Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session A41: New Materials for Superconducting QubitsFocus Recordings Available
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Sponsoring Units: DQI DMP DCMP Chair: Andrew Cleland, University of Chicago Room: McCormick Place W-196C |
Monday, March 14, 2022 8:00AM - 8:12AM |
A41.00001: Impedance Enhanced Nonlinearity in Titanium Nitride Quantum Circuits Alexander V Anferov, Kan-Heng Lee, Jacob M Miller, David Schuster The high kinetic inductance offered by titanium nitride has seen recent attention for use in linear inductors for superconducting qubits, parametric amplifiers, superconducting detectors and for strong coupling in hybrid systems. While typical applications focus on ultra-high impedance devices exceeding the resistance quantum, we explore the opposite end of the spectrum in search of a nonlinear system with higher operating temperature limits and power limits than a conventional Josephson Junction. Utilizing fractal geometries together with nanowire inductors, we combine reduced critical currents with increased zero point current fluctuations to enhance the strength of the intrinsic Kerr nonlinearity. We demonstrate self-Kerr strengths exceeding 1% of the device line-width, realizing a novel path to an interesting parameter space for quantum devices. |
Monday, March 14, 2022 8:12AM - 8:24AM |
A41.00002: Miniaturizing transmon qubits using van der Waals materials Abhinandan Antony, Martin V Gustafsson, Guilhem J Ribeill, Matthew E Ware, Anjaly Rajendran, Luke C Govia, Thomas Ohki, Takashi Taniguchi, Kenji Watanabe, James C Hone, Kin Chung Fong Quantum computers can potentially achieve an exponential speedup versus classical computers on certain computational tasks, as recently demonstrated in systems of superconducting qubits. However, these qubits have large footprints due to their large capacitor electrodes needed to suppress losses by avoiding dielectric materials. This tactic hinders scaling by increasing parasitic coupling among circuit components, degrading individual qubit addressability, and limiting the spatial density of qubits. Here, we take advantage of the unique properties of the van der Waals (vdW) materials to reduce the qubit area by a factor of > 1000 while preserving the required capacitance without increasing substantial loss. Our qubits combine conventional aluminum-based Josephson junctions with parallel-plate capacitors composed of crystalline layers of superconducting niobium diselenide (NbSe2) and insulating hexagonal-boron nitride (hBN). We measure a vdW transmon T1 relaxation time of 1.06 µs, which demonstrates a path to achieve high-qubit-density quantum processors with long coherence times, and illustrates the broad utility of layered heterostructures in low-loss, highcoherence quantum devices. |
Monday, March 14, 2022 8:24AM - 8:36AM |
A41.00003: Hexagonal Boron Nitride (hBN) as a Low-loss Dielectric for Superconducting Quantum Circuits and Qubits Joel I Wang, Megan Yamoah, Qing Li, Amir H Karamlou, Thao H Dinh, Bharath Kannan, Jochen Braumueller, Sarah E Muschinske, David K Kim, Alexander Melville, Bethany M Niedzielski, Kyle Serniak, Youngkyu Sung, Roni Winik, Jonilyn L Yoder, Mollie E Schwartz, Kenji Watanabe, Takashi Taniguchi, Terry P Orlando, Simon Gustavsson, Pablo Jarillo-Herrero, William D Oliver Dielectrics with low loss at microwave frequencies are imperative for high-coherence solid-state quantum computing platforms. We study the dielectric loss of hexagonal boron nitride (hBN) thin films in the microwave regime by measuring the quality factor of parallel-plate capacitors (PPCs) made of NbSe2-hBN-NbSe2 heterostructures integrated into superconducting circuits. The extracted microwave loss tangent of hBN is bounded to be at most in the mid-10-6 range in the low temperature, single-photon regime. We integrate hBN PPCs with aluminum Josephson junctions to realize transmon qubits with coherence times reaching 25 μs, consistent with the hBN loss tangent inferred from resonator measurements. The hBN PPC reduces the qubit feature size by approximately two orders of magnitude compared to conventional all-aluminum coplanar transmons. Our results establish hBN as a promising dielectric for building high-coherence quantum circuits with substantially reduced footprint and, with high energy participation that helps to reduce unwanted qubit cross-talk. |
Monday, March 14, 2022 8:36AM - 9:12AM |
