducting coplanar waveguide (CPW) resonators fabricated with high-quality TiN thin films. As evidenced in X-ray diffraction analyses, single crystalline TiN film is epitaxially sputtered on c-plane sapphire wafers by optimizing its growth condition. CPW resonators with a center line width of 10 μm are fabricated based on TiN films of various thickness. Microwave characterization is performed at millikelvin temperature in low-power and high-power regimes, respectively. The best internal quality factor Qi > 3 million in the single-photon limit and Qi > 12 million in the high power limit. Surprisingly, anomalous film thickness dependence of kinetic inductance is observed, which may originate from varied TiN stoichiometry in the films of different thickness. |
Friday, March 19, 2021 1:42PM - 1:54PM Live |
Y30.00010: Microwave Loss in High-Q Titanium Nitride Resonators Rui Zhang, Ashish Alexander, Christopher J K Richardson, Frederick C Wellstood, Benjamin Palmer We have measured the loss in a superconducting thin film titanium nitride microwave resonator from 20 mK to 1 K and at different applied microwave powers. The titanium nitride film had a superconducting transition temperature of Tc = 5.5 K and was grown in an MBE by reactively reacting evaporated titanium in a nitrogen plasma. The measured internal loss of the resonator at low power decreased by approximately a factor of 10 from 1/Q = 2.5 x 10^-6 at 20 mK to 1/Q = 2.5 x 10^-7 at a temperature of 600 mK. We compare the measured Q data to a model based on loss from the interaction of the superconducting resonator with lossy two-level systems and separately to a model we have developed based on non-equilibrium quasiparticles accumulating in regions of the TiNx film with a lower superconducting gap. To distinguish between these competing models, we will also discuss results where we apply superconducting pair-breaking infrared light directly to the resonator device and measure the loss. |
Friday, March 19, 2021 1:54PM - 2:06PM Live |
Y30.00011: Extraction and Modeling of TLS Losses in TiN Qubits Wayne Woods, Alexander Melville, Greg Calusine, Kyle Serniak, Evan Golden, Bethany Huffman, David K Kim, Arjan Sevi, Jonilyn Yoder, William Oliver Predictive modeling of microwave loss due to dielectric two-level-systems allows for design optimization and targeted fabrication-process development to improve the performance of transmon qubits. We have previously used isotropically-trenched TiN and Al resonators to extract TLS-loss characteristics of material interfaces and the Si substrate [1, 2]. Applying this knowledge to the design and modeling of TiN qubits is complicated by the additional processing needed to fabricate high-quality Josephson junctions, which can induce additional defects at the various surfaces and interfaces. In this talk, we discuss the iterative process development that reduced the effect of these additional fabrication steps. With the resulting improved process, we determined the surface-loss contributions in TiN resonators and constructed a predictive loss model for co-fabricated TiN transmon qubits. |
Friday, March 19, 2021 2:06PM - 2:18PM Live |
Y30.00012: FABRICATION APPROACHES TO 3D SUPERCONDUCTING QUBIT RESONATORS WITH A HIGH Q-FACTOR Paul Carriere, Sergey Kutsaev, Ronald Agustsson, Andrew N Cleland, Pedro Frigola, Timothy Horn, Michael Kelly, Alexander Smirnov RadiaBeam is presently developing a 3D quarter-wave resonator (QWR) with shape optimized for operation in the quantum regime. This program includes several important steps: development of a QWR cavity with the optimal shape to increase the Q-factor; increasing the Q-factor of the niobium resonator by adopting Nb surface treatment techniques from accelerating cavities; demonstration of the performance of a multi-qubit system with Josephson junctions, and improvement of fabrication techniques to reduce the complexity and costs of the system. The activities for most of the objectives are already ongoing. For example, we recently developed a resonator with the improved shape, machined it out of niobium, and tested it at a temperature of 10 mK. Other ongoing work includes the study of niobium surface treatments, and the use of advanced fabrication techniques such as additive manufacturing (AM), hydroforming or electro-hydraulic forming. In this talk, we will discuss the current and future fabrication activities of qubit cavities at RadiaBeam. |
Friday, March 19, 2021 2:18PM - 2:30PM Live |
Y30.00013: Characterization of Al-based Airbridges for Superconducting Microwave Devices Robab Najafi Jabdaraghi, Leif Grönberg, Visa Vesterinen, Mika Prunnila The realization of a co-planar waveguide (CPW) for quantum circuits requires only a single wiring layer. However, the usability of the structure is limited by parasitic slotline modes which can modify couplings and act as a loss channel in complex superconducting integrated circuits. Free-standing metallic crossovers known as airbridges [1,2] commonly provide electrical connectivity between ground planes to suppress the parasitic modes. Here, we demonstrate our latest development of Al airbridges in conjunction with Nb CPWs. Our fabrication method relies on three separate optical lithography steps with Al deposition combined with the CPW structures using re-flowed photoresist as a scaffold. We have observed that our airbridges are superconducting and mechanically robust, also in response to sonication. Characterizing the added microwave loss is work in progress. |
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