Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session Q75: Superconducting Qubits FabriactionFocus
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Sponsoring Units: DQI Chair: Mustafa Bal, Fermilab Room: Room 401/402 |
Wednesday, March 8, 2023 3:00PM - 3:36PM |
Q75.00001: Understanding Material Systems for Superconducting Qubits Invited Speaker: Russell McLellan Superconducting qubits have emerged as one of the leading hardware platforms for realizing large scale quantum processors. Our group has recently demonstrated lifetimes in superconducting qubits made from tantalum which exceed the highest of those of qubits made from niobium, aluminum, and other materials by a factor of more than three. In this talk, I will discuss our recent work to characterize the dominant sources of loss in state-of-the-art tantalum superconducting circuits. Using systematic measurements of tantalum resonators, we find the dominant source of loss at qubit operating conditions is from two-level systems present at material interfaces and surfaces. We compare the losses across samples prepared with different surface treatments, and we use surface spectroscopy to characterize the microscopic structure of these surfaces. Our work points the way to strategies for mitigating loss, as well as new opportunities in novel material systems. |
Wednesday, March 8, 2023 3:36PM - 3:48PM |
Q75.00002: Exploring on-chip qusaiparticle thermodynamics through unconfined circuit quantum electrodynamics. Byoung-moo Ann, Younghun Ryu, Seung-Bo Shim, Jinwoong Cha, Junho Suh One of the current challenges in microwave-to-optical quantum transduction is the light-induced qusaiparticle (QP) generation at superconducting elements, particularly Aluminum-based Josephson junction (JJ). Comprehensive probes of chip-scale QP thermodynamics require integrating multiple thermometers over the chip, which has remained a challenging task. Although superconducting qubits (SCQs) already present in the devices are excellent candidates for such QP thermometry, thermally activated decoherence sources blur the spectrum of SCQs, confining the operation to low qusaiparticle density regimes. In this work, we reveal that the Aluminum JJ of a transmon still conserves the QP-induced temperature dependence which well follows Josephson physics even when the transmon is strongly driven to unconfined states, and thereby the circuit is fully linearized. By measuring dispersively coupled resonators, we can extract the temperature information from JJ almost without effects from other pure dephasing sources, and consequently, the probing range can be extended to the critical temperature of Aluminum. Our approach has a variety of distinct merits such as simplicity in measurement systems, multiplexibility in readouts, and well-established fabrication processes for transmons. Thereby, one can readily scale the number of QP thermometers on different positions of one device, establishing space-time-resolved temperature probes. Thanks to the aforementioned properties, this scheme can also find applications even outside of quantum transduction. Beyond the thermometry applications, the proposed approach will lead to the development of a new method to characterize the thermal properties of JJs. |
Wednesday, March 8, 2023 3:48PM - 4:00PM |
Q75.00003: Characterizing new materials for superconducting qubits Nana Shumiya, Kevin D Crowley, Russell McLellan, Chenyu Zhou, Yichen Jia, Conan Weiland, Andi Barbour, Adrian Hunt, Irawikanari Waluyo, Kim Kisslinger, Xin Gui, AVEEK DUTTA, Alexander P Place, Esha Umbarkar, Cady Feng, Steven Hulbert, Mingzhao Liu, Andrew L Walter, Robert Cava, Andrew A Houck, Nathalie P de Leon Superconducting qubits have emerged as a leading platform for realizing a quantum processor. Significant effort in qubit design, device integration, and processor architecture have led to programmable quantum computers with dozens of qubits. However, despite these successes, qubit coherence remains as a major limiting factor in building scalable processors. One of the major sources of loss has been attributed to two level systems that are present at the material interfaces and surfaces of superconducting qubits. Recently, our group demonstrated an improvement in the lifetime of superconducting qubits made from tantalum by a factor of more than three compared to the lifetimes of qubits made from niobium, aluminum, and other materials, showing the potential of a broad material search to substantially improve qubit coherence. In this talk, I will present further recent results on exploring new material systems for superconducting qubits and understanding microscopic sources of noise and loss. |
Wednesday, March 8, 2023 4:00PM - 4:12PM |
Q75.00004: Optimizing Designs and Materials for Transmon Qubits Kevin D Crowley, Nana Shumiya, Russell McLellan, AVEEK DUTTA, Alexander P Place, Matthew Bland, Ray Chang, Esha Umbarkar, Youqi Gang, Hoang Le, Robert Cava, Nathalie P de Leon, Andrew A Houck Superconducting qubits based on Tantalum (Ta) have achieved some of the longest reported lifetimes to date. However, recent measurements of Ta lumped element (LE) resonators suggest dielectric loss at the capacitor surface may not be the only limitation for their transmon counterparts. This implies losses from other sources, such as Josephson junction interfaces and substrates, have to be mitigated if further improvements in lifetimes are to be achieved. In this talk, we discuss some progress in addressing these other loss channels, focusing especially on losses from junction interfaces. We also discuss the status of a broader materials search involving transmons and resonators, and give an outlook for further progress in optimizing transmon designs, materials, and fabrication. |