A41.00004: Granular Aluminum: a superconducting material with amenable nonlinearity for quantum circuits Invited Speaker: Ioan-Mihai Pop The electrodynamics of granular Aluminum (grAl) in the microwave domain can be modeled using an effective Josephson junction array, which exhibits intrinsically high kinetic inductance, amenable nonlinearity [1,2] and relatively low dissipation [3]. These atributes recommend grAl for various applications in quantum technology, including kinetic inductance detectors, parametric amplifiers and quantum bits. One illustration of grAl's utility in quantum circuit design is the remarkable resilience of grAl fluxonium qubits [4] to photons populating the readout resonator. This resilience allows single shot QND measurements [5] and quantum state preparation via active feedback with fidelities exceeding 90% even without using a parametric amplifier [6]. An outstanding challenge is the mitigation of non-equilibrium quasiparticles, and in particular the abatement of quasiparticle bursts due to ionizing radiation interactions with the device substrate [7]. |
Monday, March 14, 2022 9:12AM - 9:24AM |
A41.00005: A quantum Szilard engine for two-level systems coupled to a qubit Martin Spiecker, Niv Drucker, Patrick Paluch, Daria Gusenkova, Francesco Valenti, Gianluigi Catelani, Patrick Winkel, Dennis Rieger, Nicolas Gosling, Simon Günzler, Ivan Takmakov, Richard Gebauer, Oliver Sander, Alexey V Ustinov, Wolfgang Wernsdorfer, Yonatan Cohen, Ioan-Mihai Pop The innate complexity of solid state physics exposes superconducting quantum circuits to interactions with uncontrolled degrees of freedom degrading their coherence. By using a simple stabilizer code we show that a superconducting fluxonium qubit is coupled to a two-level system (TLS) environment of unknown origin, with a relatively long energy relaxation time exceeding 50 ms. Implementing the quantum Szilard engine, the active feedback control loop allows us to decide whether the qubit heats or cools its mesoscopic TLS environment. The TLSs can either be heated up to a population inversion of ~80%, that in turn inverts the qubit population during free evolution, or they can be cooled down below the 20 mK base temperature of the dilution refrigerator. We show that the TLSs and the qubit are each other's dominant loss mechanism. Understanding and mitigating TLS environments is not only crucial to improving qubit lifetimes, but also to avoiding non-Markovian qubit dynamics. |
Monday, March 14, 2022 9:24AM - 9:36AM |
A41.00006: Growth of PAMBE grown TiN//AlN//TiN Josephson junctions Austin M Thomas, Alan Kramer, Nicholas Grabon, Christopher J Richardson In the pursuit of epitaxially grown Josephson Junction (JJ) creation, this work investigates the growth of TiN//AlN//TiN tri-layer heterostructures. This material stack offers several benefits over traditional Al//AlOx//Al JJs. These include enhanced mechanical stability, potentially reduced two level system loss (TLS) due to the use a crystalline AlN barrier, and higher superconducting critical temperature, which may reduce the number of thermally excited quasiparticles. This work will be presented in two parts: materials development and growth of the tri-layer structure, and a separate presentation covering the device fabrication and electrical characteristics of the finished device. |
Monday, March 14, 2022 9:36AM - 9:48AM |
A41.00007: Characterization of PAMBE grown TiN/AlN/TiN Josephson junctions Nicholas Grabon, Austin M Thomas, Alan Kramer, Christopher J Richardson In the pursuit of epitaxially grown Josephson Junction (JJ) creation, this work investigates the growth of TiN//AlN//TiN tri-layer heterostructures. This material stack offers several benefits over traditional Al//AlOx//Al JJs. These include enhanced mechanical stability, potentially reduced two level system loss (TLS) due to the use a crystalline AlN barrier, and heightened superconducting critical temperature, which may reduce the number of thermally excited quasiparticles. This work will be presented in two parts: here, the device fabrication and electrical characteristics of JJ devices are presented, while materials development and growth of the tri-layer structure were covered before in a separate presentation. |
Monday, March 14, 2022 9:48AM - 10:00AM |
A41.00008: Exploring Losses in Tantalum Transmons Alexander P Place, AVEEK DUTTA, Kevin D Crowley, Youqi Gang, Xuan Hoang Le, Nishaad P Khedkar, Cosmin Andrei, Nathalie P de Leon, Andrew A Houck In this talk we explore the dominant loss mechanisms in state-of-the-art tantalum transmons. We iteratively modify our fabrication procedure and examine the effect on qubit coherence through temperature dependent depolarization curves and spectroscopic pulse sequences designed to saturate two-level-systems. This allows us to explore the interaction strength and corresponding loss rate of the spurious fluctuators. In order to quantify the relative strengths of quasiparticle, two-level-system, and other loss mechanisms we measure the temperature dependence of transmons in different geometric and frequency regimes. |
