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 V30: Materials and Fabrication in Superconducting Qubits I - Josephson JunctionsFocus Session Live
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Sponsoring Units: DQI Chair: Guilhem Ribeill, BBN Technology - Massachusetts |
Thursday, March 18, 2021 3:00PM - 3:12PM Live |
V30.00001: An inorganic shadow-mask platform for Josephson junction fabrication Ran Gao, Hao Deng, Feng Bao, Hsiang-Sheng Ku, Xun Jiang, Zhisheng Li, Xiaotong Ni, Jin Qin, Zhijun Song, Hantao Sun, Chengchun Tang, Tenghui Wang, Feng Wu, Wenlong Yu, Tian Xia, Gengyan Zhang, Xiaohang Zhang, Jingwei Zhou, Xing Zhu, Minghua Wang, Yingchun Wu, Xijun Li, Hui-Hai Zhao, Chunqing Deng A reliable production of high-quality Josephson junctions and pristine superconducting materials is critical in the applications of superconducting quantum circuits. The widely applied shadow-evaporation technique based on organic resist, nevertheless, imposes stringent requirements on the process thermal budget such that only low-melting-point material systems such as aluminum Josephson junctions can be fabricated. On the other hand, it also forbids any aggressive in situ treatment process on either the substrate surface or the as-deposited materials. To resolve this challenge and to further improve the process versatility of the shadow-evaporation technique, we demonstrate a simple and robust inorganic shadow-mask platform based on a silicon compound that helps to increase the thermal budget and reduce the sensitivity of mask materials to various process steps. Using the conventional aluminum junction as a proof-of-concept, we managed to complete a full process flow including mask fabrication, junction evaporation, and mask lift-off, where the yielded aluminum transmon devices revealed expected coherent properties. We believe such a technique serves as a new opportunity for device engineering and paves the way for future exploration of other superior material systems. |
Thursday, March 18, 2021 3:12PM - 3:24PM Live |
V30.00002: Fabrication parameters for frequency targeting in scalable superconducting quantum processors Nandini Muthusubramanian, Wilhelmus Duivestein, Chris Zachariadis, Matvey Finkel, Alessandro Bruno, Leonardo DiCarlo The scaling of monolithic superconducting quantum processors based on a repeating unit cell with repeated qubit frequencies presents unique challenges. Qubit frequency targeting must be preserved while incorporating 3D interconnects such as through-silicon vias (TSVs) and airbridges, increasing die form factor to wafer level, and increasing density of resonant components. We present a systematic approach to determine the causes of spread in Al-AlOx-Al Josephson junctions (apart from intrinsic variations in the tunnel barrier) with increased complexity of the quantum plane. Room-temperature resistance measurements are compared for junction arrays fabricated on four variations of substrates, namely bare silicon substrates, bare wafers with TSVs, wafers with pre-patterned superconducting base film, and base-patterned wafers with TSVs. The annealing effect of end-of-line fabrication steps for airbridges is also investigated in an effort to control global drifts in junction resistance |
Thursday, March 18, 2021 3:24PM - 3:36PM Live |
V30.00003: Highly Uniform Submicron Junction Arrays for Quantum Information Processing John Mark Kreikebaum, Larry Chen, Alexis Morvan, Jean-Loup Ville, Kevin O'Brien, Irfan Siddiqi Josephson junctions are an ubiquitous circuit element in cQED experiments. In complex devices with many such junctions, precise control over each junction’s critical current is often required, and thus variations of the junction area and tunnel barrier thickness must be sufficiently minimized. Analyzing junction array resistance distributions from many wafers, we have identified several key processing variables to improve uniformity. Nevertheless, the uniformity of as-fabricated junctions leaves much to be desired as quantum processors or single microwave photon detectors requiring 100’s or 1000’s of junctions begin to be realized. Therefore, we also present progress in implementing laser annealing techniques to fine tune junction resistances post-fabrication. |
Thursday, March 18, 2021 3:36PM - 3:48PM Live |