Wednesday, March 8, 2023 4:12PM - 4:24PM |
Q75.00005: A 200 mm Wafer Size Superconducting Qubit Foundry at MIT Lincoln Laboratory Jeffrey Knecht, Cyrus F Hirjibehedin, Kate Azar, Bethany M Niedzielski, Michael Gingras, David K Kim, Alexander Melville, William D Oliver, Meghan Schuldt, Mollie E Schwartz, Jonilyn L Yoder MIT Lincoln Laboratory has worked over the course of more than a decade to establish robust, reliable superconducting qubit fabrication processes. Recently, we have piloted a superconducting foundry model to provide access of its robust, high-yielding process to the US quantum research and development community. Initially established on 50 mm silicon wafers, we have ported our core process to the Laboratory's 90 nm capable, Class-10, 200 mm wafer size Microelectronics Laboratory (ML). This Trusted, ISO9001 facility contains automated cluster tools with improved process resolution and control, automated defect inspection and characterization for improved yield, in-line metrology, and additional real estate to accommodate multiuser runs. We will discuss the development of this 200 mm process, in particular the development of a superconducting air bridge process and of a flip-chip process for 2-D chip stacking. |
Wednesday, March 8, 2023 4:24PM - 4:36PM |
Q75.00006: Towards Design and Fabrication of Transmons using Single-Crystal Silicon Fins Tony McFadden, Aranya Goswami, Tongyu Zhao, Teun van Schijndel, Corey Rae H McRae, Raymond W Simmonds, David Pappas, Chris J Palmstrom While the invention of the transmon has fueled rapid developments in the quantum information systems field, decoherence sources inherent in the materials and scaling challenges remain limitations as the field moves to larger scale quantum processing units. Improvements in materials growth and device processing have driven improvements to transmon coherence, though little has changed in ways of the transmon designs which typically consist of a planar superconducting capacitor shunted by an aluminum/aluminum-oxide/aluminum Josephson junction. In this work, we incorporate high aspect ratio fins fabricated from low-loss, single-crystal silicon in superconducting circuits to enhance transmon coherence and scalability. Loss measurements performed on lumped element resonators fabricated using parallel plate capacitors made from silicon fins show significant improvement over conventional interdigitated capacitor designs. Efforts to incorporate aluminum-contacted silicon fins as capacitors in transmon designs will be discussed as well as efforts to thin the silicon fins to the tunneling limit to realize merged element transmons using silicon fins. |
Wednesday, March 8, 2023 4:36PM - 4:48PM |
Q75.00007: Understanding decoherence mechanisms in tantalum-based superconducting qubits Candice Kang, Larry Chen, Maria Virginia P Altoe, Kan-Heng Lee, Christian Jünger, Trevor Chistolini, D. Frank Ogletree, Chengyu Song, David I Santiago, Irfan Siddiqi Tantalum is a promising material platform for superconducting resonators and qubits, with longer coherence times than other commonly used materials including Nb and Al. In this work, we demonstrate that pure α-tantalum can be grown on silicon either with a Nb seed layer at ambient temperature, or without a seed layer at high substrate temperatures. We perform extensive materials characterization including XPS, XRD, SEM, and cross-sectional TEM to understand the characteristics of α-tantalum on silicon. We fabricate superconducting qubits on such films and correlate changes in the fabrication process with changes in both the materials properties and cryogenic device performance. The work here will provide a pathway to fabricate high coherence superconducting qubits based on tantalum on silicon. |
Wednesday, March 8, 2023 4:48PM - 5:00PM |
Q75.00008: High Sensitivity Loss Tangent Measurement of C-Plane Sapphire at mK with a High Q Nb Superconducting Radio-frequency Cavity Daniel Bafia, Anna Grassellino, Andrei Lunin, Alexander Romanenko We present results on the dielectric loss tangent of c-plane sapphire at the temperatures and fields relevant for the superconducting quantum computing regime. We utilize an ultra-high quality factor Nb superconducting resonant frequency cavity which enables unprecedented accuracy. To identify potential mechanisms that drive RF losses, the power and temperature dependence of the sapphire dielectric loss tangent are investigated. We find that the sapphire substrate degrades the quantum coherence times of our current 2-D and 3-D quantum devices, motivating further studies into identifying minimum material specifications which further reduce RF losses. |
Wednesday, March 8, 2023 5:00PM - 5:12PM |
Q75.00009: Characterization of losses in dielectric substrates for quantum computing devices using tunable superconducting RF cavity resonator. Ivan Nekrashevich, Geev Nahal, Daniil Frolov, Roman Pilipenko, Crispin Contreras-Martinez, Yuriy Pischalnikov, Vyacheslav P Yakovlev, Sergey Kazakov, Timergali Khabiboulline, Mattia Checchin, Alexander Romanenko, Anna Grassellino Among several quantum computing (QC) approaches, Josephson junction (JJ)-based superconducting quantum devices demonstrate high potential for scalability, control and, ultimately, provide a pathway to reaching practical quantum computing. However, crucial challenges need to be addressed to achieve this goal. For instance, one of the main factors limiting performance of widely used planar superconducting QC devices such as transmons is decoherence and loss due to unwanted interaction of a device with dielectric substrate. Contribution of the substrate to the total loss is even more significant in the case of 3D architectures where QC devices are placed inside 3D superconducting cavity resonators. In this case microwave EM field penetrates into the bulk of the substrate which causes significant degradation of the quality factor of the system device + resonator. |
Wednesday, March 8, 2023 5:12PM - 5:24PM |
Q75.00010: Experimental Tools for A/B Materials Testing Towards High-Coherence Superconducting Quantum Devices Corey Rae H McRae, Shaojiang Zhu, Anthony Q McFadden, John Pitten, Nicholas Materise, Nicholas Price, Cameron Kopas, Ella O Lachman, Yuvraj Mohan, Joel N Ullom, Mustafa Bal, Josh Mutus Materials-induced losses define the coherence ceiling for state-of-the-art superconducting qubits, as well as for other superconducting devices operating in the millikelvin, single-photon regime. A myriad of difficulties arise when isolating loss contributions from particular materials and interfaces in devices with complex geometries and measurement procedures in a rapidly evolving field. Harnessing the power of simple A/B testing for materials engineering would allow immediate performance improvements in superconducting devices, but requires a new set of standard experimental tools. In this talk, we propose some of these enabling tools, including a standard resonator mask set for probing the loss contributions of device interfaces, an open-source resonator data analysis codebase, and a framework for more rigorous error analysis of materials loss parameters. |
Wednesday, March 8, 2023 5:24PM - 5:36PM |
Q75.00011: Automation of quantum-chip selection using supervised machine learning. Denis Chevallier, Gerardo Jaramillo, Konstantin Nesterov, Chiara Pelletti, Michael Sinko, Kyunghoon Lee, Alexei Marchenkov Designing a scalable superconducting quantum processor is a challenging task that requires a complex frequency allocation procedure as well as elaborate simulations to mitigate crosstalk effects. However, achieving an acceptable fabrication yield for such complex structures is even more difficult. The yield analysis at the wafer level results in variations across the wafer on the order of a few percent, which is mainly attributed to variations in the junction area and tunneling thickness. However, imperfections at the chip level can limit the yield even if a single qubit fails. We developed a tool based on room-temperature junction resistance measurements to automate the classification of failure modes at the wafer and chip level, to speed up fabrication development, and to automate chip sorting. This tool relies on supervised machine learning algorithms, which facilitate fabrication-statistics extraction and the clustering of good and bad chips in a systematic way. |
Wednesday, March 8, 2023 5:36PM - 5:48PM |
Q75.00012: Gap-Engineered Transmon Qubits for Mitigation Against Quasiparticles Zachary Steffen, Haozhi Wang, Yizhou Huang, Yi-Hsiang Huang, Kungang Li, Sudeep K Dutta, Frederick C Wellstood, Benjamin S Palmer Non-equilibrium quasiparticles are a source of decoherence for superconducting transmon qubits. To trap quasiparticles and reduce their tunneling rate across the Josephson junction, we gap engineer one electrode of an asymmetric transmon by capping the Al counter-electrode of our junction with Ti, lowering the superconducting gap via the proximity effect. The T1 of a 3D transmon fabricated with this bilayer was found to drop to 1 µs. Similarly, direct IV measurements of these junctions show high sub-gap conductance. However, by adding disorder between the Al and Ti metals in the form of a thin AlOx layer, we can restore the gap to the value of thin film Al and improve the relaxation time to 32 µs. We use these results to inform the design of new transmons with low-gap trapping away from the junction. |
Wednesday, March 8, 2023 5:48PM - 6:00PM |
Q75.00013: Characterization of fabrication methods to reach high coherence superconducting quantum circuits Leon Koch, Niklas Bruckmoser, David Bunch, Tammo Sievers, Kedar E Honasoge, Thomas Luschmann, Stefan Filipp The fabrication of superconducting qubits and resonators with long coherence times and high quality factors is an important milestone on the way towards useful quantum processors. Although significant improvements in coherence time have been made over the last years, reaching qubit lifetimes well beyond 100 µs involves careful investigation of all fabrication steps. Here, we demonstrate that such high device qualities can be achieved by a combination of substrate cleaning, etching optimization and post-process sample cleaning. We optimize ion milling, ozone descumming and metal interconnecting processes. Thereby, we reach quality factors well above 3x106 for thin-film niobium CPW resonators and qubit lifetimes of over 150 µs . Additionally we present our results on surface passivation methods and etching techniques from which we expect further improvements in the quality of superconducting quantum circuits. |
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