Monday, March 14, 2022 10:00AM - 10:12AM |
A41.00009: Tantalum on sapphire and silicon substrates for superconducting quantum circuits Valentino Seferai, Paul G Baity, Joao Barbosa, Jack Brennan, Jonathan A Collins, Sergey Danilin, Hua Feng, Paniz Foshat, Cong Fu, Wridhdhisom Karar, Ciaran Lenon, Nicholas Nugent, Jharna Paul, Alessandro Casaburi, Kaveh Delfanazari, Robert Hadfield, Martin P Weides Materials science of superconducting circuits is considered with increasing importance, particularly as it directly affects qubit coherence. Appropriate nanofabrication and film growth techniques need to be developed to incorporate quality-factor engineered components. One emerging structure for superconductor ground planes and feedlines is tantalum (Ta) on a sapphire substrate, for which high coherence times were achieved for transmon qubits. The oxide formation and stoichiometry of α-phase Ta films leads to fewer sources of noise for the qubit to incoherently exchange energy with. In this presentation, we demonstrate growth techniques for deposition of Ta on heated sapphire substrates, and deposition of Ta on Si substrates using a Nb seed layer. We will also present different recipes that were used to dry etch Ta films into resonator structures, and discuss the extracted internal quality factors from these film. We discuss our investigations into fabricating Ta resonators on Si at room temperature which opens up a way to fabricate highly coherent circuits on systems without heating capabilities, and avoids thermally induced diffusion of pre-deposited materials. Finally, we detail the different dry etch chemistries that can be used and which one we have found to be optimal. |
Monday, March 14, 2022 10:12AM - 10:24AM |
A41.00010: Mitigation of dielectric loss in aluminium-based superconducting transmon qubits Janka Biznarova, Marcus Rommel, Amr Osman, Andreas Nylander, Christopher W Warren, Sandoko Kosen, Anita Fadavi Roudsari, Jonas Bylander Recent months have seen a jump in qubit coherence as a result of new materials exploration, surpassing the values that seemed to have reached a plateau in the traditional aluminium-based devices. |
Monday, March 14, 2022 10:24AM - 10:36AM |
A41.00011: High kinetic inductance resonators in the strong disorder limit Thibault Charpentier Strongly disordered superconductors are a promising playground for the fabrication of high kinetic inductance circuits and qubits. Upon increase of disorder one is able to achieve considerably large inductances with a simple fabrication process. However, when disorder is further increased thin films undergo a transition to insulation, accompanied by puzzling other features still debated to this day. It is therefore mandatory to have a better understanding of superconductivity in highly disordered films in view of application prospects. In this talk I discuss how strong disorder deeply modifies the superconducting state. By measuring the kinetic inductance of superconducting resonators made of amorphous indium oxide at very low temperature, we show that the transition to superconductivity is bosonic with a critical temperature ruled by superconducting phase fluctuations. |
Monday, March 14, 2022 10:36AM - 10:48AM |
A41.00012: Improved Flux QubitsBased onTantalum Andres E Lombo, Noah Janzen, Rui Yang, Adrian Lupascu Recently, there have been considerable efforts to improve the design and fabrication of superconducting qubits with a focus on material platforms. The use of tantalum on sapphire has led to increased relaxation and coherence times of qubits [1]. However, to realize scalable qubits compatible with industrial fabrication processes, the quality of materials and interfaces on Si substrates must be improved. We present our results on a new process for superconducting circuits using superconducting Ta films on Si substrates. The growth of Ta on Si is characterized via XPS, XRD and ToF-SIMS analysis. We then apply microscopy methods on the film to characterize the microstructure at designed surfaces and interfaces. Subsequently, we outline our process for the fabrication of superconducting qubits using Ta on Si with special attention to interfaces, as well as roughness of designed features on the chip. We present our work on several types of designs, including resonators and flux qubits. The study gives new insight into the material engineering of Ta and shows potential for improving coherence times in superconducting qubits. |
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