V30.00004: Capacitively shunted flux qubit based on epitaxially grown NbN/AlN/NbN Josephson junctions on Si substrate Sunmi Kim, Hirotaka Terai, Taro Yamashita, Wei Qiu, Tomoko Fuse, Fumiki Yoshihara, Kunihiro Inomata, Kouichi Semba In superconducting qubits composed of aluminum-based Josephson junctions (JJs), the decoherence from microscopic two-level systems in amorphous aluminum oxide has long been a concern. As an alternative material for the qubits, fully epitaxial NbN/AlN/NbN JJs are an attractive candidate with the potential to solve the above problems because of its high crystal quality, chemical stability against oxidization, and relatively high transition temperature (~ 16 K) of NbN. Early studies of superconducting qubits using epitaxially grown nitride JJs have shown significant potential, but their coherence time was limited due to dielectric loss from the MgO substrate [1]. To improve this, we have employed a Si substrate with a TiN buffer layer for the epitaxial growth of this nitride JJs [2] and fabricated a capacitively shunted flux qubit. For the dispersive readout, the qubit is coupled to a half-wavelength coplanar waveguide resonator. We characterize the qubit relaxation time and dephasing time using spin-echo in the tens of microseconds range. |
Thursday, March 18, 2021 3:48PM - 4:00PM Live |
V30.00005: Design and Characterization of a Functional Merged Element Transmon Harry Mamin, Elbert Huang, Santino Carnevale, Cihan Kurter, Bryan Trimm, Martin O Sandberg, Charles Rettner, Noel Arellano, Robert M Shelby, Mark H. Sherwood, M A Mueed, Benjamin A Madon, Aakash Pushp, Matthias Steffen, Daniel Rugar A merged element transmon (MET) is a superconducting qubit with a Josephson junction that has been engineered to act as its own parallel shunt capacitor [1]. Because it eliminates the need for a separate capacitor, it allows for a significantly more compact qubit. Furthermore, because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/Al2O3/Al junctions with long oxidations and novel processing, we have produced functional devices with EJ/EC in the low transmon regime (EJ/EC <~30). Cryogenic I-V measurements show sharp dI/dV structure with low sub-gap conduction. Qubit spectroscopy of tunable versions show a small number of avoided level crossings, suggesting the presence of two-level systems (TLS). We have observed T1 times typically in the range of 30-50 μs, with some devices exhibiting T1>100 μs over several hours. |
Thursday, March 18, 2021 4:00PM - 4:12PM Live |
V30.00006: A Merged-element Transmon RUICHEN ZHAO, Sungoh Park, Tongyu Zhao, Mustafa Bal, Corey Rae H McRae, Junling Long, David Pappas The invention of the transmon qubit has led to multiple breakthroughs in quantum information science over the past few years. To reduce the participation ratio of lossy materials, a small Josephson junctions (JJ) with a large-scale, coplanar capacitor shunt is typically employed in transmon designs. However, this method also leads to a large qubit footprint that hinders the scaleup of transmon. |
Thursday, March 18, 2021 4:12PM - 4:24PM Live |
V30.00007: Efforts towards a low loss all van der Waals parallel plate capacitor for quantum devices Abhinandan Antony, Martin V Gustafsson, Anjaly Rajendran, Guilhem Ribeill, Avishai Benyamini, Thomas A Ohki, Kin Chung Fong, James Hone Quantum devices such as qubits require ultra low loss capacitance elements. Conventional parallel plate capacitors have been unable to fulfill this need due to bulk and interfacial losses, necessitating the use of coplanar capacitors with extremely large footprints. Single crystal van der Waals materials, grown with extremely low defect densities, can be stacked to create heterostructures with ultra-clean laminated interfaces. Here we report efforts to fabricate and characterize electrically an all van der Waals (vdW) parallel plate capacitor with low loss at gigahertz frequencies. Using exfoliated vdW materials, such as niobium diselenide (NbSe2), and hexagonal boron nitride (h-BN), our process allows us to create capacitors with atomically flat, ultra clean interfaces, and dramatically reduced footprints when compared to conventional coplanar capacitors. |
Thursday, March 18, 2021 4:24PM - 4:36PM Live |
V30.00008: Reducing fabrication complexity of Josephson traveling-wave parametric amplifiers C. W. Sandbo Chang, Shuhei Tamate, Yoshiro Urade, Jimmy Shih-Chu Hung, Yasunobu Nakamura Combining high-gain and near quantum-limited noise performance over a multi-GHz bandwidth, Josephson traveling-wave parametric amplifiers (JTWPA) [1] have become essential for characterizations in circuit-QED experiments, such as simultaneous multi-qubit readout in a large-scale superconducting quantum computer. While being highly sought-after, due to the need of phase and impedance-matching structures, JTWPAs often involve multi-layer and complex fabrications which hindered their wide adoptions in laboratories. In this work, we propose a JTWPA design with matching structures implemented using coplanar lumped-element waveguides, greatly simplifying the fabrication process. Without the use of oxide layers for capacitors found in conventional JTWPA designs, our device is expected to exhibit reduced insertion loss, which could further improve quantum efficiency in measurements. Preliminary experimental results will be presented. |
Thursday, March 18, 2021 4:36PM - 4:48PM Live |
V30.00009: Continuous Measurement of Quasiparticle Trapping in Aluminum Nanobridge Josephson Junctions James Farmer, Azarin Zarassi, Darian Marie Hartsell, Eli Levenson-Falk Non-equilibrium quasiparticle (QP) populations are a significant source of decoherence in many superconducting quantum circuits. When properly phase biased, nanobridge Josephson junctions provide sub-gap bound states for QPs and so function as QP traps [1]. The occupation of a trap state by a QP alters the inductive contribution of the junction to its host circuit. Thus, the population of QPs trapped in the junction can be directly measured by probing the resonance of a capacitively shunted nanobridge with standard superconducting qubit hardware. We discuss simulated and experimental single-shot fidelity measurements of QPs trapping in the junctions of a nanobridge SQUID embedded in a superconducting resonator. We further discuss the use of our device as a tool for measuring QP properties and for testing methods of QP mitigation. |
Thursday, March 18, 2021 4:48PM - 5:00PM Live |
V30.00010: Quasiparticle Transport in Asymmetric Josephson Junctions Zachary Steffen, Kungang Li, Sudeep Dutta, Yizhou Huang, Benjamin Palmer, Frederick C Wellstood In superconducting qubits, non-equilibrium quasiparticles may be a significant source of dissipation. Investigating the behavior of quasiparticles and the loss they produce in tunnelling may lead to improved coherence times. Asymmetric transmons, with different superconducting gaps on the left and right sides of the junction, have been fabricated to improve T1 by reducing tunneling from non-equilibrium quasiparticles. We discuss low-temperature (T < 50 mK) current - voltage and differential conductance measurements of these asymmetric devices to characterize the behavior of the junctions. We observe distinct sub-gap current features that we attribute to multiple-Andreev reflections and estimate the junction transparency and loss of transmons made from these junctions. |
Thursday, March 18, 2021 5:00PM - 5:12PM Live |
V30.00011: Supercurrent in All-Van-der-Waals Josephson Tunnel Junctions Qing Li, Joel I-Jan Wang, Megan Yamoah, Kenji Watanabe, Takashi Taniguchi, Terry Philip Orlando, Simon Gustavsson, Pablo Jarillo-Herrero, William Oliver Superconducting quantum circuits have demonstrated tremendous progress over the past decade. However, current state-of-art of superconducting qubits still suffer strongly from dielectric two-level-systems (TLS), a major cause of qubit decoherence. Further advancement of qubit lifetime ultimately requires material and fabrication improvements that reduce TLS loss. |
Thursday, March 18, 2021 5:12PM - 5:24PM Live |
V30.00012: Which-path experiments on a transmon quantum processor Pedro Cruz, Joaquín Fernández-Rossier We propose quantum circuits to test the complementarity principle through which-path experiments on symmetric two-way interferometers coupled to a single detector. First, we consider the bipartite setup in which the controlled transfer of quantum information to a detector subsystem depletes interference on the probed subspace, testing the trade-off between which-path information and interference visibility. Next, we consider the quantum eraser setup, in which properly reading-out the which-path information recovers an interference pattern. Finally, we construct an N-qubit two-way interferometer to explore the quantum-to-classical transition. We start by conceiving abstract quantum circuits and discuss the problem in the quantum computing language. Then, we perform these experiments in the transmon superconducting quantum processors by IBM Q. A detailed analysis of the results is provided. Despite finding good agreement with theory, we also observe slight systematic deviations shown to be due to cross-talking. |
Thursday, March 18, 2021 5:24PM - 5:36PM Live |
V30.00013: Investigation of geometrical effects on merged-element transmon. Sungoh Park, RUICHEN ZHAO, Corey Rae H McRae, Anthony McFadden, Mustafa Bal, Tongyu Zhao, Joel Howard, Junling Long, David Pappas